Recommendation: Maybe<T>.IfNone()

Maybe<T>.UnWrap()and Maybe<T>.Match are great if you want to leave the monad and operate on values, but a lot of the benefits of the monad come with the fluent chaining of operations and transformations.

Often, as a Maybe<T> is working its way through layers of an application, there comes a point at which a default/fallback value is known and can be provided to the Maybe<T> but I don’t what to unwrap it.

Example:

Maybe<string> firstName = "sean";
Maybe<string> lastName = "wright";

// How do I supply a fallback value (in the case of `Maybe<string>.None` without unwrapping, like "unknown"?
Maybe<string> fullName = firstName.Bind(f => lastName.Map(l => $"{f} {l}"));

// later on ...

// if `fullName` was empty earlier I'd like it to have had the value "unknown" so that `uniqueFullName`
// wouldn't be just a Guid
Maybe<string> uniqueFullName = fullName.Map(n => $"{n}{Guid.NewGuid()}");

I propose something like IfNone():

public static Maybe<T> IfNone<T>(this Maybe<T> source, T fallback)
{
    if (source.HasNoValue)
    {
        return fallback;
    }

    return source;
}

Or maybe it would make sense to add an overload to Maybe<T>.Map() that took a 2nd parameter of a fallback value?

[seangwright]

Ok, I’ll make a PR for this!

I’m guessing we’ll want the full suite of extension overloads including additions to AsyncMaybeExtensions.cs?

[vkhorikov]

I’m guessing we’ll want the full suite of extension overloads including additions to AsyncMaybeExtensions.cs?

Preferrably, yes.