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

6 Mar 2016

      ...
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
#11671: Allow labels starting with uppercase with OverloadedLabels
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------

Comment (by adamgundry):

 pattern match on these overloaded constructors.

 Interesting point! Various people have been telling me I should think
 about overloaded constructors, and perhaps I should have done so before
 now...

 I played around with your example a bit and came up with the following
 construction, which isn't the most beautiful but works in GHC 8.0 (so in
 particular, I've lowercased the label names):

 {{{#!hs
 {-# LANGUAGE DataKinds, FlexibleInstances, MultiParamTypeClasses,
              ViewPatterns, ScopedTypeVariables, KindSignatures,
              TypeApplications, OverloadedLabels, TypeFamilies,
              FunctionalDependencies #-}

 import GHC.TypeLits
 import GHC.OverloadedLabels

 data Statement = Provable | Refutable
   deriving Show

 class IsPattern (tag :: Symbol) a r | tag a -> r where
     checkPattern :: a -> Maybe r

 instance IsPattern tag a r => IsLabel tag (a -> Maybe r) where
   fromLabel _ = checkPattern @tag

 instance IsPattern "truish" Statement () where
   checkPattern Provable  = Just ()
   checkPattern Refutable = Nothing

 instance IsLabel "truish" Statement where
   fromLabel _ = Provable

 test :: Statement -> Int
 test (#truish -> Just ()) = 42
 test _ = -1

 x = test #truish
 }}}

 This extends to constructors with arguments, after a fashion:

 {{{#!hs
 instance IsPattern "truthiness" Statement (Int, Bool) where
   checkPattern Provable  = Just (42, True)
   checkPattern Refutable = Nothing

 instance a ~ (Int, Bool) => IsLabel "truthiness" (a -> Statement) where
   fromLabel _ (42, True) = Provable
   fromLabel _ _          = Refutable

 test2 :: Statement -> Int
 test2 (#truthiness -> Just (k, _)) = k
 test2 _ = -1

 y = test2 (#truthiness (42, True))
 }}}

 A potential problem here is that the required `IsLabel` instances might
 conflict with "record field selector" instances. I suppose one way of
 dealing with that might be to desugar `#Foo` using a different class to
 `#foo`, but otherwise similarly. Though perhaps we should come up with a
 special desugaring for `#Foo` that works in patterns, as you suggest.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11671#comment:9
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler