strange behaviour

David Sabel
Mon, 11 Aug 2003 13:09:29 +0200

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???


----- Original Message -----
From: "Simon Marlow" <>
To: "David Sabel" <>; "Jon Fairbairn"
Cc: <>
Sent: Monday, August 11, 2003 12:40 PM
Subject: RE: strange behaviour

> > 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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