July 2003 - Haskell-Cafe

Re: Representing cyclic data structures efficiently in Haskell
by C.Reinke 08 Jul '03

08 Jul '03

> What is the best way to represent cyclic data structures in Haskell? There used to be some work on direct cyclic representations at UCL: Dynamic Cyclic Data Structures in Lazy Functional Languages Chris Clack, Stuart Clayman, David Parrott Department of Computer Science, University College London, October 1995 The link to the project, from http://www.cs.ucl.ac.uk/staff/C.Clack/research/report94.html is broken, but the paper seems to be online at http://www.cs.ucl.ac.uk/teaching/3C11/graph.ps If you want to go for this direct cyclic representation, the interplay with lazy memo functions is also interesting: J. Hughes, Lazy memo-functions, Functional Programming Languages and Computer Architecture, J-P. Jouannaud ed., Springer Verlag, LNCS 201, 129-146, 1985. (lazy memo-functions remember previous input-output pairs based only on pointer-info, which is sufficient to write functions over cyclic structures that, instead of infinitely unrolling the cycles, produce cyclic results) (not online?-( Claus

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08 Jul '03

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Representing cyclic data structures efficiently in Haskell
by Sarah Thompson 08 Jul '03

08 Jul '03

What is the best way to represent cyclic data structures in Haskell? Lists are trivial. Trees are trivial. But what if the thing you're trying to represent is shaped like an electronic circuit, with a (possibly large) number of nodes connected by multiple arrows to other nodes? In an imperative language like C++, I'd probably just model the circuit directly, with objects representing nodes and pointers representing links between nodes. A good way to model this in Haskell is less obvious, however. A simplistic approach would be to represent such a structure as a list of nodes, where each node contained a list of other connected nodes. Containing the actual nodes within the sub-lists probably isn't feasible -- some kind of reference would be necessary in order to get around chicken-and-egg problems with instantiating the data structure. But, in this case, it's not so easy to see how one could 'walk' the structure efficiently, because moving from node to node would involve quite horrible (possibly slow) lookups. In C++, I'd simply follow a pointer, which would be an extremely fast operation. I would also need to implement efficient indexes into the data structure -- flat searching will be too slow for non-trivial amounts of data. In C++, I'd implement one or more external (probably STL-based) indexes that point into the main data structure. How do people typically solve this problem at the moment? I know a few people out there have written hardware compilers in Haskell and similar languages, so there should be some experience of this in the community. I can probably work something out soon enough, but reinventing the wheel is never a great idea. Thank you in advance, -- ---------------------------------------------- / __ + / Sarah Thompson **** / / (_ _ _ _ |_ / * / / __)(_|| (_|| ) / sarah(a)telergy.com * / / + / http://findatlantis.com/ / ----------------------------------------------

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RE: Representing cyclic data structures efficiently in Haskell
by Hal Daume 07 Jul '03

07 Jul '03

>From reading your message, it seems you want a graph library of sorts. I've used FGL extensively and found it to be very nice. It even provides both a purely functional interface as well as an imperative-esque interface. It also comes with a bunch of standard algorithms built in, though I don't know enough about circuitry stuff to know whether those would be useful or not. If FGL seems like overkill to you, I have my own little mini graph library which basically looks like: - An ADT, Node: newtype Node = Node Int deriving ... - The 'Graph n e' data structure, containing: - a FiniteMap from n -> Node - an (growable?) array of type IOArray (Node,Node) e the array represents the edges in the top half (i do some index fiddling to get this efficient) and the finitemap represents the node labellings. It's a pretty stupid interface and works a lot better when the graphs are fixed size (that way you don't need to grow the array). But it works. -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume > -----Original Message----- > From: haskell-cafe-admin(a)haskell.org > [mailto:haskell-cafe-admin@haskell.org] On Behalf Of Sarah Thompson > Sent: Monday, July 07, 2003 8:04 AM > To: haskell-cafe(a)haskell.org > Subject: Representing cyclic data structures efficiently in Haskell > > > What is the best way to represent cyclic data structures in Haskell? > > Lists are trivial. Trees are trivial. But what if the thing you're > trying to represent is shaped like an electronic circuit, with a > (possibly large) number of nodes connected by multiple arrows > to other > nodes? In an imperative language like C++, I'd probably just > model the > circuit directly, with objects representing nodes and pointers > representing links between nodes. A good way to model this in > Haskell is > less obvious, however. > > A simplistic approach would be to represent such a structure > as a list > of nodes, where each node contained a list of other connected nodes. > Containing the actual nodes within the sub-lists probably > isn't feasible > -- some kind of reference would be necessary in order to get around > chicken-and-egg problems with instantiating the data > structure. But, in > this case, it's not so easy to see how one could 'walk' the structure > efficiently, because moving from node to node would involve quite > horrible (possibly slow) lookups. In C++, I'd simply follow a > pointer, > which would be an extremely fast operation. > > I would also need to implement efficient indexes into the > data structure > -- flat searching will be too slow for non-trivial amounts of > data. In > C++, I'd implement one or more external (probably STL-based) indexes > that point into the main data structure. > > How do people typically solve this problem at the moment? I > know a few > people out there have written hardware compilers in Haskell > and similar > languages, so there should be some experience of this in the > community. > I can probably work something out soon enough, but > reinventing the wheel > is never a great idea. > > Thank you in advance, > > -- > ---------------------------------------------- > / __ + / Sarah Thompson **** / > / (_ _ _ _ |_ / * / > / __)(_|| (_|| ) / sarah(a)telergy.com * / > / + / http://findatlantis.com/ / > ---------------------------------------------- > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Mailing Lists - Fax Directory - Company Databases
by Peter Cororan 05 Jul '03

05 Jul '03

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ucuz arama
by NETTEALO 05 Jul '03

05 Jul '03

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Worldwide Partners program
by The TechNews 05 Jul '03

05 Jul '03

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FFI Ptr question
by paul＠theV.net 03 Jul '03

03 Jul '03

If a C function returns a (void*), let's say: data MyObj = MyObj foreign import ccall "static myobj_ffi.h new_obj" newMyObject :: IO (Ptr MyObj) ptr <- newMyObject should I free the ptr afterwards or not? If I have another imported C function: foreign import ccall "static myobj_ffi.h print_obj" printMyObject :: Ptr MyObj -> IO () Can I safely assume the C function print_obj(void *) always get the same pointer even if I call (printMyObject ptr) multiple times? Any hints is greatly appreciated! Regards, .paul.

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by webmaster 03 Jul '03

03 Jul '03

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RE: haskell array access
by Simon Peyton-Jones 01 Jul '03

01 Jul '03

| Are there any concoctions of flags/pragmas/techniques to coax GHC into | unrolling a function (or rather an IO computation in this case) a couple | of iterations. There was another (admittedly also noddy) program that I | could get to near gcc, but only by unrolling by hand (though Ian's TH | stuff likely could have done it). GHC generally doesn't unroll recursions. The inliner runs repeatedly, so it's hard for it to know how much it unrolled "last time". (There's a paper "Secrets of the GHC inliner" on my publications page.) Simon

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