- 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):
- Looks more natural (but that might be subjective).
- Consistent with the rest of the language.
- C++ uses
args...in variadic templates. - 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]...