[Haskell-cafe] unsafeDestructiveAssign?

Job Vranish jvranish at gmail.com
Tue Aug 11 12:45:29 EDT 2009

Ga! Before to many people start flooding me responses of "This is really
dumb idea don't do it!" I would like to clarify that for the most part

I am well aware of the usual ST/IORefs as the usual solutions to data
mutability in haskell.
I very very much understand purity, and why it is a good thing, and why we
should try to stay away from IO and ST as much as possible.
I am very much away that even if I had such a function that it will probably
break everything.
I am not just trying to make things run faster.

What I am trying to do is hyper unusual and I really do need an
unsafeHorribleThings to do it.

- Job

On Tue, Aug 11, 2009 at 12:14 PM, Max Desyatov
<explicitcall at googlemail.com>wrote:

> Job Vranish <jvranish at gmail.com> writes:
> > Does anybody know if there is some unsafe IO function that would let me
> do destructive assignment?
> > Something like:
> >
> > a = 5
> > main = do
> >   veryUnsafeAndYouShouldNeverEveryCallThisFunction_DestructiveAssign a 8
> >   print a
> >> 8
> Aren't StateT or IORefs the exact thing you are looking for?
> > I'm also looking for a way to make actual copies of data.
> > so I could do something like this:
> >
> > a = Node 5 [Node 2 [], Node 5 [a]]
> > main = do
> >   b <- makeCopy a
> >   veryUnsafeAndYouShouldNeverEveryCallThisFunction_DestructiveAssign b
> (Node 0 [])
> >   -- 'a' is unchanged
> >
> > It would be even more fantastic, if the copy function was lazy.
> > I think the traverse function might actually make a copy, but I would be
> happier with something more general (doesn't
> > require membership in traversable), and more explicit (was actually
> designed for making real copies).
> Same thing, IORefs could help you.  Anyway, I can't imagine any case
> where veryUnsafeAndYouShouldNeverEveryCallThisFunction_DestructiveAssign
> could be useful with its imperative semantics as you've described.  The
> point is that Haskell is pure language and I use it because of this
> feature (not only because of this, to be exact).  I don't want to use
> any library code that brokes pure semantics and launches nuclear bombs
> behind the IO monad.  GHC is smart enough these days to do all optimised
> destructive assignments, copies and all that imperative stuff and there
> are plenty of other ways to get a performance boost without
> unsafeHorribleThings.
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