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2.12.4 define-record-type (SRFI 9)

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:

extended standard special form: define-record-type type-name (constructor-name field-tag …) predicate-name field-spec …

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 definitions:

Assigning the value of any of these identifiers has no effect on the behavior of any of their original values.

The following

(define-record-type :pare
  (kons x y)
  (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

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