while let
Similar to if let
, while let
can make awkward match
sequences
more tolerable. Consider the following sequence that increments i
:
#![allow(unused)] 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. }