2 Values and Types

A Racket value is represented by a pointer-sized value. The low bit is a mark bit: a 1 in the low bit indicates an immediate integer, a 0 indicates a (word-aligned) pointer.

A pointer Racket value references a structure that begins with a Scheme_Object sub-structure, which in turn starts with a tag that has the C type Scheme_Type. The rest of the structure, following the Scheme_Object header, is type-dependent. Racket’s C interface gives Racket values the type Scheme_Object*. (The “object” here does not refer to objects in the sense of the racket/class library.)

Examples of Scheme_Type values include scheme_pair_type and scheme_symbol_type. Some of these are implemented as instances of Scheme_Simple_Object, which is defined in "scheme.h", but extension or embedding code should never access this structure directly. Instead, the code should use macros, such as SCHEME_CAR, that provide access to the data of common Racket types.

For most Racket types, a constructor is provided for creating values of the type. For example, scheme_make_pair takes two Scheme_Object* values and returns the cons of the values.

The macro SCHEME_TYPE takes a Scheme_Object * and returns the type of the object. This macro performs the tag-bit check, and returns scheme_integer_type when the value is an immediate integer; otherwise, SCHEME_TYPE follows the pointer to get the type tag. Macros are provided to test for common Racket types; for example, SCHEME_PAIRP returns 1 if the value is a cons cell, 0 otherwise.

In addition to providing constructors, Racket defines six global constant Racket values: scheme_true, scheme_false, scheme_null, scheme_eof, scheme_void, and scheme_undefined. Each of these has a type tag, but each is normally recognized via its constant address.

An extension or embedding application can create new a primitive data type by calling scheme_make_type, which returns a fresh Scheme_Type value. To create a collectable instance of this type, allocate memory for the instance with scheme_malloc. From Racket’s perspective, the main constraint on the data format of such an instance is that the first sizeof(Scheme_Object) bytes must correspond to a Scheme_Object record; furthermore, the first sizeof(Scheme_Type) bytes must contain the value returned by scheme_make_type. Extensions with modest needs can use scheme_make_cptr, instead of creating an entirely new type.

Racket values should never be allocated on the stack, and they should never contain pointers to values on the stack. Besides the problem of restricting the value’s lifetime to that of the stack frame, allocating values on the stack creates problems for continuations and threads, both of which copy into and out of the stack.

2.1 Standard Types

The following are the Scheme_Type values for the standard types:

scheme_bool_type – the constants scheme_true and scheme_false are the only values of this type; use SCHEME_FALSEP to recognize scheme_false and use SCHEME_TRUEP to recognize anything except scheme_false; test for this type with SCHEME_BOOLP
scheme_char_type – SCHEME_CHAR_VAL extracts the character (of type mzchar); test for this type with SCHEME_CHARP
scheme_integer_type – fixnum integers, which are identified via the tag bit rather than following a pointer to this Scheme_Type value; SCHEME_INT_VAL extracts the integer; test for this type with SCHEME_INTP
scheme_double_type – flonum inexact numbers; SCHEME_FLOAT_VAL or SCHEME_DBL_VAL extracts the floating-point value; test for this type with SCHEME_DBLP
scheme_float_type – single-precision flonum inexact numbers, when specifically enabled when compiling Racket; SCHEME_FLOAT_VAL or SCHEME_FLT_VAL extracts the floating-point value; test for this type with SCHEME_FLTP
scheme_bignum_type – test for this type with SCHEME_BIGNUMP
scheme_rational_type – test for this type with SCHEME_RATIONALP
scheme_complex_type – test for this type or scheme_complex_izi_type with SCHEME_COMPLEXP
scheme_complex_izi_type – complex number with an inexact zero imaginary part (so it counts as a real number); test for this type specifically with SCHEME_COMPLEX_IZIP
scheme_char_string_type – SCHEME_CHAR_STR_VAL extracts the string as a mzchar*; the string is always nul-terminated, but may also contain embedded nul characters, and the Racket string is modified if this string is modified; SCHEME_CHAR_STRLEN_VAL extracts the string length (in characters, not counting the nul terminator); test for this type with SCHEME_CHAR_STRINGP
scheme_byte_string_type – SCHEME_BYTE_STR_VAL extracts the string as a char*; the string is always nul-terminated, but may also contain embedded nul characters, and the Racket string is modified if this string is modified; SCHEME_BYTE_STRLEN_VAL extracts the string length (in bytes, not counting the nul terminator); test for this type with SCHEME_BYTE_STRINGP
scheme_path_type – SCHEME_PATH_VAL extracts the path as a char*; the string is always nul-terminated; SCHEME_PATH_LEN extracts the path length (in bytes, not counting the nul terminator); test for this type with SCHEME_PATHP
scheme_symbol_type – SCHEME_SYM_VAL extracts the symbol’s string as a char* UTF-8 encoding (do not modify this string); SCHEME_SYM_LEN extracts the number of bytes in the symbol name (not counting the nul terminator); test for this type with SCHEME_SYMBOLP; 3m: see Cooperating with 3m for a caution about SCHEME_SYM_VAL
scheme_keyword_type – SCHEME_KEYWORD_VAL extracts the keyword’s string (without the leading hash colon) as a char* UTF-8 encoding (do not modify this string); SCHEME_KEYWORD_LEN extracts the number of bytes in the keyword name (not counting the nul terminator); test for this type with SCHEME_KEYWORDP; 3m: see Cooperating with 3m for a caution about SCHEME_KEYWORD_VAL
scheme_box_type – SCHEME_BOX_VAL extracts/sets the boxed value; test for this type with SCHEME_BOXP
scheme_pair_type – SCHEME_CAR extracts/sets the car and SCHEME_CDR extracts/sets the cdr; test for this type with SCHEME_PAIRP
scheme_mutable_pair_type – SCHEME_MCAR extracts/sets the mcar and SCHEME_MCDR extracts/sets the mcdr; test for this type with SCHEME_MPAIRP
scheme_vector_type – SCHEME_VEC_SIZE extracts the length and SCHEME_VEC_ELS extracts the array of Racket values (the Racket vector is modified when this array is modified); test for this type with SCHEME_VECTORP; 3m: see Cooperating with 3m for a caution about SCHEME_VEC_ELS
scheme_structure_type – structure instances; test for this type with SCHEME_STRUCTP
scheme_struct_type_type – structure types; test for this type with SCHEME_STRUCT_TYPEP
scheme_struct_property_type – structure type properties
scheme_input_port_type – SCHEME_INPORT_VAL extracts/sets the user data pointer; test for just this type with SCHEME_INPORTP, but use SCHEME_INPUT_PORTP to recognize all input ports (including structures with the prop:input-port property)
scheme_output_port_type – SCHEME_OUTPORT_VAL extracts/sets the user data pointer; test for just this type with SCHEME_OUTPORTP, but use SCHEME_OUTPUT_PORTP to recognize all output ports (including structures with the prop:output-port property)
scheme_thread_type – thread descriptors; test for this type with SCHEME_THREADP
scheme_sema_type – semaphores; test for this type with SCHEME_SEMAP
scheme_hash_table_type – test for this type with SCHEME_HASHTP
scheme_bucket_table_type – test for this type with SCHEME_BUCKTP
scheme_weak_box_type – test for this type with SCHEME_WEAKP; SCHEME_WEAK_PTR extracts the contained object, or NULL after the content is collected; do not set the content of a weak box
scheme_namespace_type – namespaces; test for this type with SCHEME_NAMESPACEP
scheme_cpointer_type – #<void> pointer with a type-describing Scheme_Object; SCHEME_CPTR_VAL extracts the pointer and SCHEME_CPTR_TYPE extracts the type tag object; test for this type with SCHEME_CPTRP. The tag is used when printing such objects when it’s a symbol, a byte string, a string, or a pair holding one of these in its car.

The following are the procedure types:

scheme_prim_type – a primitive procedure, possibly with data elements
scheme_closed_prim_type – an old-style primitive procedure with a data pointer
scheme_compiled_closure_type – a Racket procedure
scheme_cont_type – a continuation
scheme_escaping_cont_type – an escape continuation
scheme_case_closure_type – a case-lambda procedure
scheme_native_closure_type – a procedure with native code generated by the just-in-time compiler

The predicate SCHEME_PROCP returns 1 for all procedure types and 0 for anything else.

The following are additional number predicates:

SCHEME_NUMBERP – all numerical types
SCHEME_REALP – all non-complex numerical types, plus scheme_complex_izi_type
SCHEME_EXACT_INTEGERP – fixnums and bignums
SCHEME_EXACT_REALP – fixnums, bignums, and rationals
SCHEME_FLOATP – both single-precision (when enabled) and double-precision flonums

2.2 Global Constants

There are six global constants:

scheme_null – test for this value with SCHEME_NULLP
scheme_eof – test for this value with SCHEME_EOFP
scheme_true
scheme_false – test for this value with SCHEME_FALSEP; test against it with SCHEME_TRUEP
scheme_void – test for this value with SCHEME_VOIDP
scheme_undefined

2.3 Strings

As noted in Racket, Unicode, Characters, and Strings, a Racket character is a Unicode code point represented by a mzchar value, and character strings are mzchar arrays. Racket also supplies byte strings, which are char arrays.

For a character string s, SCHEME_CHAR_STR_VAL(s) produces a pointer to mzchars, not chars. Convert a character string to its UTF-8 encoding as byte string with scheme_char_string_to_byte_string. For a byte string bs, SCHEME_BYTE_STR_VAL(bs) produces a pointer to chars. The function scheme_byte_string_to_char_string decodes a byte string as UTF-8 and produces a character string. The functions scheme_char_string_to_byte_string_locale and scheme_byte_string_to_char_string_locale are similar, but they use the current locale’s encoding instead of UTF-8.

For more fine-grained control over UTF-8 encoding, use the scheme_utf8_decode and scheme_utf8_encode functions, which are described in String Encodings.

2.4 Value Functions

Scheme_Object*

scheme_make_char

(

mzchar ch)

Returns the character value. The ch value must be a legal Unicode code point (and not a surrogate, for example). The first 256 characters are represented by constant Racket values, and others are allocated.

Scheme_Object*

scheme_make_char_or_null

(

mzchar ch)

Like scheme_make_char, but the result is NULL if ch is not a legal Unicode code point.

Scheme_Object*

scheme_make_character

(

mzchar ch)

Returns the character value. This is a macro that directly accesses the array of constant characters when ch is less than 256.

Scheme_Object*

scheme_make_ascii_character

(

mzchar ch)

Returns the character value, assuming that ch is less than 256. (This is a macro.)

Scheme_Object*

scheme_make_integer

(

long i)

Returns the integer value; i must fit in a fixnum. (This is a macro.)

Scheme_Object*

scheme_make_integer_value

(

long i)

Returns the integer value. If i does not fit in a fixnum, a bignum is returned.

Scheme_Object*
scheme_make_integer_value_from_unsigned	(	unsigned long i)

Like scheme_make_integer_value, but for unsigned integers.

Scheme_Object*
scheme_make_integer_value_from_long_long	(	mzlonglong i)

Like scheme_make_integer_value, but for mzlonglong values (see Integers).

Scheme_Object*
scheme_make_integer_value_from_unsigned_long_long	(	umzlonglong i)

Like scheme_make_integer_value_from_long_long, but for unsigned integers.

Scheme_Object*
scheme_make_integer_value_from_long_halves	(	unsigned long hi,
		unsigned long lo)

Creates an integer given the high and low longs of a signed integer. Note that on 64-bit platforms where long long is the same as long, the resulting integer has 128 bits. (See also Integers.)

Scheme_Object*
scheme_make_integer_value_from_unsigned_long_halves	(	unsigned long hi,
		unsigned long lo)

Creates an integer given the high and low longs of an unsigned integer. Note that on 64-bit platforms where long long is the same as long, the resulting integer has 128 bits.

int		scheme_get_int_val	(	Scheme_Object* o,
				long* i)

Extracts the integer value. Unlike the SCHEME_INT_VAL macro, this procedure will extract an integer that fits in a long from a Racket bignum. If o fits in a long, the extracted integer is placed in *i and 1 is returned; otherwise, 0 is returned and *i is unmodified.

int		scheme_get_unsigned_int_val	(	Scheme_Object* o,
				unsigned long* i)

Like scheme_get_int_val, but for unsigned integers.

int		scheme_get_long_long_val	(	Scheme_Object* o,
				mzlonglong* i)

Like scheme_get_int_val, but for mzlonglong values (see Integers).

int		scheme_get_unsigned_long_long_val	(	Scheme_Object* o,
				umzlonglong* i)

Like scheme_get_int_val, but for unsigned mzlonglong values (see Integers).

Scheme_Object*

scheme_make_double

(

double d)

Creates a new floating-point value.

Scheme_Object*

scheme_make_float

(

float d)

Creates a new single-precision floating-point value. The procedure is available only when Racket is compiled with single-precision numbers enabled.

double

scheme_real_to_double

(

Scheme_Object* o)

Converts a Racket real number to a double-precision floating-point value.

Scheme_Object*		scheme_make_pair	(	Scheme_Object* carv,
				Scheme_Object* cdrv)

Makes a cons pair.

Scheme_Object*

scheme_make_byte_string

(

char* bytes)

Makes a Racket byte string from a nul-terminated C string. The bytes string is copied.

Scheme_Object*
scheme_make_byte_string_without_copying	(	char* bytes)

Like scheme_make_byte_string, but the string is not copied.

Scheme_Object*	scheme_make_sized_byte_string	(	char* bytes,
			long len,
			int copy)

Makes a byte string value with size len. A copy of bytes is made if copy is not 0. The string bytes should contain len bytes; bytes can contain the nul byte at any position, and need not be nul-terminated if copy is non-zero. However, if len is negative, then the nul-terminated length of bytes is used for the length, and if copy is zero, then bytes must be nul-terminated.

Scheme_Object*	scheme_make_sized_offset_byte_string	(	char* bytes,
			long d,
			long len,
			int copy)

Like scheme_make_sized_byte_string, except the len characters start from position d in bytes. If d is non-zero, then copy must be non-zero.

Scheme_Object*		scheme_alloc_byte_string	(	long size,
				char fill)

Allocates a new Racket byte string.

Scheme_Object*		scheme_append_byte_string	(	Scheme_Object* a,
				Scheme_Object* b)

Creates a new byte string by appending the two given byte strings.

Scheme_Object*

scheme_make_locale_string

(

char* bytes)

Makes a Racket string from a nul-terminated byte string that is a locale-specific encoding of a character string; a new string is allocated during decoding. The “locale in the name of this function thus refers to bytes, and not the resulting string (which is internally stored as UCS-4).

Scheme_Object*

scheme_make_utf8_string

(

char* bytes)

Makes a Racket string from a nul-terminated byte string that is a UTF-8 encoding. A new string is allocated during decoding. The “utf8” in the name of this function thus refers to bytes, and not the resulting string (which is internally stored as UCS-4).

Scheme_Object*		scheme_make_sized_utf8_string	(	char* bytes,
				long len)

Makes a string value, based on len UTF-8-encoding bytes (so the resulting string is len characters or less). The string bytes should contain at least len bytes; bytes can contain the nul byte at any position, and need not be null-terminated. However, if len is negative, then the nul-terminated length of bytes is used for the length.

Scheme_Object*	scheme_make_sized_offset_utf8_string	(	char* bytes,
			long d,
			long len)

Like scheme_make_sized_char_string, except the len characters start from position d in bytes.

Scheme_Object*

scheme_make_char_string

(

mzchar* chars)

Makes a Racket string from a nul-terminated UCS-4 string. The chars string is copied.

Scheme_Object*
scheme_make_char_string_without_copying	(	mzchar* chars)

Like scheme_make_char_string, but the string is not copied.

Scheme_Object*	scheme_make_sized_char_string	(	mzchar* chars,
			long len,
			int copy)

Makes a string value with size len. A copy of chars is made if copy is not 0. The string chars should contain len characters; chars can contain the nul character at any position, and need not be nul-terminated if copy is non-zero. However, if len is negative, then the nul-terminated length of chars is used for the length, and if copy is zero, then the chars must be nul-terminated.

Scheme_Object*
scheme_make_sized_offset_char_string	(	mzchar* chars,
		long d,
		long len,
		int copy)

Like scheme_make_sized_char_string, except the len characters start from position d in chars. If d is non-zero, then copy must be non-zero.

Scheme_Object*		scheme_alloc_char_string	(	long size,
				mzchar fill)

Allocates a new Racket string.

Scheme_Object*		scheme_append_char_string	(	Scheme_Object* a,
				Scheme_Object* b)

Creates a new string by appending the two given strings.

Scheme_Object*
scheme_char_string_to_byte_string	(	Scheme_Object* s)

Converts a Racket character string into a Racket byte string via UTF-8.

Scheme_Object*
scheme_byte_string_to_char_string	(	Scheme_Object* s)

Converts a Racket byte string into a Racket character string via UTF-8.

Scheme_Object*
scheme_char_string_to_byte_string_locale	(	Scheme_Object* s)

Converts a Racket character string into a Racket byte string via the locale’s encoding.

Scheme_Object*
scheme_byte_string_to_char_string_locale	(	Scheme_Object* s)

Converts a Racket byte string into a Racket character string via the locale’s encoding.

Scheme_Object*

scheme_intern_symbol

(

char* name)

Finds (or creates) the symbol matching the given nul-terminated, ASCII string (not UTF-8). The case of name is (non-destructively) normalized before interning if scheme_case_sensitive is 0.

Scheme_Object*		scheme_intern_exact_symbol	(	char* name,
				int len)

Creates or finds a symbol given the symbol’s length in UTF-8-encoding bytes. The case of name is not normalized.

Scheme_Object*		scheme_intern_exact_char_symbol	(	mzchar* name,
				int len)

Like scheme_intern_exact_symbol, but given a character array instead of a UTF-8-encoding byte array.

Scheme_Object*

scheme_make_symbol

(

char* name)

Creates an uninterned symbol from a nul-terminated, UTF-8-encoding string. The case is not normalized.

Scheme_Object*		scheme_make_exact_symbol	(	char* name,
				int len)

Creates an uninterned symbol given the symbol’s length in UTF-8-encoded bytes.

Scheme_Object*		scheme_intern_exact_keyword	(	char* name,
				int len)

Creates or finds a keyword given the keywords length in UTF-8-encoding bytes. The case of name is not normalized, and it should not include the leading hash and colon of the keyword’s printed form.

Scheme_Object*		scheme_intern_exact_char_keyword	(	mzchar* name,
				int len)

Like scheme_intern_exact_keyword, but given a character array instead of a UTF-8-encoding byte array.

Scheme_Object*		scheme_make_vector	(	long size,
				Scheme_Object* fill)

Allocates a new vector.

Scheme_Object*

scheme_box

(

Scheme_Object* v)

Creates a new box containing the value v.

Scheme_Object*

scheme_make_weak_box

(

Scheme_Object* v)

Creates a new weak box containing the value v.

Scheme_Type

scheme_make_type

(

char* name)

Creates a new type (not a Racket value).

Scheme_Object*		scheme_make_cptr	(	void* ptr,
				const Scheme_Object* typetag)

Creates a C-pointer object that encapsulates ptr and uses typetag to identify the type of the pointer. The SCHEME_CPTRP macro recognizes objects created by scheme_make_cptr. The SCHEME_CPTR_VAL macro extracts the original ptr from the Racket object, and SCHEME_CPTR_TYPE extracts the type tag. The SCHEME_CPTR_OFFSETVAL macro returns 0 for the result Racket object.

The ptr can refer to either memory managed by the garbage collector or by some other memory manager. Beware, however, of retaining a ptr that refers to memory released by another memory manager, since the enclosing memory range might later become managed by the garbage collector (in which case ptr might become an invalid pointer that can crash the garbage collector).

Scheme_Object*
scheme_make_external_cptr	(	void* ptr,
		const Scheme_Object* typetag)

Like scheme_make_cptr, but ptr is never treated as referencing memory managed by the garbage collector.

Scheme_Object*
scheme_make_offset_cptr	(	void* ptr,
		long offset,
		const Scheme_Object* typetag)

Creates a C-pointer object that encapsulates both ptr and offset. The SCHEME_CPTR_OFFSETVAL macro returns offset for the result Racket object (and the macro be used to change the offset, since it also works on objects with no offset).

The ptr can refer to either memory managed by the garbage collector or by some other memory manager; see also scheme_make_cptr.

Scheme_Object*
scheme_make_offset_external_cptr	(	void* ptr,
		long offset,
		const Scheme_Object* typetag)

Like scheme_make_offset_cptr, but ptr is never treated as referencing memory managed by the garbage collector.

void		scheme_set_type_printer	(	Scheme_Type type,
				Scheme_Type_Printer printer)

Installs a printer to be used for printing (or writing or displaying) values that have the type tag type.

The type of printer is defined as follows:

typedef void (*Scheme_Type_Printer)(Scheme_Object *v, int dis,
Scheme_Print_Params *pp);

Such a printer must print a representation of the value using scheme_print_bytes and scheme_print_string. The first argument to the printer, v, is the value to be printed. The second argument indicates whether v is printed via write or display. The last argument is to be passed on to scheme_print_bytes or scheme_print_string to identify the printing context.

void	scheme_print_bytes	(	Scheme_Print_Params* pp,
			const char* str,
			int offset,
			int len)

Writes the content of str – starting from offset and running len bytes – into a printing context determined by pp. This function is for use by a printer that is installed with scheme_set_type_printer.

void	scheme_print_string	(	Scheme_Print_Params* pp,
			const mzchar* str,
			int offset,
			int len)

Writes the content of str – starting from offset and running len characters – into a printing context determined by pp. This function is for use by a printer that is installed with scheme_set_type_printer.

void	scheme_set_type_equality	(	Scheme_Type type,
			Scheme_Equal_Proc equalp,
			Scheme_Primary_Hash_Proc hash1,
			Scheme_Secondary_Hash_Proc hash2)

Installs an equality predicate and associated hash functions for values that have the type tag type. The equalp predicate is only applied to values that both have tag type.

The type of equalp, hash1, and hash2 are defined as follows:

  typedef int (*Scheme_Equal_Proc)(Scheme_Object* obj1,
                                   Scheme_Object* obj2,
                                   void* cycle_data);
  typedef long (*Scheme_Primary_Hash_Proc)(Scheme_Object* obj,
                                           long base,
                                           void* cycle_data);
  typedef long (*Scheme_Secondary_Hash_Proc)(Scheme_Object* obj,
                                            void* cycle_data);

The two hash functions are use to generate primary and secondary keys for double hashing in an equal?-based hash table. The result of the primary-key function should depend on both obj and base.

The cycle_data argument in each case allows checking and hashing on cyclic values. It is intended for use in recursive checking or hashing via scheme_recur_equal, scheme_recur_equal_hash_key, and scheme_recur_equal_hash_key. That is, do not call plain scheme_equal, scheme_equal_hash_key, or scheme_equal_hash_key for recursive checking or hashing on sub-elements of the given value(s).

top← prev up next →

1	Overview
2	Values and Types
3	Memory Allocation
4	Namespaces and Modules
5	Procedures
6	Evaluation
7	Exceptions and Escape Continuations
8	Threads
9	Parameterizations
10	Continuation Marks
11	String Encodings
12	Bignums, Rationals, and Complex Numbers
13	Ports and the Filesystem
14	Structures
15	Security Guards
16	Custodians
17	Miscellaneous Utilities
18	Flags and Hooks
	Index

2.1	Standard Types
2.2	Global Constants
2.3	Strings
2.4	Value Functions