Hello Daniel, thank you very much for pointing out a solution.
If I understand correctly, use
zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
-- calculates the list of differences between successive elements, -- differences [x1,x2,x3,...] = [x2-x1, x3-x2, ...] -- -- We use subtract and don't swap the list arguments to zipWith -- becuase this way there is no need to handle an empty list -- specially, zipWith's definition lets (tail xs) unevaluated in that case. -- differences :: Num a => [a] -> [a] differences xs = zipWith subtract xs (tail xs)
areas :: Floating a => [a] -> [a] -> [a] areas xs ys = zipWith (\dx dy -> dx * dy/2) (differences xs) (differences ys)
-- if it should have been (y+y2)/2 above, make that Yes, you are right. That should be an sum instead of a difference.
-- sums ys, where sums ks = zipWith (+) ks -- or areas xs ys = zipWith (*) (differenses xs) (means ys) -- where means zs = map (/ 2) (sums zs)
I have changed the functions according your advice. But still I get an error: differences :: Num a => [a] -> [a] differences xs = zipWith subtract xs (tail xs) areas :: Floating a => [a] -> [a] -> [a] areas xs ys = zipWith (*) (differences xs) (means ys) where means zs = map (/ 2) (sums zs) where sums ks = zipWith (+) ks integrals xs ys = scanl (+) 0 (areas xs ys) Couldn't match expected type `[b0]' with actual type `[c0] -> [c0]' In the return type of a call of `sums' In the second argument of `map', namely `(sums zs)' In the expression: map (/ 2) (sums zs) Thomas