[GHC] #11324: Missing Kind Inference

[GHC]

#11324: Missing Kind Inference
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           Reporter:  crockeea       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
           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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 The following example doesn't compile, but I expected it to.


 {{{
 {-# LANGUAGE RankNTypes,
     ConstraintKinds, ScopedTypeVariables, KindSignatures, PolyKinds,
     DataKinds, FlexibleInstances, UndecidableInstances, TypeFamilies #-}

 module Test where

 data Proxy a
 data Tagged t s = Tag s
 type family CharOf fp :: k

 class Reflects (a :: k) where value :: Proxy a
 instance Reflects (a :: Bool)

 type MyConstraint (x :: Bool) = (x~x)

 foo :: forall fp . (MyConstraint (CharOf fp)) => Tagged fp Int
 foo= let x = value::Proxy (CharOf fp)
      in Tag 2
 }}}

 The error in 7.10.2 (unable to test with HEAD) is `Could not deduce
 (Reflects k (CharOf k fp))
       arising from a use of ‘value’`, basically that it was unable to
 figure out the kind of `CharOf fp`. I think GHC should know the kind from
 the constraint on `foo`.

 I've found two workarounds:

 {{{
 foo :: forall fp . (MyConstraint (CharOf fp)) => Tagged fp Int
 foo= let x = value::Proxy (CharOf fp :: Bin)
      in Tag 2
 }}}

 and

 {{{
 foo :: forall fp x . (MyConstraint x, x~CharOf fp) => Tagged fp Int
 foo= let x = value::Proxy x
      in Tag 2
 }}}

 but both seem unnecessary.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11324>
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