2009/08/14 John A. De Goes
Hmmm, my point (perhaps I wasn't clear), is that different effects have different commutability properties. In the case of a file system, you can commute two sequential reads from two different files.
I think this is a bad example -- it's not something that's safe in general and discredits your idea. How would the compiler even know that two files are not actually the same file? However, the idea that a programmer can specify safely commuting effects is worthwhile. One could operate in a "different files are different" IO monad where the compiler assumes that reads on files with different names are commutable.
This has no effect on the result of the computation, assuming no interference from other programs -- and if there _is_ interference from other programs, then guarantees go out the window, _with or without_ commuting.
Well, yes -- which sounds like, there are no guarantees in general. Something that works half the time leaves you with two responsibilities -- the old responsibility of the work you did when you didn't have it and the new responsibility of knowing when it applies and when it doesn't. -- Jason Dusek