RFC 0198: slice-notation

lang (syntax | slice | patterns | ranges)

Start Date: 2014-09-11
RFC PR #: rust-lang/rfcs#198
Rust Issue #: rust-lang/rust#17177

Summary

This RFC adds overloaded slice notation:

foo[] for foo.as_slice()
foo[n..m] for foo.slice(n, m)
foo[n..] for foo.slice_from(n)
foo[..m] for foo.slice_to(m)
mut variants of all the above

via two new traits, Slice and SliceMut.

It also changes the notation for range match patterns to ..., to signify that they are inclusive whereas .. in slices are exclusive.

Motivation

There are two primary motivations for introducing this feature.

Ergonomics

Slicing operations, especially as_slice, are a very common and basic thing to do with vectors, and potentially many other kinds of containers. We already have notation for indexing via the Index trait, and this RFC is essentially a continuation of that effort.

The as_slice operator is particularly important. Since we've moved away from auto-slicing in coercions, explicit as_slice calls have become extremely common, and are one of the leading ergonomic/first impression problems with the language. There are a few other approaches to address this particular problem, but these alternatives have downsides that are discussed below (see "Alternatives").

Error handling conventions

We are gradually moving toward a Python-like world where notation like foo[n] calls fail! when n is out of bounds, while corresponding methods like get return Option values rather than failing. By providing similar notation for slicing, we open the door to following the same convention throughout vector-like APIs.

Detailed design

The design is a straightforward continuation of the Index trait design. We introduce two new traits, for immutable and mutable slicing:

trait Slice<Idx, S> {
    fn as_slice<'a>(&'a self) -> &'a S;
    fn slice_from(&'a self, from: Idx) -> &'a S;
    fn slice_to(&'a self, to: Idx) -> &'a S;
    fn slice(&'a self, from: Idx, to: Idx) -> &'a S;
}

trait SliceMut<Idx, S> {
    fn as_mut_slice<'a>(&'a mut self) -> &'a mut S;
    fn slice_from_mut(&'a mut self, from: Idx) -> &'a mut S;
    fn slice_to_mut(&'a mut self, to: Idx) -> &'a mut S;
    fn slice_mut(&'a mut self, from: Idx, to: Idx) -> &'a mut S;
}

(Note, the mutable names here are part of likely changes to naming conventions that will be described in a separate RFC).

These traits will be used when interpreting the following notation:

Immutable slicing

foo[] for foo.as_slice()
foo[n..m] for foo.slice(n, m)
foo[n..] for foo.slice_from(n)
foo[..m] for foo.slice_to(m)

Mutable slicing

foo[mut] for foo.as_mut_slice()
foo[mut n..m] for foo.slice_mut(n, m)
foo[mut n..] for foo.slice_from_mut(n)
foo[mut ..m] for foo.slice_to_mut(m)

Like Index, uses of this notation will auto-deref just as if they were method invocations. So if T implements Slice<uint, [U]>, and s: Smaht<T>, then s[] compiles and has type &[U].

Note that slicing is "exclusive" (so [n..m] is the interval n <= x < m), while .. in match patterns is "inclusive". To avoid confusion, we propose to change the match notation to ... to reflect the distinction. The reason to change the notation, rather than the interpretation, is that the exclusive (respectively inclusive) interpretation is the right default for slicing (respectively matching).

Rationale for the notation

The choice of square brackets for slicing is straightforward: it matches our indexing notation, and slicing and indexing are closely related.

Some other languages (like Python and Go -- and Fortran) use : rather than .. in slice notation. The choice of .. here is influenced by its use elsewhere in Rust, for example for fixed-length array types [T, ..n]. The .. for slicing has precedent in Perl and D.

See Wikipedia for more on the history of slice notation in programming languages.

The `mut` qualifier

It may be surprising that mut is used as a qualifier in the proposed slice notation, but not for the indexing notation. The reason is that indexing includes an implicit dereference. If v: Vec<Foo> then v[n] has type Foo, not &Foo or &mut Foo. So if you want to get a mutable reference via indexing, you write &mut v[n]. More generally, this allows us to do resolution/typechecking prior to resolving the mutability.

This treatment of Index matches the C tradition, and allows us to write things like v[0] = foo instead of *v[0] = foo.

On the other hand, this approach is problematic for slicing, since in general it would yield an unsized type (under DST) -- and of course, slicing is meant to give you a fat pointer indicating the size of the slice, which we don't want to immediately deref. But the consequence is that we need to know the mutability of the slice up front, when we take it, since it determines the type of the expression.

Drawbacks

The main drawback is the increase in complexity of the language syntax. This seems minor, especially since the notation here is essentially "finishing" what was started with the Index trait.

Limitations in the design

Like the Index trait, this forces the result to be a reference via &, which may rule out some generalizations of slicing.

One way of solving this problem is for the slice methods to take self (by value) rather than &self, and in turn to implement the trait on &T rather than T. Whether this approach is viable in the long run will depend on the final rules for method resolution and auto-ref.

In general, the trait system works best when traits can be applied to types T rather than borrowed types &T. Ultimately, if Rust gains higher-kinded types (HKT), we could change the slice type S in the trait to be higher-kinded, so that it is a family of types indexed by lifetime. Then we could replace the &'a S in the return value with S<'a>. It should be possible to transition from the current Index and Slice trait designs to an HKT version in the future without breaking backwards compatibility by using blanket implementations of the new traits (say, IndexHKT) for types that implement the old ones.

Alternatives

For improving the ergonomics of as_slice, there are two main alternatives.

Coercions: auto-slicing

One possibility would be re-introducing some kind of coercion that automatically slices. We used to have a coercion from (in today's terms) Vec<T> to &[T]. Since we no longer coerce owned to borrowed values, we'd probably want a coercion &Vec<T> to &[T] now:

fn use_slice(t: &[u8]) { ... }

let v = vec!(0u8, 1, 2);
use_slice(&v)           // automatically coerce here
use_slice(v.as_slice()) // equivalent

Unfortunately, adding such a coercion requires choosing between the following:

Tie the coercion to Vec and String. This would reintroduce special treatment of these otherwise purely library types, and would mean that other library types that support slicing would not benefit (defeating some of the purpose of DST).
Make the coercion extensible, via a trait. This is opening pandora's box, however: the mechanism could likely be (ab)used to run arbitrary code during coercion, so that any invocation foo(a, b, c) might involve running code to pre-process each of the arguments. While we may eventually want such user-extensible coercions, it is a big step to take with a lot of potential downside when reasoning about code, so we should pursue more conservative solutions first.

Deref

Another possibility would be to make String implement Deref<str> and Vec<T> implement Deref<[T]>, once DST lands. Doing so would allow explicit coercions like:

fn use_slice(t: &[u8]) { ... }

let v = vec!(0u8, 1, 2);
use_slice(&*v)          // take advantage of deref
use_slice(v.as_slice()) // equivalent

There are at least two downsides to doing so, however:

It is not clear how the method resolution rules will ultimately interact with Deref. In particular, a leading proposal is that for a smart pointer s: Smaht<T> when you invoke s.m(...) only inherent methods m are considered for Smaht<T>; trait methods are only considered for the maximally-derefed value *s.

With such a resolution strategy, implementing Deref for Vec would make it impossible to use trait methods on the Vec type except through UFCS, severely limiting the ability of programmers to usefully implement new traits for Vec.
The idea of Vec as a smart pointer around a slice, and the use of &*v as above, is somewhat counterintuitive, especially for such a basic type.

Ultimately, notation for slicing seems desireable on its own merits anyway, and if it can eliminate the need to implement Deref for Vec and String, all the better.