Learning Rust – Day 4 – Understanding Modules

https://www.geekabyte.io/2020/01/learning-rust-day-4-understanding.html

by dade • 4 years ago

61 33 4 years ago

geekabyte.io

brianhorakh• 4 years ago

Der Weg ist das Ziel!

mrlonglong• 4 years ago

The way is the goal!

faitswulff2• 4 years ago

Modules confuse me greatly. I was trying to have both a lib folder and a main.rs and I couldn’t figure it out in 15-30 minutes, so I gave up and just put the module in src as a sibling to main.rs while I got stuff done - come to think of it, I’d like to revisit that.

nkozyra• 4 years ago

I haven't seen this as being discouraged. Rust does more things differently than most newer languages so the learning curve is there but the documentation is fantastic if you have the time or patience. I say this as someone who has stumbled my way through to competence/expertise on some good newer languages (zig, nim, go without RTFM very much, but I can't do that with Rust.

But there's also a relatively early moment when you get to a local maxima and suddenly it all starts coming together.

lumost2• 4 years ago

honest question, what necessitated Rust's choice of using such a different module system? Every other modern language seems to roughly map directory to module (go/python etc), or explicitly declare a folder as a module (java). C/C++ is the outlier, effectively using file as a module.

Why is Rust so different on this seemingly boring topic?

ibraheemdev• 4 years ago

The most common confusion with this is that when importing library code from your binary, you use `crate_name::...` as opposed to `crate::...`, as they are separate. You also don't have to declare the lib.rs as a module from the binary, and generally all your `mod` statements will reside in your lib.rs

faitswulff2• 4 years ago

Hey, your comment made me realize I'm an idiot and I was trying to make a lib folder at the sibling level to the src folder. That plus your tip about how to organize the files and everything makes sense now. Thanks!

ww520• 4 years ago

Agree. Module is a weak design of Rust.

mynameisash• 4 years ago

After I was comfortable enough in Rust to start writing reasonable projects, I soon encountered modules as a necessary subject, and it confused me so much. I decided to take a couple hours to read docs, play around with some drop-dead simple code that would organize code into modules as either `mod foo { }` blocks, single file modules (foo.rs) or modules with their own submodules, organized by directory (foo/mod.rs with foo/bar.rs).

It pretty much instantly became obvious how to use them, and in retrospect, I don't know why it was so confusing. I think it's a good example of how important it is to [a] apply deliberate practice/learning and [b] have empathy for others that haven't (yet) progressed to our own level of understanding.

bobbylarrybobby2• 4 years ago

Note that the "new" way to do modules with your file hierarchy is to have foo.rs instead of foo/mod.rs. foo.rs is at the same level as folder foo, foo/mod.rs is gone, and everything else is the same.

Not sure whether this is truly better, but it does make it much easier to search for the file containing the module foo -- with the old way, you would have one mod.rs per module which quickly got painful when searching.

pjmlp7• 4 years ago

Only for simple modules, if they get complex enough, one will quickly use the "old" approach with multiple files.

naavis• 4 years ago

I had exactly the same experience. It's a simple system, but felt very confusing at first.

rackjack• 4 years ago

I think the most surprising thing about Rust modules is that you need to """pre-declare""" them, especially since Rust doesn't really use function prototypes.

So when you try to move a module into its own file, you need to keep the `mod foo` in the original file.

pjmlp7• 4 years ago

Ada,

    with foo; -- Import
    use foo;  -- open namespace

Seems familiar.

8K832d7tNmiQ• 4 years ago

importing a module is the most fustrating part of writing rust code next to writing a multiple-spawn tasks.

Can someone help me how to import a module in lib/m.rs from lib.rs ? I've never seen someone asked about it.

bobbylarrybobby2• 4 years ago

Rust has two distinct semantics surrounding how items can be "in scope".

First, the `mod` keyword takes a module that the current file/scope can see and makes it available within that file/scope as if it had been defined there. This is necessary to make the module's items accessible, i.e., specifiable by their path within the module, in the current file/scope.

Then the `use` keyword takes an already-accessible member of a named scope -- a module, a type, etc -- and makes it usable by name (i.e., without scoping on each use) in the current scope.

duped2• 4 years ago

Folder structure

    src/lib.rs
        lib/
            mod.rs
            m.rs

In lib.rs

    use lib::m;

In lib/mod.rs

    pub mod m;

You need the mod.rs file in subdirectories.

duped2• 4 years ago

IMHO you should keep your src folder flat and use multiple crates in a workspace for hierarchy instead of nested modules. People forget that the compilation unit of Rust is a crate, not a source file.

Learning Rust – Day 4 – Understanding Modules

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