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7.7 while let

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

// 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