# Rust by Example

## 8.2 Closures

Closures in Rust, also called lambdas, 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());

}