#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 inaki): Replying to [comment:6 mpickering]:
If you want these values to actually look like constructors then have you tried using pattern synonyms?
Yes, indeed, that is what I do now. (For context, this is for https://github.com/haskell-gi/haskell-gi , a set of autogenerated libraries for a number of libraries in the gtk ecosystem.) The following works beautifully: {{{#!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 Truish :: IsOverloadedPattern "Truish" a => a pattern Truish <- ((checkOverloadedPattern @"Truish") -> True) where Truish = buildOverloadedPattern @"Truish" data Statement = Provable | Refutable instance IsOverloadedPattern "Truish" Statement where checkOverloadedPattern Provable = True checkOverloadedPattern Refutable = False buildOverloadedPattern = Provable test :: Statement -> Int test Truish = 42 test _ = -1 main :: IO () main = print (test Truish) }}} The problem is that this requires one pattern for each constructor one wants to overload in this way. Which are quite a few hundred/thousand for a large library like gtk (or glib, webkit, etc.). Not an issue in itself, but we cannot ask the user of the library to write them by hand, they should be automatically in the namespace when importing `GI.Gtk` or any other autogenerated binding. And we cannot bundle this with the library directly, since patterns with the same name may be easily generated by different autogenerated libraries, giving rise to name clashes. The way we solve it now is by asking the user of the library to run some command such that all possible such patterns, for all libraries used in the project, are generated in advance, and compiled into a single module. The resulting module should then be compiled as part of the users's project. Which works, but it is clunky. In similar situations (overloaded property names and overloaded signals) we can do away completely with this somewhat annoying "overloading module" by using OverloadedLabels, but I don't currently see a way of doing the same thing allowing to pattern match against the overloaded symbols. The construction in #comment:4 comes close, perhaps it can be made to work somehow. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11671#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler