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beautiful-racket/beautiful-racket-lib/br/experimental/eopl.rkt

72 lines
2.7 KiB
Racket

#lang br
(require racket/struct (for-syntax br/datum))
(provide define-datatype cases occurs-free?)
(define-macro (define-datatype BASE-TYPE _base-type-predicate?
(SUBTYPE [FIELD FIELD-PREDICATE?] ...) ...)
#'(begin
(struct BASE-TYPE () #:transparent #:mutable)
(struct SUBTYPE BASE-TYPE (FIELD ...) #:transparent #:mutable
#:guard (λ(FIELD ... name)
(unless (FIELD-PREDICATE? FIELD)
(error name (format "arg ~a is not ~a" FIELD 'FIELD-PREDICATE?))) ...
(values FIELD ...))) ...))
(define-datatype lc-exp lc-exp?
(var-exp [var symbol?])
(lambda-exp [bound-var symbol?] [body lc-exp?])
(app-exp [rator lc-exp?] [rand lc-exp?]))
#;(define-syntax (cases stx)
(syntax-case stx (else)
[(_ _base-type INPUT-VAR
[SUBTYPE (POSITIONAL-VAR ...) . _body] ...
[else . _else-body])
(inject-syntax ([#'(_subtype? ...) (suffix-id #'(SUBTYPE ...) "?")])
#'(cond
[(_subtype? INPUT-VAR) (match-let ([(list POSITIONAL-VAR ...) (struct->list INPUT-VAR)])
. _body)] ...
[else . _else-body]))]
[(_ _base-type INPUT-VAR
SUBTYPE-CASE ...)
#'(cases _base-type INPUT-VAR
SUBTYPE-CASE ...
[else (void)])]))
(define-macro-cases cases
[(_ BASE-TYPE INPUT-VAR
[SUBTYPE (POSITIONAL-VAR ...) . BODY] ...
[else . ELSE-BODY])
(with-syntax ([(SUBTYPE? ...) (suffix-id #'(SUBTYPE ...) "?")])
#'(cond
[(SUBTYPE? INPUT-VAR) (match-let ([(list POSITIONAL-VAR ...) (struct->list INPUT-VAR)])
. BODY)] ...
[else . ELSE-BODY]))]
[(_ BASE-TYPE INPUT-VAR
SUBTYPE-CASE ...)
#'(cases BASE-TYPE INPUT-VAR
SUBTYPE-CASE ...
[else (void)])])
(define (occurs-free? search-var exp)
(cases lc-exp exp
[var-exp (var) (eqv? var search-var)]
[lambda-exp (bound-var body)
(and (not (eqv? search-var bound-var))
(occurs-free? search-var body))]
[app-exp (rator rand)
(or
(occurs-free? search-var rator)
(occurs-free? search-var rand))]))
(module+ test
(require rackunit)
(check-true (occurs-free? 'foo (var-exp 'foo)))
(check-false (occurs-free? 'foo (var-exp 'bar)))
(check-false (occurs-free? 'foo (lambda-exp 'foo (var-exp 'bar))))
(check-true (occurs-free? 'foo (lambda-exp 'bar (var-exp 'foo))))
(check-true (occurs-free? 'foo (lambda-exp 'bar (lambda-exp 'zim (lambda-exp 'zam (var-exp 'foo)))))))