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{-# LANGUAGE GADTs, TypeFamilies #-}
module TF_GADT where
-- Check that type families can be declared in GADT syntax
-- and indeed *be* GADTs
data family T a
data instance T [a] where
T1 :: a -> T [a]
data instance T (Maybe a) where
T3 :: Int -> T (Maybe Int)
T4 :: a -> b -> T (Maybe (a,b))
f :: a -> T (Maybe a) -> T (Maybe a)
f x (T3 i) = T3 x
f x (T4 p q) = T4 p (snd x)
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