Your version would expect a list of two elements, you use & to indicate that you're collecting the remainder of the list as in: fun flip_all([Posn(x, y), & r

Your version would expect a list of two elements, you use & to indicate that you're collecting the remainder of the list as in: fun flip_all([Posn(x, y), & rest]): You'd need to amend the logic of the function body to iterate or recurse over the remainder. You'd also ended up with a nested list using your version but you can use & again to splice in the result, so if you wanted a recursive version of that it would be something like (skipping the base case): fun | flip_all([]): [] | flip_all([Posn(x, y), & rest]): [Posn(y, x), & flip_all(rest)] (not tested, don't have Rhombus available on this system but that should be correct) If you really want flip_first it'd be: fun flip_first([Posn(x, y), & rest]): [Posn(y, x), & rest]