Nothing, apart from the fact that languages with type systems designed more carefully than C happen to be functional languages, to one extent or another. (Thou

Nothing, apart from the fact that languages with type systems designed more carefully than C happen to be functional languages, to one extent or another. (Though by the way: having functions that evaluate to a value when executed is itself a feature that belongs to the functional paradigm, although one so trivial and common that it’s not usually thought as such. But a purely imperative/procedural way of returning values would be via out parameters or global variables.)