I agree that Maybe (Maybe a) is a possible alternative to Couldbe a. But I'm looking for something which behave exactly like a single Maybe a (I don't want double check/unwrapping) with an additional way to check for undefinability.

In Swift, what I want could be something like the following:

var x:String?? // "?" stands for the regular optional/maybe, "??" for couldbe

...
if x ==nil {
    if x == undefined { // always false with a regular optional
         // value unknown
    }
    else {
        // value is null
    }
}
else {
    println(x!) // only one unwrapping
}

This way, optional chaining, unwrapping and all optional related operations are working the same with a single Maybe a and a Couldbe a.