diff options
Diffstat (limited to 'environment.scm')
-rw-r--r-- | environment.scm | 72 |
1 files changed, 72 insertions, 0 deletions
diff --git a/environment.scm b/environment.scm new file mode 100644 index 0000000..2444059 --- /dev/null +++ b/environment.scm | |||
@@ -0,0 +1,72 @@ | |||
1 | (include "prelude/primitives.scm") | ||
2 | |||
3 | ;;; env | ||
4 | (define (enclosing-env environ) (cdr environ)) | ||
5 | (define (first-frame environ) (car environ)) | ||
6 | (define the-empty-env '()) | ||
7 | (define (empty-env? environ) (eq? environ the-empty-env)) | ||
8 | |||
9 | ;; frames | ||
10 | (define (make-frame variables values) | ||
11 | (cons variables values)) | ||
12 | (define frame-variables car) | ||
13 | (define frame-values cdr) | ||
14 | (define (add-binding-to-frame! var val frame) | ||
15 | (begin | ||
16 | (set-car! frame (cons var (frame-variables frame))) | ||
17 | (set-cdr! frame (cons val (frame-values frame))))) | ||
18 | (define (extend-env vars vals base-env) | ||
19 | (if (= (length vars) (length vals)) | ||
20 | (cons (make-frame vars vals) base-env) | ||
21 | (error "Too few or too many arguments supplied" vars vals))) | ||
22 | |||
23 | ;;; variable lookup | ||
24 | (define (lookup-variable-value var env) | ||
25 | (cond | ||
26 | [(empty-env? env) (error "Unbound variable" var)] | ||
27 | [else | ||
28 | (let ([f (first-frame env)]) | ||
29 | (let scan ([variables (frame-variables f)] | ||
30 | [values (frame-values f)]) | ||
31 | (cond | ||
32 | [(null? variables) | ||
33 | (lookup-variable-value var | ||
34 | (enclosing-env env))] | ||
35 | [(eq? var (car variables)) (car values)] | ||
36 | [else (scan (cdr variables) (cdr values))])))])) | ||
37 | |||
38 | ;;; environment modification | ||
39 | (define (set-variable-value! var val env) | ||
40 | (cond | ||
41 | [(empty-env? env) (error "Unbound variable" var)] | ||
42 | [else | ||
43 | (let ([f (first-frame env)]) | ||
44 | (let scan ([variables (frame-variables f)] | ||
45 | [values (frame-values f)]) | ||
46 | (cond | ||
47 | [(null? variables) (set-variable-value! | ||
48 | var | ||
49 | val | ||
50 | (enclosing-env env))] | ||
51 | [(eq? var (car variables)) (set-car! values val)] | ||
52 | [else (scan (cdr variables) (cdr values))])))])) | ||
53 | |||
54 | (define (define-variable! var val env) | ||
55 | (let ([f (first-frame env)]) | ||
56 | (define (scan vars vals) | ||
57 | (cond | ||
58 | [(null? vars) (add-binding-to-frame! var val f)] | ||
59 | [(eq? var (car vars)) (set-car! vals val)] | ||
60 | [else (scan (cdr vars) (cdr vals))])) | ||
61 | (scan (frame-variables f) | ||
62 | (frame-values f)))) | ||
63 | |||
64 | (define (setup-environment) | ||
65 | (let ([initial-env (extend-env | ||
66 | (primitive-names) | ||
67 | (primitive-objs) | ||
68 | the-empty-env)]) | ||
69 | (define-variable! 'true #t initial-env) | ||
70 | (define-variable! 'false #f initial-env) | ||
71 | initial-env)) | ||
72 | |||