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@khibino
Last active September 16, 2017 03:49
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{-# LANGUAGE GADTs #-}
{-# LANGUAGE MultiParamTypeClasses #-}
data WithZero
data WithoutZero
data Exp a where
ExpZero :: Exp WithZero
ExpOne :: Exp WithoutZero
Op21 :: Exp WithoutZero -> Exp WithoutZero -> Exp WithoutZero
Op2L :: Exp WithoutZero -> Exp WithZero -> Exp WithZero
Op2R :: Exp WithZero -> Exp WithoutZero -> Exp WithZero
instance Eq (Exp a) where
ExpZero == ExpZero = True
ExpOne == ExpOne = True
a `Op21` b == c `Op21` d = a == c && b == d
a `Op2L` b == c `Op2L` d = a == c && b == d
a `Op2R` b == c `Op2R` d = a == c && b == d
_ == _ = False
instance Show (Exp a) where
show = d where
d ExpZero = "0"
d ExpOne = "1"
d (a `Op21` b) = showAB a b
d (a `Op2L` b) = showAB a b
d (a `Op2R` b) = showAB a b
showAB :: Exp a -> Exp b -> String
showAB a b = "(op2 "++shows a" "++shows b")"
class Op2 a b c where
op2 :: Exp a -> Exp b -> Exp c
instance Op2 WithoutZero WithoutZero WithoutZero where
op2 = Op21
instance Op2 WithoutZero WithZero WithZero where
op2 = Op2L
instance Op2 WithZero WithoutZero WithZero where
op2 = Op2R
e1 :: Exp WithZero
e1 = op2 ExpZero ExpOne
e2 :: Exp WithoutZero
e2 = op2 ExpOne ExpOne
-- 正しく型エラーになる
-- e3 = op2 ExpZero ExpZero
main :: IO ()
main = print (e1 == e1)
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