#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 Resolution: | Keywords: | QuantifiedConstraints, wipT2893 Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by Iceland_jack: Old description:

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’?

New description: 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') id` 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#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#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 Resolution: | Keywords: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * keywords: QuantifiedConstraints, wipT2893 => QuantifiedConstraints -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14896#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler