TL;DR. One (implicit?) assumption in recent debates on `length` seems to be unfounded, because current docs for Foldable are meant to be much stronger than their actual language, unless I'm missing something. Clarifications welcome. If my understanding is right, I'd suggest somebody fixes the docs. One point is not clear to me, so at present I could not volunteer to fix them myself. In recent debates, it was assumed or implied that `length` must be equivalent to length = length . toList
we also have a kind system, so we can ignore the name. length :: f a -> Int We immediately know that values of the kind (* -> *) slot in to the value (f), with a kind checker to ensure we get it correct. Therefore, we can easily reason about the length of values of kind ((,) a)
I don't quite get how that argument is supposed to proceed. However that's meant, that seems incorrect because length is a typeclass method, so even the following strawman instance typechecks: instance Foldable ((,,) a b) where length _ = 42 I assume this should violate some law, but the relevant law seems to be forgotten. The most constraining language I can find is the following:
sum, product, maximum, and minimum should all be essentially equivalent to foldMap forms, such as sum = getSum . foldMap Sum but may be less defined.
but (a) `length` is not even mentioned, even if it's intended (b) I think those should be laws (c) using "should" and "essentially" in "should all be essentially equivalent", seems too weak. I can infer the intention, but this seems insufficient to declare Docs for `length` don't help either:
Returns the size/length of a finite structure as an Int. The default implementation is optimized for structures that are similar to cons-lists, because there is no general way to do better.
I mean, these docs use the English word "length", so that actually forbids 42, but that text is too vague to forbid 3 frankly. As I missing something? I also take issue with "may be less defined", and here I'm not sure of the intention, since that declares instance Foldable ((,,) a b) where length _ = undefined as legal. I imagine the point is about taking the length of partially undefined structures, but it's not clear to me why a custom `sum` implementation would be less defined than `sum = getSum . foldMap Sum`. Even ignoring the above instances (which I would never write down), I can't reason much about my code based on those specifications. This situation seems unfortunate. Cheers, -- Paolo G. Giarrusso - Ph.D. Student, Tübingen University http://ps.informatik.uni-tuebingen.de/team/giarrusso/