@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
-@c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2009, 2010, 2011, 2012
-@c Free Software Foundation, Inc.
+@c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2009, 2010,
+@c 2011, 2012, 2013, 2014 Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.
@node Control Mechanisms
@code{cond} ignores its boolean state; instead, @code{cond} evaluates
@var{guard} and applies the resulting procedure to the value(s) of
@var{test}, as if @var{guard} were the @var{consumer} argument of
-@code{call-with-values}. Iff the result of that procedure call is a
+@code{call-with-values}. If the result of that procedure call is a
true value, it evaluates @var{expression} and applies the resulting
procedure to the value(s) of @var{test}, in the same manner as the
@var{guard} was called.
Before moving on, we should mention that if the handler of a prompt is a
@code{lambda} expression, and the first argument isn't referenced, an abort to
-that prompt will not cause a continuation to be reified. This can be an
+that prompt will not cause a continuation to be reified. This can be an
important efficiency consideration to keep in mind.
+@cindex continuation, escape
+One example where this optimization matters is @dfn{escape
+continuations}. Escape continuations are delimited continuations whose
+only use is to make a non-local exit---i.e., to escape from the current
+continuation. Such continuations are invoked only once, and for this
+reason they are sometimes called @dfn{one-shot continuations}. A common
+use of escape continuations is when throwing an exception
+(@pxref{Exceptions}).
+
+The constructs below are syntactic sugar atop prompts to simplify the
+use of escape continuations.
+
+@deffn {Scheme Procedure} call-with-escape-continuation proc
+@deffnx {Scheme Procedure} call/ec proc
+Call @var{proc} with an escape continuation.
+
+In the example below, the @var{return} continuation is used to escape
+the continuation of the call to @code{fold}.
+
+@lisp
+(use-modules (ice-9 control)
+ (srfi srfi-1))
+
+(define (prefix x lst)
+ ;; Return all the elements before the first occurrence
+ ;; of X in LST.
+ (call/ec
+ (lambda (return)
+ (fold (lambda (element prefix)
+ (if (equal? element x)
+ (return (reverse prefix)) ; escape `fold'
+ (cons element prefix)))
+ '()
+ lst))))
+
+(prefix 'a '(0 1 2 a 3 4 5))
+@result{} (0 1 2)
+@end lisp
+@end deffn
+
+@deffn {Scheme Syntax} let-escape-continuation k body @dots{}
+@deffnx {Scheme Syntax} let/ec k body @dots{}
+Bind @var{k} within @var{body} to an escape continuation.
+
+This is equivalent to
+@code{(call/ec (lambda (@var{k}) @var{body} @dots{}))}.
+@end deffn
+
+
@node Shift and Reset
@subsubsection Shift, Reset, and All That
This manual prefers to speak of throwing and catching exceptions, since
this terminology matches the corresponding Guile primitives.
+The exception mechanism described in this section has connections with
+@dfn{delimited continuations} (@pxref{Prompts}). In particular,
+throwing an exception is akin to invoking an @dfn{escape continuation}
+(@pxref{Prompt Primitives, @code{call/ec}}).
+
@node Catch
@subsubsection Catching Exceptions
the respective calls so an application can communicate extra
information to those functions.
-If the data consists of an @code{SCM} object, care should be taken
-that it isn't garbage collected while still required. If the
-@code{SCM} is a local C variable, one way to protect it is to pass a
-pointer to that variable as the data parameter, since the C compiler
-will then know the value must be held on the stack. Another way is to
-use @code{scm_remember_upto_here_1} (@pxref{Remembering During
-Operations}).
+If the data consists of an @code{SCM} object, care should be taken that
+it isn't garbage collected while still required. If the @code{SCM} is a
+local C variable, one way to protect it is to pass a pointer to that
+variable as the data parameter, since the C compiler will then know the
+value must be held on the stack. Another way is to use
+@code{scm_remember_upto_here_1} (@pxref{Foreign Object Memory
+Management}).
@end deftypefn
@code{system-error} then it should be a list containing the
Unix @code{errno} value; If @var{key} is @code{signal} then it
should be a list containing the Unix signal number; If
-@var{key} is @code{out-of-range} or @code{wrong-type-arg},
+@var{key} is @code{out-of-range}, @code{wrong-type-arg},
+or @code{keyword-argument-error},
it is a list containing the bad value; otherwise
it will usually be @code{#f}.
@end deffn
@deftypefnx {C Function} void scm_wrong_type_arg (char *@var{subr}, int @var{argnum}, SCM @var{bad_value})
@deftypefnx {C Function} void scm_wrong_type_arg_msg (char *@var{subr}, int @var{argnum}, SCM @var{bad_value}, const char *@var{expected})
@deftypefnx {C Function} void scm_memory_error (char *@var{subr})
-Throw an error with the various keys described above.
@deftypefnx {C Function} void scm_misc_error (const char *@var{subr}, const char *@var{message}, SCM @var{args})
+Throw an error with the various keys described above.
In @code{scm_wrong_num_args}, @var{proc} should be a Scheme symbol
which is the name of the procedure incorrectly invoked. The other
HCoop / bpt / guile.git / blobdiff