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{-# LANGUAGE TypeFamilies, MultiParamTypeClasses
, FlexibleContexts, FlexibleInstances, UndecidableInstances
, TypeSynonymInstances, GeneralizedNewtypeDeriving
, OverlappingInstances
#-}
module LongWayOverlapping where
class M a where
class M a => XMLG a
instance M [a]
instance XMLG [m] where -- Generates an implication wanted: forall m. M [m]
class M a => EmbAsChild a b where
emb :: b -> [a]
instance EmbAsChild [Char] Bool where
emb _ = emb 'c'
-- This one generates an unsolvable EmbAsChild [Char] Char
-- Original problem is:
-- [w] EmbAsChild [Char] Char
-- [w] forall m. M [m]
-- Now, by converting the wanted to given and pushing it inside the implication
-- we have the following:
-- [g] EmbAsChild [Char] Char
-- [g] M [Char] <~~ The superclass of the first given!
-- [w] M [m]
-- And now OOPS we can't solve M [m] because we are supposed to delay our choice
-- as much as possible!
-- DV:
-- One possible solution is to STOP PUSHING wanteds as givens inside an implication
-- in a checking context. I think it's the best thing to do and I've implemented it.
-- In inference mode that's ok and the error message is very comprehensible, see
-- test case PushedInFlatsOverlap.hs
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