- Haskell-Cafe - Haskell.org

Multiple applications of a function
by Mark Carroll 16 Sep '01

16 Sep '01

To multiply apply a function, I'm currently using: multiplyApply f n a = (iterate f a) !! n ...is there a Prelude function I've missed that already does this? Could I be doing this better? -- Mark

1 0

Re: Application letters at the Haskell workshop: suggestion
by Ashley Yakeley 15 Sep '01

15 Sep '01

At 2001-09-15 08:31, Mark Carroll wrote: >AFAICS a simple way to get >out of this is to only have one exception type that carries no information >instead of different ones so we can't distinguish one exception from >another, but that's obviously not great. Isn't that what 'bottom' is? -- Ashley Yakeley, Seattle WA

2 1

Re: controlling resource usage
by D. Tweed 11 Sep '01

11 Sep '01

On Fri, 7 Sep 2001, Richard Uhtenwoldt wrote: > Finally, are we all in agreement that it would be a great thing if > someone invented a Haskell-like language as good as Haskell at hiding > resource usage when resource usage is unimportant, but that made it > easier to control resource usage than Haskell does when resource usage > is important? Of course it's not just in functional languages that performance demands can require rewriting shorter, more comprehensible code in less obvious ways. (My favourite example of something so completely non-intuitive is the Intel example from within a Sieve of Eratosthenes program that for(i=0;i<lim/step;++i){ prime[i*step]=0; } can on some processor setups be slower than for(i=0;i<lim/step;++i){ if(prime[i*step]!=0){ prime[i*step]=0; } } due to cache coherency issues.) In that context, there was an old suggestion of Don Knuth's that program source code should in the idealized future not just contain the final, optimized code (with perhaps the original code in comments) but as a kind of set of layers each representing the same semantic program but with increasing orders of optimisation, and links between `things' annotated with a label placed by the programmer explaining why the transformation between an unoptimised version `thing' and an optimised `thing'. The nice bit about this would be that the development environment would be equipped with a subsystem that could almost always establish (possibly using the the label on the link) if the two things had the same semantic effect, and a naive system that could try and produce (maybe using brute force pattern modification) the changes to the optimised version corresponding to changes to the unoptimised version. The nice thing about this is that it would allow changes to the code (due to say changing requirements, or ideas for better algorithms) to be made first to the simple version and then the areas of the optimised version that needed changing would be automatically brought to the programmers attention. (The system for trying to change the optimised code automatically is just there to stop the system becoming really annoying every time you do something something like int i=1; _______\ int i=2; int j=2; / int j=1; to the unoptimised code. The reason I think this would be a good idea is because I don't really object that much to having to optimise code that's performance critical, I just dislike the way that I never manage to keep both the original and optimised version in sync and so end up having to maintain the optimised code directly. I suspect that such a system is still at least 25 years away from feasiblity though :-( ___cheers,_dave________________________________________________________ www.cs.bris.ac.uk/~tweed/pi.htm |tweed's law: however many computers email: tweed(a)cs.bris.ac.uk | you have, half your time is spent work tel: (0117) 954-5250 | waiting for compilations to finish.

4 7

Reproducing lazy evaluation in Scheme
by David Barton 10 Sep '01

10 Sep '01

All right, I know this is a stupid question. I know the models of evaluation in call by name and call by value languages; however, I am kind of puzzled by trying to reproduce call by name in a call by value language using force and delay. If I put a delay around the right side of each equation and around each expression in a function call (*all* function calls?), and a force call around each variable reference, would that get me call by name semantics? Am I completely screwed up here? Is something more necessary? I'm trying to get the gumption up to try a proof by substitution of this idea, but I cringe at the formula manipulation....... Dave Barton <*> dlb(a)averstar.com )0( http://www.averstar.com/~dlb

1 0

Pickling
by Mark Carroll 09 Sep '01

09 Sep '01

Am I right in thinking that there aren't currently any standard libraries for converting values to bitstreams and back, to be saved in files or squirted over network sockets or whatever to be resurrected later or elsewhere? I'd guess it would be hard to write, too - I've not noticed any introspection. How do people normally efficiently pass complex Haskell data structures to other co-operating Haskell processes? (I am thinking of something a bit like Java's serialisation.) -- Mark

5 6

Algebraic types and `objects'
by S.J.Thompson 06 Sep '01

06 Sep '01

To follow up Cagdas Ozgenc's message of Wednesday, I have attached a series of files which show a gradual transformation (or refactoring) of an algebraic type of shapes into a Shape type, built using type classes and existential types, which more corresponds to an OO approach. Hope this is useful. Simon

1 0

algebric types and db integration
by Cagdas Ozgenc 05 Sep '01

05 Sep '01

Hi, I am wondering how one would store instances of algebric types in a database. For example: data Shape = Circle Float | Rectangle Float Float | Triangle Float Float Float All three constructors are representing different schemas. Hence I won't be able to store them in a single table. I think algebric types seem to represent an inheritance hierarchy in terms of OO terms, Shape being the base class, and Circle, Rectangle, Triangle being its subclasses. Is this a correct way of looking at it? How do you suggest I conceptually map it to a relational database? Thanks

2 1

Using Set: Hypergraph type
by Eray Ozkural 04 Sep '01

04 Sep '01

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Greetings all! I'm a new Haskell user. I've used Haskell for a number of small hacks and I can say that this is a brilliant language. I was just working on a hypergraph implementation in haskell. A hypergraph, for those not familiar with it, is a family of sets which are subsets of a vertex set X. I went straight along the mathematical definition, and I decided to try the Edison library that came along with my ghc-5.00.2 Without further ado, here is the code: type Hypergraph n = Set (Set n) - -- hgraph constructor - -- takes a list of edges hgraph edge_list = unionManySets (map mkSet edge_list) My trouble is that this does not seem to work well. Perhaps it's my lack of experience wih Edison, but the types do not seem to be right and the following expression fails to evaluate: Hypergraph> elementOf (mkSet [1]) (hgraph [ [1,2], [1] ]) I expect "True" in response to this query. Anyway, I think this is not ghc specific. (I enabled -fglasgow-exts, no difference. I get a lot of type errors...) What do you think is wrong with this code? It now seems to me that I may not be able to use Edison for my ADT, so how would you recommend me to go about coding this mathematical definition? All I need is to be able to define a set of sets in a convenient way; it also has to be halfway efficient. Should I implement this with lists, or is there an alternative set implementation which allows me to accomplish a decent hypergraph definition? [*] Regards, [*] I could also use a mix of arrays and lists it seems, similar to the edge list and pin list representations used in imperative code. But that does not give me the interface a set implementation would :) - -- Eray Ozkural (exa) <erayo(a)cs.bilkent.edu.tr> Comp. Sci. Dept., Bilkent University, Ankara www: http://www.cs.bilkent.edu.tr/~erayo GPG public key fingerprint: 360C 852F 88B0 A745 F31B EA0F 7C07 AE16 874D 539C -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.6 (GNU/Linux) Comment: For info see http://www.gnupg.org iD8DBQE7lKJQfAeuFodNU5wRAtyBAJ0SCRLAV5u4IBkt3B2VIYB04l9LNACeNL0Y GE9TPhrWoPY6rH58ZDSdweg= =IdNC -----END PGP SIGNATURE-----

3 7

Re: Using Set: Hypergraph type
by Marcin 'Qrczak' Kowalczyk 04 Sep '01

04 Sep '01

Tue, 4 Sep 2001 21:12:41 +0300, Eray Ozkural <erayo(a)cs.bilkent.edu.tr> pisze: > Thanks for the suggestion, but isn't this a bit inefficient? Ordinarily one > would like to compare two elements of a set by reference, not by content. There is no such thing in Haskell. mkSet [mkSet [1, 2]] is indistinguishable from mkSet [mkSet [1, 2]] even if {1,2} sets were created independently. Whether representations of particular mkSet [1, 2] objects are shared is an implementation detail. Well, there are compiler-dependent unsafe functions which could detect sharing, but they are not normally used. Functional programming is about values, not about objects with identities. You could implement a "tagged object" type, like data Tagged a = Tagged Integer a give objects unique numbers as they are created, and make Eq and Ord instances which compare these numbers only, ignoring the real value. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

1 0

Re: Using Set: Hypergraph type
by Marcin 'Qrczak' Kowalczyk 04 Sep '01

04 Sep '01

Tue, 4 Sep 2001 20:28:55 +0300, Eray Ozkural <erayo(a)cs.bilkent.edu.tr> pisze: > Okay, I guess I got it now. The (types of) elements of a Set ought > to be instances of Ord. So I presume Edison supports only flat sets. You can add an inefficient implementation of Ord (Set a) yourself: instance Ord a => Ord (Set a) where compare s1 s2 = compare (toOrdList s1) (toOrdList s2) toOrdList seems to be exported only module Collection. If more than one module does this, they can't be used together. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

1 0