对于一些使用情况，匹配很尴尬。例如：
| // Make `optional` of type `Option` |
| let optional = Some(7); |
| match optional { |
| Some(i) => { |
| println!("This is a really long string and `{:?}`", i); |
| // ^ Needed 2 indentations just so we could destructure |
| // `i` from the option. |
| }, |
| _ => {}, |
| // ^ Required because `match` is exhaustive. Doesn't it seem |
| // like wasted space? |
| }; |
| fn main() { |
| // All have type `Option` |
| let number = Some(7); |
| let letter: Option = None; |
| let emoticon: Option = None; |
| // The `if let` construct reads: "if `let` destructures `number` into |
| // `Some(i)`, evaluate the block (`{}`). Else do nothing. |
| if let Some(i) = number { |
| println!("Matched {:?}!", i); |
| } |
| // If you need to specify a failure, use an else: |
| if let Some(i) = letter { |
| println!("Matched {:?}!", i); |
| } else { |
| // Destructure failed. Change the failure case. |
| println!("Didn't match a number. Let's go with a letter!"); |
| }; |
| // Provide an altered failing condition. |
| let i_like_letters = false; |
| if let Some(i) = emoticon { |
| println!("Matched {:?}!", i); |
| // Destructure failed. Evaluated the condition to see if this branch |
| // should be taken. |
| } else if i_like_letters { |
| println!("Didn't match a number. Let's go with a letter!"); |
| // The condition evaluated false. This branch is the default. |
| } else { |
| println!("I don't like letters. Let's go with an emoticon :)!"); |
| }; |
| } |
