[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):

 The classification of predicates is done by `classifyPredType`, in the
 Type module. From the definition of that function, any predicate not
 headed by a class, equality predicate, or tuple is "irreducible". Examples
 of these include predicates headed by type families or predicates headed
 by variables. Both of these possibilities require `ConstraintKinds`.

 Here is a full program that exhibits the problem:

 {{{
 {-# LANGUAGE TemplateHaskell, PolyKinds, ConstraintKinds #-}

 module Irred where

 import Language.Haskell.TH

 data Proxy a = Proxy

 foo :: a b => Proxy a -> b
 foo = undefined

 $( do info <- reify 'foo
       reportWarning (show info)
       return [] )
 }}}

 GHC 7.6.3 reports

 {{{
     Can't represent irreducible predicates in Template Haskell: a b
 }}}

 Producing this error was admittedly harder than I thought. It turns out
 that !TcSplice and !DsMeta take rather different routes to translating
 into the TH syntax. (This is, of course, because !TcSplice is translating
 from Core while !DsMeta is translating from Haskell.) Only !TcSplice
 checks the classification of predicates. Using a TH quote with an
 "irreducible" predicate produces a `ClassP`, even on a type like `foo`'s
 type, above.

 I hope this helps!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7021#comment:11>
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