Demonstration of how to extend System.Random using extension methods in F#.

gistfile1.fs

open System

(* Low-level F# implementation *)
let newRandomChar getRandomInt (chars : char[]) =
    let idx = getRandomInt()
    chars.[idx]

let newRandomWord (getRandomChar : unit -> char) length =
    let chars =
        Array.init length (fun _ -> getRandomChar())
    String(chars)

(* Extension methods for System.Random *)
type System.Random
with
    member this.NextChar(chars : char[]) =
        let getRandomInt() = this.Next(chars.Length)
        newRandomChar getRandomInt chars

    member this.NextWord(length, chars) =
        let getRandomChar() = this.NextChar(chars)
        newRandomWord getRandomChar length

(* Testing *)
let chars =
    [| [| 'a' .. 'z' |]
       [| 'A' .. 'Z' |]
       [| '0' .. '9' |]
    |]
    |> Array.concat

let rnd = new System.Random()
for i in 1..10 do
    printfn "%i: %s" i (rnd.NextWord(8, chars))

Very helpful Johann! Now I get what you meant. I have written C# extension methods, but this is the first time seeing an F# version. Cool! :)