Re: [GHC] #12553: Reference kind in a type instance declaration defined in another instance declaration

30 Aug 2016

      #12553: Reference kind in a type instance declaration defined in another instance
declaration
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Iceland_jack):

 If I constrain the types `Domain a :: Sig Type -> Type`, `Internal a ::
 Type` this fails

 {{{#!hs
 -- • Expected kind ‘Sig Type -> Type’,
 --   but ‘dom’ has kind ‘Sig a -> Type’
 -- • In the type ‘dom’
 --   In the type instance declaration for ‘Domain’
 --   In the instance declaration for ‘Syntactic (AST dom (Full a))’
 --
 -- • Expected a type, but ‘a1’ has kind ‘a’
 -- • In the type ‘a’
 --   In the type instance declaration for ‘Internal’
 --   In the instance declaration for ‘Syntactic (AST dom (Full a))’

 instance Syntactic (AST dom (Full a)) where
   type Domain   (AST dom (Full a)) = dom
   type Internal (AST dom (Full a)) = a
 }}}

 and I have to restrict the kind of `AST` to

 {{{#!hs
 data AST :: (Sig Type -> Type) -> (Sig Type -> Type) where
 }}}

 Boo! :)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12553#comment:2
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler