[swift-evolution] Removing enumerated?

Xiaodi Wu xiaodi.wu at gmail.com
Tue Jan 31 16:54:25 CST 2017

On Tue, Jan 31, 2017 at 1:16 PM, Ben Cohen via swift-evolution <
swift-evolution at swift.org> wrote:

> I think whether enumerated() is justified as a method on Sequence is one
> example of a wider question which definitely needs some discussion, which
> is: where should the standard library draw the line in providing
> convenience functions that can easily be composed from other functions in
> the std lib? Here’s another example:
> SE-100
> <https://github.com/apple/swift-evolution/blob/master/proposals/0100-add-sequence-based-init-and-merge-to-dictionary.md> is
> a proposal to add an init to Dictionary from a sequence of key/value pairs.
> It’s a commonly requested feature, and IMO much needed and should be added
> as soon as we move to the appropriate phase in Swift’s evolution.
> Another commonly requested Dictionary feature is similar: add a
> Dictionary.init that takes a sequence, and a closure that maps that
> sequence to keys. This is useful, for example, when you have a sequence of
> objects that you frequently need to index into via one property on those
> objects, so you want to build a fast lookup cache using that property.
> Now, if we implement SE-100, that second request can be easily composed.
> It would be something like Dictionary(sequence.lazy.map { (key:
> $0.someProperty, value: $0) } )
> Some people look at that line of code and think sure, that’s what I’d do
> and it’s easy enough that the second helper shouldn’t be added as it’s
> superfluous. Others look at it and say that it is unreadable clever-code FP
> nonsense, and we should just add the helper method because most programmers
> wouldn’t be able to read or write that easily.
> As we expand (and maybe contract :) the standard library, this kind of
> question comes up a lot, so it is worth setting out some criteria for
> judging these “helper” methods. Here’s my take on such a list (warning:
> objectivity and subjectivity blended together in the below).
> *1. Is it truly a frequent operation?*
> The operation needs to carry its weight. Even once we have ABI stability,
> so the size of the std lib becomes less of a concern as it could ship as
> part of the OS, we still need to keep helper method growth under control.
> APIs bristling with methods like an over-decorated Xmas tree are bad for
> usability. As mentioned in the String manifesto, String+Foundation
> currently has over 200 methods/properties. Helpers are no good if you can’t
> find them to use them.
> Someone mentioned that they actually don’t find themselves using
> enumerated() all that often. I suspect enumerated in its current form isn’t
> all that useful. In a quick (and non-scientific) review of search results
> for its use on GitHub, nearly all the examples I see of it are either 1)
> misuse – see more below, or 2) use of it to perform the equivalent of
> in-place map where the index is used to subscript into the array and
> replace it with a transformed element.

So, in general, I agree with your overarching points. But I have to push
back on this part:

Although it's clearly a misuse when one assumes `startIndex == 0` while
working with a generic Collection, it's been said on this list many times
that `0..<array.count` is a perfectly legitimate spelling for the indices
of a concrete `Array<T>`. That is, it's not an unwarranted assumption, but
rather a semantic guarantee that the first index of an array is 0. For the
same reason, using `enumerated()` on an _array_ for indices is not a misuse.

With that in mind, I took the time to do a systematic review of how
`enumerated()` is used. I took the top trending Swift language repositories
over the last 90(?--whatever the longest choice in the dropdown menu was)
days, and searched for uses of this API. Here are the results:

# Hero
Multiple uses: all on arrays; all correct

# iina
Multiple uses: two on arrays, one `string.utf8CString.enumerated()`; all

# Sharaku
No uses

# swift-algorithm-club
Multiple uses: some on arrays and some on `string.characters`; all correct

# Files
No uses
One example in README: correct

# awesome-ios
No uses

# Alamofire
One use: correct

# ODUIThreadGuard
No uses

# Vapor
One use: extension on Collection, *** incorrectly assumes `startIndex == 0`

# Charts
Multiple uses: all on arrays; all correct

# Moya
No uses

# LocalizationKit_iOS
No usages

# SwifterSwift
Multiple uses: almost all on arrays, one
`array.reversed().lazy.enumerated()`; all correct

# ios-oss
Multiple uses: all on arrays; all correct

# RxSwift
Multiple uses: all on arrays; all correct

# SwiftyJSON
No uses

# SwiftLint
Multiple uses: all on arrays; all correct

> I think the std lib needs an in-place map, and if enumerated() is removed,
> this one most-common use case should be added at the same time.
> *2. Is the helper more readable? Is the composed equivalent obvious at
> a glance**?*
> When an operation is very common, the big win with a helper method is it
> creates a readable vocabulary. If enumerated() is very common, and everyone
> sees it everywhere, it makes that code easier to read at a glance.
> That said, I think that the alternative – zip(0…, sequence) – is just as
> readable once you are familiar with zip and ranges, two concepts that, IMO
> at least, it is important that every Swift programmer learn about early on.
> I would even go so far as to say that enumerated is harmful if it
> discourages new users from discovering zip.

As others have already chimed in, `zip(0..., sequence)` is not yet
possible; as I mentioned in another thread, I was once a big fan of `0...`
but now think there are some issues that may need ironing our or may not be
easily solvable with it. In either case, I think my survey above shows that
`enumerated()` is indeed quite common. It is familiar to those coming from
at least one other popular language, Python.

> OTOH, an example that I think is strongly justified on readability grounds
> is Sequence.all(match:), a function that checks that every element in a
> sequence matches a predicate. This is by far the most common extension for
> Sequence on GitHub. Yes, you can write this as !seq.contains(!predicate)
> but that far less readable – especially since it requires you write it with
> a closure that !’s the predicate i.e. !seq.contains { !predicate($0) },
> because we don’t have a ! for composing with predicate functions (though
> maybe we should).
> *3. Does the helper have the flexibility to cover all common cases?*
> This, I think, is where enumerated() really starts to fall down.
> Sometimes you want to start from not-zero, so maybe it should be
> Sequence.enumerated(startingFrom: Int = 0)

Python, which has `enumerate`, has indeed extended that function to allow
this. We could trivially do the same.

Sometimes you want non-integer indices so maybe we should add indexed().

That has been floated and, like Jacob, I think it's a fine addition.

> Sometimes you want it the other way around (not for a for loop but for
> passing the elements directly into a mapping function) so now you need a
> flip.

Flip the order of the number and the value in the tuple? Wouldn't you just
use `map` for that?

> Sometimes you want to enumeration to run backwards in some way.
> Less of a problem if you’re already accustomed to composing your
> enumeration:
> Enumerate starting from 1: zip(1…, c)
> Enumerate indices: zip(c.indices, c)
> Need the pair to be the other way around: zip(c, 0…)
> Need the enumeration to run the other way: zip((0..<c.count).reversed,c)
> or zip(0…,c).reversed() or zip(0…,c.reversed()).

As shown above, people do indeed compose with `enumerated()`.

> Similarly, for the Dictionary helper – what if you want a sequence, and a
> closure to map keys to values, rather than values to keys?
> (zip also needs to retain its collection-ness, which is filed as SR-3760,
> a great starter proposal if anyone wants to take it on :)
> *4. Is there a correctness trap with the composed equivalent? Is there a
> correctness trap with the helper?*
> As others noted on the thread, enumerated() used for indices encourages a
> possible correctness error:
> for (idx, element) = someSlice.enumerated() { } // slices aren’t zero
> based!
> Now, chances are that since out-of-bounds subscripting traps on arrays,
> this bug would be caught early on. But maybe not – especially if you ended
> up using it on an UnsafeBufferPointer slice (perhaps dropped in for
> performance reasons to replace an Array) and now all of a sudden you have a
> memory access violation that could go undetected.
> On the flip side: many operations on collections that use indices are hard
> to get right, especially ones that involve mutating as you go, like
> removing elements from a collection based on some criteria, where you have
> to work around index invalidation or fencepost errors. For this reason, the
> std lib probably ought to have a RangeReplaceableCollection.remove(where:)
> method so people don’t have to reinvent it and risk getting it wrong.
> *5. Is there a performance trap with the composed equivalent? Or with the
> helper?*
> The composed example of Dictionary from a sequence+closure, you need to
> remember the .lazy part in sequence.lazy.map to avoid creating a temporary
> array for no reason. A helper method might lift that burden from the user.
> remove(where:) can easily be accidentally quadtratic, or perform needless
> element copies when there’s little or nothing to remove.
> Counter example: the fact that map is eager and chaining it creates
> temporary arrays needlessly is a performance problem caused by introducing
> it as a helper method.
> *6. Does the helper actually encourage misuse?*
> This is a very common pattern when searching GitHub for uses of
> enumerated():
> for (index, _) in collection.enumerated() {
>     mutate collect[index] in-place i.e. collection[index] += 1
> }.
> (or even they assign the element then don’t use it, for which specific
> case we don’t currently emit a warning)
> What the user really needs is just:
> for index in collection.indices { etc. }
> I expect if the standard way to do enumerated was with zip, people
> wouldn’t do this as often. In-place map would be even better.

As the data show above, people use `enumerated()` correctly far more often
than they use it incorrectly. In the one place where it's erroneously used,
it was an extension on Collection; it's likely that the only instances that
actually used that extension method were collections with zero-based

> *8. Can a native implementation be made more performant than the
> equivalent composition?*
> Dictionary.mapValues(transform: (Value)->T) can be implemented internally
> much more efficiently than just composing it with map and the key/value
> sequence initializer, because the layout of the hash table storage can be
> re-used in the new dictionary, even when the Value type is different.
> On Jan 31, 2017, at 6:24 AM, Chris Eidhof via swift-evolution <
> swift-evolution at swift.org> wrote:
> Hey everyone,
> I've organized a number of Swift workshops over the last two years. There
> are a couple of things that keep coming up, and a couple of mistakes that I
> see people making over and over again. One of them is that in almost every
> workshop, there's someone who thinks that `enumerated()` returns a list of
> (index, element) pairs. This is only true for arrays. It breaks when using
> array slices, or any other kind of collection. In our workshops, I
> sometimes see people doing something like `x.reversed().enumerated()`,
> where `x` is an array, and somehow it produces behavior they don't
> understand.
> A few ways I think this could be improved:
> - Move enumerated to Array
> - Change enumerated to return `(Index, Iterator.Element)` (this would mean
> we at least need to move it to collection)
> - Remove enumerated
> - Keep things as is
> In any case, just wanted to share my experience (gained from teaching
> people).
> --
> Chris Eidhof
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