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#lang eopl
;; Exercise 4.37 [★★] Call-by-value-result is a variation on call-by-reference. In call-by-value-result, the actual
;; parameter must be a variable. When a parameter is passed, the formal parameter is bound to a new reference
;; initialized to the value of the actual parameter, just as in call-by-value. The procedure body is then executed
;; normally. When the procedure body returns, however, the value in the new reference is copied back into the reference
;; denoted by the actual parameter. This may be more efficient than call-by-reference because it can improve memory
;; locality. Implement call-by-value-result and write a program that produces different answers using
;; call-by-value-result and call-by-reference.
;; 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" identifier "=" expression "in" expression) let-exp]
[expression ("proc" "(" identifier ")" expression) proc-exp]
[expression ("(" expression identifier ")") call-exp]
[expression ("letrec" (arbno identifier "(" identifier ")" "=" expression) "in" expression) letrec-exp]
[expression ("begin" expression (arbno ";" expression) "end") begin-exp]
[expression ("set" identifier "=" expression) assign-exp]
[expression ("newpair" "(" expression "," expression ")") newpair-exp]
[expression ("left" "(" expression ")") left-exp]
[expression ("setleft" expression "=" expression) setleft-exp]
[expression ("right" "(" expression ")") right-exp]
[expression ("setright" expression "=" expression) setright-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 [bvar symbol?]
[body expression?]
[env environment?]])
(define mutpair?
(lambda (v)
(reference? v)))
(define make-pair
(lambda (val1 val2)
(let ([ref1 (newref val1)])
(let ([ref2 (newref val2)])
(define left
(lambda (p)
(deref p)))
(define right
(lambda (p)
(deref (+ 1 p))))
(define setleft
(lambda (p val)
(setref! p val)))
(define setright
(lambda (p val)
(setref! (+ 1 p) val)))
(define-datatype expval expval?
[num-val [value number?]]
[bool-val [boolean boolean?]]
[proc-val [proc proc?]]
[mutpair-val [p mutpair?]])
(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->mutpair
(lambda (v)
(cases expval v
[mutpair-val (ref) ref]
[else (expval-extractor-error 'mutable-pair v)])))
;; Environments.
(define reference?
(lambda (v)
(integer? v)))
(define-datatype environment environment?
[extend-env [bvar symbol?]
[bval reference?]
[saved-env environment?]]
[extend-env-rec* [proc-names (list-of symbol?)]
[b-vars (list-of symbol?)]
[proc-bodies (list-of expression?)]
[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-sym)
(cases environment env
[empty-env () (eopl:error 'apply-env "No binding for ~s" search-sym)]
[extend-env (bvar bval saved-env) (if (eqv? search-sym bvar)
(apply-env saved-env search-sym))]
[extend-env-rec* (p-names b-vars p-bodies saved-env)
(cond [(location search-sym p-names) => (lambda (n)
(newref (proc-val (procedure (list-ref b-vars n)
(list-ref p-bodies n)
[else (apply-env saved-env search-sym)])])))
;; 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)))
(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 val)
(cases proc proc1
[procedure (var body saved-env) (let* ([new-val (newref (deref val))]
[new-env (extend-env var new-val saved-env)])
(setref! val (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 (expval->num (value-of exp1 env))]
[val2 (expval->num (value-of exp2 env))])
(num-val (- val1 val2)))]
[zero?-exp (exp1) (let ([val1 (expval->num (value-of exp1 env))])
(if (zero? val1)
(bool-val #t)
(bool-val #f)))]
[if-exp (exp0 exp1 exp2) (if (expval->bool (value-of exp0 env))
(value-of exp1 env)
(value-of exp2 env))]
[let-exp (id rhs body) (let ([val (value-of rhs env)])
(value-of body (extend-env id (newref val) env)))]
[proc-exp (var body) (proc-val (procedure var body env))]
[call-exp (rator var) (let ([proc (expval->proc (value-of rator env))]
[arg (apply-env env var)])
(apply-procedure proc arg)
(num-val 777))]
[letrec-exp (p-names b-vars p-bodies letrec-body) (value-of letrec-body
(extend-env-rec* p-names b-vars p-bodies env))]
[begin-exp (exp1 exps) (letrec ([value-of-begins (lambda (e1 es)
(let ([v1 (value-of e1 env)])
(if (null? es)
(value-of-begins (car es) (cdr es)))))])
(value-of-begins exp1 exps))]
[assign-exp (x e) (begin (setref! (apply-env env x) (value-of e env))
(num-val 27))]
[newpair-exp (exp1 exp2) (let ([v1 (value-of exp1 env)]
[v2 (value-of exp2 env)])
(mutpair-val (make-pair v1 v2)))]
[left-exp (exp1) (let ([v1 (value-of exp1 env)])
(let ([p1 (expval->mutpair v1)])
(left p1)))]
[setleft-exp (exp1 exp2) (let ([v1 (value-of exp1 env)]
[v2 (value-of exp2 env)])
(let ([p (expval->mutpair v1)])
(begin (setleft p v2)
(num-val 82))))]
[right-exp (exp1) (let ([v1 (value-of exp1 env)])
(let ([p1 (expval->mutpair v1)])
(right p1)))]
[setright-exp (exp1 exp2) (let ([v1 (value-of exp1 env)]
[v2 (value-of exp2 env)])
(let ([p (expval->mutpair v1)])
(begin (setright p v2)
(num-val 83))))])))
(define value-of-program
(lambda (pgm)
(cases program pgm
[a-program (body) (value-of body (empty-env))])))
;; Interface.
(define run
(lambda (string)
(value-of-program (scan&parse string))))
(provide num-val bool-val run)