从F#函数多次退出 [英] Multiple Exits From F# Function

问题描述

我可以在C ++中轻松做到这一点(注意:我没有测试它的正确性-只是为了说明我正在尝试做的事情):

I could do this easily in C++ (note: I didn't test this for correctness--it's only to illustrate what I'm trying to do):

   const int BadParam = -1;
   const int Success = 0;

   int MyFunc(int param)
   {
      if(param < 0)
      {
         return BadParam;
      }

      //normal processing

      return Success;
   }

但是我不知道如何在F#的早期退出例程.我想做的是在输入错误的情况下退出该功能，但如果输入正常，则继续操作.我是不是在缺少F#的某些基本属性，还是因为我只是在学习FP而以错误的方式解决问题? failwith 是我唯一的选择吗?

But I cannot figure out how to exit a routine early in F#. What I want to do is to exit the function on a bad input but continue if the input is ok. Am I missing some fundamental property of F# or am I approaching the problem in the wrong way since I'm just learning FP? Is a failwith my only option here?

这是我到目前为止所掌握的内容，并且可以正常编译:

This is what I've got so far and it compiles ok:

   #light

   module test1

       (* Define how many arguments we're expecting *)
       let maxArgs = 2;;
       (* The indices of the various arguments on the command line *)
       type ProgArguments =
           | SearchString = 0
           | FileSpec = 1;;

       (* Various errorlevels which the app can return and what they indicate *)
       type ProgReturn =
           | Success = 0
           | WrongNumberOfArgumentsPassed = 1;;

       [<EntryPoint>]
       let main (args:string[]) =

           printfn "args.Length is %d" args.Length

           let ProgExitCode = if args.Length <> maxArgs then
                                   printfn "Two arguments must be passed"
                                   int ProgReturn.WrongNumberOfArgumentsPassed
                                   (* Want to exit "main" here but how? *)
                               else
                                   int ProgReturn.Success

           let searchstring, filespec  = args.[int ProgArguments.SearchString],args.[int ProgArguments.FileSpec];

           printfn "searchstring is %s" searchstring
           printfn "filespec is %s" filespec

           ProgExitCode;;

是否有FP处理这种情况的方法?

Is there an FP way of dealing with this sort of thing?

推荐答案

在我看来，匹配表达式是早期退出的F#类似物，用于指出错误条件并分别处理它们.对于您的示例，我会写:

In my opinion, match expressions are the F# analogue of early-exit for calling out erroneous conditions and handling them separately. For your example, I'd write:

 [<EntryPoint>]
 let main (args:string[]) =
     printfn "args.Length is %d" args.Length
     match args with
     | [| searchstring; filespace |] -> 
       // much code here ...
       int Success
     | _ -> printfn "Two arguments must be passed"
       int WrongNumberOfArgumentsPassed

这很好地分隔了错误情况.通常，如果需要从某事物的中间退出，请拆分函数，然后将错误情况放在match中.在功能语言中，小函数的数量实际上没有限制.

This separates the error case nicely. In general, if you need to exit from the middle of something, split functions and then put the error case in a match. There's really no limit to how small functions should be in a functional language.

顺便说一句，将区分联合用作整数常量集有点不可思议.如果您喜欢这种惯用语，请注意，在引用它们时不需要包括类型名称.

As an aside, your use of discriminated unions as sets of integer constants is a little weird. If you like that idiom, be aware that you don't need to include the type name when referring to them.

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