I like Purescript's named instances
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Sorry, I think it's Idris that does this.
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Bottom line: I don't know what the best thing is yet. But Haskell is quite painful in many ways, so I choose to believe that there is some better option out there waiting to be found. :)
I haven't heard any arguments against named instances, but would love to hear some.
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By named instances do you mean non-coherent instances? Ed Kmett did a loong talk "Type Classes vs. The World" which outlined some of the problems. I don't necessarily agree - I'm interested in exploring the modular type classes approach, but it brings up important points.
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You definitely want to avoid Scala's situation where you never are sure what instance you have in scope at any time - and the instance resolution algorithm is mysterious and confusing.
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Coherent instances also tend to end up with people relying on it when designing their APIs, so you need to figure out how to train people out relying on that and provide alternatives.
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For example you'll need some way of statically ensuring that hash maps use a consistent hashing implementation over their life, lest all craziness ensue. There are solutions to this, but it does end up looking less 'pretty' than what Haskellers are typically used to.
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Type classes can be used in the way they do because they violate modularity, assuming global coherence across packages. But once you remove, and perhaps even encourage people to break global coherence, you need to have a way to get back what you traded away.
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Scala uses implicits, not named instances. I don't think Ed's talk about type class coherence said anything about named instances, if I recall correctly. His point is that no matter which "path" you choose when solving constraints, you should end up at the same point.
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Scala's implicits are regularly used as a non-coherent type class system. And you have to name the instances that you declare. It's not a specific feature - more a combination of a bunch of smaller features: docs.scala-lang.org/tour/implicit-
Ah, hm, I think this is different to what I think of when I say "named instance". I need to sit down and work this out!