Re: Deprecate Foldable for Either

a whole universe of nicely polymorphic functions unavailable for them to use without additional effort.

At least in terms of Either, I do not see a "universe of nicely polymorphic functions" here. I'd say applying a Foldable on a complex data structure build of all sorts of type constructors that involve Either (and tuples) is highly counterintuitive. For a start, the "sums-of-products" representation of data types is not stable under our Foldable instances. data D a = C1 a | C2 a a deriving (Foldable) gives you a completely different Foldable instance than its sums-of-product representation Either a (a, a) Andreas On 02.03.2017 19:12, Kris Nuttycombe wrote:

As with all of these discussions, the point of having Either be Foldable is not that you're going to call foldMap on an Either value directly. Instead, it is that you be able to pass whatever sort of foldable thing you choose (which Either certainly is) to a function that only requires foldability of its input. By removing or damaging the Foldable instance for Either, you don't simply prevent people from encountering problems that will be resolved the first time they test their software - you make a whole universe of nicely polymorphic functions unavailable for them to use without additional effort.

In particular, the idea that one should make all such functions partial by throwing an error is repugnant.

Kris

On Thu, Mar 2, 2017 at 10:35 AM, Francesco Ariis mailto:fa-ml@ariis.it> wrote:

On Thu, Mar 02, 2017 at 05:19:26PM +0000, Oliver Charles wrote: > Personally, I think it would be a shame to lose foldMap on EIther. I > frequently foldMap over Maybe values (where mempty is suitable in case of > "failure"), and I can certainly see myself doing the same thing with Either.

I am not trying to be polemic, just to see where the community stands: regarding Either/Maybe: do you have use cases for length (sum) too?

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