evalMonad.hs

type EvalSt = State Env

eval :: Expr -> EvalSt Int
eval (Var v) = do
  env <- get
  return (head [val |(x,val) <- env, x == v])
eval (N n) = return n
eval (Add e1 e2) = do
  v1 <- eval e1 
  v2 <- eval e2
  return (v1+v2)
eval (Assign x e) = do
  v <- eval e
  put $ (x,v):env
  return v