扁平地图歧视工会

我正在使用Github上的一个名为OneOf的库。

基本上你有一种类型可以是许多类型中的一种，但是以一种静态安全的方式。

下面是我想要实现的一个例子。
我有A<T1, T2, T3, T4>型和B<T1, T2, T3>型，我想将B型"flapmap"转换为A型。因为A型可以接受来自B的任何一个T，这应该是可能的。

编译器强迫我在赋值给A之前从B中提取每个T，正如你可以看到下面愚蠢的x => x lambdas。

我也不想以A<B<T1, T2, T3>, T4>那样的结尾。
有人能想到这些OneOf类型的SelectMany吗?

using OneOf;
using System;
using System.IO;
namespace ScratchPad
{
    class Program
    {
        struct BadRequest { }
        struct Error { }
        struct NotFound { }
        static void Main(string[] arg)
        {
            string result = GetFile(@"c:'data'foo.txt").Match(
                text => text,
                badRequest => "filepath cannot be null",
                notFound => "filepath does not exist",
                error => "an error occurred"
            );
            Console.WriteLine(result);
        }
        static OneOf<string, BadRequest, NotFound, Error> GetFile(string filepath)
        {
            OneOf<string, BadRequest, NotFound, Error> response = new BadRequest();
            if (filepath != null)
            {
                // How can I make the type from ReadText() automatically convert to the type of the response local variable, without having to write these silly lambda?
                response = ReadText(filepath).Match<OneOf<string, BadRequest, NotFound, Error>>(x => x, x => x, x => x); 
            }
            return response;
        }
        static OneOf<string, NotFound, Error> ReadText(string filepath)
        {
            OneOf<string, NotFound, Error> response = new NotFound();
            try
            {
                if (File.Exists(filepath))
                {
                    response = File.ReadAllText(filepath);
                }
            }
            catch
            {
                response = new Error();
            }
            return response;
        }
    }
}

public static class OneOfExtensions
{
  public static OneOf<T1, T2, T3, T4> ConvertOneOf<T1, T2, T3, T4>(this OneOf<T1, T2, T3> oneOf)
  {
    return oneOf.Match<OneOf<T1, T2, T3, T4>>(x => x, x => x, x=> x);
  }
  public static OneOf<T1, T2, T3, T4> ConvertOneOf<T1, T2, T3, T4>(this OneOf<T1, T2, T4> oneOf)
  {
    return oneOf.Match<OneOf<T1, T2, T3, T4>>(x => x, x => x, x=> x);
  }
  public static OneOf<T1, T2, T3, T4> ConvertOneOf<T1, T2, T3, T4>(this OneOf<T1, T3, T4> oneOf)
  {
    return oneOf.Match<OneOf<T1, T2, T3, T4>>(x => x, x => x, x=> x);
  }
  public static OneOf<T1, T2, T3, T4> ConvertOneOf<T1, T2, T3, T4>(this OneOf<T2, T3, T4> oneOf)
  {
    return oneOf.Match<OneOf<T1, T2, T3, T4>>(x => x, x => x, x=> x);
  }
}