data Exp a b c = Lit a | Bop b (Exp a b c) (Exp a b c) | Occ c |
                 Set v (Exp a b c) | Let c (Exp a b c) (Exp a b c)

data Var = Var Char

type Expr = Exp Val Opr Var

data Stmt = Simple Expr | If Expr Stmt Stmt | Block [Stmt] | While Expr Stmt


type Val = Either Int Bool

type State = Var -> Val

type Cmd   = State -> State


eval :: Expr -> Env -> Val

tran :: Stmt -> Env -> Cmd





























tt, ff :: Val
tt = Right True
ff = Right False

num :: Int -> Val
num = Left

litb :: Bool -> Expr Val b c
litn :: Int  -> Expr Val b c
litn = Lit . Left
litb = Lit . Right


type Env = [(Var, Val)]
type Tnv = [(Var, Tipe)]

data Tipe = TInt | TBool
data TBop = (Tipe, Tipe, Tipe)
data TSig = [(Opr,TSig)]

same x@(Just x') (Just y') = if x'==y' then x else Nothing
same _ _ = Nothing

tchk :: Expr -> Tnv -> Maybe Tipe
tchk (Lit (Left  _)) tnv = TInt
tchk (Lit (Right _)) tnv = TInt
tchk (Occ x)         tnv = tnv x
tchk (Bop b
tchk (Set x e)       tnv = same (tnv x) (tchk tnv e)