The point GP was making is that you can also define the function sqrt(x) as a function from reals to pairs of reals, and say that the sqrt(4) is the pair (-2, 2). With a proper definition of squaring a pair of reals, you could even have this be the inverse of the square function (we could define square((x, y)) = x²-xy+y², so square((-2,2))=4-4+4=4).

Of course, these are still functions which take exactlt one element in their input set (the reals for sqrt, or pairs of reals for square) and return for each a single element in their output sets (the pairs of reals for sqrt, or the reals for square).