Fix leaky behavior of `scm_take_TAGvector ()'.

[bpt/guile.git] / libguile / eval.c

/* Copyright (C) 1995,1996,1997,1998,1999,2000,2001,2002,2003,2004,2005,2006,2007,2008,2009
 * Free Software Foundation, Inc.
 * 
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public License
 * as published by the Free Software Foundation; either version 3 of
 * the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301 USA
 */

\f

/* SECTION: This code is compiled once.
 */

#ifdef HAVE_CONFIG_H
#  include <config.h>
#endif

#include <alloca.h>

#include "libguile/__scm.h"

#include <assert.h>
#include "libguile/_scm.h"
#include "libguile/alist.h"
#include "libguile/async.h"
#include "libguile/continuations.h"
#include "libguile/debug.h"
#include "libguile/deprecation.h"
#include "libguile/dynwind.h"
#include "libguile/eq.h"
#include "libguile/feature.h"
#include "libguile/fluids.h"
#include "libguile/futures.h"
#include "libguile/goops.h"
#include "libguile/hash.h"
#include "libguile/hashtab.h"
#include "libguile/lang.h"
#include "libguile/list.h"
#include "libguile/macros.h"
#include "libguile/modules.h"
#include "libguile/objects.h"
#include "libguile/ports.h"
#include "libguile/print.h"
#include "libguile/procprop.h"
#include "libguile/programs.h"
#include "libguile/root.h"
#include "libguile/smob.h"
#include "libguile/srcprop.h"
#include "libguile/stackchk.h"
#include "libguile/strings.h"
#include "libguile/threads.h"
#include "libguile/throw.h"
#include "libguile/validate.h"
#include "libguile/values.h"
#include "libguile/vectors.h"
#include "libguile/vm.h"

#include "libguile/eval.h"
#include "libguile/private-options.h"

\f


static SCM unmemoize_exprs (SCM expr, SCM env);
static SCM canonicalize_define (SCM expr);
static SCM *scm_lookupcar1 (SCM vloc, SCM genv, int check);
static SCM unmemoize_builtin_macro (SCM expr, SCM env);
static void ceval_letrec_inits (SCM env, SCM init_forms, SCM **init_values_eol);
static SCM ceval (SCM x, SCM env);
static SCM deval (SCM x, SCM env);

\f

/* {Syntax Errors}
 *
 * This section defines the message strings for the syntax errors that can be
 * detected during memoization and the functions and macros that shall be
 * called by the memoizer code to signal syntax errors.  */


/* Syntax errors that can be detected during memoization: */

/* Circular or improper lists do not form valid scheme expressions.  If a
 * circular list or an improper list is detected in a place where a scheme
 * expression is expected, a 'Bad expression' error is signalled.  */
static const char s_bad_expression[] = "Bad expression";

/* If a form is detected that holds a different number of expressions than are
 * required in that context, a 'Missing or extra expression' error is
 * signalled.  */
static const char s_expression[] = "Missing or extra expression in";

/* If a form is detected that holds less expressions than are required in that
 * context, a 'Missing expression' error is signalled.  */
static const char s_missing_expression[] = "Missing expression in";

/* If a form is detected that holds more expressions than are allowed in that
 * context, an 'Extra expression' error is signalled.  */
static const char s_extra_expression[] = "Extra expression in";

/* The empty combination '()' is not allowed as an expression in scheme.  If
 * it is detected in a place where an expression is expected, an 'Illegal
 * empty combination' error is signalled.  Note: If you encounter this error
 * message, it is very likely that you intended to denote the empty list.  To
 * do so, you need to quote the empty list like (quote ()) or '().  */
static const char s_empty_combination[] = "Illegal empty combination";

/* A body may hold an arbitrary number of internal defines, followed by a
 * non-empty sequence of expressions.  If a body with an empty sequence of
 * expressions is detected, a 'Missing body expression' error is signalled.
 */
static const char s_missing_body_expression[] = "Missing body expression in";

/* A body may hold an arbitrary number of internal defines, followed by a
 * non-empty sequence of expressions.  Each the definitions and the
 * expressions may be grouped arbitraryly with begin, but it is not allowed to
 * mix definitions and expressions.  If a define form in a body mixes
 * definitions and expressions, a 'Mixed definitions and expressions' error is
 * signalled.  */
static const char s_mixed_body_forms[] = "Mixed definitions and expressions in";
/* Definitions are only allowed on the top level and at the start of a body.
 * If a definition is detected anywhere else, a 'Bad define placement' error
 * is signalled.  */
static const char s_bad_define[] = "Bad define placement";

/* Case or cond expressions must have at least one clause.  If a case or cond
 * expression without any clauses is detected, a 'Missing clauses' error is
 * signalled.  */
static const char s_missing_clauses[] = "Missing clauses";

/* If there is an 'else' clause in a case or a cond statement, it must be the
 * last clause.  If after the 'else' case clause further clauses are detected,
 * a 'Misplaced else clause' error is signalled.  */
static const char s_misplaced_else_clause[] = "Misplaced else clause";

/* If a case clause is detected that is not in the format
 *   (<label(s)> <expression1> <expression2> ...)
 * a 'Bad case clause' error is signalled.  */
static const char s_bad_case_clause[] = "Bad case clause";

/* If a case clause is detected where the <label(s)> element is neither a
 * proper list nor (in case of the last clause) the syntactic keyword 'else',
 * a 'Bad case labels' error is signalled.  Note: If you encounter this error
 * for an else-clause which seems to be syntactically correct, check if 'else'
 * is really a syntactic keyword in that context.  If 'else' is bound in the
 * local or global environment, it is not considered a syntactic keyword, but
 * will be treated as any other variable.  */
static const char s_bad_case_labels[] = "Bad case labels";

/* In a case statement all labels have to be distinct.  If in a case statement
 * a label occurs more than once, a 'Duplicate case label' error is
 * signalled.  */
static const char s_duplicate_case_label[] = "Duplicate case label";

/* If a cond clause is detected that is not in one of the formats
 *   (<test> <expression1> ...) or (else <expression1> <expression2> ...)
 * a 'Bad cond clause' error is signalled.  */
static const char s_bad_cond_clause[] = "Bad cond clause";

/* If a cond clause is detected that uses the alternate '=>' form, but does
 * not hold a recipient element for the test result, a 'Missing recipient'
 * error is signalled.  */
static const char s_missing_recipient[] = "Missing recipient in";

/* If in a position where a variable name is required some other object is
 * detected, a 'Bad variable' error is signalled.  */
static const char s_bad_variable[] = "Bad variable";

/* Bindings for forms like 'let' and 'do' have to be given in a proper,
 * possibly empty list.  If any other object is detected in a place where a
 * list of bindings was required, a 'Bad bindings' error is signalled.  */
static const char s_bad_bindings[] = "Bad bindings";

/* Depending on the syntactic context, a binding has to be in the format
 * (<variable> <expression>) or (<variable> <expression1> <expression2>).
 * If anything else is detected in a place where a binding was expected, a
 * 'Bad binding' error is signalled.  */
static const char s_bad_binding[] = "Bad binding";

/* Some syntactic forms don't allow variable names to appear more than once in
 * a list of bindings.  If such a situation is nevertheless detected, a
 * 'Duplicate binding' error is signalled.  */
static const char s_duplicate_binding[] = "Duplicate binding";

/* If the exit form of a 'do' expression is not in the format
 *   (<test> <expression> ...)
 * a 'Bad exit clause' error is signalled.  */
static const char s_bad_exit_clause[] = "Bad exit clause";

/* The formal function arguments of a lambda expression have to be either a
 * single symbol or a non-cyclic list.  For anything else a 'Bad formals'
 * error is signalled.  */
static const char s_bad_formals[] = "Bad formals";

/* If in a lambda expression something else than a symbol is detected at a
 * place where a formal function argument is required, a 'Bad formal' error is
 * signalled.  */
static const char s_bad_formal[] = "Bad formal";

/* If in the arguments list of a lambda expression an argument name occurs
 * more than once, a 'Duplicate formal' error is signalled.  */
static const char s_duplicate_formal[] = "Duplicate formal";

/* If the evaluation of an unquote-splicing expression gives something else
 * than a proper list, a 'Non-list result for unquote-splicing' error is
 * signalled.  */
static const char s_splicing[] = "Non-list result for unquote-splicing";

/* If something else than an exact integer is detected as the argument for
 * @slot-ref and @slot-set!, a 'Bad slot number' error is signalled.  */
static const char s_bad_slot_number[] = "Bad slot number";


/* Signal a syntax error.  We distinguish between the form that caused the
 * error and the enclosing expression.  The error message will print out as
 * shown in the following pattern.  The file name and line number are only
 * given when they can be determined from the erroneous form or from the
 * enclosing expression.
 *
 * <filename>: In procedure memoization:
 * <filename>: In file <name>, line <nr>: <error-message> in <expression>.  */

SCM_SYMBOL (syntax_error_key, "syntax-error");

/* The prototype is needed to indicate that the function does not return.  */
static void
syntax_error (const char* const, const SCM, const SCM) SCM_NORETURN;

static void 
syntax_error (const char* const msg, const SCM form, const SCM expr)
{
  SCM msg_string = scm_from_locale_string (msg);
  SCM filename = SCM_BOOL_F;
  SCM linenr = SCM_BOOL_F;
  const char *format;
  SCM args;

  if (scm_is_pair (form))
    {
      filename = scm_source_property (form, scm_sym_filename);
      linenr = scm_source_property (form, scm_sym_line);
    }

  if (scm_is_false (filename) && scm_is_false (linenr) && scm_is_pair (expr))
    {
      filename = scm_source_property (expr, scm_sym_filename);
      linenr = scm_source_property (expr, scm_sym_line);
    }

  if (!SCM_UNBNDP (expr))
    {
      if (scm_is_true (filename))
        {
          format = "In file ~S, line ~S: ~A ~S in expression ~S.";
          args = scm_list_5 (filename, linenr, msg_string, form, expr);
        }
      else if (scm_is_true (linenr))
        {
          format = "In line ~S: ~A ~S in expression ~S.";
          args = scm_list_4 (linenr, msg_string, form, expr);
        }
      else
        {
          format = "~A ~S in expression ~S.";
          args = scm_list_3 (msg_string, form, expr);
        }
    }
  else
    {
      if (scm_is_true (filename))
        {
          format = "In file ~S, line ~S: ~A ~S.";
          args = scm_list_4 (filename, linenr, msg_string, form);
        }
      else if (scm_is_true (linenr))
        {
          format = "In line ~S: ~A ~S.";
          args = scm_list_3 (linenr, msg_string, form);
        }
      else
        {
          format = "~A ~S.";
          args = scm_list_2 (msg_string, form);
        }
    }

  scm_error (syntax_error_key, "memoization", format, args, SCM_BOOL_F);
}


/* Shortcut macros to simplify syntax error handling. */
#define ASSERT_SYNTAX(cond, message, form)              \
  { if (SCM_UNLIKELY (!(cond)))                 \
      syntax_error (message, form, SCM_UNDEFINED); }
#define ASSERT_SYNTAX_2(cond, message, form, expr)      \
  { if (SCM_UNLIKELY (!(cond)))                 \
      syntax_error (message, form, expr); }

static void error_unbound_variable (SCM symbol) SCM_NORETURN;
static void error_defined_variable (SCM symbol) SCM_NORETURN;

\f

/* {Ilocs}
 *
 * Ilocs are memoized references to variables in local environment frames.
 * They are represented as three values:  The relative offset of the
 * environment frame, the number of the binding within that frame, and a
 * boolean value indicating whether the binding is the last binding in the
 * frame.
 *
 * Frame numbers have 11 bits, relative offsets have 12 bits.
 */

#define SCM_ILOC00              SCM_MAKE_ITAG8(0L, scm_tc8_iloc)
#define SCM_IFRINC              (0x00000100L)
#define SCM_ICDR                (0x00080000L)
#define SCM_IDINC               (0x00100000L)
#define SCM_IFRAME(n)           ((long)((SCM_ICDR-SCM_IFRINC)>>8) \
                                 & (SCM_UNPACK (n) >> 8))
#define SCM_IDIST(n)            (SCM_UNPACK (n) >> 20)
#define SCM_ICDRP(n)            (SCM_ICDR & SCM_UNPACK (n))
#define SCM_IDSTMSK             (-SCM_IDINC)
#define SCM_IFRAMEMAX           ((1<<11)-1)
#define SCM_IDISTMAX            ((1<<12)-1)
#define SCM_MAKE_ILOC(frame_nr, binding_nr, last_p) \
  SCM_PACK ( \
    ((frame_nr) << 8) \
    + ((binding_nr) << 20) \
    + ((last_p) ? SCM_ICDR : 0) \
    + scm_tc8_iloc )

void
scm_i_print_iloc (SCM iloc, SCM port)
{
  scm_puts ("#@", port);
  scm_intprint ((long) SCM_IFRAME (iloc), 10, port);
  scm_putc (SCM_ICDRP (iloc) ? '-' : '+', port);
  scm_intprint ((long) SCM_IDIST (iloc), 10, port);
}

#if (SCM_DEBUG_DEBUGGING_SUPPORT == 1)

SCM scm_dbg_make_iloc (SCM frame, SCM binding, SCM cdrp);

SCM_DEFINE (scm_dbg_make_iloc, "dbg-make-iloc", 3, 0, 0,
            (SCM frame, SCM binding, SCM cdrp),
            "Return a new iloc with frame offset @var{frame}, binding\n"
            "offset @var{binding} and the cdr flag @var{cdrp}.")
#define FUNC_NAME s_scm_dbg_make_iloc
{
  return SCM_MAKE_ILOC ((scm_t_bits) scm_to_unsigned_integer (frame, 0, SCM_IFRAMEMAX),
                        (scm_t_bits) scm_to_unsigned_integer (binding, 0, SCM_IDISTMAX),
                        scm_is_true (cdrp));
}
#undef FUNC_NAME

SCM scm_dbg_iloc_p (SCM obj);

SCM_DEFINE (scm_dbg_iloc_p, "dbg-iloc?", 1, 0, 0, 
          (SCM obj),
            "Return @code{#t} if @var{obj} is an iloc.")
#define FUNC_NAME s_scm_dbg_iloc_p
{
  return scm_from_bool (SCM_ILOCP (obj));
}
#undef FUNC_NAME

#endif

\f

/* {Evaluator byte codes (isyms)}
 */

#define ISYMNUM(n)              (SCM_ITAG8_DATA (n))

/* This table must agree with the list of SCM_IM_ constants in tags.h */
static const char *const isymnames[] =
{
  "#@and",
  "#@begin",
  "#@case",
  "#@cond",
  "#@do",
  "#@if",
  "#@lambda",
  "#@let",
  "#@let*",
  "#@letrec",
  "#@or",
  "#@quote",
  "#@set!",
  "#@define",
  "#@apply",
  "#@call-with-current-continuation",
  "#@dispatch",
  "#@slot-ref",
  "#@slot-set!",
  "#@delay",
  "#@future",
  "#@call-with-values",
  "#@else",
  "#@arrow",
  "#@nil-cond",
  "#@bind"
};

void
scm_i_print_isym (SCM isym, SCM port)
{
  const size_t isymnum = ISYMNUM (isym);
  if (isymnum < (sizeof isymnames / sizeof (char *)))
    scm_puts (isymnames[isymnum], port);
  else
    scm_ipruk ("isym", isym, port);
}

\f

/* The function lookup_symbol is used during memoization: Lookup the symbol in
 * the environment.  If there is no binding for the symbol, SCM_UNDEFINED is
 * returned.  If the symbol is a global variable, the variable object to which
 * the symbol is bound is returned.  Finally, if the symbol is a local
 * variable the corresponding iloc object is returned.  */

/* A helper function for lookup_symbol: Try to find the symbol in the top
 * level environment frame.  The function returns SCM_UNDEFINED if the symbol
 * is unbound and it returns a variable object if the symbol is a global
 * variable.  */
static SCM
lookup_global_symbol (const SCM symbol, const SCM top_level)
{
  const SCM variable = scm_sym2var (symbol, top_level, SCM_BOOL_F);
  if (scm_is_false (variable))
    return SCM_UNDEFINED;
  else
    return variable;
}

static SCM
lookup_symbol (const SCM symbol, const SCM env)
{
  SCM frame_idx;
  unsigned int frame_nr;

  for (frame_idx = env, frame_nr = 0;
       !scm_is_null (frame_idx);
       frame_idx = SCM_CDR (frame_idx), ++frame_nr)
    {
      const SCM frame = SCM_CAR (frame_idx);
      if (scm_is_pair (frame))
        {
          /* frame holds a local environment frame */
          SCM symbol_idx;
          unsigned int symbol_nr;

          for (symbol_idx = SCM_CAR (frame), symbol_nr = 0;
               scm_is_pair (symbol_idx);
               symbol_idx = SCM_CDR (symbol_idx), ++symbol_nr)
            {
              if (scm_is_eq (SCM_CAR (symbol_idx), symbol))
                /* found the symbol, therefore return the iloc */
                return SCM_MAKE_ILOC (frame_nr, symbol_nr, 0);
            }
          if (scm_is_eq (symbol_idx, symbol))
            /* found the symbol as the last element of the current frame */
            return SCM_MAKE_ILOC (frame_nr, symbol_nr, 1);
        }
      else
        {
          /* no more local environment frames */
          return lookup_global_symbol (symbol, frame);
        }
    }

  return lookup_global_symbol (symbol, SCM_BOOL_F);
}


/* Return true if the symbol is - from the point of view of a macro
 * transformer - a literal in the sense specified in chapter "pattern
 * language" of R5RS.  In the code below, however, we don't match the
 * definition of R5RS exactly:  It returns true if the identifier has no
 * binding or if it is a syntactic keyword.  */
static int
literal_p (const SCM symbol, const SCM env)
{
  const SCM variable = lookup_symbol (symbol, env);
  if (SCM_UNBNDP (variable))
    return 1;
  if (SCM_VARIABLEP (variable) && SCM_MACROP (SCM_VARIABLE_REF (variable)))
    return 1;
  else
    return 0;
}


/* Return true if the expression is self-quoting in the memoized code.  Thus,
 * some other objects (like e. g. vectors) are reported as self-quoting, which
 * according to R5RS would need to be quoted.  */
static int
is_self_quoting_p (const SCM expr)
{
  if (scm_is_pair (expr))
    return 0;
  else if (scm_is_symbol (expr))
    return 0;
  else if (scm_is_null (expr))
    return 0;
  else return 1;
}


SCM_SYMBOL (sym_three_question_marks, "???");

static SCM
unmemoize_expression (const SCM expr, const SCM env)
{
  if (SCM_ILOCP (expr))
    {
      SCM frame_idx;
      unsigned long int frame_nr;
      SCM symbol_idx;
      unsigned long int symbol_nr;

      for (frame_idx = env, frame_nr = SCM_IFRAME (expr);
           frame_nr != 0; 
           frame_idx = SCM_CDR (frame_idx), --frame_nr)
        ;
      for (symbol_idx = SCM_CAAR (frame_idx), symbol_nr = SCM_IDIST (expr);
           symbol_nr != 0;
           symbol_idx = SCM_CDR (symbol_idx), --symbol_nr)
        ;
      return SCM_ICDRP (expr) ? symbol_idx : SCM_CAR (symbol_idx);
    }
  else if (SCM_VARIABLEP (expr))
    {
      const SCM sym = scm_module_reverse_lookup (scm_env_module (env), expr);
      return scm_is_true (sym) ? sym : sym_three_question_marks;
    }
  else if (scm_is_simple_vector (expr))
    {
      return scm_list_2 (scm_sym_quote, expr);
    }
  else if (!scm_is_pair (expr))
    {
      return expr;
    }
  else if (SCM_ISYMP (SCM_CAR (expr)))
    {
      return unmemoize_builtin_macro (expr, env);
    }
  else
    {
      return unmemoize_exprs (expr, env);
    }
}


static SCM
unmemoize_exprs (const SCM exprs, const SCM env)
{
  SCM r_result = SCM_EOL;
  SCM expr_idx = exprs;
  SCM um_expr;

  /* Note that due to the current lazy memoizer we may find partially memoized
   * code during execution.  In such code we have to expect improper lists of
   * expressions: On the one hand, for such code syntax checks have not yet
   * fully been performed, on the other hand, there may be even legal code
   * like '(a . b) appear as an improper list of expressions as long as the
   * quote expression is still in its unmemoized form.  For this reason, the
   * following code handles improper lists of expressions until memoization
   * and execution have been completely separated.  */
  for (; scm_is_pair (expr_idx); expr_idx = SCM_CDR (expr_idx))
    {
      const SCM expr = SCM_CAR (expr_idx);

      /* In partially memoized code, lists of expressions that stem from a
       * body form may start with an ISYM if the body itself has not yet been
       * memoized.  This isym is just an internal marker to indicate that the
       * body still needs to be memoized.  An isym may occur at the very
       * beginning of the body or after one or more comment strings.  It is
       * dropped during unmemoization.  */
      if (!SCM_ISYMP (expr))
        {
          um_expr = unmemoize_expression (expr, env);
          r_result = scm_cons (um_expr, r_result);
        }
    }
  um_expr = unmemoize_expression (expr_idx, env);
  if (!scm_is_null (r_result))
    {
      const SCM result = scm_reverse_x (r_result, SCM_UNDEFINED);
      SCM_SETCDR (r_result, um_expr);
      return result;
    }
  else
    {
      return um_expr;
    }
}


/* Rewrite the body (which is given as the list of expressions forming the
 * body) into its internal form.  The internal form of a body (<expr> ...) is
 * just the body itself, but prefixed with an ISYM that denotes to what kind
 * of outer construct this body belongs: (<ISYM> <expr> ...).  A lambda body
 * starts with SCM_IM_LAMBDA, for example, a body of a let starts with
 * SCM_IM_LET, etc.
 *
 * It is assumed that the calling expression has already made sure that the
 * body is a proper list.  */
static SCM
m_body (SCM op, SCM exprs)
{
  /* Don't add another ISYM if one is present already. */
  if (SCM_ISYMP (SCM_CAR (exprs)))
    return exprs;
  else
    return scm_cons (op, exprs);
}


/* The function m_expand_body memoizes a proper list of expressions forming a
 * body.  This function takes care of dealing with internal defines and
 * transforming them into an equivalent letrec expression.  The list of
 * expressions is rewritten in place.  */ 

/* This is a helper function for m_expand_body.  If the argument expression is
 * a symbol that denotes a syntactic keyword, the corresponding macro object
 * is returned, in all other cases the function returns SCM_UNDEFINED.  */ 
static SCM
try_macro_lookup (const SCM expr, const SCM env)
{
  if (scm_is_symbol (expr))
    {
      const SCM variable = lookup_symbol (expr, env);
      if (SCM_VARIABLEP (variable))
        {
          const SCM value = SCM_VARIABLE_REF (variable);
          if (SCM_MACROP (value))
            return value;
        }
    }

  return SCM_UNDEFINED;
}

/* This is a helper function for m_expand_body.  It expands user macros,
 * because for the correct translation of a body we need to know whether they
 * expand to a definition. */ 
static SCM
expand_user_macros (SCM expr, const SCM env)
{
  while (scm_is_pair (expr))
    {
      const SCM car_expr = SCM_CAR (expr);
      const SCM new_car = expand_user_macros (car_expr, env);
      const SCM value = try_macro_lookup (new_car, env);

      if (SCM_MACROP (value) && SCM_MACRO_TYPE (value) == 2)
        {
          /* User macros transform code into code.  */
          expr = scm_call_2 (SCM_MACRO_CODE (value), expr, env);
          /* We need to reiterate on the transformed code.  */
        }
      else
        {
          /* No user macro: return.  */
          SCM_SETCAR (expr, new_car);
          return expr;
        }
    }

  return expr;
}

/* This is a helper function for m_expand_body.  It determines if a given form
 * represents an application of a given built-in macro.  The built-in macro to
 * check for is identified by its syntactic keyword.  The form is an
 * application of the given macro if looking up the car of the form in the
 * given environment actually returns the built-in macro.  */
static int
is_system_macro_p (const SCM syntactic_keyword, const SCM form, const SCM env)
{
  if (scm_is_pair (form))
    {
      const SCM car_form = SCM_CAR (form);
      const SCM value = try_macro_lookup (car_form, env);
      if (SCM_BUILTIN_MACRO_P (value))
        {
          const SCM macro_name = scm_macro_name (value);
          return scm_is_eq (macro_name, syntactic_keyword);
        }
    }

  return 0;
}

static SCM
macroexp (SCM x, SCM env)
{
  SCM res, proc, orig_sym;

  /* Don't bother to produce error messages here.  We get them when we
     eventually execute the code for real. */

 macro_tail:
  orig_sym = SCM_CAR (x);
  if (!scm_is_symbol (orig_sym))
    return x;

  {
    SCM *proc_ptr = scm_lookupcar1 (x, env, 0);
    if (proc_ptr == NULL)
      {
        /* We have lost the race. */
        goto macro_tail;
      }
    proc = *proc_ptr;
  }
  
  /* Only handle memoizing macros.  `Acros' and `macros' are really
     special forms and should not be evaluated here. */

  if (!SCM_MACROP (proc)
      || (SCM_MACRO_TYPE (proc) != 2 && !SCM_BUILTIN_MACRO_P (proc)))
    return x;

  SCM_SETCAR (x, orig_sym);  /* Undo memoizing effect of lookupcar */
  res = scm_call_2 (SCM_MACRO_CODE (proc), x, env);

  if (scm_ilength (res) <= 0)
    /* Result of expansion is not a list.  */
    return (scm_list_2 (SCM_IM_BEGIN, res));
  else
    {
      /* njrev: Several queries here: (1) I don't see how it can be
         correct that the SCM_SETCAR 2 lines below this comment needs
         protection, but the SCM_SETCAR 6 lines above does not, so
         something here is probably wrong.  (2) macroexp() is now only
         used in one place - scm_m_generalized_set_x - whereas all other
         macro expansion happens through expand_user_macros.  Therefore
         (2.1) perhaps macroexp() could be eliminated completely now?
         (2.2) Does expand_user_macros need any critical section
         protection? */

      SCM_CRITICAL_SECTION_START;
      SCM_SETCAR (x, SCM_CAR (res));
      SCM_SETCDR (x, SCM_CDR (res));
      SCM_CRITICAL_SECTION_END;

      goto macro_tail;
    }
}

\f
/* Start of the memoizers for the standard R5RS builtin macros.  */

static SCM scm_m_quote (SCM xorig, SCM env);
static SCM scm_m_begin (SCM xorig, SCM env);
static SCM scm_m_if (SCM xorig, SCM env);
static SCM scm_m_set_x (SCM xorig, SCM env);
static SCM scm_m_and (SCM xorig, SCM env);
static SCM scm_m_or (SCM xorig, SCM env);
static SCM scm_m_case (SCM xorig, SCM env);
static SCM scm_m_cond (SCM xorig, SCM env);
static SCM scm_m_lambda (SCM xorig, SCM env);
static SCM scm_m_letstar (SCM xorig, SCM env);
static SCM scm_m_do (SCM xorig, SCM env);
static SCM scm_m_quasiquote (SCM xorig, SCM env);
static SCM scm_m_delay (SCM xorig, SCM env);
static SCM scm_m_generalized_set_x (SCM xorig, SCM env);
#if 0  /* Futures are disabled, see "futures.h".  */
static SCM scm_m_future (SCM xorig, SCM env);
#endif
static SCM scm_m_define (SCM x, SCM env);
static SCM scm_m_letrec (SCM xorig, SCM env);
static SCM scm_m_let (SCM xorig, SCM env);
static SCM scm_m_at (SCM xorig, SCM env);
static SCM scm_m_atat (SCM xorig, SCM env);
static SCM scm_m_atslot_ref (SCM xorig, SCM env);
static SCM scm_m_atslot_set_x (SCM xorig, SCM env);
static SCM scm_m_apply (SCM xorig, SCM env);
static SCM scm_m_cont (SCM xorig, SCM env);
#if SCM_ENABLE_ELISP
static SCM scm_m_nil_cond (SCM xorig, SCM env);
static SCM scm_m_atfop (SCM xorig, SCM env);
#endif /* SCM_ENABLE_ELISP */
static SCM scm_m_atbind (SCM xorig, SCM env);
static SCM scm_m_at_call_with_values (SCM xorig, SCM env);
static SCM scm_m_eval_when (SCM xorig, SCM env);


static void
m_expand_body (const SCM forms, const SCM env)
{
  /* The first body form can be skipped since it is known to be the ISYM that
   * was prepended to the body by m_body.  */
  SCM cdr_forms = SCM_CDR (forms);
  SCM form_idx = cdr_forms;
  SCM definitions = SCM_EOL;
  SCM sequence = SCM_EOL;

  /* According to R5RS, the list of body forms consists of two parts: a number
   * (maybe zero) of definitions, followed by a non-empty sequence of
   * expressions.  Each the definitions and the expressions may be grouped
   * arbitrarily with begin, but it is not allowed to mix definitions and
   * expressions.  The task of the following loop therefore is to split the
   * list of body forms into the list of definitions and the sequence of
   * expressions.  */ 
  while (!scm_is_null (form_idx))
    {
      const SCM form = SCM_CAR (form_idx);
      const SCM new_form = expand_user_macros (form, env);
      if (is_system_macro_p (scm_sym_define, new_form, env))
        {
          definitions = scm_cons (new_form, definitions);
          form_idx = SCM_CDR (form_idx);
        }
      else if (is_system_macro_p (scm_sym_begin, new_form, env))
        {
          /* We have encountered a group of forms.  This has to be either a
           * (possibly empty) group of (possibly further grouped) definitions,
           * or a non-empty group of (possibly further grouped)
           * expressions.  */
          const SCM grouped_forms = SCM_CDR (new_form);
          unsigned int found_definition = 0;
          unsigned int found_expression = 0;
          SCM grouped_form_idx = grouped_forms;
          while (!found_expression && !scm_is_null (grouped_form_idx))
            {
              const SCM inner_form = SCM_CAR (grouped_form_idx);
              const SCM new_inner_form = expand_user_macros (inner_form, env);
              if (is_system_macro_p (scm_sym_define, new_inner_form, env))
                {
                  found_definition = 1;
                  definitions = scm_cons (new_inner_form, definitions);
                  grouped_form_idx = SCM_CDR (grouped_form_idx);
                }
              else if (is_system_macro_p (scm_sym_begin, new_inner_form, env))
                {
                  const SCM inner_group = SCM_CDR (new_inner_form);
                  grouped_form_idx
                    = scm_append (scm_list_2 (inner_group,
                                              SCM_CDR (grouped_form_idx)));
                }
              else
                {
                  /* The group marks the start of the expressions of the body.
                   * We have to make sure that within the same group we have
                   * not encountered a definition before.  */
                  ASSERT_SYNTAX (!found_definition, s_mixed_body_forms, form);
                  found_expression = 1;
                  grouped_form_idx = SCM_EOL;
                }
            }

          /* We have finished processing the group.  If we have not yet
           * encountered an expression we continue processing the forms of the
           * body to collect further definition forms.  Otherwise, the group
           * marks the start of the sequence of expressions of the body.  */
          if (!found_expression)
            {
              form_idx = SCM_CDR (form_idx);
            }
          else
            {
              sequence = form_idx;
              form_idx = SCM_EOL;
            }
        }
      else
        {
          /* We have detected a form which is no definition.  This marks the
           * start of the sequence of expressions of the body.  */
          sequence = form_idx;
          form_idx = SCM_EOL;
        }
    }

  /* FIXME: forms does not hold information about the file location.  */
  ASSERT_SYNTAX (scm_is_pair (sequence), s_missing_body_expression, cdr_forms);

  if (!scm_is_null (definitions))
    {
      SCM definition_idx;
      SCM letrec_tail;
      SCM letrec_expression;
      SCM new_letrec_expression;

      SCM bindings = SCM_EOL;
      for (definition_idx = definitions;
           !scm_is_null (definition_idx);
           definition_idx = SCM_CDR (definition_idx))
        {
          const SCM definition = SCM_CAR (definition_idx);
          const SCM canonical_definition = canonicalize_define (definition);
          const SCM binding = SCM_CDR (canonical_definition);
          bindings = scm_cons (binding, bindings);
        };

      letrec_tail = scm_cons (bindings, sequence);
      /* FIXME: forms does not hold information about the file location.  */
      letrec_expression = scm_cons_source (forms, scm_sym_letrec, letrec_tail);
      new_letrec_expression = scm_m_letrec (letrec_expression, env);
      SCM_SETCAR (forms, new_letrec_expression);
      SCM_SETCDR (forms, SCM_EOL);
    }
  else
    {
      SCM_SETCAR (forms, SCM_CAR (sequence));
      SCM_SETCDR (forms, SCM_CDR (sequence));
    }
}

SCM_SYNTAX (s_and, "and", scm_i_makbimacro, scm_m_and);
SCM_GLOBAL_SYMBOL (scm_sym_and, s_and);

static SCM
scm_m_and (SCM expr, SCM env SCM_UNUSED)
{
  const SCM cdr_expr = SCM_CDR (expr);
  const long length = scm_ilength (cdr_expr);

  ASSERT_SYNTAX (length >= 0, s_bad_expression, expr);

  if (length == 0)
    {
      /* Special case:  (and) is replaced by #t. */
      return SCM_BOOL_T;
    }
  else
    {
      SCM_SETCAR (expr, SCM_IM_AND);
      return expr;
    }
}

static SCM
unmemoize_and (const SCM expr, const SCM env)
{
  return scm_cons (scm_sym_and, unmemoize_exprs (SCM_CDR (expr), env));
}


SCM_SYNTAX (s_begin, "begin", scm_i_makbimacro, scm_m_begin);
SCM_GLOBAL_SYMBOL (scm_sym_begin, s_begin);

static SCM
scm_m_begin (SCM expr, SCM env SCM_UNUSED)
{
  const SCM cdr_expr = SCM_CDR (expr);
  /* Dirk:FIXME:: An empty begin clause is not generally allowed by R5RS.
   * That means, there should be a distinction between uses of begin where an
   * empty clause is OK and where it is not.  */
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);

  SCM_SETCAR (expr, SCM_IM_BEGIN);
  return expr;
}

static SCM
unmemoize_begin (const SCM expr, const SCM env)
{
  return scm_cons (scm_sym_begin, unmemoize_exprs (SCM_CDR (expr), env));
}


SCM_SYNTAX (s_case, "case", scm_i_makbimacro, scm_m_case);
SCM_GLOBAL_SYMBOL (scm_sym_case, s_case);
SCM_GLOBAL_SYMBOL (scm_sym_else, "else");

static SCM
scm_m_case (SCM expr, SCM env)
{
  SCM clauses;
  SCM all_labels = SCM_EOL;

  /* Check, whether 'else is a literal, i. e. not bound to a value. */
  const int else_literal_p = literal_p (scm_sym_else, env);

  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 2, s_missing_clauses, expr);

  clauses = SCM_CDR (cdr_expr);
  while (!scm_is_null (clauses))
    {
      SCM labels;

      const SCM clause = SCM_CAR (clauses);
      ASSERT_SYNTAX_2 (scm_ilength (clause) >= 2, 
                       s_bad_case_clause, clause, expr);

      labels = SCM_CAR (clause);
      if (scm_is_pair (labels))
        {
          ASSERT_SYNTAX_2 (scm_ilength (labels) >= 0,
                           s_bad_case_labels, labels, expr);
          all_labels = scm_append (scm_list_2 (labels, all_labels));
        }
      else if (scm_is_null (labels))
        {
          /* The list of labels is empty.  According to R5RS this is allowed.
           * It means that the sequence of expressions will never be executed.
           * Therefore, as an optimization, we could remove the whole
           * clause.  */
        }
      else
        {
          ASSERT_SYNTAX_2 (scm_is_eq (labels, scm_sym_else) && else_literal_p,
                           s_bad_case_labels, labels, expr);
          ASSERT_SYNTAX_2 (scm_is_null (SCM_CDR (clauses)),
                           s_misplaced_else_clause, clause, expr);
        }

      /* build the new clause */
      if (scm_is_eq (labels, scm_sym_else))
        SCM_SETCAR (clause, SCM_IM_ELSE);

      clauses = SCM_CDR (clauses);
    }

  /* Check whether all case labels are distinct. */
  for (; !scm_is_null (all_labels); all_labels = SCM_CDR (all_labels))
    {
      const SCM label = SCM_CAR (all_labels);
      ASSERT_SYNTAX_2 (scm_is_false (scm_c_memq (label, SCM_CDR (all_labels))),
                       s_duplicate_case_label, label, expr);
    }

  SCM_SETCAR (expr, SCM_IM_CASE);
  return expr;
}

static SCM
unmemoize_case (const SCM expr, const SCM env)
{
  const SCM um_key_expr = unmemoize_expression (SCM_CADR (expr), env);
  SCM um_clauses = SCM_EOL;
  SCM clause_idx;

  for (clause_idx = SCM_CDDR (expr);
       !scm_is_null (clause_idx);
       clause_idx = SCM_CDR (clause_idx))
    {
      const SCM clause = SCM_CAR (clause_idx);
      const SCM labels = SCM_CAR (clause);
      const SCM exprs = SCM_CDR (clause);

      const SCM um_exprs = unmemoize_exprs (exprs, env);
      const SCM um_labels = (scm_is_eq (labels, SCM_IM_ELSE))
        ? scm_sym_else
        : scm_i_finite_list_copy (labels);
      const SCM um_clause = scm_cons (um_labels, um_exprs);

      um_clauses = scm_cons (um_clause, um_clauses);
    }
  um_clauses = scm_reverse_x (um_clauses, SCM_UNDEFINED);

  return scm_cons2 (scm_sym_case, um_key_expr, um_clauses);
}


SCM_SYNTAX (s_cond, "cond", scm_i_makbimacro, scm_m_cond);
SCM_GLOBAL_SYMBOL (scm_sym_cond, s_cond);
SCM_GLOBAL_SYMBOL (scm_sym_arrow, "=>");

static SCM
scm_m_cond (SCM expr, SCM env)
{
  /* Check, whether 'else or '=> is a literal, i. e. not bound to a value. */
  const int else_literal_p = literal_p (scm_sym_else, env);
  const int arrow_literal_p = literal_p (scm_sym_arrow, env);

  const SCM clauses = SCM_CDR (expr);
  SCM clause_idx;

  ASSERT_SYNTAX (scm_ilength (clauses) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (clauses) >= 1, s_missing_clauses, expr);

  for (clause_idx = clauses;
       !scm_is_null (clause_idx);
       clause_idx = SCM_CDR (clause_idx))
    {
      SCM test;

      const SCM clause = SCM_CAR (clause_idx);
      const long length = scm_ilength (clause);
      ASSERT_SYNTAX_2 (length >= 1, s_bad_cond_clause, clause, expr);

      test = SCM_CAR (clause);
      if (scm_is_eq (test, scm_sym_else) && else_literal_p)
        {
          const int last_clause_p = scm_is_null (SCM_CDR (clause_idx));
          ASSERT_SYNTAX_2 (length >= 2,
                           s_bad_cond_clause, clause, expr);
          ASSERT_SYNTAX_2 (last_clause_p,
                           s_misplaced_else_clause, clause, expr);
          SCM_SETCAR (clause, SCM_IM_ELSE);
        }
      else if (length >= 2
               && scm_is_eq (SCM_CADR (clause), scm_sym_arrow)
               && arrow_literal_p)
        {
          ASSERT_SYNTAX_2 (length > 2, s_missing_recipient, clause, expr);
          ASSERT_SYNTAX_2 (length == 3, s_extra_expression, clause, expr);
          SCM_SETCAR (SCM_CDR (clause), SCM_IM_ARROW);
        }
      /* SRFI 61 extended cond */
      else if (length >= 3
               && scm_is_eq (SCM_CADDR (clause), scm_sym_arrow)
               && arrow_literal_p)
        {
          ASSERT_SYNTAX_2 (length > 3, s_missing_recipient, clause, expr);
          ASSERT_SYNTAX_2 (length == 4, s_extra_expression, clause, expr);
          SCM_SETCAR (SCM_CDDR (clause), SCM_IM_ARROW);
        }
    }

  SCM_SETCAR (expr, SCM_IM_COND);
  return expr;
}

static SCM
unmemoize_cond (const SCM expr, const SCM env)
{
  SCM um_clauses = SCM_EOL;
  SCM clause_idx;

  for (clause_idx = SCM_CDR (expr);
       !scm_is_null (clause_idx);
       clause_idx = SCM_CDR (clause_idx))
    {
      const SCM clause = SCM_CAR (clause_idx);
      const SCM sequence = SCM_CDR (clause);
      const SCM test = SCM_CAR (clause);
      SCM um_test;
      SCM um_sequence;
      SCM um_clause;

      if (scm_is_eq (test, SCM_IM_ELSE))
        um_test = scm_sym_else;
      else
        um_test = unmemoize_expression (test, env);

      if (!scm_is_null (sequence) && scm_is_eq (SCM_CAR (sequence),
                                              SCM_IM_ARROW))
        {
          const SCM target = SCM_CADR (sequence);
          const SCM um_target = unmemoize_expression (target, env);
          um_sequence = scm_list_2 (scm_sym_arrow, um_target);
        }
      else
        {
          um_sequence = unmemoize_exprs (sequence, env);
        }

      um_clause = scm_cons (um_test, um_sequence);
      um_clauses = scm_cons (um_clause, um_clauses);
    }
  um_clauses = scm_reverse_x (um_clauses, SCM_UNDEFINED);

  return scm_cons (scm_sym_cond, um_clauses);
}


SCM_SYNTAX (s_define, "define", scm_i_makbimacro, scm_m_define);
SCM_GLOBAL_SYMBOL (scm_sym_define, s_define);

/* Guile provides an extension to R5RS' define syntax to represent function
 * currying in a compact way.  With this extension, it is allowed to write
 * (define <nested-variable> <body>), where <nested-variable> has of one of
 * the forms (<nested-variable> <formals>), (<nested-variable> . <formal>),  
 * (<variable> <formals>) or (<variable> . <formal>).  As in R5RS, <formals>
 * should be either a sequence of zero or more variables, or a sequence of one
 * or more variables followed by a space-delimited period and another
 * variable.  Each level of argument nesting wraps the <body> within another
 * lambda expression.  For example, the following forms are allowed, each one
 * followed by an equivalent, more explicit implementation.
 * Example 1:
 *   (define ((a b . c) . d) <body>)  is equivalent to
 *   (define a (lambda (b . c) (lambda d <body>)))
 * Example 2:
 *   (define (((a) b) c . d) <body>)  is equivalent to
 *   (define a (lambda () (lambda (b) (lambda (c . d) <body>))))
 */
/* Dirk:FIXME:: We should provide an implementation for 'define' in the R5RS
 * module that does not implement this extension.  */
static SCM
canonicalize_define (const SCM expr)
{
  SCM body;
  SCM variable;

  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 2, s_missing_expression, expr);

  body = SCM_CDR (cdr_expr);
  variable = SCM_CAR (cdr_expr);
  while (scm_is_pair (variable))
    {
      /* This while loop realizes function currying by variable nesting.
       * Variable is known to be a nested-variable.  In every iteration of the
       * loop another level of lambda expression is created, starting with the
       * innermost one.  Note that we don't check for duplicate formals here:
       * This will be done by the memoizer of the lambda expression.  */
      const SCM formals = SCM_CDR (variable);
      const SCM tail = scm_cons (formals, body);

      /* Add source properties to each new lambda expression:  */
      const SCM lambda = scm_cons_source (variable, scm_sym_lambda, tail);

      body = scm_list_1 (lambda);
      variable = SCM_CAR (variable);
    }
  ASSERT_SYNTAX_2 (scm_is_symbol (variable), s_bad_variable, variable, expr);
  ASSERT_SYNTAX (scm_ilength (body) == 1, s_expression, expr);

  SCM_SETCAR (cdr_expr, variable);
  SCM_SETCDR (cdr_expr, body);
  return expr;
}

/* According to Section 5.2.1 of R5RS we first have to make sure that the
   variable is bound, and then perform the `(set! variable expression)'
   operation.  However, EXPRESSION _can_ be evaluated before VARIABLE is
   bound.  This means that EXPRESSION won't necessarily be able to assign
   values to VARIABLE as in `(define foo (begin (set! foo 1) (+ foo 1)))'.  */
static SCM
scm_m_define (SCM expr, SCM env)
{
  ASSERT_SYNTAX (SCM_TOP_LEVEL (env), s_bad_define, expr);

  {
    const SCM canonical_definition = canonicalize_define (expr);
    const SCM cdr_canonical_definition = SCM_CDR (canonical_definition);
    const SCM variable = SCM_CAR (cdr_canonical_definition);
    const SCM value = scm_eval_car (SCM_CDR (cdr_canonical_definition), env);
    const SCM location
      = scm_sym2var (variable, scm_env_top_level (env), SCM_BOOL_T);

    if (SCM_REC_PROCNAMES_P)
      {
        SCM tmp = value;
        while (SCM_MACROP (tmp))
          tmp = SCM_MACRO_CODE (tmp);
        if (scm_is_true (scm_procedure_p (tmp))
            /* Only the first definition determines the name. */
            && scm_is_false (scm_procedure_property (tmp, scm_sym_name)))
          scm_set_procedure_property_x (tmp, scm_sym_name, variable);
      }

    SCM_VARIABLE_SET (location, value);

    return SCM_UNSPECIFIED;
  }
}


/* This is a helper function for forms (<keyword> <expression>) that are
 * transformed into (#@<keyword> '() <memoized_expression>) in order to allow
 * for easy creation of a thunk (i. e. a closure without arguments) using the
 * ('() <memoized_expression>) tail of the memoized form.  */
static SCM
memoize_as_thunk_prototype (const SCM expr, const SCM env SCM_UNUSED)
{
  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) == 1, s_expression, expr);

  SCM_SETCDR (expr, scm_cons (SCM_EOL, cdr_expr));

  return expr;
}


SCM_SYNTAX (s_delay, "delay", scm_i_makbimacro, scm_m_delay);
SCM_GLOBAL_SYMBOL (scm_sym_delay, s_delay);

/* Promises are implemented as closures with an empty parameter list.  Thus,
 * (delay <expression>) is transformed into (#@delay '() <expression>), where
 * the empty list represents the empty parameter list.  This representation
 * allows for easy creation of the closure during evaluation.  */
static SCM
scm_m_delay (SCM expr, SCM env)
{
  const SCM new_expr = memoize_as_thunk_prototype (expr, env);
  SCM_SETCAR (new_expr, SCM_IM_DELAY);
  return new_expr;
}

static SCM
unmemoize_delay (const SCM expr, const SCM env)
{
  const SCM thunk_expr = SCM_CADDR (expr);
  /* A promise is implemented as a closure, and when applying a
     closure the evaluator adds a new frame to the environment - even
     though, in the case of a promise, the added frame is always
     empty.  We need to extend the environment here in the same way,
     so that any ILOCs in thunk_expr can be unmemoized correctly. */
  const SCM new_env = SCM_EXTEND_ENV (SCM_EOL, SCM_EOL, env);
  return scm_list_2 (scm_sym_delay, unmemoize_expression (thunk_expr, new_env));
}


SCM_SYNTAX(s_do, "do", scm_i_makbimacro, scm_m_do);
SCM_GLOBAL_SYMBOL(scm_sym_do, s_do);

/* DO gets the most radically altered syntax.  The order of the vars is
 * reversed here.  During the evaluation this allows for simple consing of the
 * results of the inits and steps:

   (do ((<var1> <init1> <step1>)
        (<var2> <init2>)
        ... )
       (<test> <return>)
     <body>)

   ;; becomes

   (#@do (<init1> <init2> ... <initn>)
         (varn ... var2 var1)
         (<test> <return>)
         (<body>)
     <step1> <step2> ... <stepn>) ;; missing steps replaced by var
 */
static SCM
scm_m_do (SCM expr, SCM env SCM_UNUSED)
{
  SCM variables = SCM_EOL;
  SCM init_forms = SCM_EOL;
  SCM step_forms = SCM_EOL;
  SCM binding_idx;
  SCM cddr_expr;
  SCM exit_clause;
  SCM commands;
  SCM tail;

  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 2, s_missing_expression, expr);

  /* Collect variables, init and step forms. */
  binding_idx = SCM_CAR (cdr_expr);
  ASSERT_SYNTAX_2 (scm_ilength (binding_idx) >= 0,
                   s_bad_bindings, binding_idx, expr);
  for (; !scm_is_null (binding_idx); binding_idx = SCM_CDR (binding_idx))
    {
      const SCM binding = SCM_CAR (binding_idx);
      const long length = scm_ilength (binding);
      ASSERT_SYNTAX_2 (length == 2 || length == 3,
                       s_bad_binding, binding, expr);

      {
        const SCM name = SCM_CAR (binding);
        const SCM init = SCM_CADR (binding);
        const SCM step = (length == 2) ? name : SCM_CADDR (binding);
        ASSERT_SYNTAX_2 (scm_is_symbol (name), s_bad_variable, name, expr);
        ASSERT_SYNTAX_2 (scm_is_false (scm_c_memq (name, variables)),
                         s_duplicate_binding, name, expr);

        variables = scm_cons (name, variables);
        init_forms = scm_cons (init, init_forms);
        step_forms = scm_cons (step, step_forms);
      }
    }
  init_forms = scm_reverse_x (init_forms, SCM_UNDEFINED);
  step_forms = scm_reverse_x (step_forms, SCM_UNDEFINED);

  /* Memoize the test form and the exit sequence. */
  cddr_expr = SCM_CDR (cdr_expr);
  exit_clause = SCM_CAR (cddr_expr);
  ASSERT_SYNTAX_2 (scm_ilength (exit_clause) >= 1,
                   s_bad_exit_clause, exit_clause, expr);

  commands = SCM_CDR (cddr_expr);
  tail = scm_cons2 (exit_clause, commands, step_forms);
  tail = scm_cons2 (init_forms, variables, tail);
  SCM_SETCAR (expr, SCM_IM_DO);
  SCM_SETCDR (expr, tail);
  return expr;
}

static SCM
unmemoize_do (const SCM expr, const SCM env)
{
  const SCM cdr_expr = SCM_CDR (expr);
  const SCM cddr_expr = SCM_CDR (cdr_expr);
  const SCM rnames = SCM_CAR (cddr_expr);
  const SCM extended_env = SCM_EXTEND_ENV (rnames, SCM_EOL, env);
  const SCM cdddr_expr = SCM_CDR (cddr_expr);
  const SCM exit_sequence = SCM_CAR (cdddr_expr);
  const SCM um_exit_sequence = unmemoize_exprs (exit_sequence, extended_env);
  const SCM cddddr_expr = SCM_CDR (cdddr_expr);
  const SCM um_body = unmemoize_exprs (SCM_CAR (cddddr_expr), extended_env);

  /* build transformed binding list */
  SCM um_names = scm_reverse (rnames);
  SCM um_inits = unmemoize_exprs (SCM_CAR (cdr_expr), env);
  SCM um_steps = unmemoize_exprs (SCM_CDR (cddddr_expr), extended_env);
  SCM um_bindings = SCM_EOL;
  while (!scm_is_null (um_names))
    {
      const SCM name = SCM_CAR (um_names);
      const SCM init = SCM_CAR (um_inits);
      SCM step = SCM_CAR (um_steps);
      step = scm_is_eq (step, name) ? SCM_EOL : scm_list_1 (step);

      um_bindings = scm_cons (scm_cons2 (name, init, step), um_bindings);

      um_names = SCM_CDR (um_names);
      um_inits = SCM_CDR (um_inits);
      um_steps = SCM_CDR (um_steps);
    }
  um_bindings = scm_reverse_x (um_bindings, SCM_UNDEFINED);

  return scm_cons (scm_sym_do,
                   scm_cons2 (um_bindings, um_exit_sequence, um_body));
}


SCM_SYNTAX (s_if, "if", scm_i_makbimacro, scm_m_if);
SCM_GLOBAL_SYMBOL (scm_sym_if, s_if);

static SCM
scm_m_if (SCM expr, SCM env SCM_UNUSED)
{
  const SCM cdr_expr = SCM_CDR (expr);
  const long length = scm_ilength (cdr_expr);
  ASSERT_SYNTAX (length == 2 || length == 3, s_expression, expr);
  SCM_SETCAR (expr, SCM_IM_IF);
  return expr;
}

static SCM
unmemoize_if (const SCM expr, const SCM env)
{
  const SCM cdr_expr = SCM_CDR (expr);
  const SCM um_condition = unmemoize_expression (SCM_CAR (cdr_expr), env);
  const SCM cddr_expr = SCM_CDR (cdr_expr);
  const SCM um_then = unmemoize_expression (SCM_CAR (cddr_expr), env);
  const SCM cdddr_expr = SCM_CDR (cddr_expr);

  if (scm_is_null (cdddr_expr))
    {
      return scm_list_3 (scm_sym_if, um_condition, um_then);
    }
  else
    {
      const SCM um_else = unmemoize_expression (SCM_CAR (cdddr_expr), env);
      return scm_list_4 (scm_sym_if, um_condition, um_then, um_else);
    }
}


SCM_SYNTAX (s_lambda, "lambda", scm_i_makbimacro, scm_m_lambda);
SCM_GLOBAL_SYMBOL (scm_sym_lambda, s_lambda);

/* A helper function for memoize_lambda to support checking for duplicate
 * formal arguments: Return true if OBJ is `eq?' to one of the elements of
 * LIST or to the cdr of the last cons.  Therefore, LIST may have any of the
 * forms that a formal argument can have:
 *   <rest>, (<arg1> ...), (<arg1> ...  .  <rest>) */
static int
c_improper_memq (SCM obj, SCM list)
{
  for (; scm_is_pair (list); list = SCM_CDR (list))
    {
      if (scm_is_eq (SCM_CAR (list), obj))
        return 1;
    }
  return scm_is_eq (list, obj);
}

static SCM
scm_m_lambda (SCM expr, SCM env SCM_UNUSED)
{
  SCM formals;
  SCM formals_idx;
  SCM cddr_expr;
  int documentation;
  SCM body;
  SCM new_body;

  const SCM cdr_expr = SCM_CDR (expr);
  const long length = scm_ilength (cdr_expr);
  ASSERT_SYNTAX (length >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (length >= 2, s_missing_expression, expr);

  /* Before iterating the list of formal arguments, make sure the formals
   * actually are given as either a symbol or a non-cyclic list.  */
  formals = SCM_CAR (cdr_expr);
  if (scm_is_pair (formals))
    {
      /* Dirk:FIXME:: We should check for a cyclic list of formals, and if
       * detected, report a 'Bad formals' error.  */
    }
  else
    {
      ASSERT_SYNTAX_2 (scm_is_symbol (formals) || scm_is_null (formals),
                       s_bad_formals, formals, expr);
    }

  /* Now iterate the list of formal arguments to check if all formals are
   * symbols, and that there are no duplicates.  */
  formals_idx = formals;
  while (scm_is_pair (formals_idx))
    {
      const SCM formal = SCM_CAR (formals_idx);
      const SCM next_idx = SCM_CDR (formals_idx);
      ASSERT_SYNTAX_2 (scm_is_symbol (formal), s_bad_formal, formal, expr);
      ASSERT_SYNTAX_2 (!c_improper_memq (formal, next_idx),
                       s_duplicate_formal, formal, expr);
      formals_idx = next_idx;
    }
  ASSERT_SYNTAX_2 (scm_is_null (formals_idx) || scm_is_symbol (formals_idx),
                   s_bad_formal, formals_idx, expr);

  /* Memoize the body.  Keep a potential documentation string.  */
  /* Dirk:FIXME:: We should probably extract the documentation string to
   * some external database.  Otherwise it will slow down execution, since
   * the documentation string will have to be skipped with every execution
   * of the closure.  */
  cddr_expr = SCM_CDR (cdr_expr);
  documentation = (length >= 3 && scm_is_string (SCM_CAR (cddr_expr)));
  body = documentation ? SCM_CDR (cddr_expr) : cddr_expr;
  new_body = m_body (SCM_IM_LAMBDA, body);

  SCM_SETCAR (expr, SCM_IM_LAMBDA);
  if (documentation)
    SCM_SETCDR (cddr_expr, new_body);
  else
    SCM_SETCDR (cdr_expr, new_body);
  return expr;
}

static SCM
unmemoize_lambda (const SCM expr, const SCM env)
{
  const SCM formals = SCM_CADR (expr);
  const SCM body = SCM_CDDR (expr);

  const SCM new_env = SCM_EXTEND_ENV (formals, SCM_EOL, env);
  const SCM um_formals = scm_i_finite_list_copy (formals);
  const SCM um_body = unmemoize_exprs (body, new_env);

  return scm_cons2 (scm_sym_lambda, um_formals, um_body);
}


/* Check if the format of the bindings is ((<symbol> <init-form>) ...).  */
static void
check_bindings (const SCM bindings, const SCM expr)
{
  SCM binding_idx;

  ASSERT_SYNTAX_2 (scm_ilength (bindings) >= 0,
                   s_bad_bindings, bindings, expr);

  binding_idx = bindings;
  for (; !scm_is_null (binding_idx); binding_idx = SCM_CDR (binding_idx))
    {
      SCM name;         /* const */

      const SCM binding = SCM_CAR (binding_idx);
      ASSERT_SYNTAX_2 (scm_ilength (binding) == 2,
                       s_bad_binding, binding, expr);

      name = SCM_CAR (binding);
      ASSERT_SYNTAX_2 (scm_is_symbol (name), s_bad_variable, name, expr);
    }
}


/* The bindings, which must have the format ((v1 i1) (v2 i2) ... (vn in)), are
 * transformed to the lists (vn ... v2 v1) and (i1 i2 ... in).  That is, the
 * variables are returned in a list with their order reversed, and the init
 * forms are returned in a list in the same order as they are given in the
 * bindings.  If a duplicate variable name is detected, an error is
 * signalled.  */
static void
transform_bindings (
  const SCM bindings, const SCM expr,
  SCM *const rvarptr, SCM *const initptr )
{
  SCM rvariables = SCM_EOL;
  SCM rinits = SCM_EOL;
  SCM binding_idx = bindings;
  for (; !scm_is_null (binding_idx); binding_idx = SCM_CDR (binding_idx))
    {
      const SCM binding = SCM_CAR (binding_idx);
      const SCM cdr_binding = SCM_CDR (binding);
      const SCM name = SCM_CAR (binding);
      ASSERT_SYNTAX_2 (scm_is_false (scm_c_memq (name, rvariables)),
                       s_duplicate_binding, name, expr);
      rvariables = scm_cons (name, rvariables);
      rinits = scm_cons (SCM_CAR (cdr_binding), rinits);
    }
  *rvarptr = rvariables;
  *initptr = scm_reverse_x (rinits, SCM_UNDEFINED);
}


SCM_SYNTAX(s_let, "let", scm_i_makbimacro, scm_m_let);
SCM_GLOBAL_SYMBOL(scm_sym_let, s_let);

/* This function is a helper function for memoize_let.  It transforms
 * (let name ((var init) ...) body ...) into
 * ((letrec ((name (lambda (var ...) body ...))) name) init ...)
 * and memoizes the expression.  It is assumed that the caller has checked
 * that name is a symbol and that there are bindings and a body.  */
static SCM
memoize_named_let (const SCM expr, const SCM env SCM_UNUSED)
{
  SCM rvariables;
  SCM variables;
  SCM inits;

  const SCM cdr_expr = SCM_CDR (expr);
  const SCM name = SCM_CAR (cdr_expr);
  const SCM cddr_expr = SCM_CDR (cdr_expr);
  const SCM bindings = SCM_CAR (cddr_expr);
  check_bindings (bindings, expr);

  transform_bindings (bindings, expr, &rvariables, &inits);
  variables = scm_reverse_x (rvariables, SCM_UNDEFINED);

  {
    const SCM let_body = SCM_CDR (cddr_expr);
    const SCM lambda_body = m_body (SCM_IM_LET, let_body);
    const SCM lambda_tail = scm_cons (variables, lambda_body);
    const SCM lambda_form = scm_cons_source (expr, scm_sym_lambda, lambda_tail);

    const SCM rvar = scm_list_1 (name);
    const SCM init = scm_list_1 (lambda_form);
    const SCM body = m_body (SCM_IM_LET, scm_list_1 (name));
    const SCM letrec_tail = scm_cons (rvar, scm_cons (init, body));
    const SCM letrec_form = scm_cons_source (expr, SCM_IM_LETREC, letrec_tail);
    return scm_cons_source (expr, letrec_form, inits);
  }
}

/* (let ((v1 i1) (v2 i2) ...) body) with variables v1 .. vn and initializers
 * i1 .. in is transformed to (#@let (vn ... v2 v1) (i1 i2 ...) body).  */
static SCM
scm_m_let (SCM expr, SCM env)
{
  SCM bindings;

  const SCM cdr_expr = SCM_CDR (expr);
  const long length = scm_ilength (cdr_expr);
  ASSERT_SYNTAX (length >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (length >= 2, s_missing_expression, expr);

  bindings = SCM_CAR (cdr_expr);
  if (scm_is_symbol (bindings))
    {
      ASSERT_SYNTAX (length >= 3, s_missing_expression, expr);
      return memoize_named_let (expr, env);
    }

  check_bindings (bindings, expr);
  if (scm_is_null (bindings) || scm_is_null (SCM_CDR (bindings)))
    {
      /* Special case: no bindings or single binding => let* is faster. */
      const SCM body = m_body (SCM_IM_LET, SCM_CDR (cdr_expr));
      return scm_m_letstar (scm_cons2 (SCM_CAR (expr), bindings, body), env);
    }
  else
    {
      /* plain let */
      SCM rvariables;
      SCM inits;
      transform_bindings (bindings, expr, &rvariables, &inits);

      {
        const SCM new_body = m_body (SCM_IM_LET, SCM_CDR (cdr_expr));
        const SCM new_tail = scm_cons2 (rvariables, inits, new_body);
        SCM_SETCAR (expr, SCM_IM_LET);
        SCM_SETCDR (expr, new_tail);
        return expr;
      }
    }
}

static SCM
build_binding_list (SCM rnames, SCM rinits)
{
  SCM bindings = SCM_EOL;
  while (!scm_is_null (rnames))
    {
      const SCM binding = scm_list_2 (SCM_CAR (rnames), SCM_CAR (rinits));
      bindings = scm_cons (binding, bindings);
      rnames = SCM_CDR (rnames);
      rinits = SCM_CDR (rinits);
    }
  return bindings;
}

static SCM
unmemoize_let (const SCM expr, const SCM env)
{
  const SCM cdr_expr = SCM_CDR (expr);
  const SCM um_rnames = SCM_CAR (cdr_expr);
  const SCM extended_env = SCM_EXTEND_ENV (um_rnames, SCM_EOL, env);
  const SCM cddr_expr = SCM_CDR (cdr_expr);
  const SCM um_inits = unmemoize_exprs (SCM_CAR (cddr_expr), env);
  const SCM um_rinits = scm_reverse_x (um_inits, SCM_UNDEFINED);
  const SCM um_bindings = build_binding_list (um_rnames, um_rinits);
  const SCM um_body = unmemoize_exprs (SCM_CDR (cddr_expr), extended_env);

  return scm_cons2 (scm_sym_let, um_bindings, um_body);
}


SCM_SYNTAX(s_letrec, "letrec", scm_i_makbimacro, scm_m_letrec);
SCM_GLOBAL_SYMBOL(scm_sym_letrec, s_letrec);

static SCM
scm_m_letrec (SCM expr, SCM env)
{
  SCM bindings;

  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 2, s_missing_expression, expr);

  bindings = SCM_CAR (cdr_expr);
  if (scm_is_null (bindings))
    {
      /* no bindings, let* is executed faster */
      SCM body = m_body (SCM_IM_LETREC, SCM_CDR (cdr_expr));
      return scm_m_letstar (scm_cons2 (SCM_CAR (expr), SCM_EOL, body), env);
    }
  else
    {
      SCM rvariables;
      SCM inits;
      SCM new_body;

      check_bindings (bindings, expr);
      transform_bindings (bindings, expr, &rvariables, &inits);
      new_body = m_body (SCM_IM_LETREC, SCM_CDR (cdr_expr));
      return scm_cons2 (SCM_IM_LETREC, rvariables, scm_cons (inits, new_body));
    }
}

static SCM
unmemoize_letrec (const SCM expr, const SCM env)
{
  const SCM cdr_expr = SCM_CDR (expr);
  const SCM um_rnames = SCM_CAR (cdr_expr);
  const SCM extended_env = SCM_EXTEND_ENV (um_rnames, SCM_EOL, env);
  const SCM cddr_expr = SCM_CDR (cdr_expr);
  const SCM um_inits = unmemoize_exprs (SCM_CAR (cddr_expr), extended_env);
  const SCM um_rinits = scm_reverse_x (um_inits, SCM_UNDEFINED);
  const SCM um_bindings = build_binding_list (um_rnames, um_rinits);
  const SCM um_body = unmemoize_exprs (SCM_CDR (cddr_expr), extended_env);

  return scm_cons2 (scm_sym_letrec, um_bindings, um_body);
}



SCM_SYNTAX (s_letstar, "let*", scm_i_makbimacro, scm_m_letstar);
SCM_GLOBAL_SYMBOL (scm_sym_letstar, s_letstar);

/* (let* ((v1 i1) (v2 i2) ...) body) with variables v1 .. vn and initializers
 * i1 .. in is transformed into the form (#@let* (v1 i1 v2 i2 ...) body).  */
static SCM
scm_m_letstar (SCM expr, SCM env SCM_UNUSED)
{
  SCM binding_idx;
  SCM new_body;

  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 2, s_missing_expression, expr);

  binding_idx = SCM_CAR (cdr_expr);
  check_bindings (binding_idx, expr);

  /* Transform ((v1 i1) (v2 i2) ...) into (v1 i1 v2 i2 ...).  The
   * transformation is done in place.  At the beginning of one iteration of
   * the loop the variable binding_idx holds the form
   *   P1:( (vn . P2:(in . ())) . P3:( (vn+1 in+1) ... ) ),
   * where P1, P2 and P3 indicate the pairs, that are relevant for the
   * transformation.  P1 and P2 are modified in the loop, P3 remains
   * untouched.  After the execution of the loop, P1 will hold
   *   P1:( vn . P2:(in . P3:( (vn+1 in+1) ... )) )
   * and binding_idx will hold P3.  */
  while (!scm_is_null (binding_idx))
    {
      const SCM cdr_binding_idx = SCM_CDR (binding_idx);  /* remember P3 */
      const SCM binding = SCM_CAR (binding_idx);
      const SCM name = SCM_CAR (binding);
      const SCM cdr_binding = SCM_CDR (binding);

      SCM_SETCDR (cdr_binding, cdr_binding_idx);        /* update P2 */
      SCM_SETCAR (binding_idx, name);                   /* update P1 */
      SCM_SETCDR (binding_idx, cdr_binding);            /* update P1 */

      binding_idx = cdr_binding_idx;                    /* continue with P3 */
    }

  new_body = m_body (SCM_IM_LETSTAR, SCM_CDR (cdr_expr));
  SCM_SETCAR (expr, SCM_IM_LETSTAR);
  /* the bindings have been changed in place */
  SCM_SETCDR (cdr_expr, new_body);
  return expr;
}

static SCM
unmemoize_letstar (const SCM expr, const SCM env)
{
  const SCM cdr_expr = SCM_CDR (expr);
  const SCM body = SCM_CDR (cdr_expr);
  SCM bindings = SCM_CAR (cdr_expr);
  SCM um_bindings = SCM_EOL;
  SCM extended_env = env;
  SCM um_body;

  while (!scm_is_null (bindings))
    {
      const SCM variable = SCM_CAR (bindings);
      const SCM init = SCM_CADR (bindings);
      const SCM um_init = unmemoize_expression (init, extended_env);
      um_bindings = scm_cons (scm_list_2 (variable, um_init), um_bindings);
      extended_env = SCM_EXTEND_ENV (variable, SCM_BOOL_F, extended_env);
      bindings = SCM_CDDR (bindings);
    }
  um_bindings = scm_reverse_x (um_bindings, SCM_UNDEFINED);

  um_body = unmemoize_exprs (body, extended_env);

  return scm_cons2 (scm_sym_letstar, um_bindings, um_body);
}


SCM_SYNTAX (s_or, "or", scm_i_makbimacro, scm_m_or);
SCM_GLOBAL_SYMBOL (scm_sym_or, s_or);

static SCM
scm_m_or (SCM expr, SCM env SCM_UNUSED)
{
  const SCM cdr_expr = SCM_CDR (expr);
  const long length = scm_ilength (cdr_expr);

  ASSERT_SYNTAX (length >= 0, s_bad_expression, expr);

  if (length == 0)
    {
      /* Special case:  (or) is replaced by #f. */
      return SCM_BOOL_F;
    }
  else
    {
      SCM_SETCAR (expr, SCM_IM_OR);
      return expr;
    }
}

static SCM
unmemoize_or (const SCM expr, const SCM env)
{
  return scm_cons (scm_sym_or, unmemoize_exprs (SCM_CDR (expr), env));
}


SCM_SYNTAX (s_quasiquote, "quasiquote", scm_makacro, scm_m_quasiquote);
SCM_GLOBAL_SYMBOL (scm_sym_quasiquote, s_quasiquote);
SCM_GLOBAL_SYMBOL (scm_sym_unquote, "unquote");
SCM_GLOBAL_SYMBOL (scm_sym_uq_splicing, "unquote-splicing");

/* Internal function to handle a quasiquotation:  'form' is the parameter in
 * the call (quasiquotation form), 'env' is the environment where unquoted
 * expressions will be evaluated, and 'depth' is the current quasiquotation
 * nesting level and is known to be greater than zero.  */
static SCM 
iqq (SCM form, SCM env, unsigned long int depth)
{
  if (scm_is_pair (form))
    {
      const SCM tmp = SCM_CAR (form);
      if (scm_is_eq (tmp, scm_sym_quasiquote))
        {
          const SCM args = SCM_CDR (form);
          ASSERT_SYNTAX (scm_ilength (args) == 1, s_expression, form);
          return scm_list_2 (tmp, iqq (SCM_CAR (args), env, depth + 1));
        }
      else if (scm_is_eq (tmp, scm_sym_unquote))
        {
          const SCM args = SCM_CDR (form);
          ASSERT_SYNTAX (scm_ilength (args) == 1, s_expression, form);
          if (depth - 1 == 0)
            return scm_eval_car (args, env);
          else
            return scm_list_2 (tmp, iqq (SCM_CAR (args), env, depth - 1));
        }
      else if (scm_is_pair (tmp)
               && scm_is_eq (SCM_CAR (tmp), scm_sym_uq_splicing))
        {
          const SCM args = SCM_CDR (tmp);
          ASSERT_SYNTAX (scm_ilength (args) == 1, s_expression, form);
          if (depth - 1 == 0)
            {
              const SCM list = scm_eval_car (args, env);
              const SCM rest = SCM_CDR (form);
              ASSERT_SYNTAX_2 (scm_ilength (list) >= 0,
                               s_splicing, list, form);
              return scm_append (scm_list_2 (list, iqq (rest, env, depth)));
            }
          else
            return scm_cons (iqq (SCM_CAR (form), env, depth - 1),
                             iqq (SCM_CDR (form), env, depth));
        }
      else
        return scm_cons (iqq (SCM_CAR (form), env, depth),
                         iqq (SCM_CDR (form), env, depth));
    }
  else if (scm_is_vector (form))
    return scm_vector (iqq (scm_vector_to_list (form), env, depth));
  else
    return form;
}

static SCM
scm_m_quasiquote (SCM expr, SCM env)
{
  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) == 1, s_expression, expr);
  return iqq (SCM_CAR (cdr_expr), env, 1);
}


SCM_SYNTAX (s_quote, "quote", scm_i_makbimacro, scm_m_quote);
SCM_GLOBAL_SYMBOL (scm_sym_quote, s_quote);

static SCM
scm_m_quote (SCM expr, SCM env SCM_UNUSED)
{
  SCM quotee;

  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) == 1, s_expression, expr);
  quotee = SCM_CAR (cdr_expr);
  if (is_self_quoting_p (quotee))
    return quotee;

  SCM_SETCAR (expr, SCM_IM_QUOTE);
  SCM_SETCDR (expr, quotee);
  return expr;
}

static SCM
unmemoize_quote (const SCM expr, const SCM env SCM_UNUSED)
{
  return scm_list_2 (scm_sym_quote, SCM_CDR (expr));
}


/* Will go into the RnRS module when Guile is factorized.
SCM_SYNTAX (s_set_x, "set!", scm_i_makbimacro, scm_m_set_x); */
static const char s_set_x[] = "set!";
SCM_GLOBAL_SYMBOL (scm_sym_set_x, s_set_x);

static SCM
scm_m_set_x (SCM expr, SCM env SCM_UNUSED)
{
  SCM variable;
  SCM new_variable;

  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) == 2, s_expression, expr);
  variable = SCM_CAR (cdr_expr);

  /* Memoize the variable form. */
  ASSERT_SYNTAX_2 (scm_is_symbol (variable), s_bad_variable, variable, expr);
  new_variable = lookup_symbol (variable, env);
  /* Leave the memoization of unbound symbols to lazy memoization: */
  if (SCM_UNBNDP (new_variable))
    new_variable = variable;

  SCM_SETCAR (expr, SCM_IM_SET_X);
  SCM_SETCAR (cdr_expr, new_variable);
  return expr;
}

static SCM
unmemoize_set_x (const SCM expr, const SCM env)
{
  return scm_cons (scm_sym_set_x, unmemoize_exprs (SCM_CDR (expr), env));
}


\f
/* Start of the memoizers for non-R5RS builtin macros.  */


SCM_SYNTAX (s_at, "@", scm_makmmacro, scm_m_at);
SCM_GLOBAL_SYMBOL (scm_sym_at, s_at);

static SCM
scm_m_at (SCM expr, SCM env SCM_UNUSED)
{
  SCM mod, var;
  ASSERT_SYNTAX (scm_ilength (expr) == 3, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (scm_cadr (expr)) > 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_is_symbol (scm_caddr (expr)), s_bad_expression, expr);

  mod = scm_resolve_module (scm_cadr (expr));
  if (scm_is_false (mod))
    error_unbound_variable (expr);
  var = scm_module_variable (scm_module_public_interface (mod), scm_caddr (expr));
  if (scm_is_false (var))
    error_unbound_variable (expr);
  
  return var;
}

SCM_SYNTAX (s_atat, "@@", scm_makmmacro, scm_m_atat);
SCM_GLOBAL_SYMBOL (scm_sym_atat, s_atat);

static SCM
scm_m_atat (SCM expr, SCM env SCM_UNUSED)
{
  SCM mod, var;
  ASSERT_SYNTAX (scm_ilength (expr) == 3, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (scm_cadr (expr)) > 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_is_symbol (scm_caddr (expr)), s_bad_expression, expr);

  mod = scm_resolve_module (scm_cadr (expr));
  if (scm_is_false (mod))
    error_unbound_variable (expr);
  var = scm_module_variable (mod, scm_caddr (expr));
  if (scm_is_false (var))
    error_unbound_variable (expr);
  
  return var;
}

SCM_SYNTAX (s_atapply, "@apply", scm_i_makbimacro, scm_m_apply);
SCM_GLOBAL_SYMBOL (scm_sym_atapply, s_atapply);
SCM_GLOBAL_SYMBOL (scm_sym_apply, s_atapply + 1);

static SCM
scm_m_apply (SCM expr, SCM env SCM_UNUSED)
{
  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) == 2, s_missing_expression, expr);

  SCM_SETCAR (expr, SCM_IM_APPLY);
  return expr;
}

static SCM
unmemoize_apply (const SCM expr, const SCM env)
{
  return scm_list_2 (scm_sym_atapply, unmemoize_exprs (SCM_CDR (expr), env));
}


SCM_SYNTAX (s_atbind, "@bind", scm_i_makbimacro, scm_m_atbind);

/* FIXME: The following explanation should go into the documentation: */
/* (@bind ((var init) ...) body ...) will assign the values of the `init's to
 * the global variables named by `var's (symbols, not evaluated), creating
 * them if they don't exist, executes body, and then restores the previous
 * values of the `var's.  Additionally, whenever control leaves body, the
 * values of the `var's are saved and restored when control returns.  It is an
 * error when a symbol appears more than once among the `var's.  All `init's
 * are evaluated before any `var' is set.
 *
 * Think of this as `let' for dynamic scope.
 */

/* (@bind ((var1 exp1) ... (varn expn)) body ...) is memoized into
 * (#@bind ((varn ... var1) . (exp1 ... expn)) body ...).
 *
 * FIXME - also implement `@bind*'.
 */
static SCM
scm_m_atbind (SCM expr, SCM env)
{
  SCM bindings;
  SCM rvariables;
  SCM inits;
  SCM variable_idx;

  const SCM top_level = scm_env_top_level (env);

  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 2, s_missing_expression, expr);
  bindings = SCM_CAR (cdr_expr);
  check_bindings (bindings, expr);
  transform_bindings (bindings, expr, &rvariables, &inits);

  for (variable_idx = rvariables;
       !scm_is_null (variable_idx);
       variable_idx = SCM_CDR (variable_idx))
    {
      /* The first call to scm_sym2var will look beyond the current module,
       * while the second call wont.  */
      const SCM variable = SCM_CAR (variable_idx);
      SCM new_variable = scm_sym2var (variable, top_level, SCM_BOOL_F);
      if (scm_is_false (new_variable))
        new_variable = scm_sym2var (variable, top_level, SCM_BOOL_T);
      SCM_SETCAR (variable_idx, new_variable);
    }

  SCM_SETCAR (expr, SCM_IM_BIND);
  SCM_SETCAR (cdr_expr, scm_cons (rvariables, inits));
  return expr;
}


SCM_SYNTAX(s_atcall_cc, "@call-with-current-continuation", scm_i_makbimacro, scm_m_cont);
SCM_GLOBAL_SYMBOL(scm_sym_atcall_cc, s_atcall_cc);

static SCM
scm_m_cont (SCM expr, SCM env SCM_UNUSED)
{
  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) == 1, s_expression, expr);

  SCM_SETCAR (expr, SCM_IM_CONT);
  return expr;
}

static SCM
unmemoize_atcall_cc (const SCM expr, const SCM env)
{
  return scm_list_2 (scm_sym_atcall_cc, unmemoize_exprs (SCM_CDR (expr), env));
}


SCM_SYNTAX (s_at_call_with_values, "@call-with-values", scm_i_makbimacro, scm_m_at_call_with_values);
SCM_GLOBAL_SYMBOL(scm_sym_at_call_with_values, s_at_call_with_values);

static SCM
scm_m_at_call_with_values (SCM expr, SCM env SCM_UNUSED)
{
  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) == 2, s_expression, expr);

  SCM_SETCAR (expr, SCM_IM_CALL_WITH_VALUES);
  return expr;
}

static SCM
unmemoize_at_call_with_values (const SCM expr, const SCM env)
{
  return scm_list_2 (scm_sym_at_call_with_values,
                     unmemoize_exprs (SCM_CDR (expr), env));
}

SCM_SYNTAX (s_eval_when, "eval-when", scm_makmmacro, scm_m_eval_when);
SCM_GLOBAL_SYMBOL (scm_sym_eval_when, s_eval_when);
SCM_SYMBOL (sym_eval, "eval");
SCM_SYMBOL (sym_load, "load");


static SCM
scm_m_eval_when (SCM expr, SCM env SCM_UNUSED)
{
  ASSERT_SYNTAX (scm_ilength (expr) >= 3, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (scm_cadr (expr)) > 0, s_bad_expression, expr);

  if (scm_is_true (scm_memq (sym_eval, scm_cadr (expr)))
      || scm_is_true (scm_memq (sym_load, scm_cadr (expr))))
    return scm_cons (SCM_IM_BEGIN, scm_cddr (expr));
  
  return scm_list_1 (SCM_IM_BEGIN);
}

#if 0

/* See futures.h for a comment why futures are not enabled.
 */

SCM_SYNTAX (s_future, "future", scm_i_makbimacro, scm_m_future);
SCM_GLOBAL_SYMBOL (scm_sym_future, s_future);

/* Like promises, futures are implemented as closures with an empty
 * parameter list.  Thus, (future <expression>) is transformed into
 * (#@future '() <expression>), where the empty list represents the
 * empty parameter list.  This representation allows for easy creation
 * of the closure during evaluation.  */
static SCM
scm_m_future (SCM expr, SCM env)
{
  const SCM new_expr = memoize_as_thunk_prototype (expr, env);
  SCM_SETCAR (new_expr, SCM_IM_FUTURE);
  return new_expr;
}

static SCM
unmemoize_future (const SCM expr, const SCM env)
{
  const SCM thunk_expr = SCM_CADDR (expr);
  return scm_list_2 (scm_sym_future, unmemoize_expression (thunk_expr, env));
}

#endif /* futures disabled. */

SCM_SYNTAX (s_gset_x, "set!", scm_i_makbimacro, scm_m_generalized_set_x);
SCM_SYMBOL (scm_sym_setter, "setter");

static SCM
scm_m_generalized_set_x (SCM expr, SCM env)
{
  SCM target, exp_target;

  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) == 2, s_expression, expr);

  target = SCM_CAR (cdr_expr);
  if (!scm_is_pair (target))
    {
      /* R5RS usage */
      return scm_m_set_x (expr, env);
    }
  else
    {
      /* (set! (foo bar ...) baz) becomes ((setter foo) bar ... baz) */
      /* Macroexpanding the target might return things of the form
         (begin <atom>).  In that case, <atom> must be a symbol or a
         variable and we memoize to (set! <atom> ...).
      */
      exp_target = macroexp (target, env);
      if (scm_is_eq (SCM_CAR (exp_target), SCM_IM_BEGIN)
          && !scm_is_null (SCM_CDR (exp_target))
          && scm_is_null (SCM_CDDR (exp_target)))
        {
          exp_target= SCM_CADR (exp_target);
          ASSERT_SYNTAX_2 (scm_is_symbol (exp_target)
                           || SCM_VARIABLEP (exp_target),
                           s_bad_variable, exp_target, expr);
          return scm_cons (SCM_IM_SET_X, scm_cons (exp_target,
                                                   SCM_CDR (cdr_expr)));
        }
      else
        {
          const SCM setter_proc_tail = scm_list_1 (SCM_CAR (target));
          const SCM setter_proc = scm_cons_source (expr, scm_sym_setter,
                                                   setter_proc_tail);

          const SCM cddr_expr = SCM_CDR (cdr_expr);
          const SCM setter_args = scm_append_x (scm_list_2 (SCM_CDR (target),
                                                            cddr_expr));

          SCM_SETCAR (expr, setter_proc);
          SCM_SETCDR (expr, setter_args);
          return expr;
        }
    }
}


/* @slot-ref is bound privately in the (oop goops) module from goops.c.  As
 * soon as the module system allows us to more freely create bindings in
 * arbitrary modules during the startup phase, the code from goops.c should be
 * moved here.  */

SCM_SYNTAX (s_atslot_ref, "@slot-ref", scm_i_makbimacro, scm_m_atslot_ref);
SCM_SYNTAX (s_atslot_set_x, "@slot-set!", scm_i_makbimacro, scm_m_atslot_set_x);
SCM_SYMBOL (sym_atslot_ref, "@slot-ref");

static SCM
scm_m_atslot_ref (SCM expr, SCM env SCM_UNUSED)
{
  SCM slot_nr;

  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) == 2, s_expression, expr);
  slot_nr = SCM_CADR (cdr_expr);
  ASSERT_SYNTAX_2 (SCM_I_INUMP (slot_nr), s_bad_slot_number, slot_nr, expr);

  SCM_SETCAR (expr, SCM_IM_SLOT_REF);
  SCM_SETCDR (cdr_expr, slot_nr);
  return expr;
}

static SCM
unmemoize_atslot_ref (const SCM expr, const SCM env)
{
  const SCM instance = SCM_CADR (expr);
  const SCM um_instance = unmemoize_expression (instance, env);
  const SCM slot_nr = SCM_CDDR (expr);
  return scm_list_3 (sym_atslot_ref, um_instance, slot_nr);
}


/* @slot-set! is bound privately in the (oop goops) module from goops.c.  As
 * soon as the module system allows us to more freely create bindings in
 * arbitrary modules during the startup phase, the code from goops.c should be
 * moved here.  */

SCM_SYMBOL (sym_atslot_set_x, "@slot-set!");

static SCM
scm_m_atslot_set_x (SCM expr, SCM env SCM_UNUSED)
{
  SCM slot_nr;

  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) == 3, s_expression, expr);
  slot_nr = SCM_CADR (cdr_expr);
  ASSERT_SYNTAX_2 (SCM_I_INUMP (slot_nr), s_bad_slot_number, slot_nr, expr);

  SCM_SETCAR (expr, SCM_IM_SLOT_SET_X);
  return expr;
}

static SCM
unmemoize_atslot_set_x (const SCM expr, const SCM env)
{
  const SCM cdr_expr = SCM_CDR (expr);
  const SCM instance = SCM_CAR (cdr_expr);
  const SCM um_instance = unmemoize_expression (instance, env);
  const SCM cddr_expr = SCM_CDR (cdr_expr);
  const SCM slot_nr = SCM_CAR (cddr_expr);
  const SCM cdddr_expr = SCM_CDR (cddr_expr);
  const SCM value = SCM_CAR (cdddr_expr);
  const SCM um_value = unmemoize_expression (value, env);
  return scm_list_4 (sym_atslot_set_x, um_instance, slot_nr, um_value);
}


#if SCM_ENABLE_ELISP

static const char s_defun[] = "Symbol's function definition is void";

SCM_SYNTAX (s_nil_cond, "nil-cond", scm_i_makbimacro, scm_m_nil_cond);

/* nil-cond expressions have the form
 *   (nil-cond COND VAL COND VAL ... ELSEVAL)  */
static SCM
scm_m_nil_cond (SCM expr, SCM env SCM_UNUSED)
{
  const long length = scm_ilength (SCM_CDR (expr));
  ASSERT_SYNTAX (length >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (length >= 1 && (length % 2) == 1, s_expression, expr);

  SCM_SETCAR (expr, SCM_IM_NIL_COND);
  return expr;
}


SCM_SYNTAX (s_atfop, "@fop", scm_i_makbimacro, scm_m_atfop);

/* The @fop-macro handles procedure and macro applications for elisp.  The
 * input expression must have the form
 *    (@fop <var> (transformer-macro <expr> ...))
 * where <var> must be a symbol.  The expression is transformed into the
 * memoized form of either
 *    (apply <un-aliased var> (transformer-macro <expr> ...))
 * if the value of var (across all aliasing) is not a macro, or
 *    (<un-aliased var> <expr> ...)
 * if var is a macro. */
static SCM
scm_m_atfop (SCM expr, SCM env SCM_UNUSED)
{
  SCM location;
  SCM symbol;

  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 1, s_missing_expression, expr);

  symbol = SCM_CAR (cdr_expr);
  ASSERT_SYNTAX_2 (scm_is_symbol (symbol), s_bad_variable, symbol, expr);

  location = scm_symbol_fref (symbol);
  ASSERT_SYNTAX_2 (SCM_VARIABLEP (location), s_defun, symbol, expr);

  /* The elisp function `defalias' allows to define aliases for symbols.  To
   * look up such definitions, the chain of symbol definitions has to be
   * followed up to the terminal symbol.  */
  while (scm_is_symbol (SCM_VARIABLE_REF (location)))
    {
      const SCM alias = SCM_VARIABLE_REF (location);
      location = scm_symbol_fref (alias);
      ASSERT_SYNTAX_2 (SCM_VARIABLEP (location), s_defun, symbol, expr);
    }

  /* Memoize the value location belonging to the terminal symbol.  */
  SCM_SETCAR (cdr_expr, location);

  if (!SCM_MACROP (SCM_VARIABLE_REF (location)))
    {
      /* Since the location does not contain a macro, the form is a procedure
       * application.  Replace `@fop' by `@apply' and transform the expression
       * including the `transformer-macro'.  */
      SCM_SETCAR (expr, SCM_IM_APPLY);
      return expr;
    }
  else
    {
      /* Since the location contains a macro, the arguments should not be
       * transformed, so the `transformer-macro' is cut out.  The resulting
       * expression starts with the memoized variable, that is at the cdr of
       * the input expression.  */
      SCM_SETCDR (cdr_expr, SCM_CDADR (cdr_expr));
      return cdr_expr;
    }
}

#endif /* SCM_ENABLE_ELISP */


static SCM
unmemoize_builtin_macro (const SCM expr, const SCM env)
{
  switch (ISYMNUM (SCM_CAR (expr)))
    {
    case (ISYMNUM (SCM_IM_AND)):
      return unmemoize_and (expr, env);

    case (ISYMNUM (SCM_IM_BEGIN)):
      return unmemoize_begin (expr, env);

    case (ISYMNUM (SCM_IM_CASE)):
      return unmemoize_case (expr, env);

    case (ISYMNUM (SCM_IM_COND)):
      return unmemoize_cond (expr, env);

    case (ISYMNUM (SCM_IM_DELAY)):
      return unmemoize_delay (expr, env);

    case (ISYMNUM (SCM_IM_DO)):
      return unmemoize_do (expr, env);

    case (ISYMNUM (SCM_IM_IF)):
      return unmemoize_if (expr, env);

    case (ISYMNUM (SCM_IM_LAMBDA)):
      return unmemoize_lambda (expr, env);

    case (ISYMNUM (SCM_IM_LET)):
      return unmemoize_let (expr, env);

    case (ISYMNUM (SCM_IM_LETREC)):
      return unmemoize_letrec (expr, env);

    case (ISYMNUM (SCM_IM_LETSTAR)):
      return unmemoize_letstar (expr, env);

    case (ISYMNUM (SCM_IM_OR)):
      return unmemoize_or (expr, env);

    case (ISYMNUM (SCM_IM_QUOTE)):
      return unmemoize_quote (expr, env);

    case (ISYMNUM (SCM_IM_SET_X)):
      return unmemoize_set_x (expr, env);

    case (ISYMNUM (SCM_IM_APPLY)):
      return unmemoize_apply (expr, env);

    case (ISYMNUM (SCM_IM_BIND)):
      return unmemoize_exprs (expr, env);  /* FIXME */

    case (ISYMNUM (SCM_IM_CONT)):
      return unmemoize_atcall_cc (expr, env);

    case (ISYMNUM (SCM_IM_CALL_WITH_VALUES)):
      return unmemoize_at_call_with_values (expr, env);

#if 0
    /* See futures.h for a comment why futures are not enabled.
     */
    case (ISYMNUM (SCM_IM_FUTURE)):
      return unmemoize_future (expr, env);
#endif

    case (ISYMNUM (SCM_IM_SLOT_REF)):
      return unmemoize_atslot_ref (expr, env);

    case (ISYMNUM (SCM_IM_SLOT_SET_X)):
      return unmemoize_atslot_set_x (expr, env);

    case (ISYMNUM (SCM_IM_NIL_COND)):
      return unmemoize_exprs (expr, env);  /* FIXME */

    default:
      return unmemoize_exprs (expr, env);  /* FIXME */
    }
}


/* scm_i_unmemocopy_expr and scm_i_unmemocopy_body take a memoized expression
 * respectively a memoized body together with its environment and rewrite it
 * to its original form.  Thus, these functions are the inversion of the
 * rewrite rules above.  The procedure is not optimized for speed.  It's used
 * in scm_i_unmemoize_expr, scm_procedure_source, macro_print and scm_iprin1.
 *
 * Unmemoizing is not a reliable process.  You cannot in general expect to get
 * the original source back.
 *
 * However, GOOPS currently relies on this for method compilation.  This ought
 * to change.  */

SCM
scm_i_unmemocopy_expr (SCM expr, SCM env)
{
  const SCM source_properties = scm_whash_lookup (scm_source_whash, expr);
  const SCM um_expr = unmemoize_expression (expr, env);

  if (scm_is_true (source_properties))
    scm_whash_insert (scm_source_whash, um_expr, source_properties);

  return um_expr;
}

SCM
scm_i_unmemocopy_body (SCM forms, SCM env)
{
  const SCM source_properties = scm_whash_lookup (scm_source_whash, forms);
  const SCM um_forms = unmemoize_exprs (forms, env);

  if (scm_is_true (source_properties))
    scm_whash_insert (scm_source_whash, um_forms, source_properties);

  return um_forms;
}


#if (SCM_ENABLE_DEPRECATED == 1)

static SCM scm_m_undefine (SCM expr, SCM env);

SCM_SYNTAX (s_undefine, "undefine", scm_makacro, scm_m_undefine);

static SCM
scm_m_undefine (SCM expr, SCM env)
{
  SCM variable;
  SCM location;

  const SCM cdr_expr = SCM_CDR (expr);
  ASSERT_SYNTAX (SCM_TOP_LEVEL (env), "Bad undefine placement in", expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) >= 0, s_bad_expression, expr);
  ASSERT_SYNTAX (scm_ilength (cdr_expr) == 1, s_expression, expr);

  scm_c_issue_deprecation_warning
    ("`undefine' is deprecated.\n");

  variable = SCM_CAR (cdr_expr);
  ASSERT_SYNTAX_2 (scm_is_symbol (variable), s_bad_variable, variable, expr);
  location = scm_sym2var (variable, scm_env_top_level (env), SCM_BOOL_F);
  ASSERT_SYNTAX_2 (scm_is_true (location)
                   && !SCM_UNBNDP (SCM_VARIABLE_REF (location)),
                   "variable already unbound ", variable, expr);
  SCM_VARIABLE_SET (location, SCM_UNDEFINED);
  return SCM_UNSPECIFIED;
}

#endif /* SCM_ENABLE_DEPRECATED */


\f
/*****************************************************************************/
/*****************************************************************************/
/*                 The definitions for execution start here.                 */
/*****************************************************************************/
/*****************************************************************************/

SCM_GLOBAL_SYMBOL (scm_sym_enter_frame, "enter-frame");
SCM_GLOBAL_SYMBOL (scm_sym_apply_frame, "apply-frame");
SCM_GLOBAL_SYMBOL (scm_sym_exit_frame, "exit-frame");
SCM_GLOBAL_SYMBOL (scm_sym_memoize_symbol, "memoize-symbol");
SCM_GLOBAL_SYMBOL (scm_sym_trace, "trace");
SCM_SYMBOL (sym_instead, "instead");

/* A function object to implement "apply" for non-closure functions.  */
static SCM f_apply;
/* An endless list consisting of #<undefined> objects:  */
static SCM undefineds;


int
scm_badargsp (SCM formals, SCM args)
{
  while (!scm_is_null (formals))
    {
      if (!scm_is_pair (formals)) 
        return 0;
      if (scm_is_null (args)) 
        return 1;
      formals = SCM_CDR (formals);
      args = SCM_CDR (args);
    }
  return !scm_is_null (args) ? 1 : 0;
}

\f

/* The evaluator contains a plethora of EVAL symbols.  
 *
 *
 *   SCM_I_EVALIM is used when it is known that the expression is an
 *   immediate.  (This macro never calls an evaluator.)
 *
 *   SCM_I_XEVAL evaluates an expression that is expected to have its symbols already
 *   memoized.  Expressions that are not of the form '(<form> <form> ...)' are
 *   evaluated inline without calling an evaluator.
 *
 *   This macro uses ceval or deval depending on its 3rd argument.
 *
 *   SCM_I_XEVALCAR evaluates the car of an expression 'X:(Y:<form> <form> ...)',
 *   potentially replacing a symbol at the position Y:<form> by its memoized
 *   variable.  If Y:<form> is not of the form '(<form> <form> ...)', the
 *   evaluation is performed inline without calling an evaluator.
 *  
 *   This macro uses ceval or deval depending on its 3rd argument.
 *
 */

#define SCM_I_EVALIM2(x) \
  ((scm_is_eq ((x), SCM_EOL) \
    ? syntax_error (s_empty_combination, (x), SCM_UNDEFINED), 0 \
    : 0), \
   (x))

#define SCM_I_EVALIM(x, env) (SCM_ILOCP (x) \
                            ? *scm_ilookup ((x), (env)) \
                            : SCM_I_EVALIM2(x))

#define SCM_I_XEVAL(x, env, debug_p)                    \
  (SCM_IMP (x) \
   ? SCM_I_EVALIM2 (x) \
   : (SCM_VARIABLEP (x) \
      ? SCM_VARIABLE_REF (x) \
      : (scm_is_pair (x) \
         ? (debug_p \
            ? deval ((x), (env)) \
            : ceval ((x), (env))) \
         : (x))))

#define SCM_I_XEVALCAR(x, env, debug_p)                 \
  (SCM_IMP (SCM_CAR (x)) \
   ? SCM_I_EVALIM (SCM_CAR (x), (env)) \
   : (SCM_VARIABLEP (SCM_CAR (x)) \
      ? SCM_VARIABLE_REF (SCM_CAR (x)) \
      : (scm_is_pair (SCM_CAR (x)) \
         ? (debug_p \
            ? deval (SCM_CAR (x), (env)) \
            : ceval (SCM_CAR (x), (env))) \
         : (!scm_is_symbol (SCM_CAR (x)) \
            ? SCM_CAR (x) \
            : *scm_lookupcar ((x), (env), 1)))))

scm_i_pthread_mutex_t source_mutex;


/* Lookup a given local variable in an environment.  The local variable is
 * given as an iloc, that is a triple <frame, binding, last?>, where frame
 * indicates the relative number of the environment frame (counting upwards
 * from the innermost environment frame), binding indicates the number of the
 * binding within the frame, and last? (which is extracted from the iloc using
 * the macro SCM_ICDRP) indicates whether the binding forms the binding at the
 * very end of the improper list of bindings.  */
SCM *
scm_ilookup (SCM iloc, SCM env)
{
  unsigned int frame_nr = SCM_IFRAME (iloc);
  unsigned int binding_nr = SCM_IDIST (iloc);
  SCM frames = env;
  SCM bindings;
 
  for (; 0 != frame_nr; --frame_nr)
    frames = SCM_CDR (frames);

  bindings = SCM_CAR (frames);
  for (; 0 != binding_nr; --binding_nr)
    bindings = SCM_CDR (bindings);

  if (SCM_ICDRP (iloc))
    return SCM_CDRLOC (bindings);
  return SCM_CARLOC (SCM_CDR (bindings));
}


SCM_SYMBOL (scm_unbound_variable_key, "unbound-variable");

/* Call this for variables that are unfound.
 */
static void
error_unbound_variable (SCM symbol)
{
  scm_error (scm_unbound_variable_key, NULL,
             "Unbound variable: ~S",
             scm_list_1 (symbol), SCM_BOOL_F);
}

/* Call this for variables that are found but contain SCM_UNDEFINED.
 */
static void
error_defined_variable (SCM symbol)
{
  /* We use the 'unbound-variable' key here as well, since it
     basically is the same kind of error, with a slight variation in
     the displayed message.
  */
  scm_error (scm_unbound_variable_key, NULL,
             "Variable used before given a value: ~S",
             scm_list_1 (symbol), SCM_BOOL_F);
}


/* The Lookup Car Race
    - by Eva Luator

   Memoization of variables and special forms is done while executing
   the code for the first time.  As long as there is only one thread
   everything is fine, but as soon as two threads execute the same
   code concurrently `for the first time' they can come into conflict.

   This memoization includes rewriting variable references into more
   efficient forms and expanding macros.  Furthermore, macro expansion
   includes `compiling' special forms like `let', `cond', etc. into
   tree-code instructions.

   There shouldn't normally be a problem with memoizing local and
   global variable references (into ilocs and variables), because all
   threads will mutate the code in *exactly* the same way and (if I
   read the C code correctly) it is not possible to observe a half-way
   mutated cons cell.  The lookup procedure can handle this
   transparently without any critical sections.

   It is different with macro expansion, because macro expansion
   happens outside of the lookup procedure and can't be
   undone. Therefore the lookup procedure can't cope with it.  It has
   to indicate failure when it detects a lost race and hope that the
   caller can handle it.  Luckily, it turns out that this is the case.

   An example to illustrate this: Suppose that the following form will
   be memoized concurrently by two threads

       (let ((x 12)) x)

   Let's first examine the lookup of X in the body.  The first thread
   decides that it has to find the symbol "x" in the environment and
   starts to scan it.  Then the other thread takes over and actually
   overtakes the first.  It looks up "x" and substitutes an
   appropriate iloc for it.  Now the first thread continues and
   completes its lookup.  It comes to exactly the same conclusions as
   the second one and could - without much ado - just overwrite the
   iloc with the same iloc.

   But let's see what will happen when the race occurs while looking
   up the symbol "let" at the start of the form.  It could happen that
   the second thread interrupts the lookup of the first thread and not
   only substitutes a variable for it but goes right ahead and
   replaces it with the compiled form (#@let* (x 12) x).  Now, when
   the first thread completes its lookup, it would replace the #@let*
   with a variable containing the "let" binding, effectively reverting
   the form to (let (x 12) x).  This is wrong.  It has to detect that
   it has lost the race and the evaluator has to reconsider the
   changed form completely.

   This race condition could be resolved with some kind of traffic
   light (like mutexes) around scm_lookupcar, but I think that it is
   best to avoid them in this case.  They would serialize memoization
   completely and because lookup involves calling arbitrary Scheme
   code (via the lookup-thunk), threads could be blocked for an
   arbitrary amount of time or even deadlock.  But with the current
   solution a lot of unnecessary work is potentially done. */

/* SCM_LOOKUPCAR1 is what SCM_LOOKUPCAR used to be but is allowed to
   return NULL to indicate a failed lookup due to some race conditions
   between threads.  This only happens when VLOC is the first cell of
   a special form that will eventually be memoized (like `let', etc.)
   In that case the whole lookup is bogus and the caller has to
   reconsider the complete special form.

   SCM_LOOKUPCAR is still there, of course.  It just calls
   SCM_LOOKUPCAR1 and aborts on receiving NULL.  So SCM_LOOKUPCAR
   should only be called when it is known that VLOC is not the first
   pair of a special form.  Otherwise, use SCM_LOOKUPCAR1 and check
   for NULL.  I think I've found the only places where this
   applies. */

static SCM *
scm_lookupcar1 (SCM vloc, SCM genv, int check)
{
  SCM env = genv;
  register SCM *al, fl, var = SCM_CAR (vloc);
  register SCM iloc = SCM_ILOC00;
  for (; SCM_NIMP (env); env = SCM_CDR (env))
    {
      if (!scm_is_pair (SCM_CAR (env)))
        break;
      al = SCM_CARLOC (env);
      for (fl = SCM_CAR (*al); SCM_NIMP (fl); fl = SCM_CDR (fl))
        {
          if (!scm_is_pair (fl))
            {
              if (scm_is_eq (fl, var))
              {
                if (!scm_is_eq (SCM_CAR (vloc), var))
                  goto race;
                SCM_SET_CELL_WORD_0 (vloc, SCM_UNPACK (iloc) + SCM_ICDR);
                return SCM_CDRLOC (*al);
              }
              else
                break;
            }
          al = SCM_CDRLOC (*al);
          if (scm_is_eq (SCM_CAR (fl), var))
            {
              if (SCM_UNBNDP (SCM_CAR (*al)))
                error_defined_variable (var);
              if (!scm_is_eq (SCM_CAR (vloc), var))
                goto race;
              SCM_SETCAR (vloc, iloc);
              return SCM_CARLOC (*al);
            }
          iloc = SCM_PACK (SCM_UNPACK (iloc) + SCM_IDINC);
        }
      iloc = SCM_PACK ((~SCM_IDSTMSK) & (SCM_UNPACK(iloc) + SCM_IFRINC));
    }
  {
    SCM top_thunk, real_var;
    if (SCM_NIMP (env))
      {
        top_thunk = SCM_CAR (env); /* env now refers to a
                                      top level env thunk */
        env = SCM_CDR (env);
      }
    else
      top_thunk = SCM_BOOL_F;
    real_var = scm_sym2var (var, top_thunk, SCM_BOOL_F);
    if (scm_is_false (real_var))
      goto errout;

    if (!scm_is_null (env) || SCM_UNBNDP (SCM_VARIABLE_REF (real_var)))
      {
      errout:
        if (check)
          {
            if (scm_is_null (env))
              error_unbound_variable (var);
            else
              scm_misc_error (NULL, "Damaged environment: ~S",
                              scm_list_1 (var));
          }
        else 
          {
            /* A variable could not be found, but we shall
               not throw an error. */
            static SCM undef_object = SCM_UNDEFINED;
            return &undef_object;
          }
      }

    if (!scm_is_eq (SCM_CAR (vloc), var))
      {
        /* Some other thread has changed the very cell we are working
           on.  In effect, it must have done our job or messed it up
           completely. */
      race:
        var = SCM_CAR (vloc);
        if (SCM_VARIABLEP (var))
          return SCM_VARIABLE_LOC (var);
        if (SCM_ILOCP (var))
          return scm_ilookup (var, genv);
        /* We can't cope with anything else than variables and ilocs.  When
           a special form has been memoized (i.e. `let' into `#@let') we
           return NULL and expect the calling function to do the right
           thing.  For the evaluator, this means going back and redoing
           the dispatch on the car of the form. */
        return NULL;
      }

    SCM_SETCAR (vloc, real_var);
    return SCM_VARIABLE_LOC (real_var);
  }
}

SCM *
scm_lookupcar (SCM vloc, SCM genv, int check)
{
  SCM *loc = scm_lookupcar1 (vloc, genv, check);
  if (loc == NULL)
    abort ();
  return loc;
}


/* During execution, look up a symbol in the top level of the given local
 * environment and return the corresponding variable object.  If no binding
 * for the symbol can be found, an 'Unbound variable' error is signalled.  */
static SCM
lazy_memoize_variable (const SCM symbol, const SCM environment)
{
  const SCM top_level = scm_env_top_level (environment);
  const SCM variable = scm_sym2var (symbol, top_level, SCM_BOOL_F);

  if (scm_is_false (variable))
    error_unbound_variable (symbol);
  else
    return variable;
}


SCM
scm_eval_car (SCM pair, SCM env)
{
  return SCM_I_XEVALCAR (pair, env, scm_debug_mode_p);
}


SCM
scm_eval_body (SCM code, SCM env)
{
  SCM next;

 again:
  next = SCM_CDR (code);
  while (!scm_is_null (next))
    {
      if (SCM_IMP (SCM_CAR (code)))
        {
          if (SCM_ISYMP (SCM_CAR (code)))
            {
              scm_dynwind_begin (0);
              scm_i_dynwind_pthread_mutex_lock (&source_mutex);
              /* check for race condition */
              if (SCM_ISYMP (SCM_CAR (code)))
                m_expand_body (code, env);
              scm_dynwind_end ();
              goto again;
            }
        }
      else
        SCM_I_XEVAL (SCM_CAR (code), env, scm_debug_mode_p);
      code = next;
      next = SCM_CDR (code);
    }
  return SCM_I_XEVALCAR (code, env, scm_debug_mode_p);
}


/* scm_last_debug_frame contains a pointer to the last debugging information
 * stack frame.  It is accessed very often from the debugging evaluator, so it
 * should probably not be indirectly addressed.  Better to save and restore it
 * from the current root at any stack swaps.
 */

/* scm_debug_eframe_size is the number of slots available for pseudo
 * stack frames at each real stack frame.
 */

long scm_debug_eframe_size;

int scm_debug_mode_p;
int scm_check_entry_p;
int scm_check_apply_p;
int scm_check_exit_p;
int scm_check_memoize_p;

long scm_eval_stack;

scm_t_option scm_eval_opts[] = {
  { SCM_OPTION_INTEGER, "stack", 22000, "Size of thread stacks (in machine words)." },
  { 0 }
};

scm_t_option scm_debug_opts[] = {
  { SCM_OPTION_BOOLEAN, "cheap", 1,
    "*This option is now obsolete.  Setting it has no effect." },
  { SCM_OPTION_BOOLEAN, "breakpoints", 0, "*Check for breakpoints." },
  { SCM_OPTION_BOOLEAN, "trace", 0, "*Trace mode." },
  { SCM_OPTION_BOOLEAN, "procnames", 1,
    "Record procedure names at definition." },
  { SCM_OPTION_BOOLEAN, "backwards", 0,
    "Display backtrace in anti-chronological order." },
  { SCM_OPTION_INTEGER, "width", 79, "Maximal width of backtrace." },
  { SCM_OPTION_INTEGER, "indent", 10, "Maximal indentation in backtrace." },
  { SCM_OPTION_INTEGER, "frames", 3,
    "Maximum number of tail-recursive frames in backtrace." },
  { SCM_OPTION_INTEGER, "maxdepth", 1000,
    "Maximal number of stored backtrace frames." },
  { SCM_OPTION_INTEGER, "depth", 20, "Maximal length of printed backtrace." },
  { SCM_OPTION_BOOLEAN, "backtrace", 0, "Show backtrace on error." },
  { SCM_OPTION_BOOLEAN, "debug", 0, "Use the debugging evaluator." },
  /* This default stack limit will be overridden by debug.c:init_stack_limit(),
     if we have getrlimit() and the stack limit is not INFINITY. But it is still
     important, as some systems have both the soft and the hard limits set to
     INFINITY; in that case we fall back to this value.

     The situation is aggravated by certain compilers, which can consume
     "beaucoup de stack", as they say in France.

     See http://thread.gmane.org/gmane.lisp.guile.devel/8599/focus=8662 for
     more discussion. This setting is 640 KB on 32-bit arches (should be enough
     for anyone!) or a whoppin' 1280 KB on 64-bit arches.
  */
  { SCM_OPTION_INTEGER, "stack", 160000, "Stack size limit (measured in words; 0 = no check)." },
  { SCM_OPTION_SCM, "show-file-name", (unsigned long)SCM_BOOL_T,
    "Show file names and line numbers "
    "in backtraces when not `#f'.  A value of `base' "
    "displays only base names, while `#t' displays full names."},
  { SCM_OPTION_BOOLEAN, "warn-deprecated", 0,
    "Warn when deprecated features are used." },
  { 0 }, 
};


/*
 * this ordering is awkward and illogical, but we maintain it for
 * compatibility. --hwn
 */
scm_t_option scm_evaluator_trap_table[] = {
  { SCM_OPTION_BOOLEAN, "traps", 0, "Enable evaluator traps." },
  { SCM_OPTION_BOOLEAN, "enter-frame", 0, "Trap when eval enters new frame." },
  { SCM_OPTION_BOOLEAN, "apply-frame", 0, "Trap when entering apply." },
  { SCM_OPTION_BOOLEAN, "exit-frame", 0, "Trap when exiting eval or apply." },
  { SCM_OPTION_SCM, "enter-frame-handler", (unsigned long)SCM_BOOL_F, "Handler for enter-frame traps." },
  { SCM_OPTION_SCM, "apply-frame-handler", (unsigned long)SCM_BOOL_F, "Handler for apply-frame traps." },
  { SCM_OPTION_SCM, "exit-frame-handler", (unsigned long)SCM_BOOL_F, "Handler for exit-frame traps." },
  { SCM_OPTION_BOOLEAN, "memoize-symbol", 0, "Trap when memoizing a symbol." },
  { SCM_OPTION_SCM, "memoize-symbol-handler", (unsigned long)SCM_BOOL_F, "The handler for memoization." },
  { 0 }
};


SCM_DEFINE (scm_eval_options_interface, "eval-options-interface", 0, 1, 0, 
            (SCM setting),
            "Option interface for the evaluation options. Instead of using\n"
            "this procedure directly, use the procedures @code{eval-enable},\n"
            "@code{eval-disable}, @code{eval-set!} and @code{eval-options}.")
#define FUNC_NAME s_scm_eval_options_interface
{
  SCM ans;
  
  scm_dynwind_begin (0);
  scm_dynwind_critical_section (SCM_BOOL_F);
  ans = scm_options (setting,
                     scm_eval_opts,
                     FUNC_NAME);
  scm_eval_stack = SCM_EVAL_STACK * sizeof (void *);
  scm_dynwind_end ();

  return ans;
}
#undef FUNC_NAME


SCM_DEFINE (scm_evaluator_traps, "evaluator-traps-interface", 0, 1, 0, 
            (SCM setting),
            "Option interface for the evaluator trap options.")
#define FUNC_NAME s_scm_evaluator_traps
{
  SCM ans;

  
  scm_options_try (setting,
                   scm_evaluator_trap_table,
                   FUNC_NAME, 1);
  SCM_CRITICAL_SECTION_START;
  ans = scm_options (setting,
                     scm_evaluator_trap_table,
                     FUNC_NAME);

  /* njrev: same again. */
  SCM_RESET_DEBUG_MODE;
  SCM_CRITICAL_SECTION_END;
  return ans;
}
#undef FUNC_NAME



\f

/* Simple procedure calls
 */

SCM
scm_call_0 (SCM proc)
{
  if (SCM_PROGRAM_P (proc))
    return scm_c_vm_run (scm_the_vm (), proc, NULL, 0);
  else
    return scm_apply (proc, SCM_EOL, SCM_EOL);
}

SCM
scm_call_1 (SCM proc, SCM arg1)
{
  if (SCM_PROGRAM_P (proc))
    return scm_c_vm_run (scm_the_vm (), proc, &arg1, 1);
  else
    return scm_apply (proc, arg1, scm_listofnull);
}

SCM
scm_call_2 (SCM proc, SCM arg1, SCM arg2)
{
  if (SCM_PROGRAM_P (proc))
    {
      SCM args[] = { arg1, arg2 };
      return scm_c_vm_run (scm_the_vm (), proc, args, 2);
    }
  else
    return scm_apply (proc, arg1, scm_cons (arg2, scm_listofnull));
}

SCM
scm_call_3 (SCM proc, SCM arg1, SCM arg2, SCM arg3)
{
  if (SCM_PROGRAM_P (proc))
    {
      SCM args[] = { arg1, arg2, arg3 };
      return scm_c_vm_run (scm_the_vm (), proc, args, 3);
    }
  else
    return scm_apply (proc, arg1, scm_cons2 (arg2, arg3, scm_listofnull));
}

SCM
scm_call_4 (SCM proc, SCM arg1, SCM arg2, SCM arg3, SCM arg4)
{
  if (SCM_PROGRAM_P (proc))
    {
      SCM args[] = { arg1, arg2, arg3, arg4 };
      return scm_c_vm_run (scm_the_vm (), proc, args, 4);
    }
  else
    return scm_apply (proc, arg1, scm_cons2 (arg2, arg3,
                                             scm_cons (arg4, scm_listofnull)));
}

/* Simple procedure applies
 */

SCM
scm_apply_0 (SCM proc, SCM args)
{
  return scm_apply (proc, args, SCM_EOL);
}

SCM
scm_apply_1 (SCM proc, SCM arg1, SCM args)
{
  return scm_apply (proc, scm_cons (arg1, args), SCM_EOL);
}

SCM
scm_apply_2 (SCM proc, SCM arg1, SCM arg2, SCM args)
{
  return scm_apply (proc, scm_cons2 (arg1, arg2, args), SCM_EOL);
}

SCM
scm_apply_3 (SCM proc, SCM arg1, SCM arg2, SCM arg3, SCM args)
{
  return scm_apply (proc, scm_cons (arg1, scm_cons2 (arg2, arg3, args)),
                    SCM_EOL);
}

/* This code processes the arguments to apply:

   (apply PROC ARG1 ... ARGS)

   Given a list (ARG1 ... ARGS), this function conses the ARG1
   ... arguments onto the front of ARGS, and returns the resulting
   list.  Note that ARGS is a list; thus, the argument to this
   function is a list whose last element is a list.

   Apply calls this function, and applies PROC to the elements of the
   result.  apply:nconc2last takes care of building the list of
   arguments, given (ARG1 ... ARGS).

   Rather than do new consing, apply:nconc2last destroys its argument.
   On that topic, this code came into my care with the following
   beautifully cryptic comment on that topic: "This will only screw
   you if you do (scm_apply scm_apply '( ... ))"  If you know what
   they're referring to, send me a patch to this comment.  */

SCM_DEFINE (scm_nconc2last, "apply:nconc2last", 1, 0, 0, 
            (SCM lst),
            "Given a list (@var{arg1} @dots{} @var{args}), this function\n"
            "conses the @var{arg1} @dots{} arguments onto the front of\n"
            "@var{args}, and returns the resulting list. Note that\n"
            "@var{args} is a list; thus, the argument to this function is\n"
            "a list whose last element is a list.\n"
            "Note: Rather than do new consing, @code{apply:nconc2last}\n"
            "destroys its argument, so use with care.")
#define FUNC_NAME s_scm_nconc2last
{
  SCM *lloc;
  SCM_VALIDATE_NONEMPTYLIST (1, lst);
  lloc = &lst;
  while (!scm_is_null (SCM_CDR (*lloc))) /* Perhaps should be
                                          SCM_NULL_OR_NIL_P, but not
                                          needed in 99.99% of cases,
                                          and it could seriously hurt
                                          performance. - Neil */
    lloc = SCM_CDRLOC (*lloc);
  SCM_ASSERT (scm_ilength (SCM_CAR (*lloc)) >= 0, lst, SCM_ARG1, FUNC_NAME);
  *lloc = SCM_CAR (*lloc);
  return lst;
}
#undef FUNC_NAME



/* SECTION: The rest of this file is only read once.
 */

/* Trampolines
 *  
 * Trampolines make it possible to move procedure application dispatch
 * outside inner loops.  The motivation was clean implementation of
 * efficient replacements of R5RS primitives in SRFI-1.
 *
 * The semantics is clear: scm_trampoline_N returns an optimized
 * version of scm_call_N (or NULL if the procedure isn't applicable
 * on N args).
 *
 * Applying the optimization to map and for-each increased efficiency
 * noticeably.  For example, (map abs ls) is now 8 times faster than
 * before.
 */

static SCM
call_subr0_0 (SCM proc)
{
  return SCM_SUBRF (proc) ();
}

static SCM
call_subr1o_0 (SCM proc)
{
  return SCM_SUBRF (proc) (SCM_UNDEFINED);
}

static SCM
call_lsubr_0 (SCM proc)
{
  return SCM_SUBRF (proc) (SCM_EOL);
}

SCM 
scm_i_call_closure_0 (SCM proc)
{
  const SCM env = SCM_EXTEND_ENV (SCM_CLOSURE_FORMALS (proc),
                                  SCM_EOL,
                                  SCM_ENV (proc));
  const SCM result = scm_eval_body (SCM_CLOSURE_BODY (proc), env);
  return result;
}

scm_t_trampoline_0
scm_trampoline_0 (SCM proc)
{
  scm_t_trampoline_0 trampoline;

  if (SCM_IMP (proc))
    return NULL;

  switch (SCM_TYP7 (proc))
    {
    case scm_tc7_subr_0:
      trampoline = call_subr0_0;
      break;
    case scm_tc7_subr_1o:
      trampoline = call_subr1o_0;
      break;
    case scm_tc7_lsubr:
      trampoline = call_lsubr_0;
      break;
    case scm_tcs_closures:
      {
        SCM formals = SCM_CLOSURE_FORMALS (proc);
        if (scm_is_null (formals) || !scm_is_pair (formals))
          trampoline = scm_i_call_closure_0;
        else
          return NULL;
        break;
      }
    case scm_tcs_struct:
      if (SCM_OBJ_CLASS_FLAGS (proc) & SCM_CLASSF_PURE_GENERIC)
        trampoline = scm_call_generic_0;
      else if (SCM_I_OPERATORP (proc))
        trampoline = scm_call_0;
      else
        return NULL;
      break;
    case scm_tc7_smob:
      if (SCM_SMOB_APPLICABLE_P (proc))
        trampoline = SCM_SMOB_DESCRIPTOR (proc).apply_0;
      else
        return NULL;
      break;
    case scm_tc7_asubr:
    case scm_tc7_rpsubr:
    case scm_tc7_gsubr:
    case scm_tc7_pws:
    case scm_tc7_program:
      trampoline = scm_call_0;
      break;
    default:
      return NULL; /* not applicable on zero arguments */
    }
  /* We only reach this point if a valid trampoline was determined.  */

  /* If debugging is enabled, we want to see all calls to proc on the stack.
   * Thus, we replace the trampoline shortcut with scm_call_0.  */
  if (scm_debug_mode_p)
    return scm_call_0;
  else
    return trampoline;
}

static SCM
call_subr1_1 (SCM proc, SCM arg1)
{
  return SCM_SUBRF (proc) (arg1);
}

static SCM
call_subr2o_1 (SCM proc, SCM arg1)
{
  return SCM_SUBRF (proc) (arg1, SCM_UNDEFINED);
}

static SCM
call_lsubr_1 (SCM proc, SCM arg1)
{
  return SCM_SUBRF (proc) (scm_list_1 (arg1));
}

static SCM
call_dsubr_1 (SCM proc, SCM arg1)
{
  if (SCM_I_INUMP (arg1))
    {
      return (scm_from_double (SCM_DSUBRF (proc) ((double) SCM_I_INUM (arg1))));
    }
  else if (SCM_REALP (arg1))
    {
      return (scm_from_double (SCM_DSUBRF (proc) (SCM_REAL_VALUE (arg1))));
    }
  else if (SCM_BIGP (arg1))
    {
      return (scm_from_double (SCM_DSUBRF (proc) (scm_i_big2dbl (arg1))));
    }
  else if (SCM_FRACTIONP (arg1))
    {
      return (scm_from_double (SCM_DSUBRF (proc) (scm_i_fraction2double (arg1))));
    }
  SCM_WTA_DISPATCH_1_SUBR (proc, arg1, SCM_ARG1);
}

static SCM
call_cxr_1 (SCM proc, SCM arg1)
{
  return scm_i_chase_pairs (arg1, (scm_t_bits) SCM_SUBRF (proc));
}

static SCM 
call_closure_1 (SCM proc, SCM arg1)
{
  const SCM env = SCM_EXTEND_ENV (SCM_CLOSURE_FORMALS (proc),
                                  scm_list_1 (arg1),
                                  SCM_ENV (proc));
  const SCM result = scm_eval_body (SCM_CLOSURE_BODY (proc), env);
  return result;
}

scm_t_trampoline_1
scm_trampoline_1 (SCM proc)
{
  scm_t_trampoline_1 trampoline;

  if (SCM_IMP (proc))
    return NULL;

  switch (SCM_TYP7 (proc))
    {
    case scm_tc7_subr_1:
    case scm_tc7_subr_1o:
      trampoline = call_subr1_1;
      break;
    case scm_tc7_subr_2o:
      trampoline = call_subr2o_1;
      break;
    case scm_tc7_lsubr:
      trampoline = call_lsubr_1;
      break;
    case scm_tc7_dsubr:
      trampoline = call_dsubr_1;
      break;
    case scm_tc7_cxr:
      trampoline = call_cxr_1;
      break;
    case scm_tcs_closures:
      {
        SCM formals = SCM_CLOSURE_FORMALS (proc);
        if (!scm_is_null (formals)
            && (!scm_is_pair (formals) || !scm_is_pair (SCM_CDR (formals))))
          trampoline = call_closure_1;
        else
          return NULL;
        break;
      }
    case scm_tcs_struct:
      if (SCM_OBJ_CLASS_FLAGS (proc) & SCM_CLASSF_PURE_GENERIC)
        trampoline = scm_call_generic_1;
      else if (SCM_I_OPERATORP (proc))
        trampoline = scm_call_1;
      else
        return NULL;
      break;
    case scm_tc7_smob:
      if (SCM_SMOB_APPLICABLE_P (proc))
        trampoline = SCM_SMOB_DESCRIPTOR (proc).apply_1;
      else
        return NULL;
      break;
    case scm_tc7_asubr:
    case scm_tc7_rpsubr:
    case scm_tc7_gsubr:
    case scm_tc7_pws:
    case scm_tc7_program:
      trampoline = scm_call_1;
      break;
    default:
      return NULL; /* not applicable on one arg */
    }
  /* We only reach this point if a valid trampoline was determined.  */

  /* If debugging is enabled, we want to see all calls to proc on the stack.
   * Thus, we replace the trampoline shortcut with scm_call_1.  */
  if (scm_debug_mode_p)
    return scm_call_1;
  else
    return trampoline;
}

static SCM
call_subr2_2 (SCM proc, SCM arg1, SCM arg2)
{
  return SCM_SUBRF (proc) (arg1, arg2);
}

static SCM
call_lsubr2_2 (SCM proc, SCM arg1, SCM arg2)
{
  return SCM_SUBRF (proc) (arg1, arg2, SCM_EOL);
}

static SCM
call_lsubr_2 (SCM proc, SCM arg1, SCM arg2)
{
  return SCM_SUBRF (proc) (scm_list_2 (arg1, arg2));
}

static SCM 
call_closure_2 (SCM proc, SCM arg1, SCM arg2)
{
  const SCM env = SCM_EXTEND_ENV (SCM_CLOSURE_FORMALS (proc),
                                  scm_list_2 (arg1, arg2),
                                  SCM_ENV (proc));
  const SCM result = scm_eval_body (SCM_CLOSURE_BODY (proc), env);
  return result;
}

scm_t_trampoline_2
scm_trampoline_2 (SCM proc)
{
  scm_t_trampoline_2 trampoline;

  if (SCM_IMP (proc))
    return NULL;

  switch (SCM_TYP7 (proc))
    {
    case scm_tc7_subr_2:
    case scm_tc7_subr_2o:
    case scm_tc7_rpsubr:
    case scm_tc7_asubr:
      trampoline = call_subr2_2;
      break;
    case scm_tc7_lsubr_2:
      trampoline = call_lsubr2_2;
      break;
    case scm_tc7_lsubr:
      trampoline = call_lsubr_2;
      break;
    case scm_tcs_closures:
      {
        SCM formals = SCM_CLOSURE_FORMALS (proc);
        if (!scm_is_null (formals)
            && (!scm_is_pair (formals)
                || (!scm_is_null (SCM_CDR (formals))
                    && (!scm_is_pair (SCM_CDR (formals))
                        || !scm_is_pair (SCM_CDDR (formals))))))
          trampoline = call_closure_2;
        else
          return NULL;
        break;
      }
    case scm_tcs_struct:
      if (SCM_OBJ_CLASS_FLAGS (proc) & SCM_CLASSF_PURE_GENERIC)
        trampoline = scm_call_generic_2;
      else if (SCM_I_OPERATORP (proc))
        trampoline = scm_call_2;
      else
        return NULL;
      break;
    case scm_tc7_smob:
      if (SCM_SMOB_APPLICABLE_P (proc))
        trampoline = SCM_SMOB_DESCRIPTOR (proc).apply_2;
      else
        return NULL;
      break;
    case scm_tc7_gsubr:
    case scm_tc7_pws:
    case scm_tc7_program:
      trampoline = scm_call_2;
      break;
    default:
      return NULL; /* not applicable on two args */
    }
  /* We only reach this point if a valid trampoline was determined.  */

  /* If debugging is enabled, we want to see all calls to proc on the stack.
   * Thus, we replace the trampoline shortcut with scm_call_2.  */
  if (scm_debug_mode_p)
    return scm_call_2;
  else
    return trampoline;
}

/* Typechecking for multi-argument MAP and FOR-EACH.

   Verify that each element of the vector ARGV, except for the first,
   is a proper list whose length is LEN.  Attribute errors to WHO,
   and claim that the i'th element of ARGV is WHO's i+2'th argument.  */
static inline void
check_map_args (SCM argv,
                long len,
                SCM gf,
                SCM proc,
                SCM args,
                const char *who)
{
  long i;

  for (i = SCM_SIMPLE_VECTOR_LENGTH (argv) - 1; i >= 1; i--)
    {
      SCM elt = SCM_SIMPLE_VECTOR_REF (argv, i);
      long elt_len = scm_ilength (elt);

      if (elt_len < 0)
        {
          if (gf)
            scm_apply_generic (gf, scm_cons (proc, args));
          else
            scm_wrong_type_arg (who, i + 2, elt);
        }

      if (elt_len != len)
        scm_out_of_range_pos (who, elt, scm_from_long (i + 2));
    }
}


SCM_GPROC (s_map, "map", 2, 0, 1, scm_map, g_map);

/* Note: Currently, scm_map applies PROC to the argument list(s)
   sequentially, starting with the first element(s).  This is used in
   evalext.c where the Scheme procedure `map-in-order', which guarantees
   sequential behaviour, is implemented using scm_map.  If the
   behaviour changes, we need to update `map-in-order'.
*/

SCM 
scm_map (SCM proc, SCM arg1, SCM args)
#define FUNC_NAME s_map
{
  long i, len;
  SCM res = SCM_EOL;
  SCM *pres = &res;

  len = scm_ilength (arg1);
  SCM_GASSERTn (len >= 0,
                g_map, scm_cons2 (proc, arg1, args), SCM_ARG2, s_map);
  SCM_VALIDATE_REST_ARGUMENT (args);
  if (scm_is_null (args))
    {
      scm_t_trampoline_1 call = scm_trampoline_1 (proc);
      SCM_GASSERT2 (call, g_map, proc, arg1, SCM_ARG1, s_map);
      while (SCM_NIMP (arg1))
        {
          *pres = scm_list_1 (call (proc, SCM_CAR (arg1)));
          pres = SCM_CDRLOC (*pres);
          arg1 = SCM_CDR (arg1);
        }
      return res;
    }
  if (scm_is_null (SCM_CDR (args)))
    {
      SCM arg2 = SCM_CAR (args);
      int len2 = scm_ilength (arg2);
      scm_t_trampoline_2 call = scm_trampoline_2 (proc);
      SCM_GASSERTn (call,
                    g_map, scm_cons2 (proc, arg1, args), SCM_ARG1, s_map);
      SCM_GASSERTn (len2 >= 0,
                    g_map, scm_cons2 (proc, arg1, args), SCM_ARG3, s_map);
      if (len2 != len)
        SCM_OUT_OF_RANGE (3, arg2);
      while (SCM_NIMP (arg1))
        {
          *pres = scm_list_1 (call (proc, SCM_CAR (arg1), SCM_CAR (arg2)));
          pres = SCM_CDRLOC (*pres);
          arg1 = SCM_CDR (arg1);
          arg2 = SCM_CDR (arg2);
        }
      return res;
    }
  arg1 = scm_cons (arg1, args);
  args = scm_vector (arg1);
  check_map_args (args, len, g_map, proc, arg1, s_map);
  while (1)
    {
      arg1 = SCM_EOL;
      for (i = SCM_SIMPLE_VECTOR_LENGTH (args) - 1; i >= 0; i--)
        {
          SCM elt = SCM_SIMPLE_VECTOR_REF (args, i);
          if (SCM_IMP (elt)) 
            return res;
          arg1 = scm_cons (SCM_CAR (elt), arg1);
          SCM_SIMPLE_VECTOR_SET (args, i, SCM_CDR (elt));
        }
      *pres = scm_list_1 (scm_apply (proc, arg1, SCM_EOL));
      pres = SCM_CDRLOC (*pres);
    }
}
#undef FUNC_NAME


SCM_GPROC (s_for_each, "for-each", 2, 0, 1, scm_for_each, g_for_each);

SCM 
scm_for_each (SCM proc, SCM arg1, SCM args)
#define FUNC_NAME s_for_each
{
  long i, len;
  len = scm_ilength (arg1);
  SCM_GASSERTn (len >= 0, g_for_each, scm_cons2 (proc, arg1, args),
                SCM_ARG2, s_for_each);
  SCM_VALIDATE_REST_ARGUMENT (args);
  if (scm_is_null (args))
    {
      scm_t_trampoline_1 call = scm_trampoline_1 (proc);
      SCM_GASSERT2 (call, g_for_each, proc, arg1, SCM_ARG1, s_for_each);
      while (SCM_NIMP (arg1))
        {
          call (proc, SCM_CAR (arg1));
          arg1 = SCM_CDR (arg1);
        }
      return SCM_UNSPECIFIED;
    }
  if (scm_is_null (SCM_CDR (args)))
    {
      SCM arg2 = SCM_CAR (args);
      int len2 = scm_ilength (arg2);
      scm_t_trampoline_2 call = scm_trampoline_2 (proc);
      SCM_GASSERTn (call, g_for_each,
                    scm_cons2 (proc, arg1, args), SCM_ARG1, s_for_each);
      SCM_GASSERTn (len2 >= 0, g_for_each,
                    scm_cons2 (proc, arg1, args), SCM_ARG3, s_for_each);
      if (len2 != len)
        SCM_OUT_OF_RANGE (3, arg2);
      while (SCM_NIMP (arg1))
        {
          call (proc, SCM_CAR (arg1), SCM_CAR (arg2));
          arg1 = SCM_CDR (arg1);
          arg2 = SCM_CDR (arg2);
        }
      return SCM_UNSPECIFIED;
    }
  arg1 = scm_cons (arg1, args);
  args = scm_vector (arg1);
  check_map_args (args, len, g_for_each, proc, arg1, s_for_each);
  while (1)
    {
      arg1 = SCM_EOL;
      for (i = SCM_SIMPLE_VECTOR_LENGTH (args) - 1; i >= 0; i--)
        {
          SCM elt = SCM_SIMPLE_VECTOR_REF (args, i);
          if (SCM_IMP (elt))
            return SCM_UNSPECIFIED;
          arg1 = scm_cons (SCM_CAR (elt), arg1);
          SCM_SIMPLE_VECTOR_SET (args, i, SCM_CDR (elt));
        }
      scm_apply (proc, arg1, SCM_EOL);
    }
}
#undef FUNC_NAME


SCM 
scm_closure (SCM code, SCM env)
{
  SCM z;
  SCM closcar = scm_cons (code, SCM_EOL);
  z = scm_immutable_cell (SCM_UNPACK (closcar) + scm_tc3_closure,
                          (scm_t_bits) env);
  scm_remember_upto_here (closcar);
  return z;
}


scm_t_bits scm_tc16_promise;

SCM_DEFINE (scm_make_promise, "make-promise", 1, 0, 0, 
            (SCM thunk),
            "Create a new promise object.\n\n"
            "@code{make-promise} is a procedural form of @code{delay}.\n"
            "These two expressions are equivalent:\n"
            "@lisp\n"
            "(delay @var{exp})\n"
            "(make-promise (lambda () @var{exp}))\n"
            "@end lisp\n")
#define FUNC_NAME s_scm_make_promise
{
  SCM_VALIDATE_THUNK (1, thunk);
  SCM_RETURN_NEWSMOB2 (scm_tc16_promise,
                       SCM_UNPACK (thunk),
                       scm_make_recursive_mutex ());
}
#undef FUNC_NAME


static int 
promise_print (SCM exp, SCM port, scm_print_state *pstate)
{
  int writingp = SCM_WRITINGP (pstate);
  scm_puts ("#<promise ", port);
  SCM_SET_WRITINGP (pstate, 1);
  scm_iprin1 (SCM_PROMISE_DATA (exp), port, pstate);
  SCM_SET_WRITINGP (pstate, writingp);
  scm_putc ('>', port);
  return !0;
}

SCM_DEFINE (scm_force, "force", 1, 0, 0, 
            (SCM promise),
            "If the promise @var{x} has not been computed yet, compute and\n"
            "return @var{x}, otherwise just return the previously computed\n"
            "value.")
#define FUNC_NAME s_scm_force
{
  SCM_VALIDATE_SMOB (1, promise, promise);
  scm_lock_mutex (SCM_PROMISE_MUTEX (promise));
  if (!SCM_PROMISE_COMPUTED_P (promise))
    {
      SCM ans = scm_call_0 (SCM_PROMISE_DATA (promise));
      if (!SCM_PROMISE_COMPUTED_P (promise))
        {
          SCM_SET_PROMISE_DATA (promise, ans);
          SCM_SET_PROMISE_COMPUTED (promise);
        }
    }
  scm_unlock_mutex (SCM_PROMISE_MUTEX (promise));
  return SCM_PROMISE_DATA (promise);
}
#undef FUNC_NAME


SCM_DEFINE (scm_promise_p, "promise?", 1, 0, 0, 
            (SCM obj),
            "Return true if @var{obj} is a promise, i.e. a delayed computation\n"
            "(@pxref{Delayed evaluation,,,r5rs.info,The Revised^5 Report on Scheme}).")
#define FUNC_NAME s_scm_promise_p
{
  return scm_from_bool (SCM_TYP16_PREDICATE (scm_tc16_promise, obj));
}
#undef FUNC_NAME


SCM_DEFINE (scm_cons_source, "cons-source", 3, 0, 0, 
            (SCM xorig, SCM x, SCM y),
            "Create and return a new pair whose car and cdr are @var{x} and @var{y}.\n"
            "Any source properties associated with @var{xorig} are also associated\n"
            "with the new pair.")
#define FUNC_NAME s_scm_cons_source
{
  SCM p, z;
  z = scm_cons (x, y);
  /* Copy source properties possibly associated with xorig. */
  p = scm_whash_lookup (scm_source_whash, xorig);
  if (scm_is_true (p))
    scm_whash_insert (scm_source_whash, z, p);
  return z;
}
#undef FUNC_NAME


/* The function scm_copy_tree is used to copy an expression tree to allow the
 * memoizer to modify the expression during memoization.  scm_copy_tree
 * creates deep copies of pairs and vectors, but not of any other data types,
 * since only pairs and vectors will be parsed by the memoizer.
 *
 * To avoid infinite recursion due to cyclic structures, the hare-and-tortoise
 * pattern is used to detect cycles.  In fact, the pattern is used in two
 * dimensions, vertical (indicated in the code by the variable names 'hare'
 * and 'tortoise') and horizontal ('rabbit' and 'turtle').  In both
 * dimensions, the hare/rabbit will take two steps when the tortoise/turtle
 * takes one.
 *
 * The vertical dimension corresponds to recursive calls to function
 * copy_tree: This happens when descending into vector elements, into cars of
 * lists and into the cdr of an improper list.  In this dimension, the
 * tortoise follows the hare by using the processor stack: Every stack frame
 * will hold an instance of struct t_trace.  These instances are connected in
 * a way that represents the trace of the hare, which thus can be followed by
 * the tortoise.  The tortoise will always point to struct t_trace instances
 * relating to SCM objects that have already been copied.  Thus, a cycle is
 * detected if the tortoise and the hare point to the same object,
 *
 * The horizontal dimension is within one execution of copy_tree, when the
 * function cdr's along the pairs of a list.  This is the standard
 * hare-and-tortoise implementation, found several times in guile.  */

struct t_trace {
  struct t_trace *trace; /* These pointers form a trace along the stack. */
  SCM obj;               /* The object handled at the respective stack frame.*/
};

static SCM
copy_tree (
  struct t_trace *const hare,
  struct t_trace *tortoise,
  unsigned int tortoise_delay )
{
  if (!scm_is_pair (hare->obj) && !scm_is_simple_vector (hare->obj))
    {
      return hare->obj;
    }
  else
    {
      /* Prepare the trace along the stack.  */
      struct t_trace new_hare;
      hare->trace = &new_hare;

      /* The tortoise will make its step after the delay has elapsed.  Note
       * that in contrast to the typical hare-and-tortoise pattern, the step
       * of the tortoise happens before the hare takes its steps.  This is, in
       * principle, no problem, except for the start of the algorithm: Then,
       * it has to be made sure that the hare actually gets its advantage of
       * two steps.  */
      if (tortoise_delay == 0)
        {
          tortoise_delay = 1;
          tortoise = tortoise->trace;
          ASSERT_SYNTAX (!scm_is_eq (hare->obj, tortoise->obj),
                         s_bad_expression, hare->obj);
        }
      else
        {
          --tortoise_delay;
        }

      if (scm_is_simple_vector (hare->obj))
        {
          size_t length = SCM_SIMPLE_VECTOR_LENGTH (hare->obj);
          SCM new_vector = scm_c_make_vector (length, SCM_UNSPECIFIED);

          /* Each vector element is copied by recursing into copy_tree, having
           * the tortoise follow the hare into the depths of the stack.  */
          unsigned long int i;
          for (i = 0; i < length; ++i)
            {
              SCM new_element;
              new_hare.obj = SCM_SIMPLE_VECTOR_REF (hare->obj, i);
              new_element = copy_tree (&new_hare, tortoise, tortoise_delay);
              SCM_SIMPLE_VECTOR_SET (new_vector, i, new_element);
            }

          return new_vector;
        }
      else /* scm_is_pair (hare->obj) */
        {
          SCM result;
          SCM tail;

          SCM rabbit = hare->obj;
          SCM turtle = hare->obj;

          SCM copy;

          /* The first pair of the list is treated specially, in order to
           * preserve a potential source code position.  */
          result = tail = scm_cons_source (rabbit, SCM_EOL, SCM_EOL);
          new_hare.obj = SCM_CAR (rabbit);
          copy = copy_tree (&new_hare, tortoise, tortoise_delay);
          SCM_SETCAR (tail, copy);

          /* The remaining pairs of the list are copied by, horizontally,
           * having the turtle follow the rabbit, and, vertically, having the
           * tortoise follow the hare into the depths of the stack.  */
          rabbit = SCM_CDR (rabbit);
          while (scm_is_pair (rabbit))
            {
              new_hare.obj = SCM_CAR (rabbit);
              copy = copy_tree (&new_hare, tortoise, tortoise_delay);
              SCM_SETCDR (tail, scm_cons (copy, SCM_UNDEFINED));
              tail = SCM_CDR (tail);

              rabbit = SCM_CDR (rabbit);
              if (scm_is_pair (rabbit))
                {
                  new_hare.obj = SCM_CAR (rabbit);
                  copy = copy_tree (&new_hare, tortoise, tortoise_delay);
                  SCM_SETCDR (tail, scm_cons (copy, SCM_UNDEFINED));
                  tail = SCM_CDR (tail);
                  rabbit = SCM_CDR (rabbit);

                  turtle = SCM_CDR (turtle);
                  ASSERT_SYNTAX (!scm_is_eq (rabbit, turtle),
                                 s_bad_expression, rabbit);
                }
            }

          /* We have to recurse into copy_tree again for the last cdr, in
           * order to handle the situation that it holds a vector.  */
          new_hare.obj = rabbit;
          copy = copy_tree (&new_hare, tortoise, tortoise_delay);
          SCM_SETCDR (tail, copy);

          return result;
        }
    }
}

SCM_DEFINE (scm_copy_tree, "copy-tree", 1, 0, 0, 
            (SCM obj),
            "Recursively copy the data tree that is bound to @var{obj}, and return a\n"
            "the new data structure.  @code{copy-tree} recurses down the\n"
            "contents of both pairs and vectors (since both cons cells and vector\n"
            "cells may point to arbitrary objects), and stops recursing when it hits\n"
            "any other object.")
#define FUNC_NAME s_scm_copy_tree
{
  /* Prepare the trace along the stack.  */
  struct t_trace trace;
  trace.obj = obj;

  /* In function copy_tree, if the tortoise makes its step, it will do this
   * before the hare has the chance to move.  Thus, we have to make sure that
   * the very first step of the tortoise will not happen after the hare has
   * really made two steps.  This is achieved by passing '2' as the initial
   * delay for the tortoise.  NOTE: Since cycles are unlikely, giving the hare
   * a bigger advantage may improve performance slightly.  */
  return copy_tree (&trace, &trace, 2);
}
#undef FUNC_NAME


/* We have three levels of EVAL here:

   - scm_i_eval (exp, env)

     evaluates EXP in environment ENV.  ENV is a lexical environment
     structure as used by the actual tree code evaluator.  When ENV is
     a top-level environment, then changes to the current module are
     tracked by updating ENV so that it continues to be in sync with
     the current module.

   - scm_primitive_eval (exp)

     evaluates EXP in the top-level environment as determined by the
     current module.  This is done by constructing a suitable
     environment and calling scm_i_eval.  Thus, changes to the
     top-level module are tracked normally.

   - scm_eval (exp, mod_or_state)

     evaluates EXP while MOD_OR_STATE is the current module or current
     dynamic state (as appropriate).  This is done by setting the
     current module (or dynamic state) to MOD_OR_STATE, invoking
     scm_primitive_eval on EXP, and then restoring the current module
     (or dynamic state) to the value it had previously.  That is,
     while EXP is evaluated, changes to the current module (or dynamic
     state) are tracked, but these changes do not persist when
     scm_eval returns.

  For each level of evals, there are two variants, distinguished by a
  _x suffix: the ordinary variant does not modify EXP while the _x
  variant can destructively modify EXP into something completely
  unintelligible.  A Scheme data structure passed as EXP to one of the
  _x variants should not ever be used again for anything.  So when in
  doubt, use the ordinary variant.

*/

SCM 
scm_i_eval_x (SCM exp, SCM env)
{
  if (scm_is_symbol (exp))
    return *scm_lookupcar (scm_cons (exp, SCM_UNDEFINED), env, 1);
  else
    return SCM_I_XEVAL (exp, env, scm_debug_mode_p);
}

SCM 
scm_i_eval (SCM exp, SCM env)
{
  exp = scm_copy_tree (exp);
  if (scm_is_symbol (exp))
    return *scm_lookupcar (scm_cons (exp, SCM_UNDEFINED), env, 1);
  else
    return SCM_I_XEVAL (exp, env, scm_debug_mode_p);
}

SCM
scm_primitive_eval_x (SCM exp)
{
  SCM env;
  SCM transformer = scm_current_module_transformer ();
  if (SCM_NIMP (transformer))
    exp = scm_call_1 (transformer, exp);
  env = scm_top_level_env (scm_current_module_lookup_closure ());
  return scm_i_eval_x (exp, env);
}

SCM_DEFINE (scm_primitive_eval, "primitive-eval", 1, 0, 0,
            (SCM exp),
            "Evaluate @var{exp} in the top-level environment specified by\n"
            "the current module.")
#define FUNC_NAME s_scm_primitive_eval
{
  SCM env;
  SCM transformer = scm_current_module_transformer ();
  if (scm_is_true (transformer))
    exp = scm_call_1 (transformer, exp);
  env = scm_top_level_env (scm_current_module_lookup_closure ());
  return scm_i_eval (exp, env);
}
#undef FUNC_NAME


/* Eval does not take the second arg optionally.  This is intentional
 * in order to be R5RS compatible, and to prepare for the new module
 * system, where we would like to make the choice of evaluation
 * environment explicit.  */

SCM
scm_eval_x (SCM exp, SCM module_or_state)
{
  SCM res;

  scm_dynwind_begin (SCM_F_DYNWIND_REWINDABLE);
  if (scm_is_dynamic_state (module_or_state))
    scm_dynwind_current_dynamic_state (module_or_state);
  else
    scm_dynwind_current_module (module_or_state);

  res = scm_primitive_eval_x (exp);

  scm_dynwind_end ();
  return res;
}

SCM_DEFINE (scm_eval, "eval", 2, 0, 0, 
            (SCM exp, SCM module_or_state),
            "Evaluate @var{exp}, a list representing a Scheme expression,\n"
            "in the top-level environment specified by\n"
            "@var{module_or_state}.\n"
            "While @var{exp} is evaluated (using @code{primitive-eval}),\n"
            "@var{module_or_state} is made the current module when\n"
            "it is a module, or the current dynamic state when it is\n"
            "a dynamic state."
            "Example: (eval '(+ 1 2) (interaction-environment))")
#define FUNC_NAME s_scm_eval
{
  SCM res;

  scm_dynwind_begin (SCM_F_DYNWIND_REWINDABLE);
  if (scm_is_dynamic_state (module_or_state))
    scm_dynwind_current_dynamic_state (module_or_state);
  else if (scm_module_system_booted_p)
    {
      SCM_VALIDATE_MODULE (2, module_or_state);
      scm_dynwind_current_module (module_or_state);
    }
  /* otherwise if the module system isn't booted, ignore the module arg */

  res = scm_primitive_eval (exp);

  scm_dynwind_end ();
  return res;
}
#undef FUNC_NAME


/* At this point, deval and scm_dapply are generated.
 */

#define DEVAL
#include "eval.i.c"
#undef DEVAL
#include "eval.i.c"


void 
scm_init_eval ()
{
  scm_i_pthread_mutex_init (&source_mutex,
                            scm_i_pthread_mutexattr_recursive);

  scm_init_opts (scm_evaluator_traps,
                 scm_evaluator_trap_table);
  scm_init_opts (scm_eval_options_interface,
                 scm_eval_opts);
  
  scm_tc16_promise = scm_make_smob_type ("promise", 0);
  scm_set_smob_print (scm_tc16_promise, promise_print);

  undefineds = scm_list_1 (SCM_UNDEFINED);
  SCM_SETCDR (undefineds, undefineds);
  scm_permanent_object (undefineds);

  scm_listofnull = scm_list_1 (SCM_EOL);

  f_apply = scm_c_define_subr ("apply", scm_tc7_lsubr_2, scm_apply);
  scm_permanent_object (f_apply);

#include "libguile/eval.x"

  scm_add_feature ("delay");
}

/*
  Local Variables:
  c-file-style: "gnu"
  End:
*/