[Haskell-beginners] [Haskell-cafe] select :: [(Float, a)] -> a -- Weighted stochastic selection - help?

I am already using the following function pickOne xs g = (xs !! r, g') where (r, g') = Random.randomR (0, length xs - 1) g but I need to weight the items in the list, e.g. [(0.1, 'a') , (0.9, 'b')] for stochastic selection in a purely functional way, i.e. no monads. Any ideas welcome. Tim.