以下所有工作:
{-# LANGUAGE TypeFamilies #-}
type family TF a
type instance TF Int = String
type instance TF Bool = Char
data family DF a
data instance DF Int = DFInt String
data instance DF Bool = DFBool Char
type family CTF a where
CTF Int = String
CTF Bool = Char
CTF a = Double -- Overlap OK!
......但这不是(截至GHC-8.2):
data family CDF a where
CDF Int = CDFInt String
CDF Bool = CDFBool Char
CDF a = CDFOther Double …