wubwub.hs

fib :: Int -> Int
fib 0 = 1
fib 1 = 1
fib n = fib (n-1) + fib (n-2)

data Type = IntType | StringType deriving (Eq, Show)
data Expr = IntExpr Int
          | StringExpr String
          | PlusExpr Expr Expr
          | IntToString Expr
            deriving (Show)

-- instance Show Expr where
--   show (IntExpr i) = show i
--   show (PlusExpr x y) = "(" ++ show x ++ " + " ++ show y ++ ")"

infer :: Expr -> Maybe Type
infer (IntExpr x) = Just IntType
infer (StringExpr _) = Just StringType
infer (PlusExpr a b) = do _ <- check IntType a
                          _ <- check IntType b
                          Just IntType
infer (IntToString e) = do _ <- check IntType e
                           Just StringType
infer (IntToString e) = check IntType e >>= \_ -> Just StringType
infer (IntToString e) = case check IntType e of
                          Nothing -> Nothing
                          Just _ -> Just StringType

check :: Type -> Expr -> Maybe ()
check tp exp =
  case infer exp of
    Nothing -> Nothing
    Just actual -> if tp == actual then Just () else Nothing

check tp exp =
  do actual <- infer exp
     if tp == actual then Just () else Nothing

check tp exp = (infer exp) >>= f
    where f :: Type -> Maybe ()
          f = (\actual -> if tp == actual then Just () else Nothing)

-- (>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b
-- (Nothing >>= f) = Nothing
-- ((Just a) >>= f) = f a

x, y :: Maybe Int
x = Just 2
y = Nothing

-- eval :: Expr -> Int
-- eval (IntExpr i) = i
-- eval (PlusExpr x y) = eval x + eval y

main :: IO ()
main = putStrLn "hello, world"