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active/0000-module-imports.md

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+- Start Date: 2014-03-23
+- RFC PR #:
+- Rust Issue #:
+
+# Summary
+
+A proposal for making the `mod` keyword do what everybody expects, and make
+it easier to manage multiple modules in a directory. The actual semantics of
+the `use` keyword would not significantly change, but the practical use
+would be tightened.
+
+# Motivation
+
+Note: in this proposal, I omit most use of the `pub` keyword, even if is
+required. When I speak of "public", I refer only to conceptual visibility.
+
+Frequently, newcomers to the language try to put a `mod` statement in
+every file that accesses a module. Partilarly, they expect `mod` to work
+like C/C++ `#include` or python `import`. C++ `using` or python `from x import`
+only do a single duty of bringing a name into scope, but Rust's `use` does
+the equivalent of both `#include` and `using`, and there is no equivalent
+to Rust's `mod` at all.
+
+Being confusing to newcomers is, in itself, a bug. But the strange purpose
+of Rust's `mod` is also harmful even if you know what it does. Consider the
+case when you have many files in a module. Several of the files in this
+module require some shared routines. Currently, the mod.rs is required
+to not only list the `mod` statements for the "public" modules (which is
+reasonable, though some might argue in favor of `mod *;` to catch them all),
+but also all of the private details that those modules use.
+
+Apparently, the current way is not confusing if coming from Javascript,
+but Javascript is an example of good *marketing*, not good *design*.
+
+My proposal is mostly based on the way Python does it.
+
+# Detailed design
+
+## Source layout
+
+- src/main.rs
+  `mod fish;`
+  `mod bird;`
+  `mod penguin;`
+- src/fish.rs
+- src/bird.rs
+  `mod penguin;`
+- src/penguin/
+ - src/penguin/mod.rs
+   `mod foo;`
+   `mod bar;`
+   `use self::foo::popular_function;`
+   `use super::bird;`
+   `use ::fish::swim;`
+ - src/penguin/foo.rs
+   `mod bar;`
+   `mod detail;`
+ - src/penguin/bar.rs
+   `mod detail;`
+ - src/penguin/detail.rs
+   `use self::bar;` (\*)
+   `use super::fish;` (\*)
+
+(Those (\*)s would need an extra super:: in alternative #3.)
+
+## Stop
+
+Go back and look at the source layout again.
+
+I bet you (whether familiar with Rust, or only with other languages)
+understood *exactly* what that source layout means without having to read
+any documentation at all. Documentation is *never* a substitute for being
+obvious.
+
+## Suggested Approach
+
+First, introduce the (transparent) concept of a 'package'. A package is a
+collection of modules from one directory. A crate has a root package.
+A package may contain subpackages, which are also modules.
+
+One of the modules in a package is considered the root module. For the
+root package, this is main.rs or lib.rs; for subpackages this is mod.rs.
+All of the visible names in the root module are also visible when the
+package is viewed as a module. Besides the obvious subpackage case, this
+is important for locating `fn main`, and can also happen with `super::`.
+
+To avoid cycles during compilation, the modules may be completely loaded,
+or just registered to be loaded when done with the current load
+(alternatively, the "return early" method may be used).
+
+When actually loading a file, if the compiler see `mod foo` at any point -
+even `mod bar { mod foo; }` - it tries to add foo from the current
+directory. If foo/mod.rs exists, it is added as a new package (just like
+the root package was), otherwise it is added as a module.
+
+Note that any modules in a package that are not directly visible from its
+root module will not be directly visible from the package when the package
+is used as a module. With the example layout, `::penguin::detail` does not
+exist (but `::penguin::foo::detail` and `::penguin::bar::detail` do - if they
+didn't `pub` mod/use it, this has the benefits of Java's package-private).
+
+# Alternatives
+
+- Do nothing. Insist to the newcomers "it'll be less confusing later".
+- Just eliminate `mod` entirely. Automatically find the right filename
+    when `use` is used (this is basically how Java imports work).
+- Fix the meaning of `mod`, but don't think about 'package'. This could
+  work, but has a slightly pointless meaning for `self::` and `super::`
+
+# Unresolved questions
+
+All of these questions can be put off until after this RFC is implemented,
+though due attention should be given to the first.
+
+- Should `use foo::...` default to `use self::foo::...` or `use ::foo::...`
+  I'm fond of defaulting `self::`, but rust currently does `::`.
+  Perhaps it should be forbidden entirely (or gated) for a while?
+- Are there any additional improvements that could be made for multiple
+  crates? (make another RFC after this one is implemented).
+- Should `mod *;` be added so a root module need not list all the .rs files?
+- Should there be a way to not have a mod.rs at all?
+- Should `mod foo { }` be feature-gated?