RFC 0202: subslice-syntax-change

lang (syntax | slice | patterns | array)

• Start Date: 2014-08-15
• RFC PR: rust-lang/rfcs#202
• Rust Issue: rust-lang/rust#16967

Summary

Change syntax of subslices matching from ..xs to xs.. to be more consistent with the rest of the language and allow future backwards compatible improvements.

Small example:

match slice {
    [xs.., _] => xs,
    [] => fail!()
}

This is basically heavily stripped version of RFC 101.

Motivation

In Rust, symbol after .. token usually describes number of things, as in [T, ..N] type or in [e, ..N] expression. But in following pattern: [_, ..xs], xs doesn't describe any number, but the whole subslice.

I propose to move dots to the right for several reasons (including one mentioned above):

1. Looks more natural (but that might be subjective).
2. Consistent with the rest of the language.
3. C++ uses args... in variadic templates.
4. It allows extending slice pattern matching as described in RFC 101.

Detailed design

Slice matching grammar would change to (assuming trailing commas; grammar syntax as in Rust manual):

slice_pattern : "[" [[pattern | subslice_pattern] ","]* "]" ;
subslice_pattern : ["mut"? ident]? ".." ["@" slice_pattern]? ;

To compare, currently it looks like:

slice_pattern : "[" [[pattern | subslice_pattern] ","]* "]" ;
subslice_pattern : ".." ["mut"? ident ["@" slice_pattern]?]? ;

Drawbacks

Backward incompatible.

Alternatives

Don't do it at all.

Unresolved questions

Whether subslice matching combined with @ should be written as xs.. @[1, 2] or maybe in another way: xs @[1, 2]...