[GHC] #15561: TypeInType: Type error conditioned on ordering of GADT and type family definitions

[GHC]

#15561: TypeInType: Type error conditioned on ordering of GADT and type family
definitions
-------------------------------------+-------------------------------------
        Reporter:  Bj0rn             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:  TypeInType,
                                     |  TypeFamilies, GADTs
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Yes, I think it's #12088 again.  Here is what I think is going on.

 * The declaration
 {{{
 type instance UnwrapAnyWrapperLikeThing ('Wrap a :: IndexWrapper [b]) = a
 }}}
     mentions `IndexWrapper`, so the latter must be typechecked first (and
 it is).

 * `IndexWrapper` is mutually recursive with `HasIndex` and `Index`, so
 they must be typechecked together (and they are).

 * Returning to
 {{{
 type instance UnwrapAnyWrapperLikeThing ('Wrap a :: IndexWrapper [b]) = a
 }}}
     we can see that `a :: Index [b]`.  Now, if we have already processed
 {{{
 instance HasIndex [a] where
     type Index [a] = Int
 }}}
    then that mens `a :: Int`, and all is well.  (This does involve some
 type family reduction in the patterns of a `type instance`. Where does
 that happen?)

 * But if we have not yet processed the instance for `HasIndex` then we
 complain that the LHS
 {{{
 type instance UnwrapAnyWrapperLikeThing
     ('Wrap (a :: Index [b]) :: IndexWrapper [b]) = a
 }}}
     mentions `Index [b]` on the LHS.

 For now, a robust solution is to force GHC to deal with the instance of
 `HasIndex` first,(mis)-using a Template Haskell splice.  For example, this
 compiles fine, but fails if you remove the splice
 {{{
 type family UnwrapAnyWrapperLikeThing (a :: t) :: k

 class HasIndex a where
     type Index a
     emptyIndex :: IndexWrapper a

 data IndexWrapper a where
     Wrap :: Index a -> IndexWrapper a

 instance HasIndex [a] where
     type Index [a] = Int
     emptyIndex = Wrap 0

 $([d| |])   -- Empty Template Haskell top level splice

 type instance UnwrapAnyWrapperLikeThing ('Wrap a :: IndexWrapper [b]) = a
 }}}
 More grist for the #12088 mill!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15561#comment:2>
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