How to use enums as result type in PHP 8.1

Different types of error handling

• Exception
• Mixed return type
• Return argument (depreceated)
• Return tuple

No broad concensus, new language makes different decisions:

• Go, Rust - no exceptions
• Koka - an exception is just a special case of effect handlers
• Haskell, another monad (and also other things I don’t know about)

Another alternative to above mentiond techniques is the Result type from OCaml:

type ('a, 'e) result = 
    | Ok of 'a
    | Error of 'e

This defines a type result that is either Ok or Error. Further more, it carries data of type 'a or 'e, meaning, any type.

This is called algebraic data-types. The PHP enum functionality is based on this.

enum Result: mixed {
    case Ok = null;
    case Error = null;
}

TODO: Result can only carry int and string. Why?

We need generics to make it type-safe (not “mixed”). Don’t know if Psalm or PHPStan supports this yet.

Example:

function getPost(int? $courseId): Result
{
    if (is_null($courseId) {
        return Result::Error("Course id cannot be null");
    }
}