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假设我们已经有啤酒与威士忌两个模块:

module Beer = struct

      type t = BEER 

      let pour () =  

          let () = Printf.printf "... a nice head ... " in BEER

      let consume t =Printf.printf "Ha! Nothing like a good beer to quench the thirst\n"

end

module Whisky = struct

      type t = WHISKY 

      let pour () =  

          let () = Printf.printf "... 2 fingers of this ...." in WHISKY

      let consume _ = Printf.printf "... Ha! Piss of the Gods!\n"

end

如果现在想实现一个酒鬼模块, 这个酒鬼有时喜欢啤酒,有时又喜欢威士忌. 注意: 啤酒与威士忌模块实现了一样的函数(pour/consume). 所以我们可以定义一个如下的"模块类型":

module type Beverage = sig

      type t 

      val pour :unit -> t 

      val consume : t ->unit

end

然后,使用 functor建一个酒鬼模块:

module Alkie =functor (B:Beverage) -> struct

      let rec another = function       | 0 -> ()        | i -> let b = B.pour () in           let () = B.consume b in           another (i-1)  end

使用:

let menu = Hashtbl.create 3 

let () = Hashtbl.add menu "Beer" (module Beer : Beverage) let () = Hashtbl.add menu "Whisky" (module Whisky : Beverage) let () = let favourite =ref "Whisky" in     let () = Arg.parse         [("--beverage", Arg.Set_string favourite,Printf.sprintf "which beverage to use (%s)" !favourite)]       (fun s -> raise (Arg.Bad s))       ("usage:")     in       let module B = (val (Hashtbl.find menu !favourite) : Beverage)in

      let module  AlkieB = Alkie(B) in

      let () = AlkieB.another 3 in ();;

将上面代码存入文件a.ml,然后:

>ocamlc a.ml

>./a.out

... 2 fingers of this ....... Ha! Piss of the Gods! ... 2 fingers of this ....... Ha! Piss of the Gods! ... 2 fingers of this ....... Ha! Piss of the Gods!

>./a.out --beverage Beer

... a nice head ... Ha! Nothing like a good beer to quench the thirst ... a nice head ... Ha! Nothing like a good beer to quench the thirst ... a nice head ... Ha! Nothing like a good beer to quench the thirst

当然你也可以用类来实现, 只需将啤酒与威士忌对象注入酒鬼就可以了. 附代码:

classtype beverage = object

    method consume : unit -> unit end class beer = object (self :#beverage)     method consume () = Printf.printf "...drank beer\n" end let make_beer () =     let () = Printf.printf "pouring beer\n" in         let b = new beer in             (b :> beverage)  (*b是beverage的子对象,函数返回的类型是beverage *) class whisky = object(self :#beverage)     method consume () = Printf.printf "...drank whisky\n" end let make_whisky () =     let ()  = Printf.printf "pouring whisky\n" in         let b = new whisky in             (b :> beverage) class alkie p = object (self)     method another n =        if n = 0          then ()          else             let b = p () in                 let () = b#consume ()in                     self#another (n-1) end let menu = Hashtbl.create 3

let () = Hashtbl.add menu "Beer" make_beer

let () = Hashtbl.add menu "Whisky" make_whisky

let () =

    let favourite = ref "Whisky" in

    let () = Arg.parse

        [("--beverage", Arg.Set_string favourite,Printf.sprintf "which beverage to use (%s)" !favourite)]

        (fun s -> raise (Arg.Bad s))

        ("usage:") 

    in

        let p = Hashtbl.find menu !favouritein

        let a = new alkie pin   (* 注入对象 *)

            a#another 3 ;;

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