Something stronger than IncoherentInstances needed (Univalent Classes?)

[Joachim Breitner]

Hi,

in the context of the newtype wrapper I have an instance selection
problem where even IncoherentInstances is not liberal enough. Consider
this example:

Prelude> :set -XFlexibleInstances -XIncoherentInstances  -XMultiParamTypeClasses
Prelude> class Class a b where { method :: (a,b); method = undefined }
Prelude> instance Class a b => Class [a] [b]
Prelude> instance Class a a
Prelude> :t method :: ([a],[a])

<interactive>:1:1:
    Overlapping instances for Class [a1] [a1]
      arising from a use of `method'
    Matching instances:
      instance [incoherent] Class a b => Class [a] [b]
        -- Defined at <interactive>:4:10
      instance [incoherent] Class a a -- Defined at <interactive>:5:10
    In the expression: method :: ([a], [a])

As none of the two instances are more specific than the other, the
typechecker stops despite the incoherent flag.

My suggestion is to add another flag, this time to the class
declaration, marking the class as univalent (different naming
suggestions welcome, of course), indicating that it will generally not
matter which instance is selected, and in the case of overlap the
typechecker should just pick any matching instance.

Would such a feature be welcome?

I could imagine that it might be more useful in other settings as well,
e.g. if type classes are pure predicates about types, without any
methods.

Greetings,
Joachim

PS: I will be on a summer school the next two week. I have written down
the status of the newtype coercion implementation at
http://ghc.haskell.org/trac/ghc/wiki/NewtypeWrappers#Status

-- 
Joachim “nomeata” Breitner
  mail at joachim-breitner.de • http://www.joachim-breitner.de/
  Jabber: nomeata at joachim-breitner.de  • GPG-Key: 0x4743206C
  Debian Developer: nomeata at debian.org
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