Error Handling

In imperative programs, it’s normal to see throw statements or try/catch. Exceptions are discouraged in FP. Instead, the return value of a function should contain information about the possible error. It’s very similar to Rust’s Result<T,E>.

In FP, the Either type is used to signal that an operation can potentially fail. It’s quite similar to Option<T>. The difference is that the “bad” case may return an additional payload, unlike None that doesn’t bear any additional information on why None was returned.

Either has two possible variants:

Left<L> - failure
Right<R> - success

Among the typical FP functions (Map, Bind, Where, etc.), the Where function is not applicable to Either. That’s because Where only accepts a predicate that returns a boolean. It is unable to create a proper Left<L>. A workaround is to use Bind with a function that either returns Right<R> or Left<L>

Right(person)
    .Bind(CheckAge) // with Option<T> it'd be .Where(HasRightAge)
    .Bind(Greet)
    .Bind(LetIn);

Either<Rejection, Person> CheckAge(Person p)
{
    if (person.Age >= 18) return p;
    else return new Rejection("Age less than 18");
}

Checking For Errors

Typically, Either-based flows follow a track where each function may either succeed or fail. At the end of the flow, the failure scenario should be checked.

Right(person)
    .Bind(CheckAge) // with Option<T> it'd be .Where(HasRightAge)
    .Bind(Greet)
    .Bind(LetIn)
    .Match(
        Right: _ => {},
        Left: rejection =>
        {
            LogRejection(rejection); // impure function
        }
    );

Error Types

There should be a special type for the “left” (error) scenario. We could have a simple base Error with all necessary properties (like string Message) and, when needed, we could create derived error types. Such error types could contain predefined error messages for convenience.

record UserDoesntExist()
    : Error("The provided username does not exist");

Throwing Exceptions

It is OK to throw exceptions when something is wrong in the program logic. In such a case, an exception is a clear sign that some code needs to be fixed.

Additionally, throwing exceptions is alright during the initialization. In initialization requires connecting to some message bus and that connection fails, it’s an exceptional situation and the progrm should probably terminate.

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