On Wed Apr 5 16:15:48 UTC 2017, Paolo Giarrusso wrote: Thanks for all the mails, but I'm still missing an answer, so I must have done something wrong.

:-) Thanks Paolo, you've done nothing wrong IMHO. You've done the community a service by pointing out an omission in the docos. It's the same failure to communicate clearly that's given rise to the whole FTP kerfuffle.

I guess I didn't explain my question well. Or nobody thinks it's worth answering (in which case, sorry, but I'd prefer to be told I'm misguided, over having the question ignored).

I think your original question was clear. I think it is very well worth answering. That is: answering what are the 'Laws' for Foldable? I've been waiting patiently for Libraries HQ to: * explain the applicable Laws * acknowledge they're not doco'd * volunteer to fix that

... I'm really asking a different question: do length docs actually give a complete specification—it still seems to me they don't.

I agree they don't.

... But that's very confusing: can really debate on `length (a, b) = 1` have continued for so long, without noticing that `length = getSum . foldMap (Sum . const 1)` is a fundamental assumption and is not mandated by anything written down? I assume I must be missing something, which is why I asked.

The only reply I've seen [from David F, you copied below, apologies if I've missed others] doesn't give any "fundamental assumption". It only says "should be quite well-behaved" and "pretty much everybody agrees on". I'm not 'getting at' David, at least he answered. I agree with your intuition that Foldable length = length . toList But that just pushes the question back to what are the Laws for `toList`? Note that `toList` has been in Foldable 'for ever'. And instance Foldable Maybe 'for ever'. That returns a 0-or-1 length list. Contrast that `instance Foldable ((,) b)` appeared in GHC 7.8 ~2014, along with `instance Foldable Either`.

Of all incomplete docs, this matters more since `length (a, b)` is so surprising.

I'd say the surprising bit is `toList (a, b)`. `length` is just a consequence. Cue another long thread on `toList`. I'm inclined to agree with Edward K, that there's only one possible definition for `toList` -- that is, if we accept `instance Foldable ((,) b). Cue another long thread on tuple instances, and the Either instance.

The unsuspecting looking at docs lacks the facts needed to infer this wart. Spelling out implications would IMHO be even better, for the same reason people state theorems even though they can be proved, but maybe some disagree. A complete spec would be a starting point.

Yes. In particular, the conspicuous lack of docos is now wildly at variance with all the tutorial/intro texts, which explain `length` as for Lists. I plain think that `length` should be Lists-only; there should be some other name for Foldable's size.

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On 3 April 2017 at 17:59, David Feuer wrote: length should be quite well-behaved relative to foldMap:

length = getSum . foldMap (Sum . const 1)

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Thanks, that looks useful to add to docs. I was thinking of `length = length . toList`.

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Another law pretty much everyone agrees on is that *if* f is an instance of Traversable, then

foldMap = foldMapDefault

AntC