[GHC] #11534: Allow class associated types to reference functional dependencies

[GHC]

#11534: Allow class associated types to reference functional dependencies
-------------------------------------+-------------------------------------
        Reporter:  ekmett            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.3
  checker)                           |             Keywords:  TypeFamilies,
      Resolution:                    |  FunctionalDependencies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ekmett):

 Then I get stuck when I get to the

 {{{#!hs
 instance (Category p, Category q) => Category (Nat p q) where
   type Ob (Nat p q) = Functor p q
 }}}

 line mentioned.

 I can't define

 {{{#!hs
 type Fun c d f = Functor (Dom f) (Cod f) f
 }}}

 and then put that in

 {{{#!hs
 instance (Category p, Category q) => Category (Nat p q) where
   type Ob (Nat p q) = Fun p q
 }}}

 because that involves partially applying a type family.

 For other reasons, I can't eta expand `Ob` to make it a type family with
 two parameters.

 Normally I'd do precisely what you suggest


 {{{#!hs
 class Functor (f :: i -> j) where
   type Dom f :: i -> i -> *
   type Cod f :: j -> j -> *
 }}}

 and then use the class/instance trick

 {{{#!hs
 class (Functor f, Dom f ~ c, Cod f ~ d) => FunctorOf c d f | f -> c d
 instance (Functor f, Dom f ~ c, Cod f ~ d) => FunctorOf c d f
 }}}

 to get a type family I can partially apply:

 {{{#!hs
 instance (Category p, Category q) => Category (Nat p q) where
   type Ob (Nat p q) = FunctorOf p q
 }}}

 but I get stuck due to #11523 causing the compiler to loop.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11534#comment:8>
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