Rust Notes #1: If Let, Question Mark Operator

Rust Notes #1: If Let, Question Mark Operator

On my quest to learn Rust, I’m reading The Rust Programming Language. The book reveals some interesting language features that lead to more concise code.

If Let

Rust has powerful pattern matching constructs, like match. Sometimes, though, match leads to unnecessary verbosity.

In some cases, using if let instead of match leads to greater concision. For example, you could replace:

let an_option = Some(4);

match an_option  {
  Some(num) => {
    println!("{}", num);
  }
  _ => {}
}

With this variation:

let an_option = Some(4);

if let Some(num) = an_option {
  println!("{}", num);
}

The if let variant destructures the value in an_option in a concise way. Rust by Example has more examples, including how to handle error cases.

Question Mark Operator

Without some magic, handling Result and Option types means boilerplate match expressions. The question mark operator ? reduces that boilerplate by either evaluating to the unwrapped value inside OK or immediately returning and propagating the Err.

Here’s an imaginary example:

fn get_some_data() -> Result<i32, SomeErrorType>  {
    some_unreliable_service::get_data()?
}

Instead of writing code that handles and manually returns the potential error from the call to get_data, the question mark operator will automatically return and propagate the Error case.

For more examples, see the book’s section on the question mark operator.

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