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

[GHC]

#9783: Pattern synonym matcher is unnecessarily strict on unboxed continuations
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              Reporter:  cactus      |            Owner:  cactus
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Compiler    |          Version:  7.8.3
  (Type checker)                     |         Keywords:  pattern synonyms
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Moderate (less
  Unknown/Multiple                   |  than a day)
       Type of failure:  Incorrect   |       Blocked By:
  result at runtime                  |  Related Tickets:  9732
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by Dr. ERDI Gergo <gergo@…>):

 In [changeset:"474e535b6b121809a8d75df5a4c37dc574d3d302/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="474e535b6b121809a8d75df5a4c37dc574d3d302"
 In pattern synonym matchers, support unboxed continuation results (fixes
 #9783).

 This requires ensuring the continuations have arguments by adding a dummy
 Void# argument when needed. This is so that matching on a pattern synonym
 is lazy even when the result is unboxed, e.g.

     pattern P = ()
     f P = 0#

 In this case, without dummy arguments, the generated matcher's type would
 be

    $mP :: forall (r :: ?). () -> r -> r -> r

 which is called in `f` at type `() -> Int# -> Int# -> Int#`,
 so it would be strict, in particular, in the failure continuation
 of `patError`.

 We work around this by making sure both continuations have arguments:

   $mP :: forall (r :: ?). () -> (Void# -> r) -> (Void# -> r) -> r

 Of course, if `P` (and thus, the success continuation) has any arguments,
 we are only adding the extra dummy argument to the failure continuation.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9783#comment:1>
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