Closures

Closures in Rust, also called lambda expressions or lambdas, are functions that can capture the enclosing environment. For example, a closure that captures the x variable:

1. |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.

1. fn main() {
2. // Increment via closures and functions.
3. fn  function            (i: i32) -> i32 { i + 1 }

5. // Closures are anonymous, here we are binding them to references
6. // Annotation is identical to function annotation but is optional
7. // as are the `{}` wrapping the body. These nameless functions
8. // are assigned to appropriately named variables.
9. let closure_annotated = |i: i32| -> i32 { i + 1 };
10. let closure_inferred  = |i     |          i + 1  ;

12. let i = 1;
13. // Call the function and closures.
14. println!("function: {}", function(i));
15. println!("closure_annotated: {}", closure_annotated(i));
16. println!("closure_inferred: {}", closure_inferred(i));

18. // A closure taking no arguments which returns an `i32`.
19. // The return type is inferred.
20. let one = || 1;
21. println!("closure returning one: {}", one());

23. }