The patterns in eval that need to modify the state

interestingEval.hs

eval :: Expr -> EvalSt Int
eval (Var v) = do 
  env <- get
  return (head [val | (x,val) <- env, x==v])

eval (Assign x e) = do
  v <- eval e
  env <- get
  put $ (x,v):env
  return v