Re: SAFE: a Foldable proposal

On Thu, Feb 25, 2016 at 3:32 PM, Kosyrev Serge <_deepfire@feelingofgreen.ru> wrote:

Thomas Tuegel writes:

...

Because it is not Applicative? Yes. For the same reason as Set, though, I do not think it should be Foldable or Traversable. It is convenient, but unsound. Or at least, less-sound. To fold or traverse Set and Map, I think the sound thing to do is to use one of the provided functions to convert them to a type where ordering is reflected structurally.

What about adding unstructural fold/traversal under different names?

That way we could have the convenience when we truly don't care about the directionality property, and the benefits of pure folding at the same time.

That's a good idea, but I don't think it really changes anything. The chief problem with types that aren't structurally ordered is really that there are multiple valid orders. For example, if [a] is our canonical structurally-ordered type, there are at least two obvious ways to write f :: Ord a => Set a -> [a]. I don't think an unstructured version of Foldable has much benefit over simply converting the improper type to a proper one. Regards, Tom