Presumably what you want is not to abstract the program logic but to ensure it is well-formed. I would prefer too see that done with typing of expressions rather than functions, for example by adding an overloaded “maybe” keyword to C++. It’s use is made clear by the following example:

maybe int m;

maybe (m) {
// m is in scope as type int
} else {
// m is hidden from scope
}

I did say I was a dinosaur

“I commented previously in my own blog, which you can get to if you hit the TrackBack link at the foot of Andy’s article.”

Now on to this comment…)

I note that you can write code with structure much like Pavlos desires by having Maybe<X> implement an implicit cast to X that throws a MaybeIsInvalid exception when the object is invalid. Then you can write

Maybe<int> m;
try {
// use m as if it has type int
} catch MaybeIsInvalid {
// your alternative action
// (m remains in scope, but I’m not sure why it shouldn’t, Pavlos – you might want e.g. to assign to it.)
}