Method Syntax

Functions are great, but if you want to call a bunch of them on some data, it can be awkward. Consider this code:

baz(bar(foo));

We would read this left-to-right, and so we see ‘baz bar foo’. But this isn’t the order that the functions would get called in, that’s inside-out: ‘foo bar baz’. Wouldn’t it be nice if we could do this instead?

foo.bar().baz();

Luckily, as you may have guessed with the leading question, you can! Rust provides the ability to use this ‘method call syntax’ via the impl keyword.

Method calls

Here’s how it works:

struct Circle {
    x: f64,
    y: f64,
    radius: f64,
}

impl Circle {
    fn area(&self) -> f64 {
        std::f64::consts::PI * (self.radius * self.radius)
    }
}

fn main() {
    let c = Circle { x: 0.0, y: 0.0, radius: 2.0 };
    println!("{}", c.area());
}

This will print 12.566371.

We’ve made a struct that represents a circle. We then write an impl block, and inside it, define a method, area.

Methods take a special first parameter, of which there are three variants: self, &self, and &mut self. You can think of this first parameter as being the foo in foo.bar(). The three variants correspond to the three kinds of things foo could be: self if it’s a value on the stack, &self if it’s a reference, and &mut self if it’s a mutable reference. Because we took the &self parameter to area, we can use it like any other parameter. Because we know it’s a Circle, we can access the radius like we would with any other struct.

We should default to using &self, as you should prefer borrowing over taking ownership, as well as taking immutable references over mutable ones. Here’s an example of all three variants:

struct Circle {
    x: f64,
    y: f64,
    radius: f64,
}

impl Circle {
    fn reference(&self) {
       println!("taking self by reference!");
    }

    fn mutable_reference(&mut self) {
       println!("taking self by mutable reference!");
    }

    fn takes_ownership(self) {
       println!("taking ownership of self!");
    }
}

You can use as many impl blocks as you’d like. The previous example could have also been written like this:

struct Circle {
    x: f64,
    y: f64,
    radius: f64,
}

impl Circle {
    fn reference(&self) {
       println!("taking self by reference!");
    }
}

impl Circle {
    fn mutable_reference(&mut self) {
       println!("taking self by mutable reference!");
    }
}

impl Circle {
    fn takes_ownership(self) {
       println!("taking ownership of self!");
    }
}

Chaining method calls

So, now we know how to call a method, such as foo.bar(). But what about our original example, foo.bar().baz()? This is called ‘method chaining’. Let’s look at an example:

struct Circle {
    x: f64,
    y: f64,
    radius: f64,
}

impl Circle {
    fn area(&self) -> f64 {
        std::f64::consts::PI * (self.radius * self.radius)
    }

    fn grow(&self, increment: f64) -> Circle {
        Circle { x: self.x, y: self.y, radius: self.radius + increment }
    }
}

fn main() {
    let c = Circle { x: 0.0, y: 0.0, radius: 2.0 };
    println!("{}", c.area());

    let d = c.grow(2.0).area();
    println!("{}", d);
}

Check the return type:

# struct Circle;
# impl Circle {
fn grow(&self, increment: f64) -> Circle {
# Circle } }

We say we’re returning a Circle. With this method, we can grow a new Circle to any arbitrary size.

Associated functions

You can also define associated functions that do not take a self parameter. Here’s a pattern that’s very common in Rust code:

struct Circle {
    x: f64,
    y: f64,
    radius: f64,
}

impl Circle {
    fn new(x: f64, y: f64, radius: f64) -> Circle {
        Circle {
            x: x,
            y: y,
            radius: radius,
        }
    }
}

fn main() {
    let c = Circle::new(0.0, 0.0, 2.0);
}

This ‘associated function’ builds a new Circle for us. Note that associated functions are called with the Struct::function() syntax, rather than the ref.method() syntax. Some other languages call associated functions ‘static methods’.

Builder Pattern

Let’s say that we want our users to be able to create Circles, but we will allow them to only set the properties they care about. Otherwise, the x and y attributes will be 0.0, and the radius will be 1.0. Rust doesn’t have method overloading, named arguments, or variable arguments. We employ the builder pattern instead. It looks like this:

struct Circle {
    x: f64,
    y: f64,
    radius: f64,
}

impl Circle {
    fn area(&self) -> f64 {
        std::f64::consts::PI * (self.radius * self.radius)
    }
}

struct CircleBuilder {
    x: f64,
    y: f64,
    radius: f64,
}

impl CircleBuilder {
    fn new() -> CircleBuilder {
        CircleBuilder { x: 0.0, y: 0.0, radius: 1.0, }
    }

    fn x(&mut self, coordinate: f64) -> &mut CircleBuilder {
        self.x = coordinate;
        self
    }

    fn y(&mut self, coordinate: f64) -> &mut CircleBuilder {
        self.y = coordinate;
        self
    }

    fn radius(&mut self, radius: f64) -> &mut CircleBuilder {
        self.radius = radius;
        self
    }

    fn finalize(&self) -> Circle {
        Circle { x: self.x, y: self.y, radius: self.radius }
    }
}

fn main() {
    let c = CircleBuilder::new()
                .x(1.0)
                .y(2.0)
                .radius(2.0)
                .finalize();

    println!("area: {}", c.area());
    println!("x: {}", c.x);
    println!("y: {}", c.y);
}

What we’ve done here is make another struct, CircleBuilder. We’ve defined our builder methods on it. We’ve also defined our area() method on Circle. We also made one more method on CircleBuilder: finalize(). This method creates our final Circle from the builder. Now, we’ve used the type system to enforce our concerns: we can use the methods on CircleBuilder to constrain making Circles in any way we choose.