Simple interp.

Author: dvanhorn

langs/mountebank/simple-interp.rkt

@@ -0,0 +1,59 @@
+#lang racket
+
+;; type Expr = Number
+;;           | Boolean
+;;           | (list Op1 Expr)
+;;           | (list Op2 Expr)
+;;           | (list 'if Expr Expr Expr)
+;;           | (list Expr Expr)
+;;           | (list 'λ (list Id) Expr)
+;;           | Id
+
+;; type Id = Symbol
+;; type Op1 = 'sub1 | 'zero?
+;; type Op2 = '+
+
+;; type Value = Number
+;;            | Boolean
+;;            | (Value -> Value)
+
+;; Expr Env -> Value
+(define (interp e r)
+  (match e
+    [(list '+ e1 e2)
+     (+ (interp e1 r) (interp e2 r))]
+    [(list 'sub1 e1)
+     (sub1 (interp e1 r))]
+    [(list 'zero? e1)
+     (zero? (interp e1 r))]
+    [(list 'if e1 e2 e3)
+     (if (interp e1 r)
+         (interp e2 r)
+	 (interp e3 r))]
+    [(list 'λ (list x) e1)
+     (λ (v) (interp e1 (cons (cons x v) r)))]
+    [(list e1 e2)
+     ((interp e1 r) (interp e2 r))]
+    [_
+     (if (symbol? e)
+         (lookup e r)
+	 e)]))
+
+;; Id Env -> Value
+(define (lookup x r)
+  (match r
+    [(cons (cons y v) r)
+     (if (eq? x y)
+         v
+	 (lookup x r))]))
+
+(interp '(((λ (t)
+            ((λ (f) (t (λ (z) ((f f) z))))
+             (λ (f) (t (λ (z) ((f f) z))))))
+           (λ (tri)
+            (λ (n)
+             (if (zero? n)
+                 0
+                 (+ n (tri (sub1 n)))))))
+          36)
+          '())