[GHC] #9671: Allow expressions in patterns

[GHC]

#9671: Allow expressions in patterns
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:
      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 ekmett):

 {{{Lookup $"Alice" =}}} is syntactically ambiguous in the context where
 you are doing a destructuring assignment. It could be defining `$` or
 providing the marker. This seems to indicate that the marker should be a
 divider or indicator of purpose that is already reserved syntax and isn't
 a valid operator.

 `->` is taken by view patterns, but you could use (with varying degrees of
 absurdity) already reserved operators like `<-`, `\`, `;`, `..`, `=>` as
 either a separator between parameters and fields or as an indicator in a
 manner that would simply be filling a hole in the grammar, if you really
 want a visual indicator of which arguments to the pattern are inputs if we
 really do want a visual marker here.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9671#comment:16>
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