类似于if let，while let 可以使劲匹配更多序列。例如考虑以下序列：
| // Make `optional` of type `Option` |
| 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 when the destructure fails, meaning `break`. |
| _ => { break; } |
| // ^ Why should this be required? Seems superfluous. |
| } |
| } |
while let 使该序列好得更多：
| fn main() { |
| // Make `optional` of type `Option` |
| 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. |
| } |
