[GHC] #11352: Allow applying type to label

[GHC]

#11352: Allow applying type to label
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.11
  checker)                           |
      Resolution:                    |             Keywords:  ORF
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by adamgundry):

 * owner:  adamgundry =>


Comment:

 Hmm, this isn't as straightforward as I thought. We could change the type
 of `fromLabel` to be `forall x a . IsLabel x a => a`, and then implement
 `#answer` by replacing it with `fromLabel @"answer"`. That gives you the
 behaviour you want, but it means that error messages mention applications
 of `fromLabel`, e.g.
 {{{
     Could not deduce (IsLabel "y" t)
       arising from the overloaded label ‘#y’
 }}}
 becomes
 {{{
     Could not deduce (IsLabel "y" t)
         arising from a use of ‘fromLabel’
 }}}
 which is somewhat undesirable. Moreover, overloaded string/numeric
 literals are similarly incompatible with visible type application. So I'm
 inclined to leave things as they are: after all, you can always write
 `#answer :: Int`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11352#comment:2>
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