DiffArray Performance

[Simon Marlow]

> >This is one good reason they have to be protected by MVars
> 
> Forgive my stupidity, but arn't the MVar operations 
> (takeMVar, putMVar)
> IO operation, therefore the locks must be in the IO monad, therefore
> the code acting on the DiffArray should be in the IO monad... 
> otherwise they can't use the MVar calls? 

DiffArray is an example of a good use for unsafePerformIO: it uses
imperative operations to implement a pure API.  The DiffArray is made of
mutable objects under the hood, but as far as the programmer is
concerned it behaves just like a pure Array.

Cheers,
	Simon