Hello, I have two list (one with x values, the other with y values) What I want to do is a numercial integration according the following formula: Result x2 = Result x1 + ((y(x1) + y(x2))/2) * (x2 -x1) and put the result in another list. below my first try: integriereListe::(a)->(a)->(a) integriereListe [][] = [0.0] integriereListe (x:xs) (y:ys) = ((y - y2) /2) * (x2 -x) where x2 = head xs y2 = head ys but I got the failure Couldn't match type `[t0]' with `[[t0]]' In the pattern: y : ys In an equation for `integriereListe': integriereListe (x : xs) (y : ys) = ((y - y2) / 2) * (x2 - x) where x2 = head xs y2 = head ys another problem is how get the result x1 (see above) any hints are welcome. Thomas