[Haskell-cafe] Re: [Haskell-beginners] laziness and optimization
aneumann at inf.fu-berlin.de
Sat Mar 21 10:56:28 EDT 2009
You should not rely on the compiler to spot such things. As far as I
know GHC doesn't do automatic caching (in many cases that would hurt
performance, I think). Have a look at http://haskell.org/haskellwiki/
Am 21.03.2009 um 14:02 schrieb Michael Mossey:
> I understand a bit about the concept of "lazy evaluation." I think
> of that as saying an imperative language would always make one
> evaluation, whereas Haskell might make 0 evaluations. I have
> another similar situation where an imperative language would make N
> evaluations of the same expression, and I would like Haskell to
> make only 1.
> This is the situation: the graphical score editor displays
> "LayoutItems." A LayoutItem can be a single displayed entity, like
> a round notehead, or it can be composed of several entities.
> A common situation in my code is the need to determine the size and
> shape of a LayoutItem. For a fundamental item, this can be looked
> up in a table or read from the font properties. For a composite
> item, some computation is required: the code must determine the
> positions of each sub-item and compute the bounds of a shape
> containing all of them.
> It's this latter computation, finding the bounds of a composite
> item, which might come up multiple times. Consider that I ask for
> the bounds of a composite-composite item (a composite item composed
> of composite items). It will run the computation associated with
> each composite sub-item, even though it is very likely I already
> make that computation when I first constructed and placed that sub-
> In an imperative language, one might cache values for later lookup.
> This raises the problem of keeping the cache current to the current
> So I'm wondering to what extent the haskell compiler recognizes
> computations it's done before. In a purely functional language this
> should be pretty easy, right? If it sees the same expression, it
> knows it will have the same value. That's my understanding, so far.
> Beginners mailing list
> Beginners at haskell.org
-------------- next part --------------
A non-text attachment was scrubbed...
Size: 194 bytes
Desc: Signierter Teil der Nachricht
Url : http://www.haskell.org/pipermail/haskell-cafe/attachments/20090321/04a71a26/PGP.bin
More information about the Haskell-Cafe