Eval.lhs

	Evaluating expressions and commands

	(c) Simon Thompson, 1998.

>	module Eval where

>	import Types
>	import Store

>	eval :: Expr -> Store -> Int

>	eval (Lit n) st = n
>	eval (Var v) st = value st v
>	eval (Op op e1 e2) st
>	  = opValue op v1 v2
>	    where
>	    v1 = eval e1 st
>	    v2 = eval e2 st

>	opValue :: Ops -> Int -> Int -> Int

>	opValue Add = (+)
>	opValue Sub = (-) 
>	opValue Mul = (*) 
>	opValue Div = div 
>	opValue Mod = mod

>	command :: Command -> Store -> (Int,Store)

>	command Null st     = (0 , st)
>	command (Eval e) st = (eval e st , st)
>	command (Assign v e) st 
>	  = (val , newSt)
>	    where
>	    val   = eval e st
>	    newSt = update st v val