[GHC] #10708: Rejection of constant functions defined using conditional pattern matching

29 Jul 2015

      #10708: Rejection of constant functions defined using conditional pattern matching
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              Reporter:              |             Owner:
  HubertGaravel                      |
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
              Keywords:              |  Operating System:  Linux
          Architecture:  x86         |   Type of failure:  GHC rejects
                                     |  valid program
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 Hi,

 The Haskell program below is rejected, although it is correct. GHC gives
 the
 following error message:

 {{{
 HASKELL/tricky.hs:12:1:
     Multiple declarations of `Main.d3'
     Declared at: HASKELL/tricky.hs:10:1
                  HASKELL/tricky.hs:11:1
                  HASKELL/tricky.hs:12:1
 }}}

 I agree that the definition of d3 is tricky and useless, but it seems
 to be valid. This example was exctracted from Haskell code automatically
 produced by an in-house code generator.

 {{{#!hs
 data Nat = D0 | Succ Nat
   deriving (Show, Eq, Ord)

 d1 :: Nat
 d2 :: Nat
 d3 :: Nat

 d1 = (Succ D0)
 d2 | D0 == D0 = D0
 d3 | D0 /= D0 = D0
 d3 | (Succ D0) == D0 = D0
 d3 | (Succ D0) /= D0 = (Succ D0)

 main = do
  print d1
  print d2
  print d3
 }}}

 Best regards
 Hubert

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10708
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