Re: [GHC] #10079: Coercible solver regression: Couldn't match rep of () with Const () b

11 Feb 2015

      #10079: Coercible solver regression: Couldn't match rep of () with Const () b
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        Reporter:  glguy             |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.10.1-rc1
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |                Blocking:
      Blocked By:                    |  Differential Revisions:
 Related Tickets:                    |
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Description changed by glguy:

Old description:
...
Hello, I ran into what appears to be a regression in the Coercible solver
since 7.8.4. This is as small as I've managed to get my example case.
{{{
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleContexts #-}
module Bug where
import Control.Applicative
import Data.Coerce
broken :: Bizarre (->) w => w a b t -> ()
broken = getConst #. bazaar (Const #. const ())
class Profunctor p where
  (#.) :: Coercible c b => (b -> c) -> p a b -> p a c
class Bizarre p w | w -> p where
  bazaar :: Applicative f => p a (f b) -> w a b t -> f t
}}}
{{{
Bug.hs:10:36:
    Couldn't match representation of type ‘()’
                             with that of ‘Const () b’
    Relevant role signatures: type role Const representational phantom
    Relevant bindings include
      broken :: w a b t -> () (bound at Bug.hs:10:1)
    In the first argument of ‘bazaar’, namely ‘(Const #. const ())’
    In the second argument of ‘(#.)’, namely
      ‘bazaar (Const #. const ())’
    In the expression: getConst #. bazaar (Const #. const ())
}}}
New description:

 Hello, I ran into what appears to be a regression in the Coercible solver
 since 7.8.4. This is as small as I've managed to get my example case.

 {{{
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE FlexibleContexts #-}
 module Bug where

 import Control.Applicative
 import Data.Coerce

 broken :: Bizarre (->) w => w a b t -> ()
 broken = getConst #. bazaar (Const #. const ())

 class Profunctor p where
   (#.) :: Coercible c b => (b -> c) -> p a b -> p a c

 instance Profunctor (->) where
   (#.) = (.)

 class Bizarre p w | w -> p where
   bazaar :: Applicative f => p a (f b) -> w a b t -> f t
 }}}

 {{{
 Bug.hs:10:36:
     Couldn't match representation of type ‘()’
                              with that of ‘Const () b’
     Relevant role signatures: type role Const representational phantom
     Relevant bindings include
       broken :: w a b t -> () (bound at Bug.hs:10:1)
     In the first argument of ‘bazaar’, namely ‘(Const #. const ())’
     In the second argument of ‘(#.)’, namely
       ‘bazaar (Const #. const ())’
     In the expression: getConst #. bazaar (Const #. const ())
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10079#comment:1
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