[Haskell-cafe] Linking and unsafePerformIO

Jules Bean jules at jellybean.co.uk
Tue Oct 14 09:05:02 EDT 2008

Mauricio wrote:
> Hi,
> If I have a Haskell wrapper (with unsafe...)
> over a function that's never going to return
> different values and is always side-effect
> free, but can change depending on compile time
> options of its library; my program is running,
> and then the version of my library is updated
> by my distribution smart instalation system,
> which does update versions of libraries in
> use; is it possible that I get a wrong behavior
> of my program?
> I do not understand enough about package
> management to understand how running programs
> or libraries are updated, and less about how
> linking works between Haskelll and libraries
> on other languages, so I don't know if my
> program is guaranteed to stay with a single
> version of a library for each run.
> (Sure this is a weird situation, but I do
> like to think about worst cases.)

In practice that is fine, with current RTSes and so on.

In principle it's not fine. A 'constant' should be constant over all 
time, not just constant over a particular library version or sub-version 
or a particular program invocation or OS or....

Who knows, maybe some future haskell runtime will be able to perform the 
trickery you describe and will cause this to break ;)


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