Monad implementation for Lazy<T>

lazymonad.cs

// See also: http://davesquared.net/2012/12/lazy-monad-csharp.html

public static class LazyExtensions
{
    public static Lazy<TResult> Select<T, TResult>(this Lazy<T> value, Func<T, TResult> selector)
    {
        return new Lazy<TResult>(() => selector(value.Value));
    }

    public static Lazy<TResult> SelectMany<T, TResult>(this Lazy<T> value, Func<T, Lazy<TResult>> selector)
    {
        return SelectMany(value, selector, (a, b) => b);
    }

    public static Lazy<TResult> SelectMany<T, TA, TResult>(this Lazy<T> value, Func<T, Lazy<TA>> selector, Func<T, TA, TResult> resultSelector)
    {
        return new Lazy<TResult>(() =>
                                 {
                                     var first = value.Value;
                                     var second = selector(first).Value;
                                     return resultSelector(first, second);
                                 });
    }
}


public class LazyMonadTests
{
    [Test]
    public void Map()
    {
        var lazyInt = new Lazy<int>(() => 42);

        var newLazy = lazyInt.Select(x => (x * 2).ToString());

        Assert.AreEqual("84", newLazy.Value);
    }

    [Test]
    public void MapDefersExecution()
    {
        var firstCounter = 0;
        var secondCounter = 0;

        var first = new Lazy<int>(() => { firstCounter++; return 10; });
        var second = first.Select(x => { secondCounter++; return x.ToString(); });

        Assert.AreEqual("10", second.Value);
        Assert.AreEqual("10", second.Value, "subsequent calls keep same value");
        Assert.AreEqual(1, firstCounter, "init first lazy once");
        Assert.AreEqual(1, secondCounter, "init second lazy once");
    }

    [Test]
    public void SelectMany()
    {
        var first = new Lazy<int>(() => 42);
        var second = new Lazy<string>(() => "nyan");

        var third = from x in first
                    from str in second
                    select str + " cat " + x;

        Assert.AreEqual("nyan cat 42", third.Value);
    }

    [Test]
    public void SelectManyDefersExecution()
    {
        var firstCounter = 0;
        var secondCounter = 0;
        var thirdCounter = 0;
        var first = new Lazy<int>(() => { firstCounter++; return 10; });
        var second = new Lazy<string>(() => { secondCounter++; return "nyan"; });

        var third = from x in first
                    from str in second
                    let temp = thirdCounter++
                    select str + " cat " + x;

        Assert.AreEqual("nyan cat 10", third.Value);
        Assert.AreEqual(1, firstCounter);
        Assert.AreEqual(1, secondCounter);
        Assert.AreEqual(1, thirdCounter);
    }
}