Wishlist: functions with keyword args, optional args, and/or variable-arity argument (varargs) lists

[pnkfelix]

A portion of the community (and of the core team) sees one or more of the following features as important for programmer ergonomics:

• keyword-based parameters (as opposed to position-based parameters),
• optional parameters (where one provides a default value for them, usually in the parameter list),
• variable-arity functions (which can be seen as a generalization or variation on optional parameters, depending on how you look at it).

This issue is recording that we want to investigate designs for this, but not immediately. The main backwards compatibility concern is about premature commitment to library API's that would be simplified if one adds one or more of the above features. Nonetheless, We believe we can produce a reasonable 1.0 version of Rust without support for this.

(This issue is also going to collect links to all of the useful RFC PR's and/or Rust issues that contain community discussion of these features.)

[flying-sheep]

Nonetheless, We believe we produce a reasonable 1.0 version of Rust without support for this.

depends on how you see it. the API will definitely be well-considered given the constraints of no default/kw arguments.

but once those are added, i’m sure the API will be considered lacking, especially in areas where new defaults render old functions obsolete.

a good example (if the new syntax sugar wouldn’t exist) would be ImmutableSlice:

fn slice(&self, start: uint, end: uint) -> &'a [T];
fn slice_from(&self, start: uint) -> &'a [T];
fn slice_to(&self, end: uint) -> &'a [T];

slice_from and slice_to will be immediately obsolete once you can just leave out start or end from slice. i bet there are hundreds of examples more.

[pnkfelix]

@flying-sheep so then you deprecate such methods, just like today, no?

[reem]

@pnkfelix I think his argument is that we don't want to be stuck with a ton of deprecated cruft in the stdlib, but I'm still personally not too sympathetic to the need to have default arguments before 1.0.

[flying-sheep]

yeah, that’s my argument. either cruft, or lengthy deprecation formalities and rust 2.0 a year after 1.0 (semver requires a major version bump for breaking changes)

[Valloric]

While I'd prefer to have optional/keyword args before 1.0, I believe the problem with deprecated functions crufting up the API can be substantially lessened by removing such functions from the generated API docs. This is how the Qt project (and D AFAIK) handles API deprecation; the deprecated stuff continues working but developers writing new code don't see it.

Of course, the generated docs should have a setting/link/button to show the deprecated API items but it should be off by default.

I think this is also a good idea in general; just a couple of days ago I accidentally used a deprecated function because it seemed like a good pick and I didn't notice the stability color.

[sfackler]

Rustdoc's handling of deprecated items definitely needs some improvement - see rust-lang/rust#15468 for some discussion.

[aturon]

See the "Struct sugar" RFC for another take.