[GHC] #10828: TH can't represent all GADTs in its AST

[GHC]

#10828: TH can't represent all GADTs in its AST
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        Reporter:  spinda            |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Template Haskell  |                 Version:  7.10.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by goldfire):

 I believe the TH AST is fully expressive here (with one tiny caveat). It's
 just clunky.

 Take

 {{{
 data T :: k -> * -> * where
   MkT :: T x Int
 }}}

 To encode into TH, you transform the definition to this:

 {{{
 data T (a1 :: k) (a2 :: *) = forall . a2 ~ Int => MkT
 }}}

 which has a direct translation.

 The one tiny caveat is the new feature for deriving `Foldable` for GADTs,
 which distinguishes among the two forms above. (I can't find a reference
 to this change. Can you? It was described at Haskell Implementors'
 Workshop a few days ago. You, the reader, if you know the reference,
 please add it!)

 Is it worth refactoring the TH types to handle GADTs more directly?
 Perhaps.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10828#comment:1>
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