Rust by Example

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`

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`

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.

}

הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX