(define-syntax define-record-type
(syntax-rules ()
((define-record-type type
(constructor constructor-tag ...)
predicate
(field-tag accessor . more) ...)
(begin
(define type
(make-record-type 'type '(field-tag ...)))
(define constructor
(record-constructor type '(constructor-tag ...)))
(define predicate
(record-predicate type))
(define-record-field type field-tag accessor . more)
...))))
; An auxilliary macro for define field accessors and modifiers.
; This is needed only because modifiers are optional.
(define-syntax define-record-field
(syntax-rules ()
((define-record-field type field-tag accessor)
(define accessor (record-accessor type 'field-tag)))
((define-record-field type field-tag accessor modifier)
(begin
(define accessor (record-accessor type 'field-tag))
(define modifier (record-modifier type 'field-tag))))))
; Record types
; We define the following procedures:
;
; (make-record-type <type-name <field-names>) -> <record-type>
; (record-constructor <record-type<field-names>) -> <constructor>
; (record-predicate <record-type>) -> <predicate>
; (record-accessor <record-type <field-name>) -> <accessor>
; (record-modifier <record-type <field-name>) -> <modifier>
; where
; (<constructor> <initial-value> ...) -> <record>
; (<predicate> <value>) -> <boolean>
; (<accessor> <record>) -> <value>
; (<modifier> <record> <value>) -> <unspecific>
; Record types are implemented using vector-like records. The first
; slot of each record contains the record's type, which is itself a
; record.
(define (record-type record)
(record-ref record 0))
;----------------
; Record types are themselves records, so we first define the type for
; them. Except for problems with circularities, this could be defined as:
; (define-record-type :record-type
; (make-record-type name field-tags)
; record-type?
; (name record-type-name)
; (field-tags record-type-field-tags))
; As it is, we need to define everything by hand.
(define :record-type (make-record 3))
(record-set! :record-type 0 :record-type) ; Its type is itself.
(record-set! :record-type 1 ':record-type)
(record-set! :record-type 2 '(name field-tags))
; Now that :record-type exists we can define a procedure for making more
; record types.
(define (make-record-type name field-tags)
(let ((new (make-record 3)))
(record-set! new 0 :record-type)
(record-set! new 1 name)
(record-set! new 2 field-tags)
new))
; Accessors for record types.
(define (record-type-name record-type)
(record-ref record-type 1))
(define (record-type-field-tags record-type)
(record-ref record-type 2))
;----------------
; A utility for getting the offset of a field within a record.
(define (field-index type tag)
(let loop ((i 1) (tags (record-type-field-tags type)))
(cond ((null? tags)
(error "record type has no such field" type tag))
((eq? tag (car tags))
i)
(else
(loop (+ i 1) (cdr tags))))))
;----------------
; Now we are ready to define RECORD-CONSTRUCTOR and the rest of the
; procedures used by the macro expansion of DEFINE-RECORD-TYPE.
(define (record-constructor type tags)
(let ((size (length (record-type-field-tags type)))
(arg-count (length tags))
(indexes (map (lambda (tag)
(field-index type tag))
tags)))
(lambda args
(if (= (length args)
arg-count)
(let ((new (make-record (+ size 1))))
(record-set! new 0 type)
(for-each (lambda (arg i)
(record-set! new i arg))
args
indexes)
new)
(error "wrong number of arguments to constructor" type args)))))
(define (record-predicate type)
(lambda (thing)
(and (record? thing)
(eq? (record-type thing)
type))))
(define (record-accessor type tag)
(let ((index (field-index type tag)))
(lambda (thing)
(if (and (record? thing)
(eq? (record-type thing)
type))
(record-ref thing index)
(error "accessor applied to bad value" type tag thing)))))
(define (record-modifier type tag)
(let ((index (field-index type tag)))
(lambda (thing value)
(if (and (record? thing)
(eq? (record-type thing)
type))
(record-set! thing index value)
(error "modifier applied to bad value" type tag thing)))))
Records
; This implements a record abstraction that is identical to vectors,
; except that they are not vectors (VECTOR? returns false when given a
; record and RECORD? returns false when given a vector). The following
; procedures are provided:
; (record? <value>) -> <boolean>
; (make-record <size>) -> <record>
; (record-ref <record> <index>) -> <value>
; (record-set! <record> <index> <value>) -> <unspecific>
;
; These can implemented in R5RS Scheme as vectors with a distinguishing
; value at index zero, providing VECTOR? is redefined to be a procedure
; that returns false if its argument contains the distinguishing record
; value. EVAL is also redefined to use the new value of VECTOR?.
; Define the marker and redefine VECTOR? and EVAL.
(define record-marker (list 'record-marker))
(define real-vector? vector?)
(define (vector? x)
(and (real-vector? x)
(or (= 0 (vector-length x))
(not (eq? (vector-ref x 0)
record-marker)))))
; This won't work if ENV is the interaction environment and someone has
; redefined LAMBDA there.
(define eval
(let ((real-eval eval))
(lambda (exp env)
((real-eval `(lambda (vector?) ,exp))
vector?))))
; Definitions of the record procedures.
(define (record? x)
(and (real-vector? x)
(< 0 (vector-length x))
(eq? (vector-ref x 0)
record-marker)))
(define (make-record size)
(let ((new (make-vector (+ size 1))))
(vector-set! new 0 record-marker)
new))
(define (record-ref record index)
(vector-ref record (+ index 1)))
(define (record-set! record index value)
(vector-set! record (+ index 1) value))