Hacker News new | ask | show | jobs
by charlieflowers 3778 days ago
Wow, really? I learned some Haskell on the side a few years ago, and typeclasses were one of my favorite features. They allowed so much expressiveness from such small building blocks. They seemed like a cornerstone of the language.

I'm surprised to hear they're often eschewed.
2 comments
tome 3778 days ago
Highly abstract and general typeclasses for standard mathematical and algebraic concepts are extremely useful and not eschewed. With them you can do genuinely generic programming in a syntactically convenient way. Examples include: Functor, Applicative, Monad, Num, Floating, Integral, MonadState, MonadReader, Profunctor, Contravariant, ...

Typeclasses which are ad hoc, specific, and typically defined on a per project basis, are much more debatable. If you have the concept of "rendering to the screen" and you have typeclass Renderable with a render method then this is really no better than just writing separate functions for each instance of the Renderable. This isn't genuinely generic programming, it's just symbol overloading.

link
efnx 3778 days ago
Hey, you're talking about my library Renderable - and I'm totally up to finding a more idiomatic way to express the problem. It just so happened that type classes were the easiest way to do that.
link
tome 3778 days ago
Ha, sorry, didn't mean to make it sound like I was talking about anything in particular! I just made up that example on the spot. A "Renderable" thing fits into a common pattern where you see typeclasses used in this way.

(Are you talking about this library? https://hackage.haskell.org/package/renderable-0.1.0.0/docs/...

I don't actually see a Renderable class there anyway.)

link
efnx 3778 days ago
Ahh - I thought you may have read my articles on HN or Reddit - how self centered of me :)

Yes, there's no 'Renderable' typeclass but there are 'Primitive's. There are no laws that the 'Primitive' typeclass instances must obey but there are associated types that need to be satisfied. I tried to write this without a typeclass and I just couldn't get it to check, so I turned to TypeFamilies to encode the relation ship between types. I'd love to make it more idiomatic though!

[I'd love some feedback if you're so inclined](https://github.com/schell/renderable/blob/master/src/Data/Re...)

link
danidiaz 3778 days ago
I wouldn't say they are eschewed.

Haskell programmers do seem to prefer typeclasses that come equipped with some set of meaningful "laws" that help to reject "unreasonable" instances.

And even without that, they are very useful for data type conversions and the like.

link