EOPL 第3章の Exercise 3.30

練習問題 3.30 からは動的束縛（動的スコープ）の実装。まずはプロシージャを単純に　(lambda (args env) ...) で表現する実装。クロージャよりも簡単。

; Exercise 3.30
(load "r5rs.scm")
(load "define-datatype.scm")
(load "sllgen.scm")
(load "./2.eopl.scm") ; for extend-env, empty-env

(define true-value (lambda () 1))
(define false-value (lambda () 0))
(define true-value?
  (lambda (x)
    (not (= 0 x))))

(define eval-program
  (lambda (pgm)
    (cases program pgm
      (a-program (body)
        (eval-expression body (init-env))))))

(define eval-expression
  (lambda (exp env)
    (cases expression exp
      (lit-exp (datum) datum)
      (var-exp (id) (apply-env env id))
      (primapp-exp (prim rands)
        (let ((args (eval-rands rands env)))
          (apply-primitive prim args)))
      (if-exp (test-exp true-exp false-exp)
        (if (true-value? (eval-expression test-exp env))
          (eval-expression true-exp env)
          (eval-expression false-exp env)))
      (let-exp (ids rands body)
        (let ((args (eval-rands rands env)))
          (eval-expression body (extend-env ids args env))))
      (proc-exp (ids body)
        (lambda (args env)
          (eval-expression body (extend-env ids args env))))
      (app-exp (rator rands)
        (let ((proc (eval-expression rator env))
              (args (eval-rands rands env)))
          (proc args env)))
      )))

(define eval-rands
  (lambda (rands env)
    (map (lambda (x) (eval-rand x env)) rands)))

(define eval-rand
  (lambda (rand env)
    (eval-expression rand env)))

(define apply-primitive
  (lambda (prim args)
    (cases primitive prim
      (add-prim () (+ (car args) (cadr args)))
      (subtract-prim () (- (car args) (cadr args)))
      (mult-prim () (* (car args) (cadr args)))
      (incr-prim () (+ (car args) 1))
      (decr-prim () (- (car args) 1))
      )))

(define init-env
  (lambda ()
    (extend-env
      '(i v x)
      '(1 5 10)
      (empty-env))))

(define scanner-spec-3.30
  '((white-sp
      (whitespace) skip)
    (comment
      ("%" (arbno (not #\newline))) skip)
    (identifier
      (letter (arbno (or letter digit "?"))) symbol)
    (number
      (digit (arbno digit)) number)))

(define grammar-3.30
  '((program
      (expression)
      a-program)
    (expression
      (number)
      lit-exp)
    (expression
      (identifier)
      var-exp)
    (expression
      (primitive "(" (separated-list expression ",") ")" )
      primapp-exp)
    (expression
      ("if" expression "then" expression "else" expression)
      if-exp)
    (expression
      ("let" (arbno identifier "=" expression) "in" expression)
      let-exp)
    (expression
      ("proc" "(" (separated-list identifier ",") ")" expression)
      proc-exp)
    (expression
      ("(" expression (arbno expression) ")")
      app-exp)
    (primitive ("+") add-prim)
    (primitive ("-") subtract-prim)
    (primitive ("*") mult-prim)
    (primitive ("add1") incr-prim)
    (primitive ("sub1") decr-prim)
    ))

(define scan&parse
  (sllgen:make-string-parser
    scanner-spec-3.30
    grammar-3.30))

(sllgen:make-define-datatypes scanner-spec-3.30 grammar-3.30)

(define run
  (lambda (string)
    (eval-program
      (scan&parse string))))

(define read-eval-print
  (sllgen:make-rep-loop "-->" eval-program
    (sllgen:make-stream-parser
      scanner-spec-3.30
      grammar-3.30)))