FastEndpoints usage with Ardalis.Result package

program.cs

using Ardalis.Result;
using FastEndpoints;
using FastEndpoints.Swagger;

var bld = WebApplication.CreateBuilder();
bld.Services
   .AddFastEndpoints()
   .SwaggerDocument();

var app = bld.Build();
app.UseAuthorization()
   .UseFastEndpoints()
   .UseSwaggerGen();
app.Run();

sealed class Request
{
    public bool IsHappyPath { get; set; }
}

sealed class Response
{
    public string Message { get; set; }
}

sealed class TestEndpoint : Endpoint<Request, Response>
{
    public override void Configure()
    {
        Get("test/{IsHappyPath}");
        AllowAnonymous();
    }

    public override async Task HandleAsync(Request r, CancellationToken c)
    {
        var result = HelloService.SayHello(r.IsHappyPath);

        await this.SendResponse(result, r => new Response
        {
            Message = r.Value
        });
    }
}

sealed class HelloService
{
    public static Result<string> SayHello(bool isHappyPath)
    {
        if (!isHappyPath)
        {
            return Result<string>.Invalid(new List<ValidationError>
            {
                new ValidationError
                {
                    Identifier = nameof(Request.IsHappyPath),
                    ErrorMessage = "I am unhappy!"
                }
            });
        }
        return Result<string>.Success("hello world...");
    }
}

static class ArdalisResultsExtensions
{
    public static async Task SendResponse<TResult, TResponse>(this IEndpoint ep, TResult result, Func<TResult, TResponse> mapper) where TResult : Ardalis.Result.IResult
    {
        switch (result.Status)
        {
            case ResultStatus.Ok:
                await ep.HttpContext.Response.SendAsync(mapper(result));
                break;

            case ResultStatus.Invalid:
                result.ValidationErrors.ForEach(e =>
                    ep.ValidationFailures.Add(new(e.Identifier, e.ErrorMessage)));

                await ep.HttpContext.Response.SendErrorsAsync(ep.ValidationFailures);
                break;
        }
    }
}