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#lang eopl
;; HW05a - Environments
;; Part 3: Rib-Cage Implementation
;;
;; CSI-310 Programming Languages
;; Due: 2013-02-25
;; Name: Ben Burwell
;; empty environment
(define empty-env
(lambda () '(())
))
;; extend environment
(define extend-env
(lambda (sym-val-lst env)
(cons sym-val-lst env)))
;; helper
(define in-sym-list?
(lambda (item lst)
(cond
[ (equal? (length lst) 0) #f ]
[ (equal? (caar lst) item) #t ]
[ else (in-sym-list? item (cdr lst)) ]
)))
(define get-val
(lambda (sym lst)
(cond
[ (equal? (length lst) 0) 'err ]
[ (equal? (caar lst) sym) (cadar lst) ]
[ else (get-val sym (cdr lst)) ]
)))
;; apply environment
(define apply-env
(lambda (env sym)
(cond
[ (in-sym-list? sym (car env)) (get-val sym (car env)) ]
[ (equal? (length (car env)) 0) (eopl:error "No binding for" sym) ]
[ else (apply-env (cdr env) sym) ]
)))
;; tests
(apply-env (extend-env '((a 5) (b 9)) (extend-env '((a 1) (b 2) (c 3)) (empty-env))) 'a)
(apply-env (extend-env '((a 5) (b 9)) (extend-env '((a 1) (b 2) (c 3)) (empty-env))) 'b)
(apply-env (extend-env '((a 5) (b 9)) (extend-env '((a 1) (b 2) (c 3)) (empty-env))) 'c)
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