Proposal: add a foldable law

Edward Kmett ekmett at gmail.com
Sun May 6 23:39:36 UTC 2018 

Fair point. I was more thinking about the fact that you don't necessarily
visit all of the a's because you can use the combinators you have there to
generate an unbounded number of combination of them than the injectivity
concern.

On Sun, May 6, 2018 at 7:28 PM, David Feuer <david.feuer at gmail.com> wrote:

> I don't understand. Something like
>
> data Foo a where
>   Foo :: Num a => a -> Foo a
>
> has a perfectly good "forgetful" injection:
>
> toTrav (Foo a) = Identity a
>
> This is injective because of class coherence.
>
> On Sun, May 6, 2018, 6:04 PM Edward Kmett <ekmett at gmail.com> wrote:
>
>> You can get stuck with contravariance in some fashion on the backswing,
>> though, just from holding onto instance constraints about the type 'a'.
>> e.g. If your Foldable data type captures a Num instance for 'a', it could
>> build fresh 'a's out of the ones you have lying around.
>>
>> There isn't a huge difference between that and just capturing the member
>> of Num that you use.
>>
>> -Edward
>>
>> On Sun, May 6, 2018 at 3:37 PM, David Feuer <david.feuer at gmail.com>
>> wrote:
>>
>>> The question, of course, is what we actually want to require. The strong
>>> injectivity law prohibits certain instances, yes. But it's not obvious, a
>>> priori, that those are "good" instances. Should a Foldable instance be
>>> allowed contravariance? Maybe that's just too weird. Nor is it remotely
>>> clear to me that the enumeration-based instance you give (that simply
>>> ignores the Foldable within) is something we want to accept. If we want
>>> Foldable to be as close to Traversable as possible while tolerating types
>>> that restrict their arguments in some fashion (i.e., things that look kind
>>> of like decorated lists) then I think the strong injectivity law is the way
>>> to go. Otherwise we need something else. I don't think your new law is
>>> entirely self-explanatory. Perhaps you can break it down a bit?
>>>
>>> On Sun, May 6, 2018, 1:56 PM Gershom B <gershomb at gmail.com> wrote:
>>>
>>>> An amendment to the below, for clarity. There is still a problem, and
>>>> the fix I suggest is still the fix I suggest.
>>>>
>>>> The contravarient example `data Foo a = Foo [a] (a -> Int)` is as I
>>>> described, and passes quantification and almost-injectivity (as I suggested
>>>> below), but not strong-injectivity (as proposed by David originally), and
>>>> is the correct example to necessitate the fix.
>>>>
>>>> However, in the other two cases, while indeed they have instances that
>>>> pass the quantification law (and the almost-injectivity law I suggest),
>>>> these instances are more subtle than one would imagine. In other words, I
>>>> wrote that there was an “obvious” foldable instance. But the instances, to
>>>> pass the laws, are actually somewhat nonobvious. Furthermore, the technique
>>>> to give these instances can _also_ be used to construct a type that allows
>>>> them to pass strong-injectivity
>>>>
>>>> In particular, these instances are not the ones that come from only
>>>> feeding the elements of the first component into the projection function of
>>>> the second component. Rather, they arise from the projection function alone.
>>>>
>>>> So for `data Store f a b = Store (f a) (a -> b)`, then we have a
>>>> Foldable instance for any enumerable type `a` that just foldMaps over every
>>>> `b` produced by the function as mapped over every `a` in the enumeration,
>>>> and the first component is discarded. I.e. we view the function as “an
>>>> a-indexed container of b” and fold over it by knowledge of the index.
>>>> Similarly for the `data Foo a = Foo [Int] (Int -> a)` case.
>>>>
>>>> So, while in general `r -> a` is not traversable, in the case when
>>>> there is _any_ full enumeration on `r` (i.e., when `r` is known), then it
>>>> _is_ able to be injected into something traversable, and hence these
>>>> instances also pass the strong-injectivity law.
>>>>
>>>> Note that if there were universal quantification on `Store` then we’d
>>>> have `Coyoneda` and the instance that _just_ used the `f a` in the first
>>>> component (as described in Pudlák's SO post) would be the correct one, and
>>>> furthermore that instance would pass all three versions of the law.
>>>>
>>>> Cheers,
>>>> Gershom
>>>>
>>>>
>>>> On May 6, 2018 at 2:37:12 AM, Gershom B (gershomb at gmail.com) wrote:
>>>>
>>>> Hmm… I think Pudlák's Store as given in the stackoveflow post is a
>>>> genuine example of where the two laws differ. That’s unfortunate.
>>>>
>>>> The quantification law allows the reasonable instance given in the
>>>> post. Even with clever use of GADTs I don’t see how to produce a type to
>>>> fulfill the injectivity law, though I’m not ruling out the possibility
>>>> altogether.
>>>>
>>>> We can cook up something even simpler with the same issue,
>>>> unfortunately.
>>>>
>>>> data Foo a = Foo [Int] (Int -> a)
>>>>
>>>> Again, there doesn’t seem to be a way to produce a GADT with an
>>>> injection that also has traversable. But there is an obvious foldable
>>>> instance, and it again passes the quantification law.
>>>>
>>>> The problem is that injectivity is too strong, but we need to get
>>>> “almost” there for the law to work. We hit the same problem in fact if we
>>>> have an `a` in any nontraversable position or structure, even of we have
>>>> some other ones lying around. So also failing is:
>>>>
>>>> data Foo a = Foo [a] (a -> Int).
>>>>
>>>> I guess not only is the invectivity law genuinely stronger, it really
>>>> is _too_ strong.
>>>>
>>>> What we want is the “closest thing” to an injection. I sort of know how
>>>> to say this, but it results in something with the same complicated
>>>> universal quantification statement (sans GenericSet) that you already
>>>> dislike in the quantification law.
>>>>
>>>> So given  “a GADT `u a` and function `toTrav :: forall a. f a -> u a`”
>>>> we no longer require `toTrav` to be injective and instead require:
>>>>
>>>> `forall (g :: forall a. f a -> Maybe a), exists (h :: forall a. u a ->
>>>> Maybe a)  such that g === h . toTrav`.
>>>>
>>>> In a sense, rather than requiring a global retract, we instead require
>>>> that each individual “way of getting an `a`” induces a local retract.
>>>>
>>>> This is certainly a more complicated condition than “injective”. On the
>>>> other hand it still avoids the ad-hoc feeling of `GenericSet` that Edward
>>>> has been concerned about.
>>>>
>>>> —Gershom
>>>>
>>>>
>>>> On May 6, 2018 at 12:41:11 AM, David Feuer (david.feuer at gmail.com)
>>>> wrote:
>>>>
>>>> Two more points:
>>>>
>>>> People have previously considered unusual Foldable instances that this
>>>> law would prohibit. See for example Petr Pudlák's example instance for
>>>> Store f a [*]. I don't have a very strong opinion about whether such things
>>>> should be allowed, but I think it's only fair to mention them.
>>>>
>>>> If the Committee chooses to accept the proposal, I suspect it would be
>>>> reasonable to add that if the type is also a Functor, then it should be
>>>> possible to write a Traversable instance compatible with the Functor and
>>>> Foldable instances. This would subsume the current foldMap f = fold . fmap
>>>> f law.
>>>>
>>>> [*] https://stackoverflow.com/a/12896512/1477667
>>>>
>>>> On Sat, May 5, 2018, 10:37 PM Edward Kmett <ekmett at gmail.com> wrote:
>>>>
>>>>> I actually don't have any real objection to something like David's
>>>>> version of the law.
>>>>>
>>>>> Unlike the GenericSet version, it at first glance feels like it
>>>>> handles the GADT-based cases without tripping on the cases where the law
>>>>> doesn't apply because it doesn't just doesn't type check. That had been my
>>>>> major objection to Gershom's law.
>>>>>
>>>>> -Edward
>>>>>
>>>>> On Sat, May 5, 2018 at 5:09 PM, David Feuer <david.feuer at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> I have another idea that might be worth considering. I think it's a
>>>>>> lot simpler than yours.
>>>>>>
>>>>>> Law: If t is a Foldable instance, then there must exist:
>>>>>>
>>>>>> 1. A Traversable instance u and
>>>>>> 2. An injective function
>>>>>>        toTrav :: t a -> u a
>>>>>>
>>>>>> Such that
>>>>>>
>>>>>>     foldMap @t = foldMapDefault . toTrav
>>>>>>
>>>>>> I'm pretty sure this gets at the point you're trying to make.
>>>>>>
>>>>>>
>>>>>> On May 3, 2018 11:58 AM, "Gershom B" <gershomb at gmail.com> wrote:
>>>>>>
>>>>>> This came up before (see the prior thread):
>>>>>> https://mail.haskell.org/pipermail/libraries/2015-
>>>>>> February/024943.html
>>>>>>
>>>>>> The thread at that time grew rather large, and only at the end did I
>>>>>> come up with what I continue to think is a satisfactory formulation of
>>>>>> the law.
>>>>>>
>>>>>> However, at that point nobody really acted to do anything about it.
>>>>>>
>>>>>> I would like to _formally request that the core libraries committee
>>>>>> review_ the final version of the law as proposed, for addition to
>>>>>> Foldable documentation:
>>>>>>
>>>>>> ==
>>>>>> Given a fresh newtype GenericSet = GenericSet Integer deriving (Eq,
>>>>>> Ord), where GenericSet is otherwise fully abstract:
>>>>>>
>>>>>> forall (g :: forall a. f a -> Maybe a), (x :: f GenericSet).
>>>>>> maybe True (`Foldable.elem` x) (g x) =/= False
>>>>>> ==
>>>>>>
>>>>>> The intuition is: "there is no general way to get an `a` out of `f a`
>>>>>> which cannot be seen by the `Foldable` instance". The use of
>>>>>> `GenericSet` is to handle the case of GADTs, since even parametric
>>>>>> polymorphic functions on them may at given _already known_ types have
>>>>>> specific behaviors.
>>>>>>
>>>>>> This law also works over infinite structures.
>>>>>>
>>>>>> It rules out "obviously wrong" instances and accepts all the instances
>>>>>> we want to that I am aware of.
>>>>>>
>>>>>> My specific motivation for raising this again is that I am rather
>>>>>> tired of people saying "well, Foldable has no laws, and it is in base,
>>>>>> so things without laws are just fine." Foldable does a have a law we
>>>>>> all know to obey. It just has been rather tricky to state. The above
>>>>>> provides a decent way to state it. So we should state it.
>>>>>>
>>>>>> Cheers,
>>>>>> Gershom
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>>>>>>
>>>>>>
>>>>>>
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