[swift-evolution] Pitch: Compound name `foo(:)` for nullary functions

[rintaro ishizaki]

+1

I prefer foo(:) because it matches with empty dictional literal [:].

Also, foo(_) potentially introduce a confusion.
Remember that this syntax can be appear in a pattern.

```
struct S {
    let val: Int
}
func newS() -> S { return S(val: 0) }
func newS(_ val: Int) -> S { return S(val: val) }

func ~= (pat: () -> S, obj: S) -> Bool { return pat().val == obj.val }

let obj = S(val: 0)
switch obj {
case newS(_): // Is this a pattern???
    break
default:
    break
}
```



2017-02-22 16:05 GMT+09:00 Jacob Bandes-Storch via swift-evolution <
swift-evolution at swift.org>:

> Evolutioniers,
>
> *Compound name syntax* — foo(_:), foo(bar:), foo(bar:baz:) — is used to
> disambiguate references to functions. (You might've used it inside a
> #selector expression.) But there's currently no compound name for a
> function with no arguments.
>
>     func foo() {}  // no compound syntax for this one :(
>     func foo(_ bar: Int) {}  // foo(_:)
>     func foo(bar: Int) {}  // foo(bar:)
>     func foo(bar: String, baz: Double) {}  // foo(bar:baz:)
>
> Given these four functions, only the first one has no compound name
> syntax. And the simple reference "let myfn = foo" is ambiguous because it
> could refer to any of the four. A workaround is to specify a contextual
> type, e.g. "let myfn = foo as () -> Void".
>
> I filed SR-3550 <https://bugs.swift.org/browse/SR-3550> for this a while
> ago, and there was some discussion in JIRA about it. I'd like to continue
> exploring solutions here and then write up a formal proposal.
>
> To kick off the discussion, *I'd like to propose foo(:) for nullary
> functions.*
>
> Advantages:
> - the colon marks a clear similarity to the foo(bar:) form when argument
> labels are present.
> - cutely parallels the empty dictionary literal, [:].
>
> Disadvantages:
> - violates intuition about one-colon-per-argument.
> - the parallel between #selector(foo(:)) and @selector(foo) is not quite
> as obvious as between #selector(foo(_:)) and @selector(foo:).
>
>
> For the sake of discussion, another option would be *foo(_)*. This was my
> original choice, and I like that the number of colons matches the number of
> parameters. However, it's a little less obvious as a function reference. It
> would preclude _ from acting as an actual identifier, and might conflict
> with pattern-matching syntax (although it appears functions can't be
> compared with ~= anyway).
>
>
> Looking forward to everyone's bikeshed color ideas,
> Jacob
>
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>
>
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