[GHC] #11671: Allow labels starting with uppercase with OverloadedLabels

[GHC]

#11671: Allow labels starting with uppercase with OverloadedLabels
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        Reporter:  inaki             |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1-rc1
  (Parser)                           |
      Resolution:                    |             Keywords:  ORF
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 inaki):

 Just to elaborate on my motivation for asking this: in bindings to C
 libraries (say gtk), due to the requirement of uniqueness of identifiers,
 one ends up with rather long identifiers for constants, which are
 typically represented by different constructors for a certain sum type.
 See for example:
 https://hackage.haskell.org/package/gtk-0.14.2/docs/Graphics-UI-Gtk-
 General-Enums.html#t:TreeViewColumnSizing

 It would be much more convenient to be able to say
 {{{#!hs
 treeViewColumnSetSizing tvc #Fixed
 }}}
 rather than the current, much more verbose
 {{{#!hs
 treeViewColumnSetSizing tvc TreeViewColumnFixed
 }}}

 The former is just as typesafe, but much more pleasant to read and use, I
 think, and here it is fairly natural to expect the overloaded label to
 start with uppercase. It can just as easily be made to work with
 lowercase, but I think that from the point of view of the user of the
 library the version starting with uppercase is more natural.

 (As a side remark: the above is still unnecessarily verbose, with further
 use of OverloadedLabels we can shorten it to
 {{{#!hs
 set tvc [#sizing := #Fixed]
 }}}
 which is close to Python levels of convenience, while being perfectly type
 safe. This already works well in the haskell-gi autogenerated bindings for
 gtk, only that I need to replace `#Fixed` -> `#fixed` due to the parser
 limitation.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11671#comment:2>
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