http://nenono.hatenablog.com/entry/2014/06/24/215137

gistfile1.txt

open FsRandom

let search goal state =
   // 入力から文字種を得る
   let goalSize = String.length goal
   let input = Seq.distinct goal |> Seq.toArray
   let inputSize = Array.length input
   use e =
      Utility.chooseOne inputSize
      |> Random.map (fun index -> input.[index])
      |> Seq.ofRandom <| state
      |> fun s -> s.GetEnumerator ()
   let rec loop (buffer:System.Text.StringBuilder) =
      if buffer.ToString () = goal then
         ()
      else
         e.MoveNext () |> ignore
         let c = e.Current
         printf "%c" c
         if buffer.Length >= goalSize then
            buffer.Remove (0, 1) |> ignore
         loop (buffer.Append (c))
   loop (new System.Text.StringBuilder ())

search "twitter" Utility.defaultState