Rust by Example

Closures

Closures are functions that can capture the enclosing environment. For example, a closure that captures the x variable:

|val| val + x

The syntax and capabilities of closures make them very convenient for on the fly usage. Calling a closure is exactly like calling a function. However, both input and return types can be inferred and input variable names must be specified.

Other characteristics of closures include:

• using || instead of () around input variables.

• optional body delimination ({}) for a single expression (mandatory otherwise).

• the ability to capture the outer environment variables.

fn main() {

// Increment via closures and functions.

fn function(i: i32) -> i32 { i + 1 }

// Closures are anonymous, here we are binding them to references

// Annotation is identical to function annotation but is optional

// as are the `{}` wrapping the body. These nameless functions

// are assigned to appropriately named variables.

let closure_annotated = |i: i32| -> i32 { i + 1 };

let closure_inferred  = |i     |          i + 1  ;

let i = 1;

// Call the function and closures.

println!("function: {}", function(i));

println!("closure_annotated: {}", closure_annotated(i));

println!("closure_inferred: {}", closure_inferred(i));

// A closure taking no arguments which returns an `i32`.

// The return type is inferred.

let one = || 1;

println!("closure returning one: {}", one());

}

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