[GHC] #15198: `Language.Haskell.TH.Syntax.reify` returns * rather than Constraint

30 May 2018


      #15198: `Language.Haskell.TH.Syntax.reify` returns * rather than Constraint
-------------------------------------+-------------------------------------
           Reporter:  benzrf         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.4.4
          Component:  Template       |           Version:  8.4.2
  Haskell                            |
           Keywords:  reify          |  Operating System:  Linux
  constraint constraintkinds         |
       Architecture:  x86_64         |   Type of failure:  Incorrect result
  (amd64)                            |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This module is sufficient to demonstrate the mistake:

 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TemplateHaskell #-}
 module Bug where

 import Data.Kind
 import Data.Proxy
 import Language.Haskell.TH

 foo :: forall (k :: Constraint). Proxy k
 foo = Proxy

 return [] -- delimit declaration groups

 main :: IO ()
 main = putStrLn $(do
   VarI _ ty _ <- reify 'foo
   let p = pprint ty
   [| p |])
 }}}

 Running this in GHC 8.0.2 correctly prints out `forall (k_0 :: Constraint)
 . Data.Proxy.Proxy k_0`, but GHC 8.2.2 and 8.4.2 both print `forall (k_0
 :: *) . Data.Proxy.Proxy k_0`.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15198
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