[GHC] #9783: Pattern synonym matcher is unnecessarily strict on unboxed continuations

7 Nov 2014

      #9783: Pattern synonym matcher is unnecessarily strict on unboxed continuations
-------------------------------------+-------------------------------------
       Reporter:  cactus             |                   Owner:  cactus
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:  7.10.1
      Component:  Compiler (Type     |                 Version:  7.8.3
  checker)                           |        Operating System:
       Keywords:  pattern synonyms   |  Unknown/Multiple
   Architecture:  Unknown/Multiple   |         Type of failure:  Incorrect
     Difficulty:  Moderate (less     |  result at runtime
  than a day)                        |               Test Case:
     Blocked By:                     |                Blocking:
Related Tickets:  9732               |  Differential Revisions:
-------------------------------------+-------------------------------------
 As discovered while investigating #9732, if you have something like

 {{{
 {-# LANGUAGE PatternSynonyms, MagicHash #-}
 import GHC.Base

 pattern P = True

 f :: Bool -> Int#
 f P = 42#
 }}}

 `f` is compiled into

 {{{
 Main.f :: GHC.Types.Bool -> GHC.Prim.Int#
 [LclIdX, Str=DmdType]
 Main.f =
   letrec {
     f_apU :: GHC.Types.Bool -> GHC.Prim.Int#
     [LclId, Str=DmdType]
     f_apU =
       \ (ds_dq1 :: GHC.Types.Bool) ->
         break<2>()
         let {
           fail_dq2 :: GHC.Prim.Void# -> GHC.Prim.Int#
           [LclId, Str=DmdType]
           fail_dq2 =
             \ (ds_dq3 [OS=OneShot] :: GHC.Prim.Void#) ->
               Control.Exception.Base.patError
                 @ GHC.Prim.Int# "unboxed.hs:7:1-9|function f"# } in
         case fail_dq2 GHC.Prim.void# of wild_00 { __DEFAULT ->
         (case break<1>() 42 of wild_00 { __DEFAULT ->
          Main.$mP @ GHC.Prim.Int# ds_dq1 wild_00
          })
           wild_00
         }; } in
   f_apU
 }}}

 Note how `fail_dq2` is applied on `void#` _before_ the pattern match,
 meaning the following expression:

 {{{
 I# (f True)
 }}}

 will fail with

 {{{
 *** Exception: unboxed.hs:7:1-9: Non-exhaustive patterns in function f
 }}}

 This is because the the type of `P`'s matcher, instantiated for its use in
 `f`, is

 {{{
 $mP :: Bool -> Int# -> Int# -> Int#
 }}}

 so of course it is strict both on the success and the failure
 continuation.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9783
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