1;;; interpret.ss
2;;; Copyright (C) 1996 R. Kent Dybvig
3;;; from "The Scheme Programming Language, 2ed" by R. Kent Dybvig
4
5;;; Permission is hereby granted, free of charge, to any person obtaining a
6;;; copy of this software and associated documentation files (the "Software"),
7;;; to deal in the Software without restriction, including without limitation
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9;;; and/or sell copies of the Software, and to permit persons to whom the
10;;; Software is furnished to do so, subject to the following conditions:
11;;;
12;;; The above copyright notice and this permission notice shall be included in
13;;; all copies or substantial portions of the Software.
14;;;
15;;; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
16;;; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
17;;; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
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21;;; DEALINGS IN THE SOFTWARE.
22
23(define interpret #f)
24(let ()
25  ;; primitive-environment is an environment containing a small
26  ;; number of primitive procedures; it can be extended easily
27  ;; to include additional primitives.
28  (define primitive-environment
29    (list (cons 'apply apply)
30          (cons 'assq assq)
31          (cons 'call/cc call/cc)
32          (cons 'car car)
33          (cons 'cadr cadr)
34          (cons 'caddr caddr)
35          (cons 'cadddr cadddr)
36          (cons 'cddr cddr)
37          (cons 'cdr cdr)
38          (cons 'cons cons)
39          (cons 'eq? eq?)
40          (cons 'list list)
41          (cons 'map map)
42          (cons 'memv memv)
43          (cons 'null? null?)
44          (cons 'pair? pair?)
45          (cons 'read read)
46          (cons 'set-car! set-car!)
47          (cons 'set-cdr! set-cdr!)
48          (cons 'symbol? symbol?)))
49
50  ;; new-env returns a new environment from a formal parameter
51  ;; specification, a list of actual parameters, and an outer
52  ;; environment.  The symbol? test identifies "improper"
53  ;; argument lists.  Environments are association lists,
54  ;; associating variables with values.
55  (define new-env
56    (lambda (formals actuals env)
57      (cond
58        ((null? formals) env)
59        ((symbol? formals) (cons (cons formals actuals) env))
60        (else
61         (cons (cons (car formals) (car actuals))
62               (new-env (cdr formals) (cdr actuals) env))))))
63
64  ;; lookup finds the value of the variable var in the environment
65  ;; env, using assq.  Assumes var is bound in env.
66  (define lookup
67    (lambda (var env)
68      (cdr (assq var env))))
69
70  ;; assign is similar to lookup but alters the binding of the
71  ;; variable var in the environment env by changing the cdr of
72  ;; association pair
73  (define assign
74    (lambda (var val env)
75      (set-cdr! (assq var env) val)))
76
77  ;; exec evaluates the expression, recognizing all core forms.
78  (define exec
79    (lambda (exp env)
80      (cond
81        ((symbol? exp) (lookup exp env))
82        ((pair? exp)
83         (case (car exp)
84           ((quote) (cadr exp))
85           ((lambda)
86            (lambda vals
87              (let ((env (new-env (cadr exp) vals env)))
88                (let loop ((exps (cddr exp)))
89                   (if (null? (cdr exps))
90                       (exec (car exps) env)
91                       (begin
92                          (exec (car exps) env)
93                          (loop (cdr exps))))))))
94           ((if)
95            (if (exec (cadr exp) env)
96                (exec (caddr exp) env)
97                (exec (cadddr exp) env)))
98           ((set!)
99            (assign (cadr exp)
100                    (exec (caddr exp) env)
101                    env))
102           (else
103            (apply (exec (car exp) env)
104                   (map (lambda (x) (exec x env))
105                        (cdr exp))))))
106        (else exp))))
107
108  ;; interpret starts execution with the primitive environment.
109  (set! interpret
110    (lambda (exp)
111      (exec exp  primitive-environment))))
112