fsharp-learning/for-fun-and-profit/folding.fsx

let printInt = printfn "%d"

let product n =
        let initialValue = 1
        let action productSoFar x = productSoFar * x
        [1..n] |> List.fold action initialValue

printInt (product 10)

let sumOfOdds n =
        let initialValue = 0
        let action sumSoFar x = if x % 2 = 0 then sumSoFar else sumSoFar + x
        [1..n] |> List.fold action initialValue

printInt (sumOfOdds 10)

let alternatingSum n =
        let initialValue = (true, 0)
        let action (isNeg, sumSoFar) x = if isNeg then (false, sumSoFar - x) else (true, sumSoFar + x)
        [1..n] |> List.fold action initialValue |> snd // `snd` means the second in a tuple

printInt (alternatingSum 100)

// Now we can factor out the commonality and re-write these functions like so:
let iterAct ini act n =
        [1..n] |> List.fold act ini

let product2 = iterAct 1 (fun x y -> x * y)
printInt (product2 10)

let sumOfOdds2 = iterAct 0 (fun x y -> if y % 2 = 0 then x else x + y)
printInt (sumOfOdds2 10)