[GHC] #12592: Explicit type signature for type classes fails

12 Sep 2016


      #12592: Explicit type signature for type classes fails
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 GHC 8.0.1: Consider the [https://hackage.haskell.org/package/indexed-0.1
 indexed classes]:

 {{{#!hs
 class IxFunctor f where
   imap :: (a -> b) -> f j k a -> f j k b

 class IxFunctor m => IxPointed m where
   ireturn :: a -> m i i a

 class IxPointed m => IxApplicative m where
   iap :: m i j (a -> b) -> m j k a -> m i k b

 class IxFunctor w => IxCopointed w where
   iextract :: w i i a -> a

 class IxApplicative m => IxMonad m where
   ibind :: (a -> m j k b) -> m i j a -> m i k b

 class IxMonad m => IxMonadFree f m | m -> f where
   iwrap :: f i j (m j k a) -> m i k a
 }}}

 GHCi gives their kinds:

 {{{
 IxFunctor     :: (k -> i -> * -> *)                       -> Constraint
 IxPointed     :: (k -> k -> * -> *)                       -> Constraint
 IxApplicative :: (k -> k -> * -> *)                       -> Constraint
 IxCopointed   :: (k -> k -> * -> *)                       -> Constraint
 IxMonad       :: (k -> k -> * -> *)                       -> Constraint
 IxMonadFree   :: (k -> k -> * -> *) -> (k -> k -> * -> *) -> Constraint
 }}}

 so I attempt to make them explicit:

 {{{#!hs
 type IxFunct i = i -> i -> Type -> Type

 class IxFunctor (f :: IxFunct i) where
   imap :: (a -> b) -> f j k a -> f j k b
 }}}

 Works fine, but if I add it to any of the others it fails:

 {{{#!hs
 -- tZRa.hs:15:17: error: …
 --     • Expected kind ‘i’, but ‘i’ has kind ‘*’
 --     • In the first argument of ‘w’, namely ‘i’
 --       In the type signature:
 --         iextract :: w i i a -> a
 --       In the class declaration for ‘IxCopointed’
 -- Compilation failed.
 class IxFunctor w => IxCopointed (w :: IxFunct i) where
   iextract :: w i i a -> a

 -- ...
 class IxMonad m => IxMonadFree (f :: IxFunct i) (m :: IxFunct i) | m -> f
 where
   iwrap :: f i j (m j k a) -> m i k a
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12592
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