传播可选参数

本文介绍了传播可选参数的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

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

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"
```

arg2