Hi, I have a computation where a function is always applied to the previous result. However, this function may not return a value (it involves finding a root numerically, and there may be no zero on the interval). The whole problem has a parameter c0, and the function is also parametrized by the number of steps that have been taken previously. To make things concrete, type Failmessage = Int -- this might be something more complex data Result a = Root a | Failure Failmessage -- guess I could use Either too f :: Double -> Int -> Double 0 -> Result Double f c0 0 _ = c0 f c0 j x = {- computation using x, parameters calculated from c0 and j -} Then c1 = f c0 0 c0 c2 = f c0 1 c1 c3 = f c0 2 c2 ... up to cn. I would like to 1) stop the computation when a Failure occurs, and store that failure 2) keep track of intermediate results up to the point of failure, ie have a list [c1,c2,c3,...] at the end, which would go to cn in the ideal case of no failure. I think that a monad would be the cleanest way to do this. I think I could try writing one (it would be a good exercise, I haven't written a monad before). I would like to know if there is a predefined one which would work. Thank you, Tamas