The crucial design choice is that you can't get a Doodad by just saying oh, I'm sure this is a Doodad, I will validate later. You have to parse the thing you've got to get a Doodad if that's what you meant, and the parsing can fail because maybe it isn't one.

    let almost_pi: Rational = "22/7".parse().unwrap();

Here the example is my realistic::Rational. The actual Pi isn't a Rational number so we can't represent it, but 22 divided by 7 is a pretty good approximation considering.

I agree that many languages don't provide a nice API for this, but what I don't see (and maybe you have examples) is languages which do provide a nice API but call it validate. To me that naming would make no sense, but if you've got examples I'll look at them.