复制工会案例但在F＃中使用不同的值

Question

在F＃中，我想基于现有实例构造一个有区别的联合的实例（正确的术语？）。例如：

type union Currency =
    | Dollar of int
    | Euro of int

let lowPrice = Dollar 100 (* or, it could be *) let lowPrice = Euro 100
let highPrice = (* of the same union case as lowPrice but with value 200 *)

我可以使用哪些代码代替注释来创建该效果？

Answer 1

你可以做到

let highPrice =
    let n = 200
    match lowPrice with
    | Dollar _ -> Dollar n
    | Euro _ -> Euro n

但计量单位可能更好。

修改

或者，也许你想要

type MoneyType = Dollar | Euro
type Currency = Currency of MoneyType * int
let lowPrice = Currency(Dollar, 100)
let highPrice = 
    match lowPrice with
    | Currency(kind, _) -> Currency(kind, 200)

Answer 2

我认为对于这类问题，使用度量单位更合适 - 比如

[<Measure>] type Dollar
[<Measure>] type Euro

let lowprice = 100<Dollar>
let inline _highprice (newv:int) (oldv:int<'t>) : int<'t> = 
    LanguagePrimitives.Int32WithMeasure newv
let highprice = _highprice 200 lowprice

转换功能有点棘手，但它会做你想做的事情

Answer 3

您可以使用Reflection基于同一联合案例的现有值创建新的联合案例值。为了实现这一点，只需将实例成员Same添加到您的区分联合，它首先从实例self派生特定的联合案例，然后通过相同的联合案例构造一个新实例，但现在填充{ {1}}：

newVal

现在将其应用于open Microsoft.FSharp.Reflection type Currency = | Dollar of int | Euro of int member self.Same newVal : Currency = FSharpValue.MakeUnion(fst (FSharpValue.GetUnionFields(self, typeof<Currency>)), [|newVal|]) |> unbox 以下的值

lowPrice

你会得到let lowPrice = Euro(100) let highPrice = lowPrice.Same 200