unsafePerformIO safety.

[David Brown]

I've noticed quite a few pages referencing constructs such as:

  var :: MVar ([Foo])
  var = unsafePerformIO (newMVar ([]))

and the likes.  Is there a danger of different uses of 'var' getting
new MVars instead of all sharing one.

Having a reliable way to create a piece of global state would be very
convenient.

Dave