SelectMany

using System;
using System.Threading.Tasks;

namespace SelectMany
{
	public static class SelectManyExtensions
	{
		public static Func<TResult> SelectMany<T1, T2, TResult>( this Func<T1> f1, Func<T1, Func<T2>> selector,
			Func<T1, T2, TResult> resultSelector )
		{
			return () =>
			{
				var v1 = f1();
				var f2 = selector( v1 );
				var v2 = f2();
				return resultSelector( v1, v2 );
			};
		}

		public static async Task<TResult> SelectMany<T1, T2, TResult>( this Task<T1> t1, Func<T1, Task<T2>> selector,
			Func<T1, T2, TResult> resultSelector )
		{
			var v1 = await t1;
			var t2 = selector( v1 );
			var v2 = await t2;
			return resultSelector( v1, v2 );
		}

		public static TResult? SelectMany<T1, T2, TResult>( this T1? n1, Func<T1, T2?> selector,
			Func<T1, T2, TResult> resultSelector )
			where T1 : struct
			where T2 : struct
			where TResult : struct
		{
			if ( n1 == null )
			{
				return null;
			}

			var v1 = n1.Value;

			var n2 = selector( v1 );
			if ( n2 == null )
			{
				return null;
			}

			var result = resultSelector( v1, n2.Value );
			return result;
		}

		public static Lazy<TResult> SelectMany<T1, T2, TResult>( this Lazy<T1> l1, Func<T1, Lazy<T2>> selector,
			Func<T1, T2, TResult> resultSelector )
		{
			return new Lazy<TResult>( () =>
			{
				var v1 = l1.Value;
				var l2 = selector( v1 );
				return resultSelector( v1, l2.Value );
			} );
		}
	}

	public class Tests
	{
		public Func<int> F1 = () => 1;
		public Func<int> F2 = () => 2;

		public void Func()
		{
			var z = from x in F1
					from y in F2
					select x + y;
		}

		public void Tasks()
		{
			var z = from x in Task.FromResult( 1 )
					from y in Task.FromResult( 2 )
					select x + y;
		}

		public int? N1 = 1;
		public int? N2 = 2;

		public void Nullables()
		{
			var z = from x in N1
					from y in N2
					select x + y;
		}

		public Lazy<int> L1 = new Lazy<int>( () => 1 );
		public Lazy<int> L2 = new Lazy<int>( () => 2 );

		public void Lazies()
		{
			var z = from x in L1
					from y in L2
					select x + y;
		}
	}
}