[GHC] #13809: TH-reified data family instances have a paucity of kinds

9 Jun 2017

      #13809: TH-reified data family instances have a paucity of kinds
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           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Template       |           Version:  8.0.1
  Haskell                            |
           Keywords:  TypeFamilies   |  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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 Consider this data family (and instances):

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 module Foo where

 data family Foo a
 data instance Foo ((f :: *        -> *) (a ::       *))
 data instance Foo ((f :: (* -> *) -> *) (a :: (* -> *)))
 }}}

 These are two data family instances that GHC distinguishes by the kinds of
 their type parameters. However, Template Haskell does not give me the same
 insight that GHC has, because if I call `Language.Haskell.TH.reify ''Foo`,
 I get this:

 {{{#!hs
 FamilyI
   (DataFamilyD
      Foo.Foo [ KindedTV a_6989586621679025989 StarT ] (Just StarT))
   [ DataInstD
       []
       Foo.Foo
       [ AppT (VarT f_6989586621679026001) (VarT a_6989586621679026000) ]
       Nothing
       []
       []
   , DataInstD
       []
       Foo.Foo
       [ AppT (VarT f_6989586621679026007) (VarT a_6989586621679026006) ]
       Nothing
       []
       []
   ]
 }}}

 Note that neither `f` nor `a` have a kind signature in either instance!
 This makes it completely impossible to tell which is which (aside from the
 order, which is brittle). It would make my life a lot easier if TH were to
 include kind signatures for each type variable in a data family instance.
 I can see two ways to accomplish this:

 1. Include a `[TyVarBndr]` field in `DataInstD` and `NewtypeInstD` where
 each `TyVarBndr` is a `KindedTV`.
 2. Walk over the `Type`s in a `DataInstD`/`NewtypeInstD` and ensure that
 every occurrence of a `VarT` is surrounded with `SigT` to indicate its
 kind.

 While (1) is arguably the cleaner solution, since it makes the kinds easy
 to discover, it is a breaking change. Therefore, I'm inclined to implement
 option (2) instead.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13809
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