# 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:

fn main() { baz(bar(foo)); }
`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?

fn main() { foo.bar().baz(); }
`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()); }
```struct Circle {
x: f64,
y: f64,
}

impl Circle {
fn area(&self) -> f64 {
}
}

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:

fn main() { 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!"); } } }
```struct Circle {
x: f64,
y: 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:

fn main() { 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!"); } } }
```struct Circle {
x: f64,
y: 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); }
```struct Circle {
x: f64,
y: f64,
}

impl Circle {
fn area(&self) -> f64 {
}

fn grow(&self, increment: f64) -> Circle {
}
}

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:

fn main() { struct Circle; impl Circle { fn grow(&self, increment: f64) -> Circle { Circle } } }
`fn grow(&self, increment: f64) -> 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); }
```struct Circle {
x: f64,
y: f64,
}

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

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 `Circle`s, 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); }
```struct Circle {
x: f64,
y: f64,
}

impl Circle {
fn area(&self) -> f64 {
}
}

struct CircleBuilder {
x: f64,
y: 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
}

self
}

fn finalize(&self) -> Circle {
}
}

fn main() {
let c = CircleBuilder::new()
.x(1.0)
.y(2.0)
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 `Circle`s in any way we choose.