On this blog post I am going to talk about Macros, well Rust Macros, not the type of Macros you see on the picture above, so if you came in here looking for a tutorial on how much carbs, fats and proteins to ingest every day I am sorry to disappoint you. But back lets get back on track here, I would like to talk about specifically what are Macros? What type of Macros there are in Rust? And when to use them?

So the plan here is to start by defining what are Macros in a more general context and within the Rust language. Once we understand what they are we are going to take a deeper dive into the different types of macros we have in the Rust Programming Language and maybe throw in some examples in there, because theory without practice just does not stick. And to finish off the post I am going to talk about when to use Macros and some knows use cases out there.

What are Macros?

In a more general context a Macro is just a shorthand for a programming term: macroinstruction. In the most basic terms, a macro is a way to automate a simple task. They can be used in a variety of different ways, from Microsoft Excel Macros to Automating tasks on your operating system.

What are Macros in the context of Rust programming?

A quote directly out of the Rust Book, "Fundamentally, macros are a way of writing code that writes other code, which is known as metaprogramming." A lot of times in our day to day we write the exact same code over and over again and Macros are useful for reducing the amount of code you have to write and maintain, also helps to keep your code cleaner (if you use it right), which is also one of the roles of functions. However, macros have some additional powers that functions don’t, that we will get into it later in the blog post.

Let's look at an example. If you have been using Rust for more than a day you probably used the Macro println!, which as you may know prints a formatted text and add a \n (new line) at the end for you.

fn main() {
    println!("Hello there.")
}

But have you ever stoped to think what goes on behind the scenes when you write that? Have you ever thought about what you would have to do to actually print a text to the console with the println! macro? Well, if you would like to print a list of text without using the println! this is one way you could do it.

// use Write trait that contains write() function  
use std::io::Write;  

fn main() {  
    std::io::stdout().write(b"Hello, world!\n").unwrap();  
}

Can you imagine having to write all that every time you needed to print one single line of text to the console? No thanks!

Well, and if you are curious, this is what the println! macro definition actually looks like:

#[macro_export]  
#[stable(feature = "rust1", since = "1.0.0")]  
#[cfg_attr(not(test), rustc_diagnostic_item = "println_macro")]  
#[allow_internal_unstable(print_internals, format_args_nl)]  
macro_rules! println {  
    () => {  
        $crate::print!("\n")  
    };  
    ($($arg:tt)*) => {{  
        $crate::io::_print($crate::format_args_nl!($($arg)*));  
    }};  
}

Reference: https://doc.rust-lang.org/src/std/macros.rs.html#101

Even the macro is using Macros! Wow! But well, after you have that down, you life you get a lot easier for sure.

Different Types of Macros

Rust has two types of macros:

1. Declarative macros sometimes referred to as "macros by example", “macro_rules! macros,” or just plain “macros”. Declarative macros enable you to write something similar to a match expression that operates on the Rust code you provide as arguments. It uses the code you provide to generate code that replaces the macro invocation.

We have already looked at a real example on how the println! is defined, but now lets look at how to define one of our own.

// use macro_rules! {}
macro_rules! add{
    // match like arm for macro
    ($a:expr,$b:expr)=>{
        // macro expand to this code
        {
            // $a and $b will be templated using the value/variable provided to macro
            $a+$b
        }
    }
}

fn main(){
    // call to macro, $a=1 and $b=2
    add!(1,2);
}

This code creates a macro that adds two numbers. The annotation [[macro_rules!]](https://doc.rust-lang.org/rust-by-example/macros.html) is used with the name of the macro, add, and the body of the macro.

The macro doesn’t really add two numbers, it just replaces itself with the code to add two numbers at compile time. Very similar to a match statement in this case the macro definition will look for the pattern that matches the parameters passed.

1. Procedural macros allow you to operate on the abstract syntax tree (AST) of the Rust code it is given. Procedural macros act more like functions in a sense. They accept some code as an input, operate on the code, and outputs some other piece of code matching against any pattern and replacing the code just like declarative macros do.

The three kinds of procedural macros:

• Attribute-like macros

• Custom derive macros

• Function-like macros (not covered on this blog post, unfortunetely)

Let's take a quick look into each of them…

To write a procedural macro, first we start by creating a project using cargo new my-macro-lib --lib. Once the project is ready, update the Cargo.toml to notify cargo the project will create procedural macros.

# Cargo.toml
[lib]
proc-macro = true

Now we are all set to venture into procedural macros.

Attribute macros:

Attribute-like macros helps you to create a custom attribute that attaches itself to an item and allows you to manipulate that item. Ohh and It can also take arguments!

#[some_attribute(some_argument)]
fn my_task(){
// some code
}

In the above code, some_attribute is an attribute macro. It manipulates the function my_task.

The function that defines a procedural macro takes a TokenStream as an input and produces a TokenStream as an output. The TokenStream type is defined by the proc_macro crate that we included above in the Cargo.toml and represents a sequence of tokens. This is the core of the macro: the source code that the macro is operating on makes up the input TokenStream, and the code the macro produces is the output TokenStream. To write a procedural macro, we also need to write our parser to parse TokenStream. The Rust community has a very good crate, [syn](https://docs.rs/syn/1.0.53/syn/), for parsing TokenStream.

Custo Derive macros:

As you’ve seen Rust provides a mechanism called “derive” that lets you implement traits easily. Let's look at a quick example:

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

That is a lot simpler then, don't you think?

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)  
    }  
}

Rust includes several traits that you can derive in his codebase, but it also lets you define your own.

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

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();  
}

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

Let's start by writing what the use of our macro would look like from the user's perspective. Our src/main.rs would look like this:

#[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();  
}

With that we would expect the output:

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

Great. Now that we know what we want let's write the procedural macro. At the moment, procedural macros need to be in their own library crate. 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 --lib

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" }

Now let's look at the source of our hello-world-derive crate, here's one way it could look like:

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_derive_input(&s).unwrap();

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

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

There is a lot going on here I know. Here we are making use of two crates: [syn](https://crates.io/crates/syn) and [quote](https://crates.io/crates/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. At the moment, the only thing you can do with a TokenStream is convert it to a string, as far as I know at least (I may be wrong).

What we really need is to be able to parse Rust code into something usable. This is where syn comes to play. The other crate we've introduced is quote. It's essentially the dual of syn as it will make generating Rust code really easy. Remember? Input and Output…

What we are doing here is we are taking a String of the Rust code for the type we are deriving, parsing it using syn, constructing the implementation of hello_world (using quote), then passing 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, unfortunately. In this case, I am trying to keep it as simple as possible.

Awesome, we got this far, now let’s write impl_hello_world(&ast).

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

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. If you have worked with any type of parsing before, you may be familiar with ASTs. We are able to get the name of the type using ast.ident. The quote! macro (I know, a macro within a macro, feels we are in that inception movie, anyway…) lets us write up the Rust code that we wish to return and convert it into Tokens. quote! lets 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 = "0.11.11"  
quote = "0.3.15"

[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

That is it! We have built our first macro. I know it doesn't really do much, but we were able to show what it can do and now the ball is on your court.

When to use Macros?

One of the advantages of using macros is that they don’t evaluate their arguments eagerly like functions do, which is one of the motivations to use macros other than functions.

A general rule of thumb is that macros can be used in situations where functions fail to provide the desired solution, where you have code that is quite repetitive, or in cases where you need to inspect the structure of your types and generate code at compile time. Taking examples from real use cases, Rust macros are used in a lot of cases, such as the following:

• Augmenting the language syntax by creating custom Domain-Specific Languages (DSLs)

• Writing compile time serialization code, like serde does

• Moving computation to compile-time, thereby reducing runtime overhead

• Writing and generating boilerplate code for repetitive tasks.

Lately I have been diving a lot into game development, specially with Unreal Engine and C++ and I guess that is why Macros were brought up to my attention so much. On game development with Unreal and C++ macros are a huge part of the development, they not only help keep you code clean and to the point, but also improve you productivity by a lot. Just like they do it in Rust.

Conclusion

In this blog post we covered what are Macros in a more general context and also in the Rust programming language, we looked at the different types of Macros and also at some simple implementation examples and to finish it off we talked about when to use macros to help solve your coding problems and how it can help you to keep you code clean, organised and keep your productivity high.

I hope I was able to spike your interest on this topic and I curious to see how you will use macros from now on. Feel free to send me so I can take a look!

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