Interpreting an Imperative Language

Here is the interpreter:

module Imperative where

data E = Con Int | Var String | Prim String E E
data S = Skip | Assign String E | Block String S | Seq S S |
         While E S

eval e env =
  case e of
    Con n -> n
    Var s -> look s env
    Prim op e1 e2 -> prim op (eval e1 env) (eval e2 env)

prim s v1 v2 =
  if s=="+" then v1+v2
  else if s=="-" then v1-v2
  else if s=="*" then v1*v2
  else error ("Bad operator "++s)

exec s env =
  case s of
    Skip -> env
    Assign x e -> assign x (eval e env) env
    Block x s -> tail (exec s ((x,0):env))
    Seq s1 s2 -> exec s2 (exec s1 env)
    While e s -> while e s env

while e s env =
  if eval e env==0 then env else while e s (exec s env)

look s env = 
  case env of
    nv:env' ->
      case nv of
        (n,v) -> if s==n then v else look s env'

assign x y env =
  case env of
    nv:env' ->
      case nv of
        (n,v) -> if x==n then (n,y):env' else nv:assign x y env'

test = exec (Block "n"
              (Seq (Assign "n" (Con 6))
	           (Seq (Assign "ans" (Con 1))
		        (While (Var "n")
			  (Seq (Assign "ans" (Prim "*" (Var "ans") (Var "n")))
			       (Assign "n" (Prim "-" (Var "n") (Con 1))))))))
            [("ans",0)]