while let - Rust By Example
Description: A description

# while let

Similar to `if let`, `while let` can make awkward `match` sequences more tolerable. Consider the following sequence that increments `i`:

    # #![allow(unused_variables)]
    #fn main() {
    // Make `optional` of type `Option<i32>`
    let mut optional = Some(0);

    // Repeatedly try this test.
    loop {
        match optional {
            // If `optional` destructures, evaluate the block.
            Some(i) => {
                if i > 9 {
                    println!("Greater than 9, quit!");
                    optional = None;
                } else {
                    println!("`i` is `{:?}`. Try again.", i);
                    optional = Some(i + 1);
                }
                // ^ Requires 3 indentations!
            },
            // Quit the loop when the destructure fails:
            _ => { break; }
            // ^ Why should this be required? There must be a better way!
        }
    }
    #}

Using `while let` makes this sequence much nicer:

    fn main() {
        // Make `optional` of type `Option<i32>`
        let mut optional = Some(0);

        // This reads: "while `let` destructures `optional` into
        // `Some(i)`, evaluate the block (`{}`). Else `break`.
        while let Some(i) = optional {
            if i > 9 {
                println!("Greater than 9, quit!");
                optional = None;
            } else {
                println!("`i` is `{:?}`. Try again.", i);
                optional = Some(i + 1);
            }
            // ^ Less rightward drift and doesn't require
            // explicitly handling the failing case.
        }
        // ^ `if let` had additional optional `else`/`else if`
        // clauses. `while let` does not have these.
    }

### See also:

`enum`, `Option`, and the RFC