[swift-evolution] When to use argument labels, part DEUX

Dave Abrahams dabrahams at apple.com
Sat Feb 6 23:31:03 CST 2016

on Sat Feb 06 2016, plx <swift-evolution at swift.org> wrote:

> I like these revisions quite a bit.
>> On Feb 5, 2016, at 3:32 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> Given all the awesome feedback I've gotten on this thread, I went back
>> to the drawing board and came up with something new; I think this one
>> works.  The previously-stated goals still apply:
>> * describe when and where to use argument labels
>> * require labels in many of the cases people have asked for them
>> * are understandable by humans (this means relatively simple)
>> * preserve important semantics communicated by existing APIs.
>> Please keep in mind that it is a non-goal to capture considerations we
>> think have a bearing on good names (such as relatedness of parameters):
>> it's to create simple guidelines that have the right effect in nearly
>> all cases.
>> A. When arguments can't be usefully distinguished from one another, none
>>   should have argument labels, e.g. min(x,y), zip(x,y,z).  
>> B. Otherwise,
>>  1. At the call site, a first parameter that has no argument label must
>>     form part of a grammatical phrase that starts with the basename, less
>>     any trailing nouns.  
>>       print(x)
>>       a.contains(b)
>>       a.mergeWith(b)
>>       a.addGestureRecognizer(x)
>>            ^~~~~~~~~~~~~~~~~ trailing noun
>>     This phrase must have the correct semantic implications, so, e.g.
>>       a.dismiss(b)           // no, unless a is really dismissing b
>>       a.dismissAnimated(b)   // no, not grammatical
>>       a.dismiss(animated: b) // yes, using a label
>>  2. If the first argument is part of a prepositional phrase, put the
>>     parenthesis immediately after the preposition. 
>>       a.encodeWith(b)
>>       a.moveFrom(b, to: c)
>>     Thus, if words are required for any reason between the preposition
>>     and the first argument, they go into the first argument label.
>>       a.tracksWith(mediaType: b, composer: c)
>>       a.moveTo(x: 22, y: 99)
>> Notes: 
>> a. I would recommend prepositions other than "with" in nearly all
>>   cases, but that's not the point of these rules.
>> b. I can understand the aesthetic appeal of
>>    a.move(from: b, to: c)
> I agree with the aesthetic and also not wanting to complicate the rules to cover it. 
> I also think this is a bad example, though, b/c in practice one would
> *hope* that such calls would look more like:
> container.move(element, from: currentIndex, to: destinationIndex)
> …and whatnot; 

I don't understand how this one works.  In most cases the call would
have lots of redundancy:

  container.move(container[currentIndex], from: currentIndex, to: destinationIndex)
  ^^^^^^^^^      ^^^^^^^^^ ~~~~~~~~~~~~         ~~~~~~~~~~~~ 

Why make the caller come up with the element value here?

> I suspect that the `move(from:to:)` case may be getting strong
> reactions in part b/c it’s actually an unusually-poorly-named method
> to begin with.
> Also, as these are just guidelines, in third party code we can deviate
> from them (at our own peril, but we can...).
>>   but I believe it is not a clear enough improvement to justify
>>   additional complexity in the guidelines.
>> Questions:
>> 1. I'm not expecting these guidelines to make everybody optimally happy,
>>   all the time, but they shouldn't be harmful.  Are there any cases for
>>   which they produce results you couldn't live with?
>> 2. Are there any cases where you'd be confused about how to apply these
>>   guidelines?
> I do find some of the rules worded in ways that are hard to apply in
> the presence of trailing nouns; I’m not sure the guidelines are strong
> enough here to force the “right” outcome.
> Consider this example:
> // not-intended: 
> a.tracksWithMediaCharacteristic(b, composer: c)
> …applying our guidelines:
> - `tracks with media characteristic b` is a grammatical phrase

I went through a series of thoughts on this:

1. I'm not sure it is grammatical (I'll have to consult with a

2. I understand why it sounds natural: this is the somewhat misleading
   result of using a single-character identifier in the example.  It's like
   talking about a hypothetical "person A" and "person B".  You wouldn't think
   "Person Joe"  was grammatical.

3. Yeah, but "umbrellas with color yellow" is perfectly fine, if
   slightly unnatural, and it has the same grammatical structure.  My
   wife points out that it's like something you would say in poetry to
   make rhyme or meter work.

4. Again I need to consult with a linguist, but I now have hope that it
   actually is grammatical, meaning the whole “skipping trailing nouns
   on the base name” wrinkle can be eliminated from the guidelines.

> - `tracks with b` is (arguably also) a grammatical phrase 

Yes, this was the interpretation for which I introduced the “skipping
trailing nouns on the base name” wrinkle.

> - `b` is “part of” the "noun phrase" "tracks with media characteristic
>    b"
> - `b` is “part of” the “prepositional phrase” “with media characteristic b"

All true.

> …and although I think this is fine under a “strict” application of the
> guidelines—as `b` is part of a prepositional phrase, we *must* split
> the base name after the preposition, at which point we are allowed to
> “restore" the `mediaCharacteristic:` label if we wish

I don't think of it as "restoring."  It's simply that the phrase you're
encoding is "tracks with media characteristic b" and b is part of a
prepositional phrase, so you put a parenthesis after the preposition:

  "tracks with (media characteristic b"

and then you apply camel case and everything falls into place.

> —I don’t know in practice that the “strict” interpretation will be
> what people naturally opt to use.

Well, that's the intention of the guideline.  I think if we give people
examples to follow they will understand, but no matter *how* we phrase
the guidelines there's always the risk that some people will choose to
interpret them in unintended ways.

> Taking it from the other direction, the way the guidelines are phrased
> I worry bit that you wind up with:
> // not intended
> a.add(subview: v) (or add(v)) // instead of `a.addSubview(v)`

That's allowed as the guidelines were originally posted, but with the
modification to the guidelines that form B1 is to be preferred over form
B2 when both are allowed... which I swore I posted something about in
this thread in response to an excellent point someone made, but now
can't find.  

Maybe I dreamt it; until I hear otherwise, *you* get credit for making
the excellent point :-)

> a.add(layoutGuide: g) (or add(g)) // instead of `a.addLayoutGuide(g)`
> a.add(gestureRecognizer: r) (or add(r)) // instead of
> `a.addGestureRecognizer(r)`
> …b/c the rule that we should ignore such trailing nouns means those
> are *exactly* as-grammatical as the “intended” imports (it’s “add v”
> vs “add v”, after all, under the guidelines), so at least within the
> proposed guidelines there’s no apparent reason to prefer the
> `addSubview` form vis-a-vis the `add(subview:)` (or `add(_)`) form(s).
> Perhaps I just overlooked something from the non-label guidelines in
> this response, but it’d seem the omit-needless-words would also steer
> you towards `add()`, with or without a first-argument label.

It might appear to, but the words are very much needed in this case
because adding a gesture recognizer is not the same kind of operation as
adding an observer or subview.

> So overall these guidelines seem more than adequate to resolve
> labeling disputes if we are already dealing with "well-named”
> functions but may be a bit weak to steer naming *decisions*.

The guidelines in this particular thread are just trying to deal with
the labels .

> But, for guidelines, that might still be OK?

Not because they're guidelines, but because of the scope of what we're
trying to address here.

>> Thanks in advance for all your valuable input!
>> P.S. Doug is presently working on generating new importer results, based
>>     on these guidelines, for your perusal.  They should be ready soon.
>> -- 
>> -Dave
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