F# 从并集大小写中提取值

在一个Fsharp应用程序中，我定义了几个联合案例类型

type A = A of String
type B = B of String
type C = C of String

我想定义一个函数来从union case实例中提取值

let getValue( ctor: String-> 'a) = 
   ...implementation here

有没有办法完成这样的任务？ 谢谢。

假设您有：

type A = A of string
type B = B of string
type C = C of string

let a = A "hello"
let b = B "world"
let c = C "!"

提取这些值的方法有很多，以下是一些：

单个展开器

let getValueA (A v) = v
let getValueB (B v) = v
let getValueC (C v) = v

let valueOfA = getValueA a
let valueOfB = getValueB b
let valueOfC = getValueC c

type IWrapped<'a> =
    abstract getValue: 'a

type A = A of string with interface IWrapped<string> with member t.getValue = let (A x) = t in x        
type B = B of string with interface IWrapped<string> with member t.getValue = let (B x) = t in x
type C = C of string with interface IWrapped<string> with member t.getValue = let (C x) = t in x

let valueOfA = (a :> IWrapped<string>).getValue

方法重载

type T =
    static member getValue (A v) = v
    static member getValue (B v) = v
    static member getValue (C v) = v

let valueOfA = T.getValue a
let valueOfB = T.getValue b
let valueOfC = T.getValue c

type GetValue = GetValue with
    static member ($) (GetValue, (A v)) = v
    static member ($) (GetValue, (B v)) = v
    static member ($) (GetValue, (C v)) = v

let inline getValue x : string = GetValue $ x

let valueOfA = getValue a
let valueOfB = getValue b
let valueOfC = getValue c

功能过载

type T =
    static member getValue (A v) = v
    static member getValue (B v) = v
    static member getValue (C v) = v

let valueOfA = T.getValue a
let valueOfB = T.getValue b
let valueOfC = T.getValue c

type GetValue = GetValue with
    static member ($) (GetValue, (A v)) = v
    static member ($) (GetValue, (B v)) = v
    static member ($) (GetValue, (C v)) = v

let inline getValue x : string = GetValue $ x

let valueOfA = getValue a
let valueOfB = getValue b
let valueOfC = getValue c

反射

open Microsoft.FSharp.Reflection
let getValue a =  
    FSharpValue.GetUnionFields (a, a.GetType())
        |> snd
        |> Seq.head
        :?> string

let valueOfA = getValue a
let valueOfB = getValue b
let valueOfC = getValue c

重新设计您的DU

type A = A
type B = B
type C = C

type MyDU<'a> = MyDU of 'a * string

let a = MyDU (A, "hello")
let b = MyDU (B, "world")
let c = MyDU (C, "!"    )

let getValue (MyDU (_, v)) = v

let valueOfA = getValue a
let valueOfB = getValue b

类型A=A
B型=B型
C型=C型
键入MyDU.IWrapped）.getValue

我看不到一种明显的方法可以不经过反射就做到这一点，尽管可能存在一种方法。我认为最近也有类似的方法，否则您可以扩展示例代码吗？字符串的类型A不是有效的F#代码。@Gustavo我已更正了问题中的语法错误。我想做的是不要使用歧视工会。谢谢你的回答。我一直在寻找一个没有很多模式匹配的函数来从不同类型中提取值。看来反射是唯一的解决方案？我有点惊讶，F#不支持这个看似简单的场景。不，它不支持，而且有道理，DU可以用多种方式定义，而不仅仅是一个大小写、单个字符串参数。如果你所有的DU都有相同的结构，为什么不改变你的设计来反映这一点呢？看到更新了。我在关注这个特殊的博客，只是觉得有比接口更好的方法来实现“值”功能。在这种情况下，你让他们实现一个接口。继续阅读博客，它解释了如何实现接口。我确实看到了实现接口是如何工作的，但这是很多重复的代码。