module ArgParse

type ParseDef<'T> = {
    initState : 'T
    switches  : Map<string, ('T -> 'T)>
    values    : Map<string, (string -> 'T -> 'T)>
    other     : (string -> 'T -> 'T)
    error     : (string -> 'T -> 'T)
  } with

  member this.Parse(argv:string[]) =
    let isSwitch f = this.switches.ContainsKey f
    let isValue f = this.values.ContainsKey f
    let isEither f = isSwitch f || isValue f
    let rec parse args state =
      match args with
      | flag        :: []   when flag |> isEither -> state |> this.error flag        |> parse []
      | flag        :: tail when flag |> isSwitch -> state |> this.switches.[flag]   |> parse tail
      | flag :: arg :: tail when flag |> isValue  -> state |> this.values.[flag] arg |> parse tail
      |         arg :: tail                       -> state |> this.other arg         |> parse tail
      |                []                         -> state

    this.initState |> parse (List.ofArray argv)