Hacker News new | ask | show | jobs
by pauldirac137 3727 days ago
Modules are strictly more powerful than Haskell type classes (at least, more powerful than naive type classes; there are so many extensions that I'm not sure that is still the case). Modular implicits are described here: http://arxiv.org/pdf/1512.01895.pdf. The main problem with modules as typeclasses is that it's pretty painful to use in practice, and modular implicits fix that, giving a solution quite reminiscent of Haskell type classes. I don't know Scala implicits well enough to comment, but the authors feel their version avoids most of the problems in Scala's implicits (they jokingly refer to it as "a more explicit kind of implicit").
1 comments
lomnakkus 3726 days ago
AFAIUI modular implicits don't give you global uniqueness. While this can be "worked around" in some cases[0], it's not really possible in the general case, and global uniqueness is pretty essential for sanity when writing higher-level code[1].

[0] The paper gives set-union as an example.

[1] https://www.youtube.com/watch?v=hIZxTQP1ifo . (Sorry for throwing video at you -- there's probably a write-up somewhere, but I'm in a bit of a rush, and this was the thing that came to mind.)

link
airless_bar 3726 days ago
> [1] https://www.youtube.com/watch?v=hIZxTQP1ifo

Now we just need a Haskell compiler which actually bothers to provide the "pretty essential for sanity" for global uniqueness. Currently, I count zero compilers in common use which do.

link
lomnakkus 3726 days ago
Not sure what you mean. Could you elaborate?
link
airless_bar 3726 days ago
GHC gives you none of the guarantees Kmett keeps boasting about.
link
lomnakkus 3726 days ago
How so?

(EDIT: Just to forestall the inevitable: Yes, there are some weird and dangerous extensions you can enable, but mostly it's pretty clear that they're dangerous. It doesn't seem to be a problem in practice.)

link
airless_bar 3725 days ago
> How so?

Because they never bothered to actually implement the checks to guarantee global uniqueness in GHC?

> (EDIT: Just to forestall the inevitable: Yes, there are some weird and dangerous extensions you can enable, but mostly it's pretty clear that they're dangerous. It doesn't seem to be a problem in practice.)

No weird and dangerous extensions in use. Vanilla Haskell.

link