[GHC] #8779: Exhaustiveness checks for pattern synonyms

[GHC]

#8779: Exhaustiveness checks for pattern synonyms
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        Reporter:  nomeata           |                Owner:  mpickering
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.8.1
  checker)                           |             Keywords:
      Resolution:                    |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2669
       Wiki Page:                    |
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Comment (by simonpj):

 Reid wants the pattern-match completeness check to be able to "see
 through" the definition of a pattern synonym.   That is, he wants the
 client of `LL` in comment:51 to be able to see that `LL` is no more than
 `(Left (Left _))`.

 But currently pattern synonyms are set up to exploit ''abstraction''.  All
 the client knows is the type of `LL`; and the names and types of a
 matching and building function for it.  We have no mechanism at all for
 exposing `LL`'s implementation in an interface file, to client modules.

 That's not quite true: we have the unfolding for `LL`'s matching function.
 So in certain cases, where `LL` is simple, the unfolding tells you all
 about it.  But that's pretty fragile (it might change if `LL` got a bit
 bigger) and it's not information that is easy for the pattern-match
 overlap checker to exploit --- for example the unfolding of the matcher
 might be cluttered with stuff to do with re-boxing arguments that had been
 UNPACKed.

 I'm not arguing for a particular way forward here; just trying to explain
 why the context makes it hard to do what Reid wants.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8779#comment:52>
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