[GHC] #7021: Tuple (and other exotic predicates) not yet handled in Template Haskell

[GHC]

#7021: Tuple (and other exotic predicates) not yet handled in Template Haskell
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        Reporter:        |            Owner:
  goldfire               |           Status:  new
            Type:        |        Milestone:  7.8.1
  feature request        |          Version:  7.5
        Priority:        |         Keywords:  ConstraintKinds
  normal                 |  TemplateHaskell
       Component:        |     Architecture:  Unknown/Multiple
  Template Haskell       |       Difficulty:  Unknown
      Resolution:        |       Blocked By:
Operating System:        |  Related Tickets:
  Unknown/Multiple       |
 Type of failure:        |
  None/Unknown           |
       Test Case:        |
        Blocking:        |
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Comment (by yoeight):

 Here's an implementation by introducing a new TH.Pred data constructor
 (let's say TypeT) as suggested by Richard
 [http://www.haskell.org/pipermail/ghc-devs/2014-January/003629.html here]:

 So, this is what I have in mind

 {{{#!haskell
   TypeT Type
 }}}

 By using the description's snippet, I got:

 {{{
 VarI Tuple.foo (ForallT [PlainTV a_1627398569] [TypeP (AppT (AppT (TupleT
 2) (AppT (ConT GHC.Show.Show) (VarT a_1627398569))) (AppT (ConT
 GHC.Read.Read) (VarT a_1627398569)))] (VarT a_1627398569)) Nothing (Fixity
 9 InfixL)
 }}}

 What do you think ?

 Richard also suggested that we could make Pred a type synonym of TH.Type.
 Unfortunately, I have no idea how to express equality constraint only by
 using TH.Type

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7021#comment:3>
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