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exercise-4.x-implicit-refs-lang.rkt
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exercise-4.x-implicit-refs-lang.rkt
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#lang eopl
;; Grammar.
(define the-lexical-spec
'([whitespace (whitespace) skip]
[comment ("%" (arbno (not #\newline))) skip]
[identifier (letter (arbno (or letter digit "_" "-" "?"))) symbol]
[number (digit (arbno digit)) number]
[number ("-" digit (arbno digit)) number]))
(define the-grammar
'([program (expression) a-program]
[expression (number) const-exp]
[expression ("-" "(" expression "," expression ")") diff-exp]
[expression ("zero?" "(" expression ")") zero?-exp]
[expression ("if" expression "then" expression "else" expression) if-exp]
[expression (identifier) var-exp]
[expression ("let" (arbno identifier "=" expression) "in" expression) let-exp]
[expression ("proc" "(" (separated-list identifier ",") ")" expression) proc-exp]
[expression ("(" expression (arbno expression) ")") call-exp]
[expression ("letrec" (arbno identifier "(" (separated-list identifier ",") ")" "=" expression) "in" expression)
letrec-exp]
[expression ("begin" expression (arbno ";" expression) "end") begin-exp]
[expression ("set" identifier "=" expression) assign-exp]
[expression ("setdynamic" identifier "=" expression "during" expression) dynamic-assign-exp]
[expression ("ref" identifier) ref-exp]
[expression ("deref" "(" expression ")") deref-exp]
[expression ("setref" "(" expression "," expression ")") setref-exp]))
(sllgen:make-define-datatypes the-lexical-spec the-grammar)
(define scan&parse
(sllgen:make-string-parser the-lexical-spec the-grammar))
;; Data structures.
(define-datatype proc proc?
[procedure [bvars (list-of symbol?)]
[body expression?]
[env environment?]])
(define-datatype expval expval?
[num-val [value number?]]
[bool-val [boolean boolean?]]
[proc-val [proc proc?]]
[ref-val [ref reference?]])
(define expval-extractor-error
(lambda (variant value)
(eopl:error 'expval-extractors
"Looking for a ~s, found ~s"
variant
value)))
(define expval->num
(lambda (v)
(cases expval v
[num-val (num) num]
[else (expval-extractor-error 'num v)])))
(define expval->bool
(lambda (v)
(cases expval v
[bool-val (bool) bool]
[else (expval-extractor-error 'bool v)])))
(define expval->proc
(lambda (v)
(cases expval v
[proc-val (proc) proc]
[else (expval-extractor-error 'proc v)])))
(define expval->ref
(lambda (v)
(cases expval v
[ref-val (ref) ref]
[else (expval-extractor-error 'ref v)])))
;; Environments.
(define reference?
(lambda (v)
(integer? v)))
(define-datatype environment environment?
[empty-env]
[extend-env [bvar symbol?]
[bval reference?]
[saved-env environment?]])
(define location
(lambda (sym syms)
(cond [(null? syms) #f]
[(eqv? sym (car syms)) 0]
[(location sym (cdr syms)) => (lambda (n)
(+ n 1))]
[else #f])))
(define apply-env
(lambda (env search-var)
(cases environment env
[empty-env () (eopl:error 'apply-env "No binding for ~s" search-var)]
[extend-env (bvar bval saved-env) (if (eqv? search-var bvar)
bval
(apply-env saved-env search-var))])))
;; Store.
(define the-store 'uninitialized)
(define empty-store
(lambda ()
'()))
(define initialize-store!
(lambda ()
(set! the-store (empty-store))))
(define newref
(lambda (val)
(let ([next-ref (length the-store)])
(set! the-store (append the-store (list val)))
next-ref)))
(define deref
(lambda (ref)
(list-ref the-store ref)))
(define report-invalid-reference
(lambda (ref the-store)
(eopl:error 'setref
"illegal reference ~s in store ~s"
ref
the-store)))
(define setref!
(lambda (ref val)
(set! the-store
(letrec ([setref-inner (lambda (store1 ref1)
(cond [(null? store1) (report-invalid-reference ref the-store)]
[(zero? ref1) (cons val (cdr store1))]
[else (cons (car store1) (setref-inner (cdr store1) (- ref1 1)))]))])
(setref-inner the-store ref)))))
;; Interpreter.
(define apply-procedure
(lambda (proc1 args)
(cases proc proc1
[procedure (vars body saved-env) (let ([new-env (let loop ([vars vars]
[args args]
[env saved-env])
(if (null? vars)
env
(loop (cdr vars)
(cdr args)
(extend-env (car vars)
(newref (car args))
env))))])
(value-of body new-env))])))
(define value-of
(lambda (exp env)
(cases expression exp
[const-exp (num) (num-val num)]
[var-exp (var) (deref (apply-env env var))]
[diff-exp (exp1 exp2) (let ([val1 (value-of exp1 env)]
[val2 (value-of exp2 env)])
(let ([num1 (expval->num val1)]
[num2 (expval->num val2)])
(num-val (- num1 num2))))]
[zero?-exp (exp1) (let ([val1 (value-of exp1 env)])
(let ([num1 (expval->num val1)])
(if (zero? num1)
(bool-val #t)
(bool-val #f))))]
[if-exp (exp1 exp2 exp3) (let ([val1 (value-of exp1 env)])
(if (expval->bool val1)
(value-of exp2 env)
(value-of exp3 env)))]
[let-exp (vars exps body) (let ([body-env (let loop ([vars vars]
[exps exps]
[env1 env])
(if (null? vars)
env1
(loop (cdr vars)
(cdr exps)
(extend-env (car vars)
(newref (value-of (car exps) env))
env1))))])
(value-of body body-env))]
[proc-exp (vars body) (proc-val (procedure vars body env))]
[call-exp (rator rands) (let ([proc (expval->proc (value-of rator env))]
[args (map (lambda (rand)
(value-of rand env))
rands)])
(apply-procedure proc args))]
[letrec-exp (p-names b-vars p-bodies letrec-body) (let* ([refs (map (lambda (p-name)
(newref 0))
p-names)]
[rec-env (let loop ([p-names p-names]
[refs refs]
[env env])
(if (null? p-names)
env
(loop (cdr p-names)
(cdr refs)
(extend-env (car p-names)
(car refs)
env))))])
(for-each (lambda (ref b-vars p-body)
(setref! ref
(proc-val (procedure b-vars
p-body
rec-env))))
refs
b-vars
p-bodies)
(value-of letrec-body rec-env))]
[begin-exp (exp1 exps) (letrec ([value-of-begins (lambda (e1 es)
(let ([v1 (value-of e1 env)])
(if (null? es)
v1
(value-of-begins (car es) (cdr es)))))])
(value-of-begins exp1 exps))]
[assign-exp (var exp1)
(setref! (apply-env env var) (value-of exp1 env))
(num-val 27)]
[dynamic-assign-exp (var exp body) (let* ([ref (apply-env env var)]
[new-val (value-of exp env)]
[saved-val (deref ref)])
(setref! ref new-val)
(let ([result (value-of body env)])
(setref! ref saved-val)
result))]
[ref-exp (var) (ref-val (apply-env env var))]
[deref-exp (exp) (let* ([val (value-of exp env)]
[ref (expval->ref val)])
(deref ref))]
[setref-exp (exp1 exp2) (let* ([val1 (value-of exp1 env)]
[val2 (value-of exp2 env)]
[ref (expval->ref val1)])
(setref! ref val2)
(num-val 23))])))
(define value-of-program
(lambda (pgm)
(initialize-store!)
(cases program pgm
[a-program (exp1) (value-of exp1 (empty-env))])))
;; Interface.
(define run
(lambda (string)
(value-of-program (scan&parse string))))
(provide num-val bool-val run)