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% The Rust Crates and Modules Guide

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When a project starts getting large, it's considered a good software engineering practice to split it up into a bunch of smaller pieces, and then fit them together. It's also important to have a well-defined interface, so that some of your functionality is private, and some is public. To facilitate these kinds of things, Rust has a module system.

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# Basic terminology: Crates and Modules

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Rust has two distinct terms that relate to the module system: "crate" and "module." A crate is synonymous with a 'library' or 'package' in other languages. Hence "Cargo" as the name of Rust's package management tool: you ship your crates to others with Cargo. Crates can produce an executable or a shared library, depending on the project.

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Each crate has an implicit "root module" that contains the code for that crate. You can then define a tree of sub-modules under that root module. Modules allow you to partition your code within the crate itself.

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As an example, let's make a "phrases" crate, which will give us various phrases in different languages. To keep things simple, we'll stick to "greetings" and "farewells" as two kinds of phrases, and use English and Japanese (日本語） as two languages for those phrases to be in. We'll use this module layout:

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```text
+-----------+
+---| greetings |
| +-----------+
+---------+ |
| english |---+
+---------+ | +-----------+
| +---| farewells |
+---------+ | +-----------+
| phrases |---+
+---------+ | +-----------+
| +---| greetings |
+----------+ | +-----------+
| japanese |---+
+----------+ |
| +-----------+
+---| farewells |
+-----------+
```

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In this example, `phrases` is the name of our crate. All of the rest are modules. You can see that they form a tree, branching out from the crate "root", which is the root of the tree: `phrases` itself.

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Now that we have a plan, let's define these modules in code. To start, generate a new crate with Cargo:

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```bash $ cargo new phrases $ cd phrases ```