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{-# LANGUAGE Haskell2010 #-}
{-# LANGUAGE TypeFamilies #-}
-- #8883
module T8883 where
type family PF a :: * -> *
class Regular a where
from :: a -> PF a a
-- For fold we infer following type signature:
--
-- fold :: (Functor (PF a), Regular a) => (PF a b -> b) -> a -> b
--
-- However, this signature requires FlexibleContexts since the first
-- type-class constraint is not of the form (class type-variable) nor
-- (class (type-variable type1 type2 ... typen)). Since this extension
-- is not enabled compilation should fail.
fold f = f . fmap (fold f) . from
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