Ok, thanks for all your comments.
But I think that the examples haven't much to do
with the core-to-core optimisations, because I've also used
a modified version --modified be me ;-) -- of ghc5.04.3,
where only a modified simplifying is done. The modifications
are small: Do Inlining only for values, do only trivial eta-expansion
(not for case, let and so on) , and some other "small" things.
But with this modified compiler, the same problem comes up.
I didn't modify things after simplifying, espcially generating STG-Code
or transforming (optimising) this code.
I think it has something to do with the STG-Code generation or
with normal code generation, and it depends on the fact, if
the function is exported or not.
Any hints in this direction???
Thanks,
David
----- Original Message -----
From: "Simon Marlow"
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.
Cheers, Simon
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