Re: Haskell Foldable Wast

What will the signature of things like 'mapM_' be then? Will there be two versions, one with a Foldable constraint and one with a Traversable constraint? I don't think base will be the area with the most impact here, by the way. The biggest impact will be libraries using these classes, where suddenly more class constraints need to be added to the signature. Erik On 22 February 2016 at 08:33, Augustsson, Lennart wrote:

I don’t believe the practical consequences will be severe. I know you do, but you have provided no proof.

I did an experiment and removed it, and recompiled base. And it hardly needed any changes.

From: Libraries [mailto:libraries-bounces@haskell.org] On Behalf Of Edward Kmett Sent: 21 February 2016 23:51 To: Jeremy Cc: Haskell Libraries Subject: Re: Haskell Foldable Wast

That leads to a ton of ever MORE nonsensical consequences like not being to weaken calls to mapM (which uses Traversable) to mapM_(which needs only Foldable) or doubling the number of combinators we have all over again for random prescriptive reasons, right after we just starting finally healing the last source of needless duplication (Applicative not being a superclass of Monad).

-Edward

On Sun, Feb 21, 2016 at 7:57 AM, Jeremy wrote:

Marcin Mrotek wrote

...

I think that, as far as Foldable is concerned, a tuple is equivalent to Identity, so this instance is indeed useless. However, Foldable is a superclass of Traversable (and it wouldn't make much sense to make these classes unrelated, as one can always define folds with `traverse`), so I've always found it a necessary evil.

Perhaps the case of tuple is evidence that Foldable should *not* be a superclass of Traversable?

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