Re: [GHC] #11700: pattern match bug

2 May 2016

      #11700: pattern match bug
-------------------------------------+-------------------------------------
        Reporter:  TobyGoodwin       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  https://github.com/TobyGoodwin/odd-
                                     |  ghc-pattern-bug
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by osa1):

 Hm, this is a weird one. I still don't understand the typechecker so can't
 fix
 this myself, but I did some debugging. Here's a simpler function that
 fails:

 {{{#!haskell
 fn2 :: Key User -> (Entity Message, Entity Folder) -> IO ()
 fn2 usr cluster =
   let (Entity msgKey msg, Entity fldrKey fldr) = cluster
   in do
     print $ messageName msg
     print $ folderName fldr
 }}}

 Interestingly, the error message is about {{{Folder}}}, but if I replace
 {{{Message}}} with an empty tuple.

 {{{#!haskell
 fn2 :: Key User -> ((), Entity Folder) -> IO ()
 fn2 usr cluster =
   let ((), Entity fldrKey fldr) = cluster
   in do
     print $ folderName fldr
 }}}

 It works. I also tried swapping the tuple elements:

 {{{#!haskell
 fn2 :: Key User -> (Entity Folder, Entity Message) -> IO ()
 fn2 usr cluster =
   let (Entity fldrKey fldr, Entity msgKey msg) = cluster
   in do
     print $ messageName msg
     print $ folderName fldr
 }}}

 and it failed with an error message about {{{Message}}}:

 {{{
 Message.hs:45:25: error:
     • Couldn't match expected type ‘Message’ with actual type ‘Message’
     • In the first argument of ‘messageName’, namely ‘msg’
       In the second argument of ‘($)’, namely ‘messageName msg’
       In a stmt of a 'do' block: print $ messageName msg
 }}}

 It seems like some kind of side-effect is happening when type checking the
 first pattern.

 Oh, I also tried replacing the {{{in}}} part with {{{return ()}}}, and it
 worked:

 {{{#!haskell
 fn2 :: Key User -> (Entity Folder, Entity Message) -> IO ()
 fn2 usr cluster =
   let (Entity fldrKey fldr, Entity msgKey msg) = cluster
   in do
     return ()
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11700#comment:1
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler