[GHC] #12853: Unpromoted tuples in TH in correctly accepted by tthe type checker

18 Nov 2016

      #12853: Unpromoted tuples in TH in correctly accepted by tthe type checker
-------------------------------------+-------------------------------------
           Reporter:  erikd          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This issue was discovered when writing tests for #11629.

 Test case:
 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE FlexibleInstances #-}
 module T11629 where

 import Control.Monad
 import Language.Haskell.TH

 class C (a :: Bool)
 class D (a :: (Bool, Bool))
 class E (a :: [Bool])

 instance C True
 instance C 'False

 instance D '(True, False)
 instance D '(False, True)

 instance E '[True, False]
 instance E '[False, True]

 do
   let getType (InstanceD _ _ ty _) = ty
       getType _                    = error "getType: only defined for
 InstanceD"

       failMsg a ty1 ty2 = fail $ "example " ++ a
         ++ ": ty1 /= ty2, where\n ty1 = "
         ++ show ty1 ++ "\n ty2 = " ++ show ty2

       withoutSig (ForallT tvs cxt ty) = ForallT tvs cxt (withoutSig ty)
       withoutSig (AppT ty1 ty2)       = AppT (withoutSig ty1) (withoutSig
 ty2)
       withoutSig (SigT ty ki)         = withoutSig ty
       withoutSig ty                   = ty

   -- test #2: type quotations and reified types should agree wrt
   -- promoted tuples.
   ty1 <- [t| D '(True, False) |]
   ty2 <- [t| D (False, True)  |]
   ClassI _ insts <- reify ''D
   let [ty1', ty2'] = map (withoutSig . getType) insts

   when (ty1 /= ty1') $ failMsg "C" ty1 ty1'
   when (ty2 /= ty2') $ failMsg "D" ty2 ty2'

   return []
 }}}

 According to @RyanlGlScott in https://phabricator.haskell.org/D2188#79228
 the code `D (True, False)` in regular Haskell code would be rejected as a
 Kind error, whereas when its in Oxford brackets like `[t| D (True, False)
 |]`, it manages to sneak through the type checker.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12853
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