unwrap_or_else in Rust
Tags: rust, programming, tutorial, codenewbie
Published: 2024-06-08 07:21:53

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**


```rust
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**.

```rust
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**.

If you find this interesting and you want to get started with Rust, you can check this [FREE playlist on YouTube](https://www.youtube.com/watch?v=R33h77nrMqc&list=PLPoSdR46FgI412aItyJhj2bF66cudB6Qs&index=1&ab_channel=FrancescoCiulla):