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{-# OPTIONS_GHC -fno-warn-redundant-constraints #-}
{-# LANGUAGE TypeFamilies, EmptyDataDecls, FlexibleContexts #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
module NonLinearLHS where
type family E a b
type instance E a a = [a]
foo :: E [Int] (E Int Int) -> Int
foo = sum . concat
data family F a b
data instance F a a = MkF [a]
goo :: F Int Int -> F Bool Bool
goo (MkF xs) = MkF $ map odd xs
-- HList-like type equality
data True; data False;
type family EqTy a b
type instance EqTy a a = True
class EqTyP a b result
instance (EqTy a b ~ isEq, Proxy isEq result) => EqTyP a b result
class Proxy inp out
instance (result ~ True) => Proxy True result
instance (result ~ False) => Proxy notTrue result
testTrue :: EqTyP Int Int r => r
testTrue = undefined
testFalse :: EqTyP Int Bool r => r
testFalse = undefined
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