interpreter.hs

data Expr = Const Int 
          | Var String 
	  | Plus Expr Expr
	  | Minus Expr Expr
	  | Mul Expr Expr

type EnvType = [(String, Int)]

fetch_error :: String -> String
fetch_error str = "No variable of name " ++ str ++ " found!"

fetch_value :: EnvType -> String -> Maybe Int
fetch_value [] var     = Nothing
fetch_value ((name, value):xs) var = 
	if name == var then Just value 
	else fetch_value xs var

eval :: EnvType -> Expr -> Int
eval env (Const x) = x
eval env (Var str) = case fetch_value env str of
			Just x -> x
			Nothing -> error . fetch_error $ str
eval env (Plus left right)  = (eval env left) + (eval env right)
eval env (Minus left right) = (eval env left) - (eval env right)
eval env (Mul left right)   = (eval env left) * (eval env right)

env :: EnvType
env = [("x", 2), ("y", 3)] 

expression :: Expr
expression = (Mul 
		(Mul 
			(Plus (Var "x") (Var "y")) 
			(Minus (Const 24) (Const 12))) 
		(Minus (Const 79) (Var "y")))

evaluation :: Int
evaluation = eval env expression

instance Show Expr where  
    show (Const x) = show x
    show (Var str) = str
    show (Plus left right) = "(" ++ (show left) ++ " + " ++ (show right) ++ ")"
    show (Minus left right) = "(" ++ (show left) ++ " - " ++ (show right) ++ ")"
    show (Mul left right) = "(" ++ (show left) ++ " * " ++ (show right) ++ ")"