如何参数化此函数以接受函数和 TextIO.closeOut 作为输入？答案

【问题标题】：How can I parametrize this function to accept a function and TextIO.closeOut as input?如何参数化此函数以接受函数和 TextIO.closeOut 作为输入？
【发布时间】：2020-04-22 07:11:33
【问题描述】：

是否可以将此辅助函数参数化以接受TextIO.closeOut outstream 和readFileList xs outstream n 作为输入？或者我必须在 TextIO 中使用那个 append 函数来获得一个不那么难看的函数体吗？是否可以匹配它们的类型？

两者本质上是一样的，唯一的区别是NONE情况下的第二个返回函数。

    fun readFileList (x::xs) outstream n =
    case xs of
        [] => (let
              val instream = TextIO.openIn x
              val readline = TextIO.inputLine instream
              fun aux readline n =
                      case readline of
                          NONE => (TextIO.closeIn instream; TextIO.closeOut outstream)
                        | SOME s => (TextIO.output(outstream, (getLineWriteCode s n));
                                     aux (TextIO.inputLine instream) (n + 1))
              in
                  aux readline n
              end)
      | _ => (let
             val instream = TextIO.openIn x
             val readline = TextIO.inputLine instream
             fun aux readline n =
                 case readline of
                     NONE => (TextIO.closeIn instream; readFileList xs outstream n)
                   | SOME s => (TextIO.output(outstream, (getLineWriteCode s n));
                                aux (TextIO.inputLine instream) (n + 1))
            in
                aux readline n
            end)

【问题讨论】：

标签： function functional-programming sml smlnj

【解决方案1】：

只需要使用部分应用程序。不知道为什么花了这么长时间才弄清楚，但它确实有效。我唯一想知道的是匿名 lambda 函数。我可以在里面放一个通配符之类的东西吗？还是这样就可以了？

fun readFileList (x::xs) n outstream =  
    let
        val part = readFileList xs n
        val instream = TextIO.openIn x
        val readline = TextIO.inputLine instream
        fun aux readline n function =
            case readline of
                NONE => (TextIO.closeIn instream; function outstream)
              | SOME s => (TextIO.output(outstream, (getLineWriteCode s n));
                         aux (TextIO.inputLine instream) (n + 1) (fn s => ()))
    in
    case xs of
        [] => (aux readline n TextIO.closeOut)
      | _ => (aux readline n part)
    end

【讨论】：