[Haskell-cafe] Theory about uncurried functions

Lambda calculus is a nice theory in which every function always has one input and one output. Functions with multiple arguments can be simulated because functions are first class and hence a function can "return" a function. Multiple outputs cannot be done, one must embed these outputs in some data type, e.g. using a tuple, or one must use continuation passing style. Now, does a similar theory exist of functions that always have one input and one output, but these inputs and outputs are *always* tuples? Or maybe this does not make any sense?