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{-# LANGUAGE Haskell2010 #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
-- this is needed because |FamHelper a x| /< |Fam a x|
module ShouldCompile where
import Data.Kind (Type)
class Cls a where
type Fam a b :: Type
-- Multiple defaults!
type Fam a x = FamHelper a x
type family FamHelper a x
type instance FamHelper a Bool = Maybe a
type instance FamHelper a Int = (String, a)
instance Cls Int where
-- Gets type family from default
inc :: (Fam a Bool ~ Maybe Int, Fam a Int ~ (String, Int)) => a -> Fam a Bool -> Fam a Int -> Fam a Bool
inc _proxy (Just x) (_, y) = Just (x + y + 1)
inc _proxy Nothing (_, y) = Just y
foo :: Maybe Int -> (String, Int) -> Maybe Int
foo = inc (undefined :: Int)
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