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{-# LANGUAGE NoMonomorphismRestriction #-}
{-# LANGUAGE TypeFamilies #-}
-- Type error message looks like
-- TF.hs:12:11:
-- Couldn't match expected type `Memo d'
-- against inferred type `Memo d1'
-- NB: `Memo' is a (non-injective) type function
--
-- Note the "NB", which helps point out the problem
module Foo where
class Fun d where
type Memo d :: * -> *
abst :: (d -> a) -> Memo d a
appl :: Memo d a -> (d -> a)
f :: (Fun d) => Memo d a -> Memo d a -- (1)
f = abst . appl
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