Re: Proposal: Add the missing instances for Traversable (Either b) and Traversable ((, ) b)

Edward and comrades, On 28 Jul 2012, at 02:58, Edward Kmett wrote:

Fair enough. I'll try to switch my development environment around to something new enough that I can assemble a patch.

Any chance of throwing in the trivial instances for Const? (And how about composition?) Or maybe they can wait for a more coherent presentation of the "Polynomial Functor Kit" (Const, Identity, closure under sum and product). It's a bit of a tangent, but the context I have in mind is that all the polynomials are Foldable, Traversable and (the much neglected) HalfZippable (the idea and the name I learned from Roland Backhouse). class Functor f => HalfZippable f where halfZip :: f a -> f b -> Maybe (f (a, b)) which is a zip that can abort in case of shape mismatch. If a functor is both Foldable and HalfZippable, then its (least) fixpoint has a decidable equality (you try to zip each node together, and if you succeed, you foldMap the equality test over the paired children), and moreover, its free monad (chucking in a representation of "free variables") has a unification algorithm. The Const instances may seem useless in isolation, but as part of the kit, they're vital (allowing us to add labels to tree structures). A HalfZippable proposal might show up in a bit, but it would be good to sort out the rest of the Foldable/Traversable components. Cheers Conor

-Edward

On Fri, Jul 27, 2012 at 9:06 PM, Ross Paterson wrote: On Sat, Jul 28, 2012 at 01:27:47AM +0100, Edward Kmett wrote:

...

The following instances are missing and have only one sensible definition. I've been bitten by their lack repeatedly and there is no place outside of base that they can live without needlessly being orphaned.

I would like to propose adding the following instances to Data.Foldable and Data.Traversable.

instance Foldable (Either e) where foldMap f (Right m) = f m foldMap _ (Left _) = mempty instance Traversable (Either e) where traverse _ (Left e) = pure (Left e) traverse f (Right x) = Right <$> f x instance Foldable ((,) e) where foldMap f (_,x) = f x instance Traversable ((,) e) where traverse f (e,x) = (,) e <$> f x

I had thought honestly thought we'd already added them long ago.

You proposed them in January 2011 and everyone agreed (after changing the pair instance to the strict versions given here) but I don't think you sent a patch to Ian.

http://thread.gmane.org/gmane.comp.lang.haskell.libraries/15196

It's certainly overdue.

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