[GHC] #13041: Type classes in Backpack signatures are dodgy

29 Dec 2016

      #13041: Type classes in Backpack signatures are dodgy
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           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:
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 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
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