Understanding Closures in Rust.
By: Andrew Pritchard
Tags: Programming, Functional Programming, Rust, Closures, Ownership

Summary

Closures are a combination of a function pointer ( fn ) and a context.

) and a context. A closure with no context is just a function pointer.

A closure which has an immutable context belongs to Fn .

. A closure which has a mutable context belongs to FnMut .

. A closure that owns its context belongs to FnOnce .

Understanding the different types of closures in Rust.

Unlike some other languages, Rust is explicit about our use of the self parameter. We have to specify self to be the first parameter of a function signature when we are implementing a struct:

struct MyStruct {

text: &'static str,

number: u32,

} impl MyStruct {

fn new (text: &'static str, number: u32) -> MyStruct {

MyStruct {

text: text,

number: number,

}

} // We have to specify that 'self' is an argument.

fn get_number (&self) -> u32 {

self.number

}

// We can specify different kinds of ownership and mutability of self.

fn inc_number (&mut self) {

self.number += 1;

}

// There are three different types of 'self'

fn destructor (self) {

println!("Destructing {}", self.text);

}

}

As a result, the following two styles are identical:

obj.get_number();

MyStruct::get_number(&obj);

This is in contrast to other languages where self (or this ) is often implied. Simply associating a function with an object or structure in these languages can imply that the first argument is self . Demonstrated above, we have four options for self : an immutable reference, a mutable reference, an owned value, or to not use self as an argument at all.

As a result, self implies some sort of context for the execution of the function. it is explicit in Rust, but often implicit elsewhere.

Also in this post we will use the following functions:

fn is_fn <A, R>(_x: fn(A) -> R) {}

fn is_Fn <A, R, F: Fn(A) -> R> (_x: &F) {}

fn is_FnMut <A, R, F: FnMut(A) -> R> (_x: &F) {}

fn is_FnOnce <A, R, F: FnOnce(A) -> R> (_x: &F) {}

The only purpose of these functions is to typecheck. For example, if is_FnMut(&func) compiles, then we know that func belongs to the FnMut trait.

No Context and the fn (lowercase f) type

With this in mind, consider some examples of closures using MyStruct above:

let obj1 = MyStruct::new("Hello", 15);

let obj2 = MyStruct::new("More Text", 10); let closure1 = |x: &MyStruct| x.get_number() + 3;

assert_eq!(closure1(&obj1), 18);

assert_eq!(closure1(&obj2), 13);

This is about as simple as we can get. This closure adds three to the number of any object of type MyStruct it has been given. It can be executed anywhere without any issues, and the compiler will not give you any trouble. We can quite easily write closure1 like this instead:

// It doesn't matter what code appears here, the function will behave

// exactly the same. fn func1 (x: &MyStruct) -> u32 {

x.get_number() + 3

}

assert_eq!(func1(&obj1), 18);

assert_eq!(func1(&obj2), 13);

This function does not depend on its context. It will behave exactly the same no matter what happens before or after it. We can use func1 and closure1 (almost) interchangeably.

When a closure does not depend on context at all, the type of our closure is fn :

// compiles successfully.

is_fn(closure1);

is_Fn(&closure1);

is_FnMut(&closure1);

is_FnOnce(&closure1);

Immutable context and the Fn (Capital F) trait

Compared to the above, we can add a context to a closure.

let obj1 = MyStruct::new("Hello", 15);

let obj2 = MyStruct::new("More Text", 10); // obj1 is borrowed by the closure immutably.

let closure2 = |x: &MyStruct| x.get_number() + obj1.get_number();

assert_eq!(closure2(&obj2), 25); // We can borrow obj1 again immutably...

assert_eq!(obj1.get_number(), 15); // But we can't borrow it mutably.

// obj1.inc_number(); // ERROR

closure2 depends on the value of obj1 and contains information about the surrounding scope. In this case, closure2 will borrow obj1 so that it can use it in the function body. We can still borrow obj1 immutably, but if we were attempt to mutate obj1 afterwards, we would get a borrowing error.

If we try to rewrite our closure using fn syntax, everything we need to know inside of the function must be passed to it as an argument, so we add an additional argument to represent the context of the function:

struct Context<'a>(&'a MyStruct); let obj1 = MyStruct::new("Hello", 15);

let obj2 = MyStruct::new("More Text", 10); let ctx = Context(&obj1); fn func2 (context: &Context, x: &MyStruct) -> u32 {

x.get_number() + context.0.get_number()

}

Which behaves almost identically to our closure:

assert_eq!(func2(&ctx, &obj2), 25); // We can borrow obj1 again immutably...

assert_eq!(obj1.get_number(), 15); // But we can't borrow it mutably.

// obj1.inc_number(); // ERROR

Note that the Context struct contains an immutable reference to MyStruct indicating that we won’t be able to modify it inside the function.

When we call closure1 it is implied that we pass the surrounding context as an argument to the closure, like we had to do it with our fn . Like in some other languages where we don’t have to specify that we pass self as an argument, Rust doesn’t need us to explicitly specify that we pass our context as an argument.

When a closure takes a context as an immutable reference, we say that it implements the Fn trait. That tells us that we can call our function multiple times without modifying the context:

// Does not compile:

// is_fn(closure2); // Compiles successfully:

is_Fn(&closure2);

is_FnMut(&closure2);

is_FnOnce(&closure2);

Mutable context and the FnMut trait

If we modify obj1 inside the closure, we get different results:

let mut obj1 = MyStruct::new("Hello", 15);

let obj2 = MyStruct::new("More Text", 10); // obj1 is borrowed by the closure mutably.

let mut closure3 = |x: &MyStruct| {

obj1.inc_number();

x.get_number() + obj1.get_number()

};

assert_eq!(closure3(&obj2), 26);

assert_eq!(closure3(&obj2), 27);

assert_eq!(closure3(&obj2), 28); // We can't borrow obj1 mutably or immutably

// assert_eq!(obj1.get_number(), 18); // ERROR

// obj1.inc_number(); // ERROR

This time we can’t borrow obj1 mutably or immutably. We also have to annotate the closure as mut . If we wish to rewrite this function using fn syntax, we get the following:

struct Context<'a>(&'a mut MyStruct); let mut obj1 = MyStruct::new("Hello", 15);

let obj2 = MyStruct::new("More Text", 10); let mut ctx = Context(&mut obj1); // obj1 is borrowed by the closure mutably.

fn func3 (context: &mut Context, x: &MyStruct) -> u32 {

context.0.inc_number();

x.get_number() + context.0.get_number()

};

This behaves the same way as closure3 :

assert_eq!(func3(&mut ctx, &obj2), 26);

assert_eq!(func3(&mut ctx, &obj2), 27);

assert_eq!(func3(&mut ctx, &obj2), 28); // We can't borrow obj1 mutably or immutably

// assert_eq!(obj1.get_number(), 18); // ERROR

// obj1.inc_number(); // ERROR

Note that we have to pass our context with a mutable reference. This indicates that we may get different results every time we call our function.

When a closure takes it’s context using a mutable reference, we say that it belongs to the FnMut trait:

// Does not compile:

// is_fn(closure3);

// is_Fn(&closure3); // Compiles successfully:

is_FnMut(&closure3);

is_FnOnce(&closure3);

Owned Context:

For our last example we’ll take ownership of obj1 :

let obj1 = MyStruct::new("Hello", 15);

let obj2 = MyStruct::new("More Text", 10); // obj1 is owned by the closure

let closure4 = |x: &MyStruct| {

obj1.destructor();

x.get_number()

};

We have to check the type of closure4 before we use it:

// Does not compile:

// is_fn(closure4);

// is_Fn(&closure4);

// is_FnMut(&closure4); // Compiles successfully:

is_FnOnce(&closure4);

Now we can check the behavior of it:

assert_eq!(closure4(&obj2), 10); // We can't call closure4 twice...

// assert_eq!(closure4(&obj2), 10); //ERROR // We can't borrow obj1 mutably or immutably

// assert_eq!(obj1.get_number(), 15); // ERROR

// obj1.inc_number(); // ERROR

In this example, we can only call the function once. Once we have called it the first time, we have destroyed obj1 , so it no longer exists for the second call. Rust gives us an error about using a value after it has been moved. That’s why we have to check the types beforehand.

Writing this with an fn we get the following:

struct Context(MyStruct); let obj1 = MyStruct::new("Hello", 15);

let obj2 = MyStruct::new("More Text", 10); let ctx = Context(obj1); // obj1 is owned by the closure

fn func4 (context: Context, x: &MyStruct) -> u32 {

context.0.destructor();

x.get_number()

};

Which, as expected, behaves the same as our closure:

assert_eq!(func4(ctx, &obj2), 10); // We can't call func4 twice...

// assert_eq!(func4(ctx, &obj2), 10); //ERROR // We can't borrow obj1 mutably or immutably

// assert_eq!(obj1.get_number(), 15); // ERROR

// obj1.inc_number(); // ERROR

When we write our closure using fn we have to use a Context struct that owns it’s value. When a closure takes ownership of it’s context, we say that it implements FnOnce . We can only call the function once, because after that, the context has been destroyed.

Conclusion