[GHC] #14112: bang patterns on pattern synonyms? (left vs right hand sides)

[GHC]

#14112: bang patterns on pattern synonyms? (left vs right hand sides)
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        Reporter:  carter            |                Owner:  RyanGlScott
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3896
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"5dd6b13c6e2942976aa3b5f4906ff7d0f959272d/ghc"
 5dd6b13c/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="5dd6b13c6e2942976aa3b5f4906ff7d0f959272d"
 Disallow bang/lazy patterns in the RHSes of implicitly bidirectional
 patsyns

 Summary:
 GHC was allowing implicitly bidirectional pattern synonyms with bang
 patterns and irrefutable patterns in the RHS, like so:

 ```lang=haskell
 pattern StrictJust a = Just !a
 ```

 This has multiple problems:

 1. `Just !a` isn't a valid expression, so it feels strange to allow it in
 an
    implicitly bidirectional pattern synonym.
 2. `StrictJust` doesn't provide the strictness properties one would expect
    from a strict constructor. (One could imagine a design where the
    `StrictJust` builder infers a bang pattern for its pattern variable,
 but
    accomplishing this inference in a way that accounts for all possible
    patterns on the RHS, including other pattern synonyms, is somewhat
    awkward, so we do not pursue this design.)

 We nip these issues in the bud by simply disallowing bang/irrefutable
 patterns
 on the RHS.

 Test Plan: make test TEST="T14112 unidir"

 Reviewers: simonpj, austin, bgamari

 Reviewed By: simonpj

 Subscribers: rwbarton, thomie

 GHC Trac Issues: #14112

 Differential Revision: https://phabricator.haskell.org/D3896
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14112#comment:13>
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