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

4 Mar 2016

      #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):

 Thinking more about this, I came up with a small worry: having such
 overloaded constructors makes it very tempting to ask if it is possible to
 pattern match on these overloaded constructors. Simply desugaring to
 `fromLabel ...` seems to preclude this from working.

 Just for fun, I tried to come up with a desugaring that allows for pattern
 matching too, but I encountered a parsing problem when trying to
 explicitly apply types in a pattern, the following is the closest I could
 get:

 {{{#!hs
 {-# LANGUAGE DataKinds, FlexibleInstances,
   MultiParamTypeClasses,
   PatternSynonyms, ViewPatterns, ScopedTypeVariables, KindSignatures,
   TypeApplications #-}

 import GHC.TypeLits

 class IsOverloadedPattern (tag :: Symbol) (a :: *) where
     checkOverloadedPattern :: a -> Bool
     buildOverloadedPattern :: a

 pattern OverloadedPattern :: forall tag a. IsOverloadedPattern (tag ::
 Symbol) a => a
 pattern OverloadedPattern <- ((checkOverloadedPattern @tag @a) -> True)
 where
     OverloadedPattern = buildOverloadedPattern @tag @a

 data Statement = Provable | Refutable

 instance IsOverloadedPattern "Truish" Statement where
     checkOverloadedPattern Provable = True
     checkOverloadedPattern Refutable = False

     buildOverloadedPattern = Provable

 {-
 -- We would like to write something like:
 test :: Statement -> Int
 test #Truish = 42
 test _ = -1

 -- desugaring to
 test :: Statement -> Int
 test (OverloadedPattern @"Truish") = 42
 test _ = -1
 -}

 test2 :: Statement -> Int
 test2 Provable = 42
 test2 _ = -1

 main :: IO ()
 main = print (test2 (OverloadedPattern @"Truish"))
 }}}

 One may also worry how to pattern match on multi-parameter constructors,
 which is not supported by the construction above. Perhaps there is some
 clever way of making overloaded constructors work everywhere a normal
 constructor would work? I guess that if #8583 was implemented we could use
 that?

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11671#comment:4
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