datatype
Note: This is taken from the Chicken Wiki, where a more recent version could be available.
A facility for creating and using variant records, as described in the book "Essentials of Programming Languages" by Friedman, Wand and Haynes.
None
<macro>(define-datatype TYPENAME [PREDICATE] VARIANT ...)</macro>
Defines a record type named TYPENAME, where VARIANT ... defines one or more constructors for instances of this type. VARIANT should be of the form
VARIANT = (CONSTRUCTOR (FIELDNAME FIELDPRED) ...)
CONSTRUCTOR is the name of a constructor procedure that will be defined with as many arguments as fields are specified. (FIELDNAME FIELDPRED) ... specify the name for each field and a procedure of one argment that should return a true value for legal field values.
The optional PREDICATE should be the name of a procedure that will be defined and which returns #t when applied to an instance of this variant record.
<macro>(cases TYPENAME EXP CLAUSE ...)</macro>
A facility for matching and deconstructing the instance EXP of the variant record with the name TYPENAME. Each CLAUSE specifies a constructor with field-names and a body to execute when the constructor matches the record instance:
CLAUSE = (CONSTRUCTOR (FIELDNAME ...) BODY ...)
| (else BODY ...)<example> <expr> ; A simple binary tree with numbers:
(require-for-syntax 'datatype)
(define-datatype tree tree?
(leaf (n number?)) (branch (left tree?) (right tree?)) )
(define t (branch (branch (leaf 33) (leaf 44)) (leaf 55)))
(define (listify t)
(cases tree t (leaf (n) n) (branch (left right) (cons (listify left) (listify right))) ) )
(listify t) </expr> <result>((33 . 44) . 55)</result> </example>
Copyright (c) 2003, Felix L. Winkelmann
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