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How do I import from a sibling module?
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Closed 2 years ago.
I'm building a web service that uses Diesel to access a MySQL database. Everything is setup correctly and Diesel is generating the schema.rs file with content that reflects my database schema:
table! {
user (id) {
// ...
}
}
I created a store.rs file that resides next to main.rs. If my understanding of modules is correct, any code I put in the store.rs file, will belong to a module named store that is a child of the crate module. My intention is to put all the code that deals with database stuff in the store module. However, I can't seem to be able to use the stuff from the schema module in my store module to start doing some querying using the Diesel APIs.
I tried:
use schema;
use crate::schema;
use super::schema;
use super::schema::user;
Nothing works. The compiler always says that it cannot resolve one piece of the path or another.
What is the proper way to reference a sibling module in Rust?
In your main.rs, make sure you're setting diesel for #[macro_use] and importing the schema mod.
#[macro_use]
extern crate diesel;
mod schema;
In store.rs, you should be able to use the schema as you see fit.
use crate::schema::user;
I hope this would help.
in store.rs
use crate::schema::*;
// … any other diesel related code you want to put here
in main.rs
pub mod schema;
mod store;
I'm trying to create a crate that has a library and one or more binaries. I've looked at Rust package with both a library and a binary? and the Rust book section on crates and modules but am still running into errors when I try and compile.
I've included the relevant sections of each file (I think).
../cargo.toml:
[package]
name = "plotmote"
version = "0.1.0"
authors = ["Camden Narzt <my#nice.email>"]
[lib]
name = "lib_plotMote"
path = "src/lib.rs"
[[bin]]
name = "plotMote"
path = "src/main.rs"
lib.rs:
pub mod lib_plotMote;
lib_plotMote/mod.rs:
pub mod LogstreamProcessor;
lib_plotMote/LogstreamProcessor.rs:
pub struct LogstreamProcessor {
main.rs:
extern crate lib_plotMote;
use lib_plotMote::LogStreamProcessor;
error:
cargo build
Compiling plotmote v0.1.0 (file:///Users/camdennarzt/Developer/Rust/plotmote)
main.rs:6:5: 6:37 error: unresolved import `lib_plotMote::LogStreamProcessor`. There is no `LogStreamProcessor` in `lib_plotMote` [E0432]
This should work:
use lib_plotMote::lib_plotMote::LogStreamProcessor;
The first lib_plotMote comes from extern crate, and the second one comes from the module you have defined in the library crate:
pub mod lib_plotMote;
Therefore, the library crate contains one module which, coincidentally, has the same name as the crate itself.
Also, as #starblue has noticed, you have case mismatch in the declaration site of the structure (LogstreamProcessor) and its use site (LogStreamProcessor). This should also be fixed.
As as side note, I suggest you to follow the idiomatic naming convention and avoid camelCase in module/crate names.
All examples that I found create a lib.rs and then inside that file create mod foo;, mod bar; for each file foo.rs, bar.rs and so on.
Basically what I want is to to split my crate into multiple files but without having to introduce lots of modules for every file. Is this not possible with Rust?
In order to include another source file, it has to be its own module.
However, this does not mean that you must expose this fact in your API. For example, let's say you want to put some code in a file called hidden.rs:
pub fn inside_hidden() {}
Now in your lib.rs file, you could expose this module:
#![crate_type = "lib"]
pub mod hidden;
But if your library doesn't require sub-modules, it's probably better to keep the hidden module a secret, and instead expose its contents as members of your top-level module:
#![crate_type = "lib"]
pub use hidden::inside_hidden;
mod hidden;
This way, users won't know you have a hidden sub-module. They will see inside_hidden as if it were a member of your top-level module.
I want to have a module with multiple structs in it, each in its own file. Using a Math module as an example:
Math/
Vector.rs
Matrix.rs
Complex.rs
I want each struct to be in the same module, which I would use from my main file, like so:
use Math::Vector;
fn main() {
// ...
}
However Rust's module system (which is a bit confusing to begin with) does not provide an obvious way to do this. It seems to only allow you to have your entire module in one file. Is this un-rustic? If not, how do I do this?
Rust's module system is actually incredibly flexible and will let you expose whatever kind of structure you want while hiding how your code is structured in files.
I think the key here is to make use of pub use, which will allow you to re-export identifiers from other modules. There is precedent for this in Rust's std::io crate where some types from sub-modules are re-exported for use in std::io.
Edit (2019-08-25): the following part of the answer was written quite some time ago. It explains how to setup such a module structure with rustc alone. Today, one would usually use Cargo for most use cases. While the following is still valid, some parts of it (e.g. #![crate_type = ...]) might seem strange. This is not the recommended solution.
To adapt your example, we could start with this directory structure:
src/
lib.rs
vector.rs
main.rs
Here's your main.rs:
extern crate math;
use math::vector;
fn main() {
println!("{:?}", vector::VectorA::new());
println!("{:?}", vector::VectorB::new());
}
And your src/lib.rs:
#[crate_id = "math"];
#[crate_type = "lib"];
pub mod vector; // exports the module defined in vector.rs
And finally, src/vector.rs:
// exports identifiers from private sub-modules in the current
// module namespace
pub use self::vector_a::VectorA;
pub use self::vector_b::VectorB;
mod vector_b; // private sub-module defined in vector_b.rs
mod vector_a { // private sub-module defined in place
#[derive(Debug)]
pub struct VectorA {
xs: Vec<i64>,
}
impl VectorA {
pub fn new() -> VectorA {
VectorA { xs: vec![] }
}
}
}
And this is where the magic happens. We've defined a sub-module math::vector::vector_a which has some implementation of a special kind of vector. But we don't want clients of your library to care that there is a vector_a sub-module. Instead, we'd like to make it available in the math::vector module. This is done with pub use self::vector_a::VectorA, which re-exports the vector_a::VectorA identifier in the current module.
But you asked how to do this so that you could put your special vector implementations in different files. This is what the mod vector_b; line does. It instructs the Rust compiler to look for a vector_b.rs file for the implementation of that module. And sure enough, here's our src/vector_b.rs file:
#[derive(Debug)]
pub struct VectorB {
xs: Vec<i64>,
}
impl VectorB {
pub fn new() -> VectorB {
VectorB { xs: vec![] }
}
}
From the client's perspective, the fact that VectorA and VectorB are defined in two different modules in two different files is completely opaque.
If you're in the same directory as main.rs, you should be able to run it with:
rustc src/lib.rs
rustc -L . main.rs
./main
In general, the "Crates and Modules" chapter in the Rust book is pretty good. There are lots of examples.
Finally, the Rust compiler also looks in sub-directories for you automatically. For example, the above code will work unchanged with this directory structure:
src/
lib.rs
vector/
mod.rs
vector_b.rs
main.rs
The commands to compile and run remain the same as well.
The Rust module rules are:
A source file is just its own module (except the special files main.rs, lib.rs and mod.rs).
A directory is just a module path component.
The file mod.rs is just the directory's module.
The file matrix.rs1 in the directory math is just the module math::matrix. It's easy. What you see on your filesystem you also find in your source code. This is an one-to-one correspondence of file paths and module paths2.
So you can import a struct Matrix with use math::matrix::Matrix, because the struct is inside the file matrix.rs in a directory math. Not happy? You'd prefer use math::Matrix; very much instead, don't you? It's possible. Re-export the identifier math::matrix::Matrix in math/mod.rs with:
pub use self::math::Matrix;
There's another step to get this working. Rust needs a module declaration to load the module. Add a mod math; in main.rs. If you don't do that, you get an error message from the compiler when importing like this:
error: unresolved import `math::Matrix`. Maybe a missing `extern crate math`?
The hint is misleading here. There's no need for additional crates, except of course you really intend to write a separate library.
Add this at the top of main.rs:
mod math;
pub use math::Matrix;
The module declaration is also neccessary for the submodules vector, matrix and complex, because math needs to load them to re-export them. A re-export of an identifier only works if you have loaded the module of the identifier. This means, to re-export the identifier math::matrix::Matrix you need to write mod matrix;. You can do this in math/mod.rs. Therefore create the file with this content:
mod vector;
pub use self::vector::Vector;
mod matrix;
pub use self::matrix::Matrix;
mod complex;
pub use self::complex::Complex;
Aaaand you are done.
1Source file names usually start with a lowercase letter in Rust. That's why I use matrix.rs and not Matrix.rs.
2Java's different. You declare the path with package, too. It's redundant. The path is already evident from the source file location in the filesystem. Why repeat this information in a declaration at the top of the file? Of course sometimes it's easier to have a quick look at the source code instead of finding out the filesystem location of the file. I can understand people who say it's less confusing.
Rusts purists will probably call me a heretic and hate this solution, but this is much simpler: just do each thing in its own file, then use the "include!" macro in mod.rs:
include!("math/Matrix.rs");
include!("math/Vector.rs");
include!("math/Complex.rs");
That way you get no added nested modules, and avoid complicated export and rewrite rules.
Simple, effective, no fuss.
Alright, fought my compiler for a while and finally got it to work(thanks to BurntSushi for pointing out pub use.
main.rs:
use math::Vec2;
mod math;
fn main() {
let a = Vec2{x: 10.0, y: 10.0};
let b = Vec2{x: 20.0, y: 20.0};
}
math/mod.rs:
pub use self::vector::Vec2;
mod vector;
math/vector.rs
use std::num::sqrt;
pub struct Vec2 {
x: f64,
y: f64
}
impl Vec2 {
pub fn len(&self) -> f64 {
sqrt(self.x * self.x + self.y * self.y)
}
// other methods...
}
Other structs could be added in the same manner. NOTE: compiled with 0.9, not master.
I'd like to add in here how you include Rust files when they are deeply nested. I have the following structure:
|-----main.rs
|-----home/
|---------bathroom/
|-----------------sink.rs
|-----------------toilet.rs
How do you access sink.rs or toilet.rs from main.rs?
As others have mentioned, Rust has no knowledge of files. Instead it sees everything as modules and submodules. To access the files inside the bathroom directory you need to export them or barrel them to the top. You do this by specifying a filename with the directory you'd like to access and pub mod filename_inside_the_dir_without_rs_ext inside the file.
Example.
// sink.rs
pub fn run() {
println!("Wash my hands for 20 secs!");
}
// toilet.rs
pub fn run() {
println!("Ahhh... This is sooo relaxing.")
}
Create a file called bathroom.rs inside the home directory:
Export the filenames:
// bathroom.rs
pub mod sink;
pub mod toilet;
Create a file called home.rs next to main.rs
pub mod the bathroom.rs file
// home.rs
pub mod bathroom;
Within main.rs
// main.rs
// Note: If you mod something, you just specify the
// topmost module, in this case, home.
mod home;
fn main() {
home::bathroom::sink::run();
}
use statements can be also used:
// main.rs
// Note: If you mod something, you just specify the
// topmost module, in this case, home.
use home::bathroom::{sink, toilet};
fn main() {
sink::run();
sink::toilet();
}
Including other sibling modules (files) within submodules
In the case you'd like to use sink.rs from toilet.rs, you can call the module by specifying the self or super keywords.
// inside toilet.rs
use self::sink;
pub fn run() {
sink::run();
println!("Ahhh... This is sooo relaxing.")
}
Final Directory Structure
You'd end up with something like this:
|-----main.rs
|-----home.rs
|-----home/
|---------bathroom.rs
|---------bathroom/
|-----------------sink.rs
|-----------------toilet.rs
The structure above only works with Rust 2018 onwards. The following directory structure is also valid for 2018, but it's how 2015 used to work.
|-----main.rs
|-----home/
|---------mod.rs
|---------bathroom/
|-----------------mod.rs
|-----------------sink.rs
|-----------------toilet.rs
In which home/mod.rs is the same as ./home.rs and home/bathroom/mod.rs is the same as home/bathroom.rs. Rust made this change because the compiler would get confused if you included a file with the same name as the directory. The 2018 version (the one shown first) fixes that structure.
See this repo for more information and this YouTube video for an overall explanation.
One last thing... avoid hyphens! Use snake_case instead.
Important Note
You must barrel all the files to the top, even if deep files aren't required by top-level ones.
This means, that for sink.rs to discover toilet.rs, you'd need to barrel them by using the methods above all the way up to main.rs!
In other words, doing pub mod sink; or use self::sink; inside toilet.rs will not work unless you have exposed them all the way up to main.rs!
Therefore, always remember to barrel your files to the top!
A more rustlings method to export module, which I picked up from Github.
mod foo {
//! inner docstring comment 1
//! inner docstring comment 2
mod a;
mod b;
pub use a::*;
pub use b::*;
}
Adjusting the question's example directory and file names to conform to Rust naming conventions:
main.rs
math.rs
math/
vector.rs
matrix.rs
complex.rs
Make sure to export the public symbols (types, functions, etc.) in each of the files in the math directory by preceding them with the keyword pub.
Define math.rs:
mod vector;
pub use vector::*;
mod matrix;
pub use matrix::*;
mod complex;
pub use complex::*;
The above file keeps the sub-modules of math private but the submodules' public symbols are exported from module math. This effectively flattens the module structure.
Use math::Vector in main.rs:
mod math;
use crate::math::Vector;
fn main() {
// ...
}
I have a static library say "A.lib" which contains a function int foo(). I have another dll say "B.dll" which consumes A.lib and uses the function foo() and also exports some other functions. Is it possible to export the function int foo() (imported from A.lib) from B.dll so that it can be consumed in a third dll say "C.dll".
I want to know whether it is possible or not, I dont want workarounds like making A.lib available to the C.dll. Also, I am not concerned if this is a bad design or not.
Thanks very much for your patience to read this through.
I had the same requirement - just found a different solution:
Assuming that A.lib has an A.h (that is consumed by source files used to build B.dll e.g. assuming that A.h contains prototypes for functions contained in A.lib), just add the following in A.h:
#pragma comment(linker, "/export:_foo")
This will instruct the linker to export foo() when building B.dll. Note the leading underscore - it is there because that's the true name of the symbol for foo() contained in A.lib (use dumpbin /symbols A.lib | findstr foo to see it). In my example foo() was using the __cdecl calling convention, but if you use __stdcall() or compile as C++, you'll get different name decoration, so you'll have to adjust the #pragma statement above as a result.
It doesn't matter if A.h gets included by many source files in B.dll - the linker doesn't complain if the exact same definition is made multiple times.
One "advantage" to this approach is that you don't even have to use the __declspec(dllexport) specifier on foo() in A.lib ...
Yes, it's possible but any code example is language dependent.
(for example in C you may simply export a function with the same name and C.dll will see it)