Even though Ceylon uses tuple types to represent parameter lists, a function accepting multiple arguments (with the parameter list represented by a tuple: Calla

Even though Ceylon uses tuple types to represent parameter lists, a function accepting multiple arguments (with the parameter list represented by a tuple: Callable<Void, [Integer, String]>) is still different from a function accepting a tuple of multiple fields (with the parameter list represented by a tuple of a tuple: Callable<Void, [[Integer, String]]>). You can’t just call one as if it were the other. Ceylon works the same way as Rust here, as far as I can tell. BTW, for Rust beginners who might be concerned that o.map(f) is somehow a difficult problem, the solution in Rust is simple: o.map(|(a, b)| f(a, b)).