Task Extensions with Monad Laws

TaskExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace System.Threading.Tasks
{
    public class Workspace
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="Workspace"/> class.
        /// </summary>
        public Workspace()
        {
            0.Return()
             .Select(i => i + 10)
             .Where(i => i % 2 == 0)
             .SelectMany(i => i.Return())
             .Join(2.Return(), a => a, b => b, (a, b) => a + b)
             .Zip(4.Return(), (a, b) => a + b)
             .Apply(new Func<int, int>(i => i + 5).Return())
             .Do(i => Console.WriteLine("Result: " + i));
        }
    }

    public static class TaskExtensions
    {
        public static Task<TA> Return<TA>(this Task task, TA value)
        {
            return task.ContinueWith(t => value);
        }

        public static Task<TA> Return<TA>(this TA value)
        {
            return Task.FromResult(value);
        }

        public static Task<TA> Do<TA>(this Task<TA> source, Action<TA> action)
        {
            return source.SelectMany(a => { action(a); return source; });
        }

        public static Task<TB> Apply<TA, TB>(this Task<TA> source, Task<Func<TA, TB>> taskFunc)
        {
            return source.SelectMany(a => taskFunc.Select(func => func(a)));
        }

        public static Task<TB> SelectMany<TA, TB>(this Task<TA> source, Func<TA, Task<TB>> selector)
        {
            return source.ContinueWith(xTask => selector(xTask.Result)).Unwrap();
        }

        public static Task<TB> Select<TA, TB>(this Task<TA> source, Func<TA, TB> selector)
        {
            return source.SelectMany(x => selector(x).Return());
        }

        public static Task<TA> Where<TA>(this Task<TA> source, Func<TA, bool> predicate)
        {
            return source.SelectMany(x => predicate(x)
                ? x.Return()
                : throw new OperationCanceledException("Value doesn't pass predicate: " + x));
        }

        public static Task<TC> Zip<TA, TB, TC>(
            this Task<TA> source,
            Task<TB> other,
            Func<TA, TB, TC> selector)
        {
            return source.SelectMany(a => other.Select(b => selector(a, b)));
        }

        public static Task<TD> Join<TA, TB, TC, TD>(
            this Task<TA> source,
            Task<TB> inner,
            Func<TA, TC> outerKeySelector,
            Func<TB, TC> innerKeySelector,
            Func<TA, TB, TD> resultSelector)
        {
            Task.WaitAll(source, inner);
            return source.SelectMany(ta => inner.SelectMany(tb =>
            {
                TC outerC = outerKeySelector(ta);
                TC innerC = innerKeySelector(tb);
                return EqualityComparer<TC>.Default.Equals(outerC, innerC)
                    ? Task.FromResult(resultSelector(ta, tb))
                    : throw new OperationCanceledException("Not Equal: " + outerC + " != " + innerC);
            }));
        }

        public static Task<TD> GroupJoin<TA, TB, TC, TD>(
            this Task<TA> source,
            Task<TB> inner,
            Func<TA, TC> outerKeySelector,
            Func<TB, TC> innerKeySelector,
            Func<TA, Task<TB>, TD> resultSelector)
        {
            return source.SelectMany(a => 
                resultSelector(
                    a, 
                    inner.Where(
                        b => EqualityComparer<TC>.Default.Equals(outerKeySelector(a), 
                        innerKeySelector(b))))
                .Return());
        }
    }
}

Nice!

I'd do 1 small modification on line 36. Instead of starting a new thread via Task.Factory.StartNew, I'd use Task.FromResult(value). Unless you really -want- a new thread for this?

Yeah, that would be better; Thanks