Re: [Haskell-cafe] How to expose if a constraint is satisfiable

Maybe look at https://www.reddit.com/r/haskell/comments/6k86je/constraint_unions_bringing_... (relating to the repository Oliver links to) and note also Edward Kmett's response. On Tue, May 8, 2018 at 9:52 AM, Oliver Charles wrote:

I've only skimmed the thread, so sorry if this is a red herring, but could this be helpful? https://github.com/rampion/constraint-unions/blob/master/README.md

On Tue, May 8, 2018 at 12:49 PM, Li-yao Xia wrote:

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On 05/07/2018 11:45 PM, Clinton Mead wrote:

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Hi Li-yao

I understand this issue, but I think what your describing doesn't relate to what I'm talking about uniquely.

For example:

instance {-# OVERLAPPABLE #-} Num x => C x where ... instance {-# OVERLAPPING #-} C Int where ...

is completely legal, and the instance used depends on context.

This is an issue with overlapping instances, but already GHC allows it. I don't think what I'm proposing is any worse to what's already legal.

No instance for the C type class above will be picked until x is equal to some applied type constructor; it will be more challenging to break coherence with that than using what you're proposing.

It seems difficult to make a clear point because the whole topic of coherence with Haskell type classes is already a minefield.

Li-yao _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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