Re: [Haskell-cafe] number of references to a variable
Andrew Coppin wrote: Jan-Willem Maessen wrote: Usually the clever thing you want to know is this is the sole reference to the pointed-to object. Does GHC have weak references? (I seem to recall it does...) http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-Mem-Weak.html As for the uniqueness of a reference to an object, there are alternatives to reference-counting techniques. For example, the language Clean[1] builds it into the type system (to replace monads) and they've gotten very good performance results from doing so. There are other linear logic approaches as well, though porting any of these to Haskell would take a fair deal of work to make efficient. Certainly a worthy research goal, but probably not what you had in mind :) [1] http://clean.cs.ru.nl/ -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] number of references to a variable
Alberto G. Corona wrote: * Certainly a worthy research goal, but probably not what you had in mind :) *Not at all, My interest on that was on to improve RefSerialize. Although the Clean people has done a superb job on optimization. By the way, I'm not sure if uniqueness is the cause of his performance. I tested Haskell code generated using Maybe with and without monad instance and the monad does not impose any overhead at all. Oh, I wasn't saying that monads impose any overhead. Rather, the Clean folks aren't fans of monads and so they use uniqueness typing in order to provide the same kind of solution to IO while retaining functional purity. GHC's implementation of IO is actually very similar under the hood to what Clean does, though that's not necessarily true of other monads. Even though they seem to intersect at IO, the ideas of monads and linear logics are orthogonal. The big optimization trick with linear logic is, since you know when some object is uniquely referenced, you know when it's safe to destructively update it in situ. For example, you can reverse a spine-unique list in linear time with zero space (modulo registers) by just inverting the spine pointers as you traverse it. Since it's uniquely referenced, noone else will notice so referential transparency is preserved. Doing it this way saves on allocation costs and on garbage collection. There are certainly other ways to use the information that some object is held only by a single reference, but Clean's optimizations are the first that leap to my mind. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] number of references to a variable
Is there a way to know the number of memory references for a variable?. The runtime must know it but i do not know if this available for the program trough any low level trick Thanks ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] number of references to a variable
Alberto G. Corona wrote: Is there a way to know the number of memory references for a variable?. The runtime must know it but i do not know if this available for the program trough any low level trick More precisely, the GC computes it each time it runs. (And only computes it precisely during a major pass, not the more frequent minor passes.) You can attach a finaliser to an object, and that'll allow you to know when the reference count reaches zero. But beyond that, I don't know. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] number of references to a variable
On Nov 1, 2008, at 9:38 AM, Andrew Coppin wrote: Alberto G. Corona wrote: Is there a way to know the number of memory references for a variable?. The runtime must know it but i do not know if this available for the program trough any low level trick More precisely, the GC computes it each time it runs. (And only computes it precisely during a major pass, not the more frequent minor passes.) Even this isn't quite true for most GC algorithms. The GC only needs to compute whether there is 0 or = 1 reference to a given location (with some special weasel words for stuff with finalizers defined). If you can see it, the answer is always =1, so this information is much less useful than you might think! Usually the clever thing you want to know is this is the sole reference to the pointed-to object. If that's what you're interested in, try looking up one-bit reference counting, but note that like any accurate reference counting technique it's really inefficient in practice compared to GC. [There are efficient reference counting techniques, but they defer refcount updates in various subtle ways. Also, every ref count technique requires a cycle detector.] -Jan-Willem Maessen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] number of references to a variable
Jan-Willem Maessen wrote: On Nov 1, 2008, at 9:38 AM, Andrew Coppin wrote: Alberto G. Corona wrote: Is there a way to know the number of memory references for a variable?. The runtime must know it but i do not know if this available for the program trough any low level trick More precisely, the GC computes it each time it runs. (And only computes it precisely during a major pass, not the more frequent minor passes.) Even this isn't quite true for most GC algorithms. The GC only needs to compute whether there is 0 or = 1 reference to a given location. If you can see it, the answer is always =1, so this information is much less useful than you might think! Quite right. Usually the clever thing you want to know is this is the sole reference to the pointed-to object. Does GHC have weak references? (I seem to recall it does...) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
