The ‘define-record-type’ syntax described in SRFI 9 is a slight simplification of one written for Scheme 48 by Jonathan Rees. Unlike many record-defining special forms, it does not create any new identifiers. Instead, the names of the record type, predicate, constructor, and so on are all listed explicitly in the source. This has the following advantages:
grepand the GNU Emacs tag facility will see the defining occurance of each identifier.
Type-name, contructor-name, field-tag, and predicate-name are identifiers. Field-spec has one of these two forms:
(field-tag accessor-name) (field-tag accessor-name modifier-name)
where field-tag, accessor-name, and modifier-name are each identifiers.
define-record-type is generative: each use creates a new record
type that is distinct from all existing types, including other record
types and Scheme’s predefined types. Record-type definitions may only
occur at top-level (there are two possible semantics for “internal”
record-type definitions, generative and nongenerative, and no consensus
as to which is better).
An instance of
define-record-type is equivalent to the following
#twhen given a value returned by constructor-name and
#ffor everything else.
Assigning the value of any of these identifiers has no effect on the behavior of any of their original values.
(define-record-type :pare (kons x y) pare? (x kar set-kar!) (y kdr))
defines ‘kons’ to be a constructor, ‘kar’ and ‘kdr’ to be accessors, ‘set-kar!’ to be a modifier, and ‘pare?’ to be a predicate for objects of type ‘:pare’.
(pare? (kons 1 2)) ⇒ #t (pare? (cons 1 2)) ⇒ #f (kar (kons 1 2)) ⇒ 1 (kdr (kons 1 2)) ⇒ 2 (let ((k (kons 1 2))) (set-kar! k 3) (kar k)) ⇒ 3