Replace words

open System

let rng = new Random()
let sentence = "The quick brown fox jumped over the lazy dog"
let businessWords = ["synergy"; "buzz"; "overflow"; "capital"; "corporate";]

let splitToPairs (str:string) =
    let rec pairOff arr =
        match arr with
        | h::s::t -> (h, s)::(pairOff t)
        | h::[] -> (h, "")::[]
        | [] -> []
    let words = str.Split [|' '|]
    let pairs = pairOff (List.ofArray words)
    pairs

let replaceSecondWords pairs words =
    let random (words:string list) =
        let max = words.Length
        let i = rng.Next(0, max)
        words.Item i

    let replacedPairs =
        pairs
        |> List.map(fun p ->
                    match p with
                    | (f, s) when s = "" -> (f, "")
                    | (f, s) -> (f, (random words))
                )

    replacedPairs
    |> List.map(fun p -> String.Join(" ", (fst p), (snd p)))
    |> List.fold (fun state acc -> state + " " + acc) ""


let newSentence = replaceSecondWords (splitToPairs sentence) businessWords
(newSentence)