[GHC] #12240: Common Sense for Type Classes

[GHC]

#12240: Common Sense for Type Classes
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        Reporter:  Mathnerd314       |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Mathnerd314):

 > This is only "common sense" if you discard the open world assumption

 I have not seen hide nor hair of this fabled "open world assumption",
 except a small section in Real World Haskell:
 > We can add new instances anywhere; they are not confined to the module
 where we define a typeclass. This feature of the typeclass system is
 referred to as its open world assumption.

 This refers to the ''declaration'' of instances, not instance resolution.
 If the proposal was to remove orphan instances, I would see the merit of
 bringing this up, but it does not seem relevant here.

 > (or accept incoherence),

 Consider this:
 {{{#!hs
 -- in A.hs
 class A a where f :: a -> Int
 -- in B.hs
 instance A Int where f _ = 1
 foo = f (0 :: Int)
 -- in C.hs
 instance A Int where f _ = 2
 bar = f (0 :: Int)
 -- in Main.hs
 main = print (foo,bar) -- (1,2)
 }}}

 It is a blatant violation of the Haskell Report ("A type may not be
 declared as an instance of a particular class more than once in the
 program."), yet it compiles in GHC with no extensions.

 Meanwhile, all I want to do is relax instantiation from "a single instance
 matches and no other instance unifies" to "a single instance unifies and
 no other instance unifies". This is only interesting in limited
 situations, such as the example in comment:4, and leaves all Haskell 98
 instances unchanged. I do not see how it adds any incoherence, because in
 both rules only a single instance is allowed.

 > I don't claim to have any say in whether an implementation will or will
 not be accepted, but bear in mind that new features have an ongoing cost,
 even if guarded by language flags. Feel free to work on it if you like,
 but seeking wider input first might be a good idea to avoid working on
 something that is ultimately not merged.

 I have posted on Reddit, IRC, and this bugtracker; what's left besides
 Phabricator?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12240#comment:7>
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