On Thu, Mar 08, 2007 at 04:32:01PM -0500, Brandon S. Allbery KF8NH wrote: > On Mar 8, 2007, at 16:27 , David Roundy wrote: > >The real issue for me is that DiffArrays only make any sense at all if > >you often update a subset of your array, which I never expect to do... > >so even if they were efficiently implemented to the point of zero > >overhead, they would gain me nothing, and that's almost certainly overly > >optimistic. > > But that's not my understanding of what's *supposed* to be > happening: the point of DiffArrays is is not optimizing partial > updates, it's optimizing the head at the expense of any (by intent > few or none) references that might be held elsewhere. As such, if > there are no such references the DiffArray *should* get you cheap in- > place (destructive) updates.
Ah, I see. Yes, I misunderstood. When I read about DiffArrays (ages ago), I thought they stored the old array plus differences, a mistake which makes sense given that I wasn't yet comfortable with safe encapsulation of unsafePerformIO. You're right, but the cheap destructive updates still presumably involve creating and garbage-collecting O(N) thunks, which isn't particularly cheap, in my opinion. Or if the update function holds a reference to the original array, then you'll have to store two copies of the array in a non-dense format, and we won't have gained anything, as far as the temporary goes. -- David Roundy Department of Physics Oregon State University _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
