if let

For some use cases, when matching enums, match is awkward. For example:


# #![allow(unused_variables)]
#fn main() {
// Make `optional` of type `Option<i32>`
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 is cleaner for this use case and in addition allows various failure options to be specified:

fn main() {
    // All have type `Option<i32>`
    let number = Some(7);
    let letter: Option<i32> = None;
    let emoticon: Option<i32> = None;

    // The `if let` construct reads: "if `let` destructures `number` into
    // `Some(i)`, evaluate the block (`{}`).
    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 to 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. Evaluate an `else if` condition to see if the
    // alternate failure branch should be taken:
    } else if i_like_letters {
        println!("Didn't match a number. Let's go with a letter!");
    } else {
        // The condition evaluated false. This branch is the default:
        println!("I don't like letters. Let's go with an emoticon :)!");
    }
}

In the same way, if let can be used to match any enum value:

// Our example enum
enum Foo {
    Bar,
    Baz,
    Qux(u32)
}

fn main() {
    // Create example variables
    let a = Foo::Bar;
    let b = Foo::Baz;
    let c = Foo::Qux(100);
    
    // Variable a matches Foo::Bar
    if let Foo::Bar = a {
        println!("a is foobar");
    }
    
    // Variable b does not match Foo::Bar
    // So this will print nothing
    if let Foo::Bar = b {
        println!("b is foobar");
    }
    
    // Variable c matches Foo::Qux which has a value
    // Similar to Some() in the previous example
    if let Foo::Qux(value) = c {
        println!("c is {}", value);
    }

    // Binding also works with `if let`
    if let Foo::Qux(value @ 100) = c {
        println!("c is one hundred");
    }
}

Another benefit: if let allows to match enum non-parameterized variants, even if the enum doesn't #[derive(PartialEq)], neither we implement PartialEq for it. In such case, classic if Foo::Bar==a fails, because instances of such enum are not comparable for equality. However, if let works.

Would you like a challenge? Fix the following example to use if let:

// This enum purposely doesn't #[derive(PartialEq)],
// neither we implement PartialEq for it. That's why comparing Foo::Bar==a fails below.
enum Foo {Bar}

fn main() {
    let a = Foo::Bar;

    // Variable a matches Foo::Bar
    if Foo::Bar == a {
    // ^-- this causes a compile-time error. Use `if let` instead.
        println!("a is foobar");
    }
}

See also:

enum, Option, and the RFC