Seems like you could solve that by endlessly buffering stdin? Then you'd have to keep passing higher and higher offsets to read from, and old offsets would simply return their original, old values.
x = read_guess(); y = io.read_int("what's your name: "); z = read_guess();
How would your system ensure x and z are the same?
x = read_guess(0)
y = read_name("what's your name: ", len(x))
z = read_guess(len(x)+len(y))
fn do_stuff(offset: int) -> (string, int) { x = read_guess(offset) y = read_name("what's your name: ", offset + len(x)) z = read_guess(offset+len(x)+len(y)) new_offset = offset+len(x)+len(y)+len(z) return ("name is: {y}, guess is: {x}", new_offset) }