Result type in Rust

In this blog post, we're going to see how to work with Result type in Rust

Result

Result is a type used for returning and propagating errors in Rust. It is an Enum type which has 2 variants such as Ok' andErr'. Ok represents success and containing value and Err representing error and containing an error value

Variable Initiation

    // Declaring variable with Ok() variant of Result type
    let a: Result<i32, &str> = Ok(100);

    // Declaring variable with Err() variant of Result type
    let b: Result<i32, &str> = Err("Invalid input provided");

Extracting Values from Result

Extracting values from Result can be done using various ways

if-let statement

    // Get value of Result type's Ok() variant
    if let Ok(x) = a {
        println!("Extracted value from 'a' is => {x}");
    }

    // Get value of Result type's Err() variant
    if let Err(x) = b {
        println!("Extracted value from 'b' is => {x}");
    }

match expression

    match a {
        Ok(x) => println!("Extracted Ok() variant's value from 'a' is => {x}"),
        Err(x) => println!("Extracted Err() variant's value from 'a' is => {x}"),
    }

    match b {
        Ok(x) => println!("Extracted Ok() variant's value from 'b' is => {x}"),
        Err(x) => println!("Extracted Err() variant's value from 'b' is => {x}"),
    }

expect method

expect method panics with a provided custom message when the variable is of Err variant

    println!("Value of variable a => {}", a.expect("Variable cannot be empty"));

unwrap method

unwrap method panics with a generic message when the variable is of Err variant

    println!("Value of variable a => {}", a.unwrap());

unwrap_or method

unwrap_or method returns value of Ok() variant OR specified value

    println!("Value of variable a => {}", a.unwrap_or(0));

unwrap_or_default method

unwrap_or_default method returns value of Ok() variant OR default value of underlying type

    println!("Value of variable a => {}", a.unwrap_or_default());

unwrap_or_else method

unwrap_or_else method returns value of Ok() variant OR returns the value of the function argument

    println!("Value of variable a => {}", a.unwrap_or_else(|x| x.len() as i32));

expect_err method

expect_err method panics with a provided custom message

    println!("Value of variable a => {}", b.expect_err("Variable cannot be empty"));

unwrap_err method

unwrap_err method panics with a generic message

    println!("Value of variable a => {}", b.unwrap_err());

Comparing Result Variables

It is possible to compare 2 Result variables in Rust

    if a > b {
        println!("Variable value a is greater than value of variable b");
    } else {
        println!("Variable value a is less than value of variable b");
    }

Iterating over Result

A Result can be iterated over. It is possible to call FromIterator trait's method on a Result. In case if a Result array contains Err, the resulted operation would be Err as well.

In this below example, we're creating a Result array having mixed of Ok and Err variants

    let r_arr = [Ok(10), Ok(20), Err("Invalid user input"), Ok(30)];

Collecting elements from Result array

We'll apply FromIterator trait's into_iter method and call collect method on it. This will result in Err variant

    let result_1: Result<Vec<i32>, &str> = r_arr.into_iter().collect();
    println!("Collect the elements => {result_1:?}");

Filter Result array and collect elements

We'll apply FromIterator trait's filter method and call collect method on it. This time, the result will be Result<Vec<i32>>

    let result_2: Result<Vec<i32>, &str> = r_arr.into_iter().filter(|x| x.is_ok()).collect();
    println!("Filter & collect the elements => {result_2:?}");

Sum method on Result array

We'll apply FromIterator trait's sum method and as it is having Err variant, the result will be of Err variant

    let result_3: Result<i32, &str> = r_arr.into_iter().sum();
    println!("Sum of all elements without filter => {result_3:?}");

Product method on Result array

We'll apply FromIterator trait's product method and as it is having Err variant, the result will be of Err variant

    let result_4: Result<i32, &str> = r_arr.into_iter().product();
    println!("Product of all elements without filter => {result_4:?}");

Filter & Sum method on Result array

Filter the option array and then call FromIterator trait's sum method. The result will be of Result<i32, &str> type

    let result_3: Result<i32, &str> = r_arr.into_iter().filter(|x| x.is_ok()).sum();
    println!("Sum of all elements after filter => {result_3:?}");

Filter & Product method on Result array

Filter the option array and then call FromIterator trait's product method. The result will be of Result<i32, &str> variant

    let result_4: Result<i32, &str> = r_arr.into_iter().filter(|x| x.is_ok()).product();
    println!("Product of all elements after filter => {result_4:?}");

I hope this blog post gives you a brief overview about Result type.

All the code examples can be found in this link.

Please feel free to share your feedback.

Happy reading!!!