100 exercises to learn Rust

book/src/05_ticket_v2/10_packages.md

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+# Libraries and binaries
+
+It took a bit of code to implement the `Error` trait for `TicketNewError`, didn't it?  
+A manual `Display` implementation, plus an `Error` impl block.
+
+We can remove some of the boilerplate by using [`thiserror`](https://docs.rs/thiserror/latest/thiserror/), 
+a Rust crate that provides a **procedural macro** to simplify the creation of custom error types.  
+But we're getting ahead of ourselves: `thiserror` is a third-party crate, it'd be our first dependency!  
+
+Let's take a step back to talk about Rust's packaging system before we dive into dependencies.
+
+## What is a package?
+
+A Rust package is defined by the `[package]` section in a `Cargo.toml` file, also known as its **manifest**. 
+Within `[package]` you can set the package's metadata, such as its name and version.
+
+Go check the `Cargo.toml` file in the directory of this section's exercise!
+
+## What is a crate?
+
+Inside a package, you can have one or more **crates**, also known as **targets**.  
+The two most common crate types are **binary crates** and **library crates**.
+
+### Binaries
+
+A binary is a program that can be compiled to an **executable file**.  
+It must include a function named `main`—the program's entry point. `main` is invoked when the program is executed.
+
+### Libraries
+
+Libraries, on the other hand, are not executable on their own. You can't _run_ a library, 
+but you can _import its code_ from another package that depends on it.  
+A library groups together code (i.e. functions, types, etc.) that can be leveraged by other packages as a **dependency**.  
+
+All the exercises you've solved so far have been structured as libraries, with a test suite attached to them.
+
+### Conventions
+
+There are some conventions around Rust packages that you need to keep in mind:
+
+- The package's source code is usually located in the `src` directory.
+- If there's a `src/lib.rs` file, `cargo` will infer that the package contains a library crate.
+- If there's a `src/main.rs` file, `cargo` will infer that the package contains a binary crate.
+
+You can override these defaults by explicitly declaring your targets in the `Cargo.toml` file—see
+[`cargo`'s documentation](https://doc.rust-lang.org/cargo/reference/cargo-targets.html#cargo-targets) for more details.
+
+Keep in mind that while a package can contain multiple crates, it can only contain one library crate.
+
+## Scaffolding a new package
+
+You can use `cargo` to scaffold a new package:
+
+```bash
+cargo new my-binary
+```
+
+This will create a new folder, `my-binary`, containing a new Rust package with the same name and a single 
+binary crate inside. If you want to create a library crate instead, you can use the `--lib` flag:
+
+```bash
+cargo new my-library --lib
+```
+
+## References
+
+- The exercise for this section is located in `exercises/05_ticket_v2/10_packages`