传播可选参数 [英] Propagating optional arguments

问题描述

以下代码无法编译。

type A(?arg) =
  member __.Arg : string option = arg

type B(?arg) =
  inherit A(arg) //ERROR expected type string but has type 'a option

我认为这是因为必须提供该选项的基础类型的实例，并且编译器处理传递 Some / 无基于语法。

I assume this is because an instance of the underlying type of the option must be provided, and the compiler handles passing Some/None based on syntax.

假设我的假设已被正确假设，是否存在这个解决方法？是否可以传播可选参数？

Assuming my assumption has been correctly assumed, is there a workaround for this? Is it possible to propagate optional arguments?

推荐答案

F＃规范8.13.5方法成员的可选参数

F# spec 8.13.5 Optional arguments to method members

调用者可以使用以下技术指定可选参数的值：

Callers may specify values for optional arguments by using the following techniques:

• 按名称，例如arg2 = 1.


• 通过按名称传播现有可选值，例如？arg2 = None或？arg2 = Some（3 ）或？arg2 = arg2。这在构建一个将可选参数传递给另一个的方法时非常有用。



• 使用与位置匹配的普通，未命名参数。

  • By name, such as arg2 = 1.
  • By propagating an existing optional value by name, such as ?arg2=None or ?arg2=Some(3) or ?arg2=arg2. This can be useful when building one method that passes optional arguments on to another.

  • By using normal, unnamed arguments matched by position.

    type A(?arg) =
        member __.Arg : string option = arg
    
    type B(?arg) =
        inherit A(?arg = arg) 
    
    printfn "1. %A" (B()).Arg // None
    printfn "2. %A" (B("1")).Arg // Some "1"
    
    printfn "3. %A" (A()).Arg // None
    printfn "4. %A" (A("1")).Arg // Some "1"
    

  
  

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