I wrote some code to model the keystroke-to-keystroke delay in a person typing, with pseudorandomness. There are two kinds of delays.. one is a very small delay as the person reaches for a new key (call this 'reach' delays), the other is a larger delay that represents a pause to think or to take a break (call this 'break' delays). My thought is that I could create an infinite series of reach delays and an infinite series of break delays and zipWith (+). The inifinite series of break delays would look like: - The overall pattern is a large number of zeros, in the range (N,M) followed by a single positive number, chosen from a distribution D... and then repeat. Here's some code. I used the Gen monad to gain access to the 'frequency' function, which is useful, but I had trouble figuring out how to put the whole algorithm into Gen. I also looked at Rand. The problem is that if I want to create the list by 'sequence'ing a list of monads, they need to have state that remembers how many zeros are left to go. breakSeries :: Int -> Int -> StdGen -> [Float] breakSeries lowerB upperB gen = let (n,gen1) = randomR (lowerB,upperB) gen (gen2,gen3) = split gen1 delay = generate 1 gen2 breakM in replicate n 0 ++ [delay] ++ breakSeries lowerB upperB gen3 breakM :: Gen Float breakM = frequency [ (10, choose( 1::Float , 2)) , (10, choose( 4::Float , 6)) ] test = (print . take 100 . breakSeries 2 4 ) =<< newStdGen