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3.20.scm
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3.20.scm
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(require eopl)
(define scanner-spec
'((white-sp (whitespace) skip)
(comment ("%" (arbno (not #\newline))) skip)
(identifier (letter (arbno (or letter digit))) symbol)
(number (digit (arbno digit)) number)))
(define scan&parse
(sllgen:make-string-parser scanner-spec grammar))
(define report-expval-extractor-error
(lambda (type val)
(error type "Extraction type error: " val)))
(define run
(lambda (string)
(value-of-program (a-program (scan&parse string)))))
(define value-of-program
(lambda (pgm)
(cases program pgm
(a-program (exp1)
(value-of exp1 (empty-env))))))
(define-datatype program program?
(a-program
(exp1 expression?)))
(define (identifier? x)
(and (symbol? x)
(not (eqv? x 'lambda))))
(define-datatype expval expval?
(num-val
(num number?))
(bool-val
(bool boolean?))
(list-val
(lst list?))
(proc-val
(proc proc?)))
(define expval->num
(lambda (val)
(cases expval val
(num-val (num) num)
(else (report-expval-extractor-error 'num val)))))
(define expval->bool
(lambda (val)
(cases expval val
(bool-val (bool) bool)
(else (report-expval-extractor-error 'bool val)))))
(define expval->list
(lambda (val)
(cases expval val
(list-val (lst) lst)
(else (report-expval-extractor-error 'list val)))))
(define expval->proc
(lambda (val)
(cases expval val
(proc-val (proc) proc)
(else (report-expval-extractor-error 'proc val)))))
(define expval->val
(lambda (val)
(cases expval val
(num-val (num) num)
(bool-val (bool) bool)
(list-val (lst) (map expval->val lst))
(proc-val (proc) proc))))
(define-datatype environment environment?
(empty-env)
(extend-env
(var identifier?)
(val expval?)
(old-env environment?)))
(define report-no-binding-found
(lambda (search-var)
(error 'apply-env "No binding for ~s" search-var)))
(define (apply-env env search-var)
(cases environment env
(empty-env () (report-no-binding-found search-var))
(extend-env (var val old-env)
(if (eqv? var search-var)
val
(apply-env old-env search-var)))))
(define-datatype proc proc?
(procedure
(var identifier?)
(body expression?)
(saved-env environment?)))
(define apply-procedure
(lambda (proc1 val)
(cases proc proc1
(procedure (var body saved-env)
(value-of body (extend-env var val saved-env))))))
(define grammar
'((expression (number) const-exp)
(expression ("-" "(" expression "," expression ")") diff-exp)
(expression ("+" "(" expression "," expression ")") add-exp)
(expression ("*" "(" expression "," expression ")") mul-exp)
(expression ("/" "(" expression "," expression ")") quotient-exp)
(expression ("zero?" "(" expression ")") zero?-exp)
(expression ("equal?" "(" expression "," expression ")") equal?-exp)
(expression ("less?" "(" expression "," expression ")") less?-exp)
(expression ("greater?" "(" expression "," expression ")") greater?-exp)
(expression ("minus" "(" expression ")") minus-exp)
(expression ("if" expression "then" expression "else" expression) if-exp)
(expression ("cond" (arbno expression "==>" expression) "end") cond-exp)
(expression (identifier) var-exp)
(expression ("let" (arbno identifier "=" expression) "in" expression) let-exp)
(expression ("let*" (arbno identifier "=" expression) "in" expression) let*-exp)
(expression ("cons" "(" expression "," expression ")") cons-exp)
(expression ("car" "(" expression ")") car-exp)
(expression ("cdr" "(" expression ")") cdr-exp)
(expression ("null?" "(" expression ")") null?-exp)
(expression ("emptylist") emptylist-exp)
(expression ("list" "(" (separated-list expression ",") ")") list-exp)
(expression ("unpack" (arbno identifier) "=" expression "in" expression) unpack-exp)
(expression ("proc" "(" identifier ")" expression) proc-exp)
(expression ("letproc" identifier "(" identifier ")" expression "in" expression) letproc-exp)
(expression ("(" expression expression ")") call-exp)
(expression ("print" "(" expression ")") print-exp)))
(define-datatype expression expression?
(const-exp
(num number?))
(diff-exp
(exp1 expression?)
(exp2 expression?))
(add-exp
(exp1 expression?)
(exp2 expression?))
(mul-exp
(exp1 expression?)
(exp2 expression?))
(quotient-exp
(exp1 expression?)
(exp2 expression?))
(zero?-exp
(exp1 expression?))
(equal?-exp
(exp1 expression?)
(exp2 expression?))
(less?-exp
(exp1 expression?)
(exp2 expression?))
(greater?-exp
(exp1 expression?)
(exp2 expression?))
(if-exp
(exp1 expression?)
(exp2 expression?)
(exp3 expression?))
(cond-exp
(conds (list-of expression?))
(exps (list-of expression?)))
(var-exp
(var identifier?))
(let-exp
(vars (list-of identifier?))
(exps (list-of expression?))
(body expression?))
(let*-exp
(vars (list-of identifier?))
(exps (list-of expression?))
(body expression?))
(minus-exp
(exp1 expression?))
(cons-exp
(exp1 expression?)
(exp2 expression?))
(car-exp
(exp1 expression?))
(cdr-exp
(exp1 expression?))
(null?-exp
(exp1 expression?))
(emptylist-exp)
(list-exp
(exps (list-of expression?)))
(unpack-exp
(vars (list-of identifier?))
(exp1 expression?)
(exp2 expression?))
(proc-exp
(var identifier?)
(body expression?))
(letproc-exp
(name identifier?)
(var identifier?)
(exp expression?)
(body expression?))
(call-exp
(rator expression?)
(rand expression?))
(print-exp
(exp1 expression?)))
(define value-of
(lambda (exp env)
(cases expression exp
(const-exp (num) (num-val num))
(var-exp (var) (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)))))
(add-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)))))
(mul-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)))))
(quotient-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
(quotient 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)))))
(equal?-exp (exp1 exp2)
(let ((val1 (value-of exp1 env))
(val2 (value-of exp2 env)))
(let ((num1 (expval->num val1))
(num2 (expval->num val2)))
(bool-val
(= num1 num2)))))
(less?-exp (exp1 exp2)
(let ((val1 (value-of exp1 env))
(val2 (value-of exp2 env)))
(let ((num1 (expval->num val1))
(num2 (expval->num val2)))
(bool-val
(< num1 num2)))))
(greater?-exp (exp1 exp2)
(let ((val1 (value-of exp1 env))
(val2 (value-of exp2 env)))
(let ((num1 (expval->num val1))
(num2 (expval->num val2)))
(bool-val
(> num1 num2)))))
(if-exp (exp1 exp2 exp3)
(let ((val1 (value-of exp1 env)))
(if (expval->bool val1)
(value-of exp2 env)
(value-of exp3 env))))
(cond-exp (conds exps)
(cond ((null? conds) (error 'cond "No condition matched"))
((expval->bool (value-of (car conds) env))
(value-of (car exps) env))
(else
(value-of (cond-exp (cdr conds) (cdr exps)) env))))
(let-exp (vars exps body)
(let ((vals (map (lambda (exp) (value-of exp env)) exps)))
(define (add-to-env vars vals env)
(if (null? vars)
env
(add-to-env (cdr vars) (cdr vals) (extend-env (car vars) (car vals) env))))
(value-of body (add-to-env vars vals env))))
(let*-exp (vars exps body)
(if (null? vars)
(value-of body env)
(value-of (let*-exp (cdr vars) (cdr exps) body)
(extend-env (car vars) (value-of (car exps) env) env))))
(minus-exp (exp1)
(num-val (- (expval->num (value-of exp1 env)))))
(cons-exp (exp1 exp2)
(let ((val1 (value-of exp1 env))
(val2 (value-of exp2 env)))
(let ((lst2 (expval->list val2)))
(list-val (cons val1 lst2)))))
(car-exp (exp1)
(let ((val1 (value-of exp1 env)))
(let ((lst1 (expval->list val1)))
(car lst1))))
(cdr-exp (exp1)
(let ((val1 (value-of exp1 env)))
(let ((lst1 (expval->list val1)))
(list-val (cdr lst1)))))
(null?-exp (exp1)
(let ((val1 (value-of exp1 env)))
(let ((lst1 (expval->list val1)))
(bool-val (null? lst1)))))
(emptylist-exp () (list-val '()))
(list-exp (exps)
(list-val (map (lambda (exp) (value-of exp env)) exps)))
(unpack-exp (vars exp1 exp2)
(let ((val1 (value-of exp1 env)))
(let ((lst1 (expval->list val1)))
(define (unpack-rec vars lst env)
(if (null? vars)
env
(unpack-rec (cdr vars) (cdr lst) (extend-env (car vars) (car lst) env))))
(value-of exp2 (unpack-rec vars lst1 env)))))
(proc-exp (var body)
(proc-val (procedure var body env)))
(letproc-exp (name var exp body)
(value-of body (extend-env name (proc-val (procedure var exp env)) env)))
(call-exp (rator rand)
(let ((proc (expval->proc (value-of rator env)))
(arg (value-of rand env)))
(apply-procedure proc arg)))
(print-exp (exp1)
(print (expval->val (value-of exp1 env)))
(num-val 1)))))
(equal? (expval->val (run "let f = proc (x) proc (y) -(x, y)
in ((f 3) 4)")) -1)