> On Jan 31, 2017, at 12:18 PM, Matthew Johnson <[email protected]> wrote:
>
>>
>> On Jan 31, 2017, at 1:16 PM, Ben Cohen via swift-evolution
>> <[email protected] <mailto:[email protected]>> 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).
>
> This is a great analysis and list of criteria. Thanks for putting this
> together Ben!
>
>>
>> 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.
>>
>> 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.
>
> I just did a quick search in a couple of projects and found a handful of good
> examples of valid uses of `enumerated`. I have simplified the examples from
> real world code, but I think they demonstrate the kinds of things this is
> good for.
>
> // only show the first 5 views
> for (i, view) in views.enumerated() {
> view.hidden = i >= 5
> }
>
Interesting that several of these rely on reference types as elements. This
does make the loops a lot simpler. In-place amendment of an array would be
messier, and the combination with zip definitely pushes back against my notion
that an in-place map would be a useful alternative (since you couldn’t mutate
the mutable half of the zipped collection in-place without us jumping through
some hoops).
> // apply alternating view background colors
> for (i, view) in views.enumerated() {
> view.backgroundColor = i % 2 ? bgColor1 : bgColor2
> }
>
This is an interesting one because it highlights something else I think is
missing from the std lib: Sequence.cycle, which would make an infinite sequence
by repeating a sequence, enabling what I think is probably a readability win in
some cases:
let colors = [bgColor1, bgColor2].cycle
for (color, view) in zip(colors, views) {
view.backgroundColor = color
}
Also i%2 used as a boolean – what language is this? :)
> // linear layout
> for (i, view) in views.enumerated() {
> let x = width * CGFloat(i)
> view.frame = CGRect(x: x, y: 0, width: width, height: height)
> }
>
> // deriving locations for an equally spaced gradient
> let locations = colors.enumerated.map { CGFloat($0.0) / CGFloat(colors.count
> - 1) }
>
This one feels like it’s an example of enumerate discouraging use of stride,
which hits cases 2: readability and 4: performance (unlike the others, where
the alternatives are no better or worse)
> There are other ways to accomplish similar things (use `prefix` and
> `dropFirst` comes to mind), but many reasonable people would argue that using
> `enumerated` is a very straightforward way to do a lot of things.
>
Totally. Enumerating things is certainly sometimes useful, I just don’t think
it’s useful enough to have a dedicated but inflexible method to do it.
>>
>> 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.
>
> This is a very good point. Requiring programmers to zip with a range could
> help them to learn to think a little bit differently and learn more general
> tools. That’s a good thing.
>
> The other thing it does is make the order of the index and the value more
> clear at the call site. I think this is a very important point. I always
> forget which order `enumerated` uses every time I reach to use it.
>
> Of course the current alternative is actually `zip(0..<sequence.count,
> sequence)` which is not so great.
>
Totally. zip replacing enumerated will only fly in combination with one-sided
ranges.
> In order to make `zip(0…, sequence)` a workable alternative we would need to
> grab `…` as a unary postfix operator which produces a sequence by
> incrementing values. Is that something we want to do? If the answer turns
> out to be yes, then I am in agreement with your analysis. The composed
> alternative would be superior and `enumerated` wouldn’t carry it’s weight.
>
I think based on the String discussions there’s fairly well-established support
for one-sided …
>>
>> 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)
>> Sometimes you want non-integer indices so maybe we should add indexed().
>> 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.
>> 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()).
>>
>> 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.
>>
>> 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
>>> <[email protected] <mailto:[email protected]>> 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
>>> _______________________________________________
>>> swift-evolution mailing list
>>> [email protected] <mailto:[email protected]>
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>
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