#14896: QuantifiedConstraints: GHC does doesn't discharge constraints if they are quantified -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints, wipT2893 | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This came up on a [https://www.reddit.com/r/haskell/comments/8257mz/how_quantifiedconstraints_c... reddit thread], {{{#!hs {-# Language QuantifiedConstraints #-} class (forall aa. Functor (bi aa)) => Bifunctor bi where first :: (a -> a') -> (bi a b -> bi a' b) bimap :: Bifunctor bi => (a -> a') -> (b -> b') -> (bi a b -> bi a' b') bimap f g = first f . fmap g }}} This is the type we want for `bimap` even if we mix & match `Bifunctor` and `Functor`. We already know that we can `fmap @(bi xx)` for any `xx` but this is not the inferred type. Instead GHC infers a type (tidied up) with a superfluous `Functor` constraint {{{#!hs bimap :: (Bifunctor bi, Functor (bi a)) => (a -> a') -> (b -> b') -> (bi a b -> bi a' b') }}} Indeed post-composing with a superfluous `fmap @(bi a')` incurs yet another `Functor` constraint {{{#!hs bimap :: (Bifunctor bi, Functor (bi a), Functor (bi a')) => (a -> a') -> (b -> b') -> (bi a b -> bi a' b') bimap f g = fmap id . first f . fmap g }}} A terminology question, I'm not sure how to phrase what GHC isn't doing to the `Functor` constraints: ‘discharge’? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14896 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler