[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 goldfire):

 Thanks for sharing your patch. Here are some further comments:

 * The `EqualityT` constructor has type `Type -> Type -> Type`. Yet, this
 would prevent encoding constructs like `[t| ((~) Int) |]`, which should be
 valid. I recommend putting `EqualityT` in like `ArrowT` is now, as a
 nullary (0-argument) constructor.

 * The `foldM` and `go` algorithm (lines 65-67) you use seems like
 `repTapps`. Or, am I missing something?

 * In reifying an `IrredPred`, you use `splitAppTy`... but we're not sure
 there that the type is an application. For example, it could be a nullary
 type family, like this:

 {{{
 type family ALittleSilly :: Constraint
 }}}

     I believe this would be an `IrredPred`, if there are no instances for
 the type family. (I haven't tested it.)

 I will say that I appreciate your attention to detail to GHC's coding
 style. I hope all these comments don't put you off -- we do really
 appreciate your contribution!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7021#comment:13>
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