Error Handling Basic

Roc uses Result ok err types to handle errors. The ? operator tries an operation and returns early if it fails. You can replace default errors with custom ones using ? |_| CustomError(important_info), if you don't want to do that, add ? right after ) without any spaces.

You will often see Stdout.line!("hello")? without a custom error because it's very unlikely to fail.

See also:

• Real world error handling for a more complex example.
• Desugaring ? to understand how ? is converted under the hood.

Full Code

module []

Person : { first_name : Str, last_name : Str, birth_year : U16 }

parse_person : Str -> Result Person [InvalidSentenceFormat Str, InvalidNameFormat Str, InvalidBirthYearFormat Str]
parse_person = |line|
    { before: full_name, after: birth_year_str } =
        # The NotFound error is not very informative, so we discard it with `_` and provide our own.
        Str.split_first(line, " was born in ") ? |_| InvalidSentenceFormat(line)

    { before: first_name, after: last_name } =
        Str.split_first(full_name, " ") ? |_| InvalidNameFormat(full_name)

    birth_year = Str.to_u16(birth_year_str) ? |_| InvalidBirthYearFormat(birth_year_str)

    Ok({ first_name, last_name, birth_year })

expect parse_person("George Harrison was born in 1943") == Ok({ first_name: "George", last_name: "Harrison", birth_year: 1943 })

Output

Run this from the directory that has ErrorHandlingBasic.roc in it:

$ roc test ErrorHandlingBasic.roc
0 failed and 1 passed in 102 ms.