gistfile1.txt

using System;
using System.Threading.Tasks;

namespace FunctionalExperimentation.FuncTaskCovarinat.Minimal.DelegateAdapter
{
    public static partial class F
    {
        public static FuncTask<T> ToFuncTask<T>(this Func<Task<T>> @this) => 
            new FuncTask<T>(
                ()=>@this().GetAwaiter().GetResult()
             );
        public static T GetOrThrowIfNull<T>(this FuncTask<T> @this, string message)
        {
            var value = @this(); // this might also throw exception from Task. 
            return value ?? throw new Exception(message);
        }
  
    }
    public interface IWithResult
    {
        public bool IsSuccesfull { get; }
    }
    public class SomeResult : IWithResult
    {
        public bool IsSuccesfull => true;
    }

    public class FuncTaskCovarinat
    {
        public static async Task RunAsync()
        {
            Func<Task<SomeResult>> GetSomethingLazyAsync() => () => Task.FromResult(new SomeResult());

            //Func<Task<IWithResult>> t = GetSomethingLazyAsync(); 
            //Error CS0029  Cannot implicitly convert type 'System.Func<System.Threading.Tasks.Task<FunctionalExperimentation.Practical.FuncTaskCovarinat.SomeResult>>'
            //    to 'System.Func<System.Threading.Tasks.Task<FunctionalExperimentation.Practical.FuncTaskCovarinat.IWithResult>>'

            //ok this is covariant on T
            FuncTask<IWithResult> lazyResult =  GetSomethingLazyAsync().ToFuncTask();

            SomeResult result = GetSomethingLazyAsync()
                                    .ToFuncTask()
                                    .GetOrThrowIfNull("failed to get");
        }
  
    }

    public delegate T FuncTask<out T>();
   
}