Hi, Having read some tutorials, I would like to start using Haskell "for real", but I have some questions about data structures. The mathematical description of the problem is the following: assume there is a function V(a,b,theta), where a and b can have two values, High or Low, and theta is a number between zero and n (n is given). The range of V is the real numbers. Then there is an algorithm (called value iteration, but that's not important) that takes V and produces a function of the same type, called V'. The algorithm uses a mapping that is not elementwise, ie more than the corresponding values of V are needed to compute a particular V'(a,b,theta) -- things like V(other a,b,theta) and V(a,b,theta+1), where data State = Low | High other :: State -> State other High = Low other Low = High Question 1: V can be represented as a 3-dimensional array, where the first two indices are of type State, the third is Int (<= n). What data structure do you suggest in Haskell to store V? Is there a multidimensional array or something like this? Let's call this structure TypeV. Question 2: I would like to write valueit :: TypeV -> TypeV valueit V = mapondescartesproduct [Low,High] [Low,High] [0..n] mapV where -- mapV would calculate the new V' using V -- mapV :: State -> State -> Int -> Double to fill the new data structure. How to do this sensibly? Thanks, Tamas