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

Jacob Bandes-Storch jtbandes at gmail.com
Thu Apr 6 00:40:06 CDT 2017

Now that escaping with \ has been proposed for KeyPaths, this makes me
wonder whether it would be appropriate to use "\foo()" rather than
"foo(_)"/"foo(:)" ?  It still feels a bit strange, as \foo() looks like
escaping the *result* of a call.

On Sat, Feb 25, 2017 at 1:43 PM, David Hart <david at hartbit.com> wrote:

> On 25 Feb 2017, at 00:56, Jordan Rose via swift-evolution <
> swift-evolution at swift.org> wrote:
> I don't have a *good* answer for this, but I'll vote *against* 'foo(:)'
> because that's what a lot of people think the name of 'foo(_:)' should be.
> I'd rather be able to offer fix-its for that even when you have both
> 'foo()' and 'foo(_:)' defined. I'd rather go with 'foo(_)' despite the tiny
> ambiguity in pattern contexts.
> (I'm personally in favor of killing unapplied function references
> altogether in favor of closures, on the grounds that they are overly terse,
> make type-checking more complicated, and often lead to retain cycles. Then
> we'd only need this for #selector, and it's perfectly unambiguous to use
> 'foo()' there. But I wasn't planning to fight that particular battle now,
> and it is rather annoying to require the 'as' in the meantime.)
> It is potentially going to be hard to fight that battle. I think a lot of
> functional/Haskell people love them and would be sad to see them go away (I
> plead guilty). But it isn’t a well known part of the language so I don’t
> think the general community would miss it.
> Jordan
> On Feb 21, 2017, at 23:05, Jacob Bandes-Storch <jtbandes at gmail.com> wrote:
> 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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