> Am 17.10.2017 um 20:47 schrieb Michael Ilseman via swift-evolution
> <swift-evolution@swift.org>:
>
>
>
>> On Oct 17, 2017, at 10:15 AM, Kevin Nattinger via swift-evolution
>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>>
>>> Because, in my analysis, the problem is that the method is incorrectly
>>> named. The problem affects all types that conform to Sequence and not just
>>> Set and Dictionary; elementsEqual is a distinct function from ==, and it
>>> must either continue to be distinct or cease to exist, but its name does
>>> nothing to clarify any distinction.
>>
>> In my analysis, the problem is the method's implementation. As I see it, the
>> only use for `elementsEqual` is as a replacement for `==` when two objects
>> are different types (or not known to be the same)—equal elements, and IF the
>> sequences have an order, in the same order. Could you provide an example
>> where `elementsEqual` randomly returning either true or false depending on
>> internal state alone is a legitimate and desirable result?
>>
>
> It doesn’t randomly return true or false, it consistently returns true or
> false for the *same* pair of Sequences. What *same* means, of course, is
> complicated and exists at two levels (as we have two ways of talking about
> *same*).
It is random (for unordered collections) from the standpoint of business logic.
The order of the elements of a Set, while publicly observable, cannot be
influenced by business logic because it is an implementation detail. Therefore
it is not possible to write business logic that exploits this order, i.e. it is
not possible to write business logic using `elementsEqual` on a Set.
The only exception is using the receiver as argument, i.e.
`set.elementsEqual(set)` because in this case, and in this case alone, the
result is independent of the effectively random order.
Therefore the single use case for `elementsEqual` for unordered collections is
calling `set.elementsEqual(set)` to determine whether a Set of Floats contains
NaN values. I claim that this is better written as `set.contains { $0.isNaN }`
which works nicely for a Set<Float> and is intention revealing as opposed to
using `elementsEqual`.
As I have shown this really is the *single* use case for `elementsEqual` for
unordered collections. There can be no other use case.
All other uses of `elementsEqual` involving two different collections where one
is an unordered collection have random results that have absolutely no business
meaning and therefore are bugs.
Therefore having `elementsEqual` as API for unordered collections is a source
of bugs except for a single use case which has a better solution using
`contains`. That is why I would like to split the API.
Adam expressed the concern that this required `map` to implemented separately
for the split APIs: `Iterable.map` would have to return an `Iterable` whereas
`Sequence.map` would have to return a `Sequence`.
This is an independent issue IMO, because currently we have a similar
situation: `Collection.map` does not return a `Collection` either. It returns
an Array. This is a compromise partly due to missing language features which
can simply remain when `Sequence` is split.
Actually the question for the correct return type for `map` is not a simple
one. For a `Dictionary` we would probably want to get a `Dicitonary` where the
values have been replaced by the mapped ones (this is how Smalltalk does it).
But for `Set` we typically do *not* want a `Set` because in general we will be
interested in duplicate mapped values. There are times where a `Set` would be
the correct return type but this applies to ordered collections or dictionaries
as well, so there should be a variant (or an argument with a default) allowing
to provide the desired result type or target collection.
But this is a completely separate issue which can be easily deferred until the
planned language features have arrived which might help finding a good solution
for this question. Until then we will just use the current implementation which
returns an array.
To get back to the topic at hand: I propose to differentiate between unordered
and ordered collections because I think that this is an important distinction
with tractable impact on algorithms (as shown above). The method
`elementsEqual` would be part of the ordered sequence protocol and I would
suggest the name `elementsPairwiseEqual`.
-Thorsten
>
> I apologize for not reading every email in depth in this thread (they are
> coming in faster than I can parse them), but let me try to present motivation
> for this and hopefully provide more shared understanding.
>
> We have two forms of equality we’re talking about: equality of Sequence and
> equality of the elements of Sequences in their respective ordering. `==`
> covers the former, and I’ll use the existing (harmful) name of
> `elementsEqual` for the latter.
>
> `==` conveys substitutability of the two Sequences. This does not necessarily
> entail anything about their elements, how those elements are ordered, etc.,
> it just means two Sequences are substitutable. `elementsEqual` means that the
> two Sequences produce substitutable elements. These are different concepts
> and both are independently useful.
>
> Cases:
>
> 1. Two Sequences are substitutable and produce substitutable elements when
> iterated. `==` and `elementsEqual` both return true.
>
> Example: Two arrays with the same elements in the same order.
>
>
> 2. Two Sequences are substitutable, but do not produce substitutable elements
> when iterated. `==` returns true, while `elementsEqual` returns false.
>
> Example: Two Sets that contain the same elements but in a different order.
>
> Contrived Example: Two Lorem Ipsum generators are the same generator
> (referentially equal, substitutable for the purposes of my library), but they
> sample the user’s current battery level (global state) each time they produce
> text to decide how fancy to make the faux Latin. They’re substitutable, but
> don’t generate the same sequence.
>
>
> 3. Two Sequences are not substitutable, but produce substitutable elements
> when iterated. `==` returns false, while `elementsEqual` returns true.
>
> Example: Consider two sequences that have differing identity. `==` operates
> on an identity level, `elementsEqual` operates at an element level.
>
> Contrived Example: InfiniteMonkeys and Shakespeare both produce the same
> sonnet, but they’re not substitutable for my library’s purposes.
>
>
> 4. Two Sequences are not substitutable and don’t produce substitutable
> elements when iterated. `==` and `elementsEqual` both return false.
>
> Example: `[1,2,3]` compared to `[4,5,6]`
>
>
> It is true that situations #2 and #3 are a little harder to grok, but they
> are what illustrate the subtle difference at hand. I think situation #2 is
> the most confusing, and has been the primary focus of this thread as Set
> exists and exhibits it.
>
>
> Now, onto naming. `elementsEqual` is a very poor choice of name for the
> concept of equality of elements in their respective orderings, as it doesn’t
> highlight the “in their respective orderings” part. `lexicographicallyEqual`
> highlights the ordering much better, as “abc” is not lexicographically equal
> to “cba” despite having equal elements. I think it is clearly an improvement
> over the status quo. I like something a little more explicit (e.g.
> `elementsOrderedEqual`), personally, but I don’t care that strongly. I’m just
> glad to see `elementsEqual` getting some clarification.
>
>
>
>
>>>
>>> Thanks,
>>> Jon
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