RFC 0169: use-path-as-id

lang (syntax | modules)

Start Date: 2014-07-16
RFC PR #: #169
Rust Issue #: https://github.com/rust-lang/rust/issues/16461

Summary

Change the rebinding syntax from use ID = PATH to use PATH as ID, so that paths all line up on the left side, and imported identifers are all on the right side. Also modify extern crate syntax analogously, for consistency.

Motivation

Currently, the view items at the start of a module look something like this:

mod old_code {
  use a::b::c::d::www;
  use a::b::c::e::xxx;
  use yyy = a::b::yummy;
  use a::b::c::g::zzz;
}

This means that if you want to see what identifiers have been imported, your eyes need to scan back and forth on both the left-hand side (immediately beside the use) and the right-hand side (at the end of each line). In particular, note that yummy is not in scope within the body of old_code

This RFC proposes changing the grammar of Rust so that the example above would look like this:

mod new_code {
  use a::b::c::d::www;
  use a::b::c::e::xxx;
  use a::b::yummy as yyy;
  use a::b::c::g::zzz;
}

There are two benefits we can see by comparing mod old_code and mod new_code:

As alluded to above, now all of the imported identifiers are on the right-hand side of the block of view items.
Additionally, the left-hand side looks much more regular, since one sees the straight lines of a::b:: characters all the way down, which makes the actual differences between the different paths more visually apparent.

Detailed design

Currently, the grammar for use statements is something like:

  use_decl : "pub" ? "use" [ ident '=' path
                            | path_glob ] ;

Likewise, the grammar for extern crate declarations is something like:

  extern_crate_decl : "extern" "crate" ident [ '(' link_attrs ')' ] ? [ '=' string_lit ] ? ;

This RFC proposes changing the grammar for use statements to something like:

  use_decl : "pub" ? "use" [ path "as" ident
                            | path_glob ] ;

and the grammar for extern crate declarations to something like:

  extern_crate_decl : "extern" "crate" [ string_lit "as" ] ? ident [ '(' link_attrs ')' ] ? ;

Both use and pub use forms are changed to use path as ident instead of ident = path. The form use path as ident has the same constraints and meaning that use ident = path has today.

Nothing about path globs is changed; the view items that use ident = path are disjoint from the view items that use path globs, and that continues to be the case under path as ident.

The old syntaxes "use" ident '=' path and "extern" "crate" ident '=' string_lit are removed (or at least deprecated).

Drawbacks

pub use export = import_path may be preferred over pub use import_path as export since people are used to seeing the name exported by a pub item on the left-hand side of an = sign. (See "Have distinct rebinding syntaxes for use and pub use" below.)
The 'as' keyword is not currently used for any binding form in Rust. Adopting this RFC would change that precedent. (See "Change the signaling token" below.)

Alternatives

Keep things as they are

This just has the drawbacks outlined in the motivation: the left-hand side of the view items are less regular, and one needs to scan both the left- and right-hand sides to see all the imported identifiers.

Change the signaling token

Go ahead with switch, so imported identifier is on the left-hand side, but use a different token than as to signal a rebinding.

For example, we could use @, as an analogy with its use as a binding operator in match expressions:

mod new_code {
  use a::b::c::d::www;
  use a::b::c::e::xxx;
  use a::b::yummy @ yyy;
  use a::b::c::g::zzz;
}

(I do not object to path @ ident, though I find it somehow more "line-noisy" than as in this context.)

Or, we could use =:

mod new_code {
  use a::b::c::d::www;
  use a::b::c::e::xxx;
  use a::b::yummy = yyy;
  use a::b::c::g::zzz;
}

(I do object to path = ident, since typically when = is used to bind, the identifier being bound occurs on the left-hand side.)

Or, we could use :, by (weak) analogy with struct pattern syntax:

mod new_code {
  use a::b::c::d::www;
  use a::b::c::e::xxx;
  use a::b::yummy : yyy;
  use a::b::c::g::zzz;
}

(I cannot figure out if this is genius or madness. Probably madness, especially if one is allowed to omit the whitespace around the :)

Have distinct rebinding syntaxes for `use` and `pub use`

If people really like having ident = path for pub use, by the reasoning presented above that people are used to seeing the name exported by a pub item on the left-hand side of an = sign, then we could support that by continuing to support pub use ident = path.

If we were to go down that route, I would prefer to have distinct notions of the exported name and imported name, so that:

pub use a = foo::bar; would actually import bar (and a would just be visible as an export), and then one could rebind for export and import simultaneously, like so: pub use exported_bar = foo::bar as imported_bar;

But really, is pub use foo::bar as a all that bad?

Allow `extern crate ident as ident`

As written, this RFC allows for two variants of extern_crate_decl:

extern crate old_name;
extern crate "old_name" as new_name;

These are just analogous to the current options that use = instead of as.

However, the RFC comment dialogue suggested also allowing a renaming form that does not use a string literal:

extern crate old_name as new_name;

I have no opinion on whether this should be added or not. Arguably this choice is orthogonal to the goals of this RFC (since, if this is a good idea, it could just as well be implemented with the = syntax). Perhaps it should just be filed as a separate RFC on its own.

Unresolved questions

In the revised extern crate form, is it best to put the link_attrs after the identifier, as written above? Or would it be better for them to come after the string_literal when using the extern crate string_literal as ident form?