# Procedural Macros (and custom Derive)

As you've seen throughout the rest of the book, Rust provides a mechanism called "derive" that lets you implement traits easily. For example,

    #[derive(Debug)]
    struct Point {
        x: i32,
        y: i32,
    }Run

is a lot simpler than

    struct Point {
        x: i32,
        y: i32,
    }

    use std::fmt;

    impl fmt::Debug for Point {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "Point {{ x: {}, y: {} }}", self.x, self.y)
        }
    }Run

Rust includes several traits that you can derive, but it also lets you define your own. We can accomplish this task through a feature of Rust called "procedural macros." Eventually, procedural macros will allow for all sorts of advanced metaprogramming in Rust, but today, they're only for custom derive.

Let's build a very simple trait, and derive it with custom derive.

## Hello World

So the first thing we need to do is start a new crate for our project.

    $ cargo new --bin hello-world

All we want is to be able to call `hello_world()` on a derived type. Something like this:

    #[derive(HelloWorld)]
    struct Pancakes;

    fn main() {
        Pancakes::hello_world();
    }Run

With some kind of nice output, like `Hello, World! My name is Pancakes.`.

Let's go ahead and write up what we think our macro will look like from a user perspective. In `src/main.rs` we write:

    #[macro_use]
    extern crate hello_world_derive;

    trait HelloWorld {
        fn hello_world();
    }

    #[derive(HelloWorld)]
    struct FrenchToast;

    #[derive(HelloWorld)]
    struct Waffles;

    fn main() {
        FrenchToast::hello_world();
        Waffles::hello_world();
    }Run

Great. So now we just need to actually write the procedural macro. At the moment, procedural macros need to be in their own crate. Eventually, this restriction may be lifted, but for now, it's required. As such, there's a convention; for a crate named `foo`, a custom derive procedural macro is called `foo-derive`. Let's start a new crate called `hello-world-derive` inside our `hello-world` project.

    $ cargo new hello-world-derive

To make sure that our `hello-world` crate is able to find this new crate we've created, we'll add it to our toml:

    [dependencies]
    hello-world-derive = { path = "hello-world-derive" }

As for our the source of our `hello-world-derive` crate, here's an example:

    extern crate proc_macro;
    extern crate syn;
    #[macro_use]
    extern crate quote;

    use proc_macro::TokenStream;

    #[proc_macro_derive(HelloWorld)]
    pub fn hello_world(input: TokenStream) -> TokenStream {
        // Construct a string representation of the type definition
        let s = input.to_string();

        // Parse the string representation
        let ast = syn::parse_macro_input(&s).unwrap();

        // Build the impl
        let gen = impl_hello_world(&ast);

        // Return the generated impl
        gen.parse().unwrap()
    }Run

So there is a lot going on here. We have introduced two new crates: `syn` and `quote`. As you may have noticed, `input: TokenSteam` is immediately converted to a `String`. This `String` is a string representation of the Rust code for which we are deriving `HelloWorld` for. At the moment, the only thing you can do with a `TokenStream` is convert it to a string. A richer API will exist in the future.

So what we really need is to be able to parse Rust code into something usable. This is where `syn` comes to play. `syn` is a crate for parsing Rust code. The other crate we've introduced is `quote`. It's essentially the dual of `syn` as it will make generating Rust code really easy. We could write this stuff on our own, but it's much simpler to use these libraries. Writing a full parser for Rust code is no simple task.

The comments seem to give us a pretty good idea of our overall strategy. We are going to take a `String` of the Rust code for the type we are deriving, parse it using `syn`, construct the implementation of `hello_world` (using `quote`), then pass it back to Rust compiler.

One last note: you'll see some `unwrap()`s there. If you want to provide an error for a procedural macro, then you should `panic!` with the error message. In this case, we're keeping it as simple as possible.

Great, so let's write `impl_hello_world(&ast)`.

    fn impl_hello_world(ast: &syn::MacroInput) -> quote::Tokens {
        let name = &ast.ident;
        quote! {
            impl HelloWorld for #name {
                fn hello_world() {
                    println!("Hello, World! My name is {}", stringify!(#name));
                }
            }
        }
    }Run

So this is where quotes comes in. The `ast` argument is a struct that gives us a representation of our type (which can be either a `struct` or an `enum`). Check out the docs, there is some useful information there. We are able to get the name of the type using `ast.ident`. The `quote!` macro let's us write up the Rust code that we wish to return and convert it into `Tokens`. `quote!` let's us use some really cool templating mechanics; we simply write `#name` and `quote!` will replace it with the variable named `name`. You can even do some repetition similar to regular macros work. You should check out the docs for a good introduction.

So I think that's it. Oh, well, we do need to add dependencies for `syn` and `quote` in the `cargo.toml` for `hello-world-derive`.

    [dependencies]
    syn = "0.10.5"
    quote = "0.3.10"

That should be it. Let's try to compile `hello-world`.

    error: the `#[proc_macro_derive]` attribute is only usable with crates of the `proc-macro` crate type
     --> hello-world-derive/src/lib.rs:8:3
      |
    8 | #[proc_macro_derive(HelloWorld)]
      |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Oh, so it appears that we need to declare that our `hello-world-derive` crate is a `proc-macro` crate type. How do we do this? Like this:

    [lib]
    proc-macro = true

Ok so now, let's compile `hello-world`. Executing `cargo run` now yields:

    Hello, World! My name is FrenchToast
    Hello, World! My name is Waffles

We've done it!