I have the following F# functions which makes use of a ref variable to seed and keep track of a running total, something tells me this isn't in the spirit of fp or even particular clear on its own. I'd like some direction on the clearest (possible fp, but if an imperative approach is clearer I'd be open to that) way to express this in F#. Note that selectItem implements a random weighted selection algorithm.

type WeightedItem(id: int, weight: int) =
    member self.id = id
    member self.weight = weight

let selectItem (items: WeightedItem list) (rand:System.Random) =
    let totalWeight = List.sumBy (fun (item: WeightedItem) -> item.weight) items
    let selection = rand.Next(totalWeight) + 1
    let runningWeight = ref 0
    List.find 
        (fun (item: WeightedItem) ->
            runningWeight := !runningWeight + item.weight
            !runningWeight >= selection)
        items

let items = [new WeightedItem(1,100); new WeightedItem(2,50); new WeightedItem(3,25)]
let selection = selectItem items (new System.Random())