Re: [GHC] #9359: Deriving clause failure with polymorphic kinds

24 Jul 2014

      #9359: Deriving clause failure with polymorphic kinds
-------------------------------------+-------------------------------------
              Reporter:  simonpj     |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.2
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
Description changed by simonpj:

Old description:
...
Consider
{{{
{-# Language GADTs, PolyKinds, TypeFamilies, DataKinds #-}
module Fam where
data Cmp a where
 Sup ::      Cmp a
 V   :: a -> Cmp a
 deriving (Show, Eq)
data family   CmpInterval (a :: Cmp k) (b :: Cmp k) :: *
data instance CmpInterval (V c)         Sup          = Starting c
  deriving( Show )
}}}
GHC 7.8.3 gives
{{{
Fam.hs:12:13:
    Can't make a derived instance of ‘Show (CmpInterval ('V c) 'Sup)’:
      No family instance for ‘CmpInterval ('V c) 'Sup’
    In the data instance declaration for ‘CmpInterval’
}}}
which is obviously wrong.
New description:

 Consider
 {{{
 {-# Language GADTs, PolyKinds, TypeFamilies, DataKinds #-}
 module Fam where

 data Cmp a where
  Sup ::      Cmp a
  V   :: a -> Cmp a
  deriving (Show, Eq)

 data family   CmpInterval (a :: Cmp k) (b :: Cmp k) :: *
 data instance CmpInterval (V c)         Sup          = Starting c
   deriving( Show )
 }}}
 GHC 7.8.3 gives
 {{{
 Fam.hs:12:13:
     Can't make a derived instance of ‘Show (CmpInterval ('V c) 'Sup)’:
       No family instance for ‘CmpInterval ('V c) 'Sup’
     In the data instance declaration for ‘CmpInterval’
 }}}
 which is obviously wrong.

 Reported on [http://www.haskell.org/pipermail/glasgow-haskell-
 users/2014-July/025145.html glasgow-haskell-users].

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9359#comment:1
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