[GHC] #9793: Some as-patterns could be accepted in pattern synonyms

[GHC]

#9793: Some as-patterns could be accepted in pattern synonyms
-------------------------------------+-------------------------------------
        Reporter:  cactus            |                Owner:  cactus
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  7.8.3
  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:D1666
       Wiki Page:                    |
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Comment (by simonpj):

 Hang on.  What is the ''design'' here?  Specifically, what is the
 semantics of matching?

 To put it another way, suppose I have
 {{{
 pattern P x <- x@(Right _)
 }}}
 What does it mean to match against ''that''?

 Well, to a first approximation, in a pattern, `P pat` means the same as
 `<rhs of P> with pat instead of x`.  So
 {{{
 f (P (Left y)) = e
 }}}
 means
 {{{
 f ((Left y)@(Right _)) = e
 }}}
 And what does ''that'' mean?

 Perhaps we could expand the semantics of as-patterns to be `p1 @ p2`,
 meaning match against ''both'' `p1` ''and'' `p1`.  An "and-pattern" if you
 like.  Now everything would be well-specified. And I suppose you could say
 things like
 {{{
 f ((\xs -> length xs < 10) -> True)@(y:_) = ...
 }}}
 which would match against a list of length shorter than 10, and then bind
 'y' to the head of that list.

 But this is a bigger change, and not directly related to pattern synonyms.

 So far I'm unconvinced.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9793#comment:5>
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