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{-# LANGUAGE FlexibleContexts, MultiParamTypeClasses, TypeFamilies #-}
module Foo where
data P f g r = f r :*: g r
type family TrieMapT (f :: * -> *) :: * -> (* -> *) -> * -> *
newtype PMap m1 (m2 :: * -> (* -> *) -> * -> *) k (a :: * -> *) ix = PMap (m1 k (m2 k a) ix)
type instance TrieMapT (P f g) = PMap (TrieMapT f) (TrieMapT g)
class TrieKeyT f m where
unionT :: (TrieMapT f ~ m) => (f k -> a ix -> a ix -> a ix) ->
m k a ix -> m k a ix -> m k a ix
sizeT :: (TrieMapT f ~ m) => m k a ix -> Int
instance (TrieKeyT f m1, TrieKeyT g m2) => TrieKeyT (P f g) (PMap m1 m2) where
unionT f (PMap m1) (PMap m2) = PMap (uT (\ a -> unionT (\ b -> f (a :*: b))) m1 m2)
where uT = unionT
sizeT = error "urk"
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