strange behaviour

Simon Marlow
Mon, 11 Aug 2003 11:40:14 +0100

> My point here is to know, what's the reason for the different
> behaviour, rather than discussing the
> correctness of using unsafePerformIO.

The reason is this:  GHC uses a lazy evaluation strategy, as opposed to
fully-lazy.  Under lazy evaluation, the unsafePerformIO expression in
your example will be evaluated each time main is invoked.

When -O is turned on, GHC performs some transformations on the code that
have the effect of changing the evaluation strategy to "almost
fully-lazy".  That is, there will be some more sharing, but not
necessarily as much as you would get in a compiler that implements
fully-lazy evaluation.

Indeed, even without -O, you might get a bit more sharing than you would
under pure lazy evaluation.

Of course, you should never write any code that depends on any of this.