[Haskell-cafe] working with lists of couples

Hello, i'd like to write a function that given a list like [1,2,3,4...] returns a list of couple where the first element is the corresponding element of the string, and the second is the sum of the previous elements. An example: input: [1,2,3,4] output: [(1,0)(2,1)(3,3)(4,6)] The problem could seem trivial, and here's a first solution: buildCouples = snd . foldl op (0,[]) where op (v,xs) x = (v+x,xs++[(x,v)]) The problem is that this solution is quadratic, and I want a solution that take time n (2n is not good either). Any suggestion? Thanks, Clare.