unwrap_or_else in Rust

The unwrap_or_else method is used as an Option or Result type.

Let's see an example for both.

unwrap_or_else on an Option

For Option, the unwrap_or_else method is used to provide a fallback value if the Option is None**

fn main() {
    let some_value: Option<i32> = Some(10);
    let none_value: Option<i32> = None;

    // Using unwrap_or_else on an Option
    let result_some = some_value.unwrap_or_else(|| {
        println!("some_value was None, using default value.");
        0 // Default value
    });

    let result_none = none_value.unwrap_or_else(|| {
        println!("none_value was None, using default value.");
        0 // Default value
    });

    println!("Result when Option is Some: {}", result_some);
    println!("Result when Option is None: {}", result_none);
}

Output:

none_value was None, using default value.
Result when Option is Some: 10
Result when Option is None: 0

Explanation:

In this example, we have two Option variables, some_value and none_value. The some_value is Some(10) and the none_value is None.

The unwrap_or_else method is used on both Option variables. The closure passed to unwrap_or_else is only called when the Option is None. In this case, the closure prints a message and returns a default value of 0.

unwrap_or_else on a Result

For Result, the unwrap_or_else method is used to provide a fallback value if the Result is an Err.

fn main() {
    let ok_result: Result<i32, &str> = Ok(10);
    let err_result: Result<i32, &str> = Err("An error occurred");

    // Using unwrap_or_else on a Result
    let result_ok = ok_result.unwrap_or_else(|err| {
        println!("ok_result was Err: {}, using default value.", err);
        0 // Default value
    });

    let result_err = err_result.unwrap_or_else(|err| {
        println!("err_result was Err: {}, using default value.", err);
        0 // Default value
    });

    println!("Result when Result is Ok: {}", result_ok);
    println!("Result when Result is Err: {}", result_err);
}

Output:

Error: Error message
Result when Result is Ok: 10
Result when Result is Err: 0

Explanation:

In this example, we have two Result variables, ok_result and err_result. The ok_result is Ok(10) and the err_result is Err("An error occurred").

The unwrap_or_else method is used on both Result variables. The closure passed to unwrap_or_else is only called when the Result is Err. In this case, the closure prints a message and returns a default value of 0.

Conclusion

In both examples, the unwrap_or_else method is used to provide a default value when the Option or Result is None or Err respectively.

The unwrap_or_else method takes a closure that returns the default value. The closure is only called when the Option is None or the Result is Err.

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