Minimal

gistfile1.txt

using System;
using System.Threading.Tasks;

namespace FunctionalExperimentation.SimpleRefactorings.Medium.FuncTaskCovarinat.Minimal
{
    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 GetOrThrowIfFailed<T>(this FuncTask<T> @this, string message) where T : IWithResult
        {
            var value = @this(); // this might also throw exception from Task. 
            return value?.IsSuccesfull ?? false ? 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().GetOrThrowIfFailed("failed to get");
        }

    }

    public delegate T FuncTask<out T>();

}