[swift-evolution] Opt-out for first unnamed parameter instead of opt-in

Jordan Rose jordan_rose at apple.com
Mon Feb 8 20:27:46 CST 2016

Among other things, this allows a warning or style check equivalent to Clang's -Wunused-param. You could always give the parameter a dummy name and then assign it to _, but this makes the intent clearer, IMHO, that the parameter is ignored.

On the other hand, it also destroys any way of referring to it in a doc comment, so if you want to say why it's ignored you're in trouble.


> On Feb 8, 2016, at 18:08, Dave via swift-evolution <swift-evolution at swift.org> wrote:
> If we decide to allow external names without an internal name, I’m strongly in favor of the “func doThing(with _: Int) {}“ syntax. It makes it crystal clear that we’re purposefully not assigning an internal name to that argument.
> - Dave Sweeris
>> On Feb 8, 2016, at 16:46, Jessy Catterwaul via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> Sorry, I forgot about this one:
>> 5. An external, but not internal parameter name is needed. I don’t think a colon is useful in these cases.
>> func doThing(with Int) {}
>> It also could satisfy a protocol requirement, without having to resort to this kind of underscore usage, which should become illegal:
>> func doThing(with _: Int) {}
>>> On Feb 8, 2016, at 5:32 PM, Jessy Catterwaul <mr.jessy at gmail.com <mailto:mr.jessy at gmail.com>> wrote:
>>> How often do you find yourself wanting an internal parameter name, but not an external (case 4)? I think it’s very rare.
>>> I doubt that the compiler’s AI will be able to deal with case 2.b, automatically, any time soon. That is okay with me, as long as we can manually type direct objects that would not be clear with only a preposition. 
>>> I think these represent the options for parameter names in order of prevalence. Case 3 has obviously never been available in Swift, but can be emulated with generic-sounding internal parameter names. If those types of names were not necessary, I believe all of these “opting” problems would go away. Is that incorrect? 
>>> 1. Internal is the same as external
>>> func function(int: Int) {}
>>> 2. External and internal names are different 
>>> a. func doThing(with int: Int) {}
>>> b. func doThing(withGiraffe giraffe: Any) {}
>>> 3. No parameter name is needed internally or externally
>>> func function(Int)
>>> 4. Underscore for external, name for internal
>>> func function(_ int: Int) {}
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