This is beautiful work! I see small shades of inspiration from the .Net TPL[1] and the more recent async-await paradigm[2] in C#. But, this is way more concise and beautiful implementation :-)

[1] - http://msdn.microsoft.com/en-us/library/vstudio/system.threa...

[2] - http://msdn.microsoft.com/en-us/library/vstudio/hh191443.asp... and http://msdn.microsoft.com/en-us/library/vstudio/hh156528.asp...