[swift-evolution] Referencing zero-parameter functions

[Alex Hoppen]

The idea of adding a syntax to reference zero-argument functions just like foo(arg:) is used to reference a one-parameter function has come up several times on the list. Pyry and I have put together a proposal to let foo(_) refer to a function foo without any parameters. GitHub-Link: https://github.com/ahoppen/swift-evolution/blob/reference-zero-param-func/proposals/0000-refernce-zero-param-func.md <https://github.com/ahoppen/swift-evolution/blob/reference-zero-param-func/proposals/0000-refernce-zero-param-func.md>

Comments welcome, especially if someone thinks that any of the issues listed in "Possible issues" are major or sees any other problems.

– Alex


Referencing zero-parameter functions

Proposal: SE-NNNN <https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md>
Author(s): Alex Hoppen <https://github.com/ahoppen>, Pyry Jahkola <https://github.com/pyrtsa>
Status: Draft
Review manager: TBD
 <https://github.com/ahoppen/swift-evolution/tree/reference-zero-param-func#introduction>Introduction

Since the approval of SE-0021 <https://github.com/apple/swift-evolution/blob/master/proposals/0021-generalized-naming.md> it is possible to reference a function by its argument names using the foo(arg:) syntax but there is no way to reference a zero-parameter function. This proposal adds a new syntax foo(_) to reference an overloaded function with zero parameters.

This was one point in the discussion: [Pitch] Richer function identifiers, simpler function types <http://thread.gmane.org/gmane.comp.lang.swift.evolution/15577/>
 <https://github.com/ahoppen/swift-evolution/tree/reference-zero-param-func#motivation>Motivation

Consider the following example

class Bar {
  func foo() {
  }

  func foo(arg: Int) {
  }
}
You can reference foo(arg: Int) using Bar.foo(arg:) but there is currently no syntax to reference foo() without using disambiguation by type Bar.foo() as () -> Void. We believe this is a major hole in the current disambiguation syntax.

 <https://github.com/ahoppen/swift-evolution/tree/reference-zero-param-func#proposed-solution>Proposed solution

We propose that Bar.foo(_) references the function with no parameters just as Bar.foo(arg:) references the function with one argument named arg. 

In the context of functions declarations _ already has the meaning of "there is nothing" (e.g. func foo(_ arg: Int)). Thus, we believe that _ is the right character to mean that a function has no parameters.

 <https://github.com/ahoppen/swift-evolution/tree/reference-zero-param-func#detailed-design>Detailed design

The unqualified-name grammar rule from SE-0021 <https://github.com/apple/swift-evolution/blob/master/proposals/0021-generalized-naming.md> changes to 

unqualified-name -> identifier
                  | identifier '(' ((identifier | '_') ':')+ ')'
                  | identifier '(_)'
If two overloads with zero-parameters exist with different return types, disambiguation has still to be done via as just like with the foo(arg:) syntax.

 <https://github.com/ahoppen/swift-evolution/tree/reference-zero-param-func#impact-on-existing-code>Impact on existing code

This is a purely additive feature and has no impact on existing code.

 <https://github.com/ahoppen/swift-evolution/tree/reference-zero-param-func#possible-issues>Possible issues

If Swift should ever support out-only parameters Bar.foo(_) could mean that the only out-only parameter shall be ignored. This would clash with the currently proposed syntax. However, since Swift functions may return multiple values as a tuple, we don't see this coming.

Bar.foo(_) may be mistaken for Bar.foo(_:) if there is also a one-parameter function without a label. This mistake would, however, be mostly detected by the compiler when later calling the function with an argument.

 <https://github.com/ahoppen/swift-evolution/tree/reference-zero-param-func#alternatives-considered>Alternatives considered

 <https://github.com/ahoppen/swift-evolution/tree/reference-zero-param-func#alternative-1-barfoo>Alternative 1: Bar.foo

Let Bar.foo reference the function with zero parameters only. While this works around the possible issue of ignored out-only parameters described above, this has several minor drawbacks to the proposed solution (some of these drawbacks are mutually exclusive based on possible future proposals but one always applies):

Most functions are not overloadad and using the base name only offers a shorthand way to reference these functions.
This would block the way of allowing properties with the same name as a function with zero parameters by banning Bar.foo as a function reference (could be another proposal once this one is accepted).
Bar.foo(arg:) hints that a function is referenced by its paranthesis. Bar.foo doesn't include paranthesis, which causes a subtle inconsistency.
 <https://github.com/ahoppen/swift-evolution/tree/reference-zero-param-func#alternative-2-barfoo-inside-selector>Alternative 2: Bar.foo() inside #selector

Let Bar.foo() refer to the zero-parameter function only inside #selector as it was proposed by Doug Gregor here <https://github.com/apple/swift-evolution/pull/280#discussion_r61849122>. This requires the proposal to disallow arbitrary expressions in #selector (GitHub-Link <https://github.com/ahoppen/swift-evolution/blob/arbitrary-expressions-in-selectors/proposals/0000-arbitrary-expressions-in-selectors.md>) to be approved. Issues we see are:

This gives the illusion that foo is actually called which it isn't
It doen't solve the issue of referncing a zero-parameter function in arbitrary expressions somewhere else in code.
 <https://github.com/ahoppen/swift-evolution/tree/reference-zero-param-func#future-directions>Future directions

If this proposal is accepted there is no need that Bar.foo references a function with base name foo since there is a notation with paranthesis for every argument constellation. We could disallow Bar.foo to reference a function and allow a property named foo on Bar. Bar.foo would then refer to this property.

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