#lang br (require rackunit racket/struct (for-syntax br/datum)) (provide define-datatype cases occurs-free?) #;(begin (struct lc-exp () #:transparent) (struct var-exp lc-exp (var) #:transparent #:guard (λ(var name) (unless (symbol? var) (error name (format "arg ~a not ~a" var 'symbol?))) (values var))) (struct lambda-exp lc-exp (bound-var body) #:transparent #:guard (λ(bound-var body name) (unless (symbol? bound-var) (error name (format "arg ~a not ~a" bound-var 'symbol?))) (unless (lc-exp? body) (error name (format "arg ~a not ~a" body 'lc-exp?))) (values bound-var body))) (struct app-exp lc-exp (rator rand) #:transparent #:guard (λ(rator rand name) (unless (lc-exp? rator) (error name (format "arg ~a not ~a" rator 'lc-exp?))) (unless (lc-exp? rand) (error name (format "arg ~a not ~a" rand 'lc-exp?))) (values rator rand)))) (define #'(define-datatype

( [

] ...) ...) #'(begin (struct () #:transparent #:mutable) (struct

( ...) #:transparent #:mutable #:guard (λ( ... name) (unless ( ) (error name (format "arg ~a is not ~a" '))) ... (values ...))) ...)) (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 (occurs-free? search-var exp) (cond [(var-exp? exp) (let ([var (var-exp-var exp)]) (eqv? var search-var))] [(lambda-exp? exp) (let ([bound-var (lambda-exp-bound-var exp)] [body (lambda-exp-body exp)]) (and (not (eqv? search-var bound-var)) (occurs-free? search-var body)))] [(app-exp? exp) (let ([rator (app-exp-rator exp)] [rand (app-exp-rand exp)]) (or (occurs-free? search-var rator) (occurs-free? search-var rand)))])) (define-syntax (cases stx) (syntax-case stx (else) [(_

[ ( ...) ...] ... [else ...]) (inject-syntax ([#'( ...) (map-syntax (λ(s) (format-datum '~a? s)) #'( ...))]) #'(cond [(

) (match-let ([(list ...) (struct->list )]) ...)] ... [else ...]))] [(_

...) #'(cases

... [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))])) (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))))))