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+#lang eopl
+
+;; HW05a - Environments
+;;   Part 2: Abstract Syntax Tree
+;;
+;; CSI-310 Programming Languages
+;; Due: 2013-02-25
+;; Name: Ben Burwell
+
+
+;; helpers and data types
+(define scheme-value?
+  (lambda (a) #t))
+
+(define-datatype env env?
+  (empty-env-record)
+  (extended-env-record
+   (defs (list-of (list-of scheme-value?)))
+   (prev-env env?)))
+
+;; some basics
+(define empty-env
+  (lambda ()
+    (empty-env-record)))
+
+(define extend-env
+  (lambda (defs prev-env)
+    (extended-env-record defs prev-env)))
+
+;; more helpers
+(define def-exists?
+  (lambda (sym defs)
+    (cond
+      [ (equal? (length defs) 0) #f ]
+      [ (equal? (caar defs) sym) #t ]
+      [ else (def-exists? sym (cdr defs)) ]
+      )))
+
+(define get-def
+  (lambda (sym defs)
+    (cond
+      [ (equal? (length defs) 0) 'error ]
+      [ (equal? (caar defs) sym) (cadar defs) ]
+      [ else (get-def sym (cdr defs)) ]
+      )))
+  
+;; now the fun stuff
+(define apply-env
+  (lambda (this-env sym)
+    (cases env this-env
+      (empty-env-record
+       ()
+       (eopl:error 'apply-env "No binding for ~s" sym))
+      (extended-env-record
+       (defs prev-env)
+       (if (def-exists? sym defs)
+           (get-def sym defs)
+           (apply-env prev-env sym))))))
+
+;; tests
+(apply-env (extend-env '((a 4) (b 5)) (extend-env '((a 1) (b 2) (c 3)) (empty-env))) 'a)
+\ No newline at end of file