Rust if let

if let

對於一些使用情況，匹配很尷尬。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?
};

if let 是清潔器用例，另外允許指定的各種失敗選項：

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 :)!"); }; } 

也可以參考:

enum, Option, 和 RFC