从字符串创建有区别的联合用例 [英] Create Discriminated Union Case from String

问题描述

我正在尝试从字符串创建DU案例.我看到的唯一方法是通过Microsoft.FSharp.Reflection.FSharpType.GetUnionCases枚举DU的情况，然后选择与字符串匹配的UnionCase(通过使用.Name)，然后通过使用<从中得出实际的DU情况c3>.

I'm trying to create DU cases from strings. The only way I can see doing this is by enumerating over the DU cases via Microsoft.FSharp.Reflection.FSharpType.GetUnionCases and then picking the UnionCase that matches the string (by using .Name) and then making the actual DU case out of that by using FSharpValue.MakeUnion.

难道没有一种更简单/更优雅的方法吗?在我的场景中，我有一个带有数百个关键字案例的DU.我必须从文件中读取字符串(关键字)，并从中找出类型.我通过将案例放入地图中进行了一些优化"，但我希望这样做会有更好的方法.

Isn't there an easier/more elegant way of doing this? In my scenario I have a DU with a couple of hundred cases for keywords. I have to read the strings (keywords) from a file and make the types out of them. I did some "optimization" by putting the cases into a Map but I was hoping there'd be a better way of doing this.

例如，我有以下内容:

type Keyword = 
    | FOO
    | BAR
    | BAZ
    | BLAH

let mkKeywords (file: string) =
    use sr = new StreamReader(file)

    let caseMap = 
        FSharpType.GetUnionCases(typeof<Keyword>)
        |> Array.map (fun c -> (c.Name, FSharpValue.MakeUnion(c, [||]) :?> Keyword))
        |> Map.ofArray

    [
        while not sr.EndOfStream do
            let l = sr.ReadLine().Trim()

            match caseMap.TryFind l with
            | Some c -> yield c
            | None -> failwith <| "Could not find keyword: " + l
    ] 

推荐答案

我发现了此方便的代码段.

I found this handy code snippet...

open Microsoft.FSharp.Reflection

let toString (x:'a) = 
    match FSharpValue.GetUnionFields(x, typeof<'a>) with
    | case, _ -> case.Name

let fromString<'a> (s:string) =
    match FSharpType.GetUnionCases typeof<'a> |> Array.filter (fun case -> case.Name = s) with
    |[|case|] -> Some(FSharpValue.MakeUnion(case,[||]) :?> 'a)
    |_ -> None

...这使得轻松将两行代码附加到任何DU ...

... which makes it easy to tack on two lines of code to any DU...

type A = X|Y|Z with
    override this.ToString() = FSharpUtils.toString this
    static member fromString s = FSharpUtils.fromString<A> s

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