Data Representation in Scheme
=============================

Scheme is a latently-typed language; this means that the system cannot, in general, determine the type of a given expression at compile time. Types only become apparent at run time.  Variables do not have fixed types; a variable may hold a pair at one point, an integer at the next, and a thousand-element vector later.  Instead, values, not variables, have fixed types.

In order to implement standard Scheme functions like pair? and string? and provide garbage collection, the representation of every value must contain enough information to accurately determine its type at run time.  Often, Scheme systems also use this information to determine whether a program has attempted to apply an operation to an inappropriately typed value (such as taking the car of a string).

Because variables, pairs, and vectors may hold values of any type, Scheme implementations use a uniform representation for values --- a single type large enough to hold either a complete value or a pointer to a complete value, along with the necessary typing information.

The following sections will present a simple typing system, and then make some refinements to correct its major weaknesses.  However, this is not a description of the system Guile actually uses.  It is only an illustration of the issues Guile's system must address.  We provide all the information one needs to work with Guile's data in The Libguile Runtime Environment.

A Simple Representation
-----------------------

The simplest way to meet the above requirements in C would be to represent each value as a pointer to a structure containing a type indicator, followed by a union carrying the real value.  Assuming that SCM is the name of our universal type, we can write:

     enum type { integer, pair, string, vector, ... };

     typedef struct value *SCM;

     struct value {
       enum type type;
       union {
         int integer;
         struct { SCM car, cdr; } pair;
         struct { int length; char *elts; } string;
         struct { int length; SCM  *elts; } vector;
         ...
       } value;
     };

with the ellipses replaced with code for the remaining Scheme types.

This representation is sufficient to implement all of Scheme's semantics.  If X is an SCM value:

  * To test if X is an integer, we can write X->type == integer.

  * To find its value, we can write X->value.integer.

  * To test if X is a vector, we can write X->type == vector.

  * If we know X is a vector, we can write X->value.vector.elts[0] to refer to its first element.

  * If we know X is a pair, we can write X->value.pair.car to extract its car.

Faster Integers
---------------

Unfortunately, the above representation has a serious disadvantage.  In order to return an integer, an expression must allocate a struct value, initialize it to represent that integer, and return a pointer to it.  Furthermore, fetching an integer's value requires a memory reference, which is much slower than a register reference on most processors.  Since integers are extremely common, this representation is too costly, in both time and space.  Integers should be very cheap to create and manipulate.

One possible solution comes from the observation that, on many architectures, structures must be aligned on a four-byte boundary. (Whether or not the machine actually requires it, we can write our own allocator for struct value objects that assures this is true.) In this case, the lower two bits of the structure's address are known to be zero.

This gives us the room we need to provide an improved representation for integers.  We make the following rules:

  * If the lower two bits of an SCM value are zero, then the SCM value is a pointer to a struct value, and everything proceeds as before.

  * Otherwise, the SCM value represents an integer, whose value appears in its upper bits.

Here is C code implementing this convention:

     enum type { pair, string, vector, ... };

     typedef struct value *SCM;

     struct value {
       enum type type;
       union {
         struct { SCM car, cdr; } pair;
         struct { int length; char *elts; } string;
         struct { int length; SCM  *elts; } vector;
         ...
       } value;
     };

     #define POINTER_P(x) (((int) (x) & 3) == 0)
     #define INTEGER_P(x) (! POINTER_P (x))

     #define GET_INTEGER(x)  ((int) (x) >> 2)
     #define MAKE_INTEGER(x) ((SCM) (((x) << 2) | 1))

Notice that integer no longer appears as an element of enum type, and the union has lost its integer member.  Instead, we use the POINTER_P and INTEGER_P macros to make a coarse classification of values into integers and non-integers, and do further type testing as before.

Here's how we would answer the questions posed above (again, assume X is an SCM value):

  * To test if X is an integer, we can write INTEGER_P (X).

  * To find its value, we can write GET_INTEGER (X).

  * To test if X is a vector, we can write:

            POINTER_P (X) && X->type == vector

     Given the new representation, we must make sure X is truly a pointer before we dereference it to determine its complete type.

  * If we know X is a vector, we can write X->value.vector.elts[0] to refer to its first element, as before.

  * If we know X is a pair, we can write X->value.pair.car to extract its car, just as before.

This representation allows us to operate more efficiently on integers than the first.  For example, if X and Y are known to be integers, we can compute their sum as follows:

     MAKE_INTEGER (GET_INTEGER (X) + GET_INTEGER (Y))

Now, integer math requires no allocation or memory references.  Most real Scheme systems actually use an even more efficient representation, but this essay isn't about bit-twiddling.  (Hint: what if pointers had 01 in their least significant bits, and integers had 00?)

Cheaper Pairs
-------------

However, there is yet another issue to confront.  Most Scheme heaps contain more pairs than any other type of object; Jonathan Rees says that pairs occupy 45% of the heap in his Scheme implementation, Scheme 48.  However, our representation above spends three SCM-sized words per pair --- one for the type, and two for the CAR and CDR.  Is there any way to represent pairs using only two words?

Let us refine the convention we established earlier.  Let us assert that:

  * If the bottom two bits of an SCM value are #b00, then it is a pointer, as before.

  * If the bottom two bits are #b01, then the upper bits are an integer.  This is a bit more restrictive than before.

  * If the bottom two bits are #b10, then the value, with the bottom two bits masked out, is the address of a pair.

Here is the new C code:

     enum type { string, vector, ... };

     typedef struct value *SCM;

     struct value {
       enum type type;
       union {
         struct { int length; char *elts; } string;
         struct { int length; SCM  *elts; } vector;
         ...
       } value;
     };

     struct pair {
       SCM car, cdr;
     };

     #define POINTER_P(x) (((int) (x) & 3) == 0)

     #define INTEGER_P(x)  (((int) (x) & 3) == 1)
     #define GET_INTEGER(x)  ((int) (x) >> 2)
     #define MAKE_INTEGER(x) ((SCM) (((x) << 2) | 1))

     #define PAIR_P(x) (((int) (x) & 3) == 2)
     #define GET_PAIR(x) ((struct pair *) ((int) (x) & ~3))

Notice that enum type and struct value now only contain provisions for vectors and strings; both integers and pairs have become special cases.  The code above also assumes that an int is large enough to hold a pointer, which isn't generally true.

Our list of examples is now as follows:

  * To test if X is an integer, we can write INTEGER_P (X); this is as before.

  * To find its value, we can write GET_INTEGER (X), as before.

  * To test if X is a vector, we can write:

            POINTER_P (X) && X->type == vector

     We must still make sure that X is a pointer to a struct value before dereferencing it to find its type.

  * If we know X is a vector, we can write X->value.vector.elts[0] to refer to its first element, as before.

  * We can write PAIR_P (X) to determine if X is a pair, and then write GET_PAIR (X)->car to refer to its car.

This change in representation reduces our heap size by 15%.  It also makes it cheaper to decide if a value is a pair, because no memory references are necessary; it suffices to check the bottom two bits of the SCM value.  This may be significant when traversing lists, a common activity in a Scheme system.

Again, most real Scheme systems use a slightly different implementation; for example, if GET_PAIR subtracts off the low bits of x, instead of masking them off, the optimizer will often be able to combine that subtraction with the addition of the offset of the structure member we are referencing, making a modified pointer as fast to use as an unmodified pointer.

Guile Is Hairier
----------------

We originally started with a very simple typing system --- each object has a field that indicates its type.  Then, for the sake of efficiency in both time and space, we moved some of the typing information directly into the SCM value, and left the rest in the struct value. Guile itself employs a more complex hierarchy, storing finer and finer gradations of type information in different places, depending on the object's coarser type.

In the author's opinion, Guile could be simplified greatly without significant loss of efficiency, but the simplified system would still be more complex than what we've presented above.

The Libguile Runtime Environment
================================

Here we present the specifics of how Guile represents its data.  We don't go into complete detail; an exhaustive description of Guile's system would be boring, and we do not wish to encourage people to write code which depends on its details anyway.  We do, however, present everything one need know to use Guile's data. It is assumed that the reader understands the concepts laid out in Data Representation in Scheme.

FIXME: much of this is outdated as of 1.8, we don't provide many of these macros any more. Also here we're missing sections about the evaluator implementation, which is interesting, and notes about tail recursion between scheme and c.

General Rules
-------------

Any code which operates on Guile datatypes must #include the header file <libguile.h>.  This file contains a definition for the SCM typedef (Guile's universal type, as in the examples above), and definitions and declarations for a host of macros and functions that operate on SCM values.

All identifiers declared by <libguile.h> begin with scm_ or SCM_.

The functions described here generally check the types of their SCM arguments, and signal an error if their arguments are of an inappropriate type.  Macros generally do not, unless that is their specified purpose.  You must verify their argument types beforehand, as necessary.

Macros and functions that return a boolean value have names ending in P or _p (for ``predicate'').  Those that return a negated boolean value have names starting with SCM_N.  For example, SCM_IMP (X) is a predicate which returns non-zero iff X is an immediate value (an IM).  SCM_NCONSP (X) is a predicate which returns non-zero iff X is not a pair object (a CONS).

Conservative Garbage Collection
-------------------------------

Aside from the latent typing, the major source of constraints on a Scheme implementation's data representation is the garbage collector. The collector must be able to traverse every live object in the heap, to determine which objects are not live.

There are many ways to implement this, but Guile uses an algorithm called mark and sweep.  The collector scans the system's global variables and the local variables on the stack to determine which objects are immediately accessible by the C code.  It then scans those objects to find the objects they point to, et cetera.  The collector sets a mark bit on each object it finds, so each object is traversed only once.  This process is called tracing.

When the collector can find no unmarked objects pointed to by marked objects, it assumes that any objects that are still unmarked will never be used by the program (since there is no path of dereferences from any global or local variable that reaches them) and deallocates them.

In the above paragraphs, we did not specify how the garbage collector finds the global and local variables; as usual, there are many different approaches.  Frequently, the programmer must maintain a list of pointers to all global variables that refer to the heap, and another list (adjusted upon entry to and exit from each function) of local variables, for the collector's benefit.

The list of global variables is usually not too difficult to maintain, since global variables are relatively rare.  However, an explicitly maintained list of local variables (in the author's personal experience) is a nightmare to maintain.  Thus, Guile uses a technique called conservative garbage collection, to make the local variable list unnecessary.

The trick to conservative collection is to treat the stack as an ordinary range of memory, and assume that every word on the stack is a pointer into the heap.  Thus, the collector marks all objects whose addresses appear anywhere in the stack, without knowing for sure how that word is meant to be interpreted.

Obviously, such a system will occasionally retain objects that are actually garbage, and should be freed.  In practice, this is not a problem.  The alternative, an explicitly maintained list of local variable addresses, is effectively much less reliable, due to programmer error.

To accommodate this technique, data must be represented so that the collector can accurately determine whether a given stack word is a pointer or not.  Guile does this as follows:

  * Every heap object has a two-word header, called a cell.  Some objects, like pairs, fit entirely in a cell's two words; others may store pointers to additional memory in either of the words.  For example, strings and vectors store their length in the first word, and a pointer to their elements in the second.

  * Guile allocates whole arrays of cells at a time, called heap segments.  These segments are always allocated so that the cells they contain fall on eight-byte boundaries, or whatever is appropriate for the machine's word size.  Guile keeps all cells in a heap segment initialized, whether or not they are currently in use.

  * Guile maintains a sorted table of heap segments.

Thus, given any random word W fetched from the stack, Guile's garbage collector can consult the table to see if W falls within a known heap segment, and check W's alignment.  If both tests pass, the collector knows that W is a valid pointer to a cell, intentional or not, and proceeds to trace the cell.

Note that heap segments do not contain all the data Guile uses; cells for objects like vectors and strings contain pointers to other memory areas.  However, since those pointers are internal, and not shared among many pieces of code, it is enough for the collector to find the cell, and then use the cell's type to find more pointers to trace.

Immediates vs Non-immediates
----------------------------

Guile classifies Scheme objects into two kinds: those that fit entirely within an SCM, and those that require heap storage.

The former class are called immediates.  The class of immediates includes small integers, characters, boolean values, the empty list, the mysterious end-of-file object, and some others.

The remaining types are called, not surprisingly, non-immediates. They include pairs, procedures, strings, vectors, and all other data types in Guile.

 -- Macro: int SCM_IMP (SCM X)
     Return non-zero iff X is an immediate object.

 -- Macro: int SCM_NIMP (SCM X)
     Return non-zero iff X is a non-immediate object.  This is the exact complement of SCM_IMP, above.

Note that for versions of Guile prior to 1.4 it was necessary to use the SCM_NIMP macro before calling a finer-grained predicate to determine X's type, such as SCM_CONSP or SCM_VECTORP.  This is no longer required: the definitions of all Guile type predicates now include a call to SCM_NIMP where necessary.

Immediate Datatypes
-------------------

The following datatypes are immediate values; that is, they fit entirely within an SCM value.  The SCM_IMP and SCM_NIMP macros will distinguish these from non-immediates; see Immediates vs Non-immediates for an explanation of the distinction.

Note that the type predicates for immediate values work correctly on any SCM value; you do not need to call SCM_IMP first, to establish that a value is immediate.

Integers
........

Here are functions for operating on small integers, that fit within an SCM.  Such integers are called immediate numbers, or INUMs.  In general, INUMs occupy all but two bits of an SCM.

Bignums and floating-point numbers are non-immediate objects, and have their own, separate accessors.  The functions here will not work on them.  This is not as much of a problem as you might think, however, because the system never constructs bignums that could fit in an INUM, and never uses floating point values for exact integers.

 -- Macro: int SCM_INUMP (SCM X)
     Return non-zero iff X is a small integer value.

 -- Macro: int SCM_NINUMP (SCM X)
     The complement of SCM_INUMP.

 -- Macro: int SCM_INUM (SCM X)
     Return the value of X as an ordinary, C integer.  If X is not an INUM, the result is undefined.

 -- Macro: SCM SCM_MAKINUM (int I)
     Given a C integer I, return its representation as an SCM. This function does not check for overflow.

Characters
..........

Here are functions for operating on characters.

 -- Macro: int SCM_CHARP (SCM X)
     Return non-zero iff X is a character value.

 -- Macro: unsigned int SCM_CHAR (SCM X)
     Return the value of x as a C character.  If X is not a Scheme character, the result is undefined.

 -- Macro: SCM SCM_MAKE_CHAR (int C)
     Given a C character C, return its representation as a Scheme character value.

Booleans
........

Booleans are represented as two specific immediate SCM values, SCM_BOOL_T and SCM_BOOL_F.  See Booleans, for more information.

Unique Values
.............

The immediate values that are neither small integers, characters, nor booleans are all unique values --- that is, datatypes with only one instance.

 -- Macro: SCM SCM_EOL
     The Scheme empty list object, or ``End Of List'' object, usually written in Scheme as '().

 -- Macro: SCM SCM_EOF_VAL
     The Scheme end-of-file value.  It has no standard written representation, for obvious reasons.

 -- Macro: SCM SCM_UNSPECIFIED
     The value returned by expressions which the Scheme standard says return an ``unspecified'' value.

     This is sort of a weirdly literal way to take things, but the standard read-eval-print loop prints nothing when the expression returns this value, so it's not a bad idea to return this when you can't think of anything else helpful.

 -- Macro: SCM SCM_UNDEFINED
     The ``undefined'' value.  Its most important property is that is not equal to any valid Scheme value.  This is put to various internal uses by C code interacting with Guile.

     For example, when you write a C function that is callable from Scheme and which takes optional arguments, the interpreter passes SCM_UNDEFINED for any arguments you did not receive.

     We also use this to mark unbound variables.

 -- Macro: int SCM_UNBNDP (SCM X)
     Return true if X is SCM_UNDEFINED.  Apply this to a symbol's value to see if it has a binding as a global variable.

Non-immediate Datatypes
-----------------------

A non-immediate datatype is one which lives in the heap, either because it cannot fit entirely within a SCM word, or because it denotes a specific storage location (in the nomenclature of the Revised^5 Report on Scheme).

The SCM_IMP and SCM_NIMP macros will distinguish these from immediates; see Immediates vs Non-immediates.

Given a cell, Guile distinguishes between pairs and other non-immediate types by storing special tag values in a non-pair cell's car, that cannot appear in normal pairs.  A cell with a non-tag value in its car is an ordinary pair.  The type of a cell with a tag in its car depends on the tag; the non-immediate type predicates test this value.  If a tag value appears elsewhere (in a vector, for example), the heap may become corrupted.

Note how the type information for a non-immediate object is split between the SCM word and the cell that the SCM word points to.  The SCM word itself only indicates that the object is non-immediate --- in other words stored in a heap cell.  The tag stored in the first word of the heap cell indicates more precisely the type of that object.

The type predicates for non-immediate values work correctly on any SCM value; you do not need to call SCM_NIMP first, to establish that a value is non-immediate.

Pairs
.....

Pairs are the essential building block of list structure in Scheme.  A pair object has two fields, called the car and the cdr.

It is conventional for a pair's CAR to contain an element of a list, and the CDR to point to the next pair in the list, or to contain SCM_EOL, indicating the end of the list.  Thus, a set of pairs chained through their CDRs constitutes a singly-linked list. Scheme and libguile define many functions which operate on lists constructed in this fashion, so although lists chained through the CARs of pairs will work fine too, they may be less convenient to manipulate, and receive less support from the community.

Guile implements pairs by mapping the CAR and CDR of a pair directly into the two words of the cell.

 -- Macro: int SCM_CONSP (SCM X)
     Return non-zero iff X is a Scheme pair object.

 -- Macro: int SCM_NCONSP (SCM X)
     The complement of SCM_CONSP.

 -- Function: SCM scm_cons (SCM CAR, SCM CDR)
     Allocate (``CONStruct'') a new pair, with CAR and CDR as its contents.

The macros below perform no type checking.  The results are undefined if CELL is an immediate.  However, since all non-immediate Guile objects are constructed from cells, and these macros simply return the first element of a cell, they actually can be useful on datatypes other than pairs.  (Of course, it is not very modular to use them outside of the code which implements that datatype.)

 -- Macro: SCM SCM_CAR (SCM CELL)
     Return the CAR, or first field, of CELL.

 -- Macro: SCM SCM_CDR (SCM CELL)
     Return the CDR, or second field, of CELL.

 -- Macro: void SCM_SETCAR (SCM CELL, SCM X)
     Set the CAR of CELL to X.

 -- Macro: void SCM_SETCDR (SCM CELL, SCM X)
     Set the CDR of CELL to X.

 -- Macro: SCM SCM_CAAR (SCM CELL)
 -- Macro: SCM SCM_CADR (SCM CELL)
 -- Macro: SCM SCM_CDAR (SCM CELL) ...
 -- Macro: SCM SCM_CDDDDR (SCM CELL)
     Return the CAR of the CAR of CELL, the CAR of the CDR of CELL, et cetera.

Vectors, Strings, and Symbols
.............................

Vectors, strings, and symbols have some properties in common.  They all have a length, and they all have an array of elements.  In the case of a vector, the elements are SCM values; in the case of a string or symbol, the elements are characters.

All these types store their length (along with some tagging bits) in the CAR of their header cell, and store a pointer to the elements in their CDR.  Thus, the SCM_CAR and SCM_CDR macros are (somewhat) meaningful when applied to these datatypes.

 -- Macro: int SCM_VECTORP (SCM X)
     Return non-zero iff X is a vector.

 -- Macro: int SCM_STRINGP (SCM X)
     Return non-zero iff X is a string.

 -- Macro: int SCM_SYMBOLP (SCM X)
     Return non-zero iff X is a symbol.

 -- Macro: int SCM_VECTOR_LENGTH (SCM X)
 -- Macro: int SCM_STRING_LENGTH (SCM X)
 -- Macro: int SCM_SYMBOL_LENGTH (SCM X)
     Return the length of the object X.  The result is undefined if X is not a vector, string, or symbol, respectively.

 -- Macro: SCM * SCM_VECTOR_BASE (SCM X)
     Return a pointer to the array of elements of the vector X. The result is undefined if X is not a vector.

 -- Macro: char * SCM_STRING_CHARS (SCM X)
 -- Macro: char * SCM_SYMBOL_CHARS (SCM X)
     Return a pointer to the characters of X.  The result is undefined if X is not a symbol or string, respectively.

There are also a few magic values stuffed into memory before a symbol's characters, but you don't want to know about those.  What cruft!

Note that SCM_VECTOR_BASE, SCM_STRING_CHARS and SCM_SYMBOL_CHARS return pointers to data within the respective object.  Care must be taken that the object is not garbage collected while that data is still being accessed.  This is the same as for a smob, see Remembering During Operations.

Procedures
..........

Guile provides two kinds of procedures: closures, which are the result of evaluating a lambda expression, and subrs, which are C functions packaged up as Scheme objects, to make them available to Scheme programmers.

(There are actually other sorts of procedures: compiled closures, and continuations; see the source code for details about them.)

 -- Function: SCM scm_procedure_p (SCM X)
     Return SCM_BOOL_T iff X is a Scheme procedure object, of any sort.  Otherwise, return SCM_BOOL_F.

Closures
........

[FIXME: this needs to be further subbed, but texinfo has no subsubsub]

A closure is a procedure object, generated as the value of a lambda expression in Scheme.  The representation of a closure is straightforward --- it contains a pointer to the code of the lambda expression from which it was created, and a pointer to the environment it closes over.

In Guile, each closure also has a property list, allowing the system to store information about the closure.  I'm not sure what this is used for at the moment --- the debugger, maybe?

 -- Macro: int SCM_CLOSUREP (SCM X)
     Return non-zero iff X is a closure.

 -- Macro: SCM SCM_PROCPROPS (SCM X)
     Return the property list of the closure X.  The results are undefined if X is not a closure.

 -- Macro: void SCM_SETPROCPROPS (SCM X, SCM P)
     Set the property list of the closure X to P.  The results are undefined if X is not a closure.

 -- Macro: SCM SCM_CODE (SCM X)
     Return the code of the closure X.  The result is undefined if X is not a closure.

     This function should probably only be used internally by the interpreter, since the representation of the code is intimately connected with the interpreter's implementation.

 -- Macro: SCM SCM_ENV (SCM X)
     Return the environment enclosed by X. The result is undefined if X is not a closure.

     This function should probably only be used internally by the interpreter, since the representation of the environment is intimately connected with the interpreter's implementation.

Subrs
.....

[FIXME: this needs to be further subbed, but texinfo has no subsubsub]

A subr is a pointer to a C function, packaged up as a Scheme object to make it callable by Scheme code.  In addition to the function pointer, the subr also contains a pointer to the name of the function, and information about the number of arguments accepted by the C function, for the sake of error checking.

There is no single type predicate macro that recognizes subrs, as distinct from other kinds of procedures.  The closest thing is scm_procedure_p; see Procedures.

 -- Macro: char * SCM_SNAME (X)
     Return the name of the subr X.  The result is undefined if X is not a subr.

 -- Function: SCM scm_c_define_gsubr (char *NAME, int REQ, int OPT, int REST, SCM (*FUNCTION)())
     Create a new subr object named NAME, based on the C function FUNCTION, make it visible to Scheme the value of as a global variable named NAME, and return the subr object.

     The subr object accepts REQ required arguments, OPT optional arguments, and a REST argument iff REST is non-zero.  The C function FUNCTION should accept REQ + OPT arguments, or REQ + OPT + 1 arguments if rest is non-zero.

     When a subr object is applied, it must be applied to at least REQ arguments, or else Guile signals an error.  FUNCTION receives the subr's first REQ arguments as its first REQ arguments.  If there are fewer than OPT arguments remaining, then FUNCTION receives the value SCM_UNDEFINED for any missing optional arguments.

     If RST is non-zero, then any arguments after the first REQ + OPT are packaged up as a list and passed as FUNCTION's last argument.  FUNCTION must not modify that list.  (Because when subr is called through apply the list is directly from the apply argument, which the caller will expect to be unchanged.)

     Note that subrs can actually only accept a predefined set of combinations of required, optional, and rest arguments.  For example, a subr can take one required argument, or one required and one optional argument, but a subr can't take one required and two optional arguments. It's bizarre, but that's the way the interpreter was written.  If the arguments to scm_c_define_gsubr do not fit one of the predefined patterns, then scm_c_define_gsubr will return a compiled closure object instead of a subr object.

Ports
.....

Haven't written this yet, 'cos I don't understand ports yet.

Signalling Type Errors
----------------------

Every function visible at the Scheme level should aggressively check the types of its arguments, to avoid misinterpreting a value, and perhaps causing a segmentation fault.  Guile provides some macros to make this easier.

 -- Macro: void SCM_ASSERT (int TEST, SCM OBJ, unsigned int POSITION, const char *SUBR)
     If TEST is zero, signal a ``wrong type argument'' error, attributed to the subroutine named SUBR, operating on the value OBJ, which is the POSITION'th argument of SUBR.

 -- Macro: int SCM_ARG1
 -- Macro: int SCM_ARG2
 -- Macro: int SCM_ARG3
 -- Macro: int SCM_ARG4
 -- Macro: int SCM_ARG5
 -- Macro: int SCM_ARG6
 -- Macro: int SCM_ARG7
     One of the above values can be used for POSITION to indicate the number of the argument of SUBR which is being checked. Alternatively, a positive integer number can be used, which allows to check arguments after the seventh.  However, for parameter numbers up to seven it is preferable to use SCM_ARGN instead of the corresponding raw number, since it will make the code easier to understand.

 -- Macro: int SCM_ARGn
     Passing a value of zero or SCM_ARGn for POSITION allows to leave it unspecified which argument's type is incorrect.  Again, SCM_ARGn should be preferred over a raw zero constant.

Unpacking the SCM Type
----------------------

The previous sections have explained how SCM values can refer to immediate and non-immediate Scheme objects.  For immediate objects, the complete object value is stored in the SCM word itself, while for non-immediates, the SCM word contains a pointer to a heap cell, and further information about the object in question is stored in that cell.  This section describes how the SCM type is actually represented and used at the C level.

In fact, there are two basic C data types to represent objects in Guile: SCM and scm_t_bits.

Relationship between SCM and scm_t_bits
.......................................

A variable of type SCM is guaranteed to hold a valid Scheme object.  A variable of type scm_t_bits, on the other hand, may hold a representation of a SCM value as a C integral type, but may also hold any C value, even if it does not correspond to a valid Scheme object.

For a variable X of type SCM, the Scheme object's type information is stored in a form that is not directly usable.  To be able to work on the type encoding of the scheme value, the SCM variable has to be transformed into the corresponding representation as a scm_t_bits variable Y by using the SCM_UNPACK macro.  Once this has been done, the type of the scheme object X can be derived from the content of the bits of the scm_t_bits value Y, in the way illustrated by the example earlier in this chapter (see Cheaper Pairs).  Conversely, a valid bit encoding of a Scheme value as a scm_t_bits variable can be transformed into the corresponding SCM value using the SCM_PACK macro.

Immediate objects
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A Scheme object may either be an immediate, i.e. carrying all necessary information by itself, or it may contain a reference to a cell with additional information on the heap.  Although in general it should be irrelevant for user code whether an object is an immediate or not, within Guile's own code the distinction is sometimes of importance. Thus, the following low level macro is provided:

 -- Macro: int SCM_IMP (SCM X)
     A Scheme object is an immediate if it fulfills the SCM_IMP predicate, otherwise it holds an encoded reference to a heap cell.  The result of the predicate is delivered as a C style boolean value.  User code and code that extends Guile should normally not be required to use this macro.

Summary:

  * Given a Scheme object X of unknown type, check first with SCM_IMP (X) if it is an immediate object.

  * If so, all of the type and value information can be determined from the scm_t_bits value that is delivered by SCM_UNPACK (X).

Non-immediate objects
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A Scheme object of type SCM that does not fulfill the SCM_IMP predicate holds an encoded reference to a heap cell. This reference can be decoded to a C pointer to a heap cell using the SCM2PTR macro.  The encoding of a pointer to a heap cell into a SCM value is done using the PTR2SCM macro.

 -- Macro: scm_t_cell * SCM2PTR (SCM X)
     Extract and return the heap cell pointer from a non-immediate SCM object X.

 -- Macro: SCM PTR2SCM (scm_t_cell * X)
     Return a SCM value that encodes a reference to the heap cell pointer X.

Note that it is also possible to transform a non-immediate SCM value by using SCM_UNPACK into a scm_t_bits variable. However, the result of SCM_UNPACK may not be used as a pointer to a scm_t_cell: only SCM2PTR is guaranteed to transform a SCM object into a valid pointer to a heap cell.  Also, it is not allowed to apply PTR2SCM to anything that is not a valid pointer to a heap cell.

Summary:

  * Only use SCM2PTR on SCM values for which SCM_IMP is false!

  * Don't use (scm_t_cell *) SCM_UNPACK (X)!  Use SCM2PTR (X) instead!

  * Don't use PTR2SCM for anything but a cell pointer!

Allocating Cells
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Guile provides both ordinary cells with two slots, and double cells with four slots.  The following two function are the most primitive way to allocate such cells.

If the caller intends to use it as a header for some other type, she must pass an appropriate magic value in WORD_0, to mark it as a member of that type, and pass whatever value as WORD_1, etc that the type expects.  You should generally not need these functions, unless you are implementing a new datatype, and thoroughly understand the code in <libguile/tags.h>.

If you just want to allocate pairs, use scm_cons.

 -- Function: SCM scm_cell (scm_t_bits word_0, scm_t_bits word_1)
     Allocate a new cell, initialize the two slots with WORD_0 and WORD_1, and return it.

     Note that WORD_0 and WORD_1 are of type scm_t_bits. If you want to pass a SCM object, you need to use SCM_UNPACK.

 -- Function: SCM scm_double_cell (scm_t_bits word_0, scm_t_bits word_1, scm_t_bits word_2, scm_t_bits word_3)
     Like scm_cell, but allocates a double cell with four slots.

Heap Cell Type Information
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Heap cells contain a number of entries, each of which is either a scheme object of type SCM or a raw C value of type scm_t_bits. Which of the cell entries contain Scheme objects and which contain raw C values is determined by the first entry of the cell, which holds the cell type information.

 -- Macro: scm_t_bits SCM_CELL_TYPE (SCM X)
     For a non-immediate Scheme object X, deliver the content of the first entry of the heap cell referenced by X.  This value holds the information about the cell type.

 -- Macro: void SCM_SET_CELL_TYPE (SCM X, scm_t_bits T)
     For a non-immediate Scheme object X, write the value T into the first entry of the heap cell referenced by X.  The value T must hold a valid cell type.

Accessing Cell Entries
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For a non-immediate Scheme object X, the object type can be determined by reading the cell type entry using the SCM_CELL_TYPE macro.  For each different type of cell it is known which cell entries hold Scheme objects and which cell entries hold raw C data.  To access the different cell entries appropriately, the following macros are provided.

 -- Macro: scm_t_bits SCM_CELL_WORD (SCM X, unsigned int N)
     Deliver the cell entry N of the heap cell referenced by the non-immediate Scheme object X as raw data.  It is illegal, to access cell entries that hold Scheme objects by using these macros.  For convenience, the following macros are also provided.

       * SCM_CELL_WORD_0 (X) => SCM_CELL_WORD (X, 0)

       * SCM_CELL_WORD_1 (X) => SCM_CELL_WORD (X, 1)

       * ...

       * SCM_CELL_WORD_N (X) => SCM_CELL_WORD (X, N)

 -- Macro: SCM SCM_CELL_OBJECT (SCM X, unsigned int N)
     Deliver the cell entry N of the heap cell referenced by the non-immediate Scheme object X as a Scheme object.  It is illegal, to access cell entries that do not hold Scheme objects by using these macros.  For convenience, the following macros are also provided.

       * SCM_CELL_OBJECT_0 (X) => SCM_CELL_OBJECT (X, 0)

       * SCM_CELL_OBJECT_1 (X) => SCM_CELL_OBJECT (X, 1)

       * ...

       * SCM_CELL_OBJECT_N (X) => SCM_CELL_OBJECT (X, N)

 -- Macro: void SCM_SET_CELL_WORD (SCM X, unsigned int N, scm_t_bits W)
     Write the raw C value W into entry number N of the heap cell referenced by the non-immediate Scheme value X.  Values that are written into cells this way may only be read from the cells using the SCM_CELL_WORD macros or, in case cell entry 0 is written, using the SCM_CELL_TYPE macro.  For the special case of cell entry 0 it has to be made sure that W contains a cell type information which does not describe a Scheme object.  For convenience, the following macros are also provided.

       * SCM_SET_CELL_WORD_0 (X, W) => SCM_SET_CELL_WORD (X, 0, W)

       * SCM_SET_CELL_WORD_1 (X, W) => SCM_SET_CELL_WORD (X, 1, W)

       * ...

       * SCM_SET_CELL_WORD_N (X, W) => SCM_SET_CELL_WORD (X, N, W)

 -- Macro: void SCM_SET_CELL_OBJECT (SCM X, unsigned int N, SCM O)
     Write the Scheme object O into entry number N of the heap cell referenced by the non-immediate Scheme value X.  Values that are written into cells this way may only be read from the cells using the SCM_CELL_OBJECT macros or, in case cell entry 0 is written, using the SCM_CELL_TYPE macro.  For the special case of cell entry 0 the writing of a Scheme object into this cell is only allowed if the cell forms a Scheme pair.  For convenience, the following macros are also provided.

       * SCM_SET_CELL_OBJECT_0 (X, O) => SCM_SET_CELL_OBJECT (X, 0, O)

       * SCM_SET_CELL_OBJECT_1 (X, O) => SCM_SET_CELL_OBJECT (X, 1, O)

       * ...

       * SCM_SET_CELL_OBJECT_N (X, O) => SCM_SET_CELL_OBJECT (X, N, O)

Summary:

  * For a non-immediate Scheme object X of unknown type, get the type information by using SCM_CELL_TYPE (X).

  * As soon as the cell type information is available, only use the appropriate access methods to read and write data to the different cell entries.

Basic Rules for Accessing Cell Entries
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For each cell type it is generally up to the implementation of that type which of the corresponding cell entries hold Scheme objects and which hold raw C values.  However, there is one basic rule that has to be followed: Scheme pairs consist of exactly two cell entries, which both contain Scheme objects.  Further, a cell which contains a Scheme object in it first entry has to be a Scheme pair.  In other words, it is not allowed to store a Scheme object in the first cell entry and a non Scheme object in the second cell entry.

 -- Macro: int SCM_CONSP (SCM X)
     Determine, whether the Scheme object X is a Scheme pair, i.e. whether X references a heap cell consisting of exactly two entries, where both entries contain a Scheme object.  In this case, both entries will have to be accessed using the SCM_CELL_OBJECT macros.  On the contrary, if the SCM_CONSP predicate is not fulfilled, the first entry of the Scheme cell is guaranteed not to be a Scheme value and thus the first cell entry must be accessed using the SCM_CELL_WORD_0 macro.