#13041: Type classes in Backpack signatures are dodgy -------------------------------------+------------------------------------- Reporter: ezyang | Owner: ezyang Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 (Type checker) | Keywords: backpack | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- At the moment, we allow exactly the same type class declarations in hsig files as we do in hs-boot files, and matching requires the type class definitions to match up exactly. But this is quite questionable. First, this means that I can't write this in a signature: {{{ class K a where f :: a -> Bool }}} but then implement K with a type class that provides a default. The matching code will say these are not compatible. That's not so useful. Second, it means that I can WRITE a default implementation in a signature file. That seems like a step too far: there are many things I might like to write in a specification of a type class, but what the default methods are seems like a step too far. So it seems we need to fix two things: 1. Relax matching so that you can add defaults post-facto in the implementation of the type class 2. Make hsig more restrictive w.r.t. what class definitions it will accept. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13041 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler