[GHC] #9671: Allow expressions in patterns

[GHC]

#9671: Allow expressions in patterns
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              Reporter:              |            Owner:
  Iceland_jack                       |           Status:  new
                  Type:  feature     |        Milestone:  7.10.1
  request                            |          Version:
              Priority:  normal      |         Keywords:  patterns, pattern
             Component:  Compiler    |  synonyms
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 Your examples all use constants, but I assume you are going to allow this:
 {{{
 f :: Person -> Map Person Age -> Age
 f p (Lookup p a) = a
 f p _            = error ("No info on " ++ show p)
 }}}
 Now this is jolly confusing.  In the pattern `(Lookup p a)`, `p` is a
 bound occurence while `a` is a binding occurrence.

 This distinction is made syntactically by view patterns:
 {{{
 f :: Person -> Map Person Age -> Age
 f p ((\m -> lookup m p) -> a) = a
 }}}
 Here the stuff on the left of the view-pattern arrow `->` are bound
 occurrences, while the stuff on the right is binding.

 You could even have
 {{{
 f p (Lookup p p) = p
 }}}
 meaning
 {{{
 f p1 (Lookup p1 p2) = p2
 }}}
 To me, the lack of syntactic cues for what is bound and what is binding
 seems quite serious.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9671#comment:1>
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