Re: [Haskell-cafe] Monomorphic containers, Functor/Foldable/Traversable WAS: mapM_ for bytestring

I didn't see this message and replied privately to Michael earlier, so I'm replicating my comments here. 1. Sooner or later I expect you'll want something like this: class LooseMap c el el' where lMap :: (el -> el') -> c el -> c el' It covers the case of things like hashmaps/unboxed vectors that have class constraints on elements. Although maybe LooseFunctor or LFunctor is a better name. Probably something similar for Traversable would be good also, as would a default instance in terms of Functor. 2. IMHO cMapM_ (and related) should be part of the Foldable class. This is entirely for performance reasons, but there's no downside since you can just provide a default instance. 3. I'm not entirely sure that the length* functions belong here. I understand why, and I think it's sensible reasoning, and I don't have a good argument against it, but I just don't like it. With those, and mapM_-like functions, it seems that the foldable class is halfway to being another monolithic ListLike. But I don't have any better ideas either. As to the bikeshed color, I would prefer to just call the classes Foldable/Traversable. People can use qualified imports to disambiguate when writing instances, and at call sites client code would never need Data.{Foldable|Traversable} and can just use these versions instead. I'd still want a separate name for Functor though, since it's in the Prelude, so maybe it's better to be consistent. My $.02. On Wed, Sep 11, 2013 at 3:25 PM, Michael Snoyman wrote:

That's really funny timing. I started work on a very similar project just this week:

https://github.com/snoyberg/mono-traversable

It's not refined yet, which is why I haven't discussed it too publicly, but it's probably at the point where some review would make sense. There's been a bit of a discussion on a separate Github issue[1] about it.

A few caveats:

- The names are completely up for debate, many of them could be improved. - The laws aren't documented yet, but they mirror the laws for the polymorphic classes these classes are based on. - The Data.MonoTraversable module is the main module to look at. The other two are far more nascent (though I'd definitely appreciate feedback people have on them).

I think this and mono-foldable have a lot of overlap, I'd be interested to hear what you think in particular John.

Michael

[1] https://github.com/snoyberg/classy-prelude/issues/18

On Wed, Sep 11, 2013 at 11:05 PM, John Lato wrote:

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I agree with everything Edward has said already. I went through a similar chain of reasoning a few years ago when I started using ListLike, which provides a FoldableLL class (although it uses fundeps as ListLike predates type families). ByteString can't be a Foldable instance, nor do I think most people would want it to be.

Even though I would also like to see mapM_ in bytestring, it's probably faster to have a library with a separate monomorphic Foldable class. So I just wrote one:

https://github.com/JohnLato/mono-foldable http://hackage.haskell.org/package/mono-foldable

Petr Pudlak has done some work in this area. A big problem is that foldM/mapM_ are typically implemented in terms of Foldable.foldr (or FoldableLL), but this isn't always optimal for performance. They really need to be part of the type class so that different container types can have specialized implementations. I did that in mono-foldable, using Artyom's map implementation (Artyom, please let me know if you object to this!)

pull requests, forks, etc all welcome.

John L.

On Wed, Sep 11, 2013 at 1:29 PM, Edward Kmett wrote:

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mapM_ is actually implemented in terms of Foldable, not Traversable, and its implementation in terms of folding a ByteString is actually rather slow in my experience doing so inside lens and isn't much faster than the naive version that was suggested at the start of this discussion.

But as we're not monomorphizing Foldable/Traversable, this isn't a think that is able to happen anyways.

-Edward

On Wed, Sep 11, 2013 at 2:25 PM, Henning Thielemann < lemming@henning-thielemann.de> wrote:

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On Wed, 11 Sep 2013, Duncan Coutts wrote:

For mapM etc, personally I think a better solution would be if

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ByteString and Text and other specialised containers could be an instance of Foldable/Traversable. Those classes define mapM etc but currently they only work for containers that are polymorphic in their elements, so all specialised containers are excluded. I'm sure there must be a solution to that (I'd guess with type families) and that would be much nicer than adding mapM etc to bytestring itself. We would then just provide efficient instances for Foldable/Traversable.

I'd prefer to keep bytestring simple with respect to the number of type extensions. Since you must implement ByteString.mapM anyway, you can plug this into an instance definition of Traversable ByteString.

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