[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:  new
        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):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 They might be ignored for data constructors, yes. But what about pattern
 synonyms? For instance, we have that:

 {{{
 λ> pattern Id a = a
 λ> let f (Id _) = "foo"
 λ> f undefined
 "foo"
 λ> let g !(Id _) = "bar"
 λ> g undefined
 "*** Exception: Prelude.undefined
 CallStack (from HasCallStack):
   error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err
   undefined, called at <interactive>:19:3 in interactive:Ghci9
 }}}

 That is to say, bang patterns aren't no-ops for //every// conlike, as they
 matter in the presence of pattern synonyms. This awkwardly melds with
 implicitly bidirectional pattern synonyms, since if you define this:

 {{{
 λ> pattern Foo a = Just !(Id a)
 λ> case Foo undefined of Just _ -> "wat"
 "wat"
 λ> case Just undefined of Foo _ -> "wat"
 "*** Exception: Prelude.undefined
 CallStack (from HasCallStack):
   error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err
   undefined, called at <interactive>:22:11 in interactive:Ghci11
 }}}

 Notice that the bang pattern doesn't kick in when `Foo` is used as an
 expression, only as a pattern. In order to make a `Foo` expression strict
 as a pattern, you'd have to dig underneath the `Id` pattern synonym to
 figure out whether //its// bindings are strict or not.

 All of this sounds terribly fiddly in contrast to option (A), which is
 simple to describe, implement, and even gives nice error messages.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14112#comment:9>
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