[GHC] #13116: Allow Overloaded things in patterns

[GHC]

#13116: Allow Overloaded things in patterns
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        Reporter:                    |                Owner:
  facundo.dominguez                  |
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
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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Description changed by facundo.dominguez:

@@ -18,2 +18,2 @@
-   Do    ((==) (fromLabel (proxy# :: Proxy# "foo")) -> True) -> ...
-   Don't ((==) (fromLabel (proxy# :: Proxy# "foo")) -> True) -> ...
+   Do    ((==) (fromLabel (proxy# :: Proxy# "block")) -> True) -> ...
+   Don't ((==) (fromLabel (proxy# :: Proxy# "block")) -> True) -> ...

New description:

 This ticket is motivated by the use of `OverloadedLabels` of the
 [http://hackage.haskell.org/package/choice-0.2.0 choice package].

 One can use a type `Choice "block"` with values `Do #block` and `Don't
 #block` by using `OverloadedLabels`. However, in patterns this syntax is
 forbidden.

 {{{
 case block of
   Do    #block -> ...
   Don't #block -> ...
 }}}

 It would be nice if overloaded labels desugared to something sensible in
 patterns, perhaps
 {{{
 case block of
   Do    ((==) (fromLabel (proxy# :: Proxy# "block")) -> True) -> ...
   Don't ((==) (fromLabel (proxy# :: Proxy# "block")) -> True) -> ...
 }}}

 `OverloadedLists` and `OverloadedStrings` could benefit from a similar
 solution.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13116#comment:1>
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