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

[GHC]

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

 Interesting! this works

 {{{#!hs
 class Syntactic a where
   type Domain   a :: Sig u -> Type
   type Internal a :: u

   a :: Proxy (Domain   a)
   b :: Proxy (Internal a)
   -- c :: AST (Domain a) (Full (Internal a))
 }}}

 but for some reason the method `c` breaks it.

 ----

 Functional dependencies solve this, but they change the API in a way that
 doesn't work for me:

 {{{#!hs
 class Syntactic a (dom :: Sig u -> Type) | a -> dom where
   type Internal a :: u
   desugar :: ASTF dom (Internal a) -> a
   sugar   :: a -> ASTF dom (Internal a)

 instance Syntactic (AST dom (Full a)) dom where
   type Internal (AST dom (Full a)) = a
   desugar = id
   sugar   = id
 }}}

 ----

 Here is another workaround, it's not perfect (need to recover `Domain`,
 `Internal`) but it's a start

 {{{#!hs
 class Syntactic a where
   type DomainInternal a :: Type

   desugar :: DomainInternal a -> a
   sugar   :: a -> DomainInternal a

 instance Syntactic (AST dom (Full a)) where
   type DomainInternal (AST dom (Full a)) = AST dom (Full a)
   desugar x = x
   sugar   x = x
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12553#comment:5>
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