[GHC] #8671: Rebindable syntax creates bogus warning

15 Jan 2014

      #8671: Rebindable syntax creates bogus warning
-------------------------+-------------------------------------------------
       Reporter:         |             Owner:
  thomaseding            |            Status:  new
           Type:  bug    |         Milestone:
       Priority:         |           Version:  7.6.3
  normal                 |  Operating System:  Windows
      Component:         |   Type of failure:  Incorrect warning at
  Compiler               |  compile-time
       Keywords:         |         Test Case:
   Architecture:         |          Blocking:
  Unknown/Multiple       |
     Difficulty:         |
  Unknown                |
     Blocked By:         |
Related Tickets:         |
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 {{{
 {-# LANGUAGE RebindableSyntax #-}

 import Data.Void
 import Prelude ((.), ($), Int, id, Num(..))

 (>>) :: (b -> c) -> (a -> b) -> (a -> c)
 (>>) = (.)

 return :: Void -> Void
 return = absurd

 run :: a -> (a -> b) -> b
 run x f = f x

 result :: Int
 result = run 8 $ do
     \n -> n * n
     id
     (+ 7)
     (* 2)
 }}}

 Compile with -Wall issues incorrect warnings. In fact the suggested fixes
 cause compile errors if implemented.

 {{{
 Test.hs:22:5: Warning:
     A do-notation statement discarded a result of type Int.
     Suppress this warning by saying "_ <- \ n -> (*) n n",
     or by using the flag -fno-warn-unused-do-bind

 Test.hs:23:5: Warning:
     A do-notation statement discarded a result of type Int.
     Suppress this warning by saying "_ <- id",
     or by using the flag -fno-warn-unused-do-bind

 Test.hs:24:5: Warning:
     A do-notation statement discarded a result of type Int.
     Suppress this warning by saying "_ <- (( \ x_ -> (+) x_ 7))",
     or by using the flag -fno-warn-unused-do-bind
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8671
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