December 2009 - Haskell-Cafe

Optimization with Strings ?
by Emmanuel CHANTREAU 04 Dec '09

04 Dec '09

Hello In my futur program, it use a lot of binary trees with strings (words) as leaf. There is just arround 1000 words and they will appear a lot of times. The program will possibly consume a lot of process and memory (it is a mathematics proover). I began this program in C++ but haskell has a prety good speed and memory footprint and is easier. But I don't know if it worth to do this optimization: having a dictionary to translate string words in Int. The answer depends on the automatic optimizations in GHC, because GHC could compare quickely two strings if it is the same object, so it depends if program generated by GHC have a dictionary (tree) of strings internaly. Someone knows this ? (Sorry for my english, I'm french) thank you -- Emmanuel Chantréau

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Re: [Haskell-cafe] Is Haskell a Fanatic?
by gladstein＠gladstein.com 04 Dec '09

04 Dec '09

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Mushrooms (edible/poisonous) and Haskell
by Hans van Thiel 04 Dec '09

04 Dec '09

Hi All, About six months ago I uploaded version 0.6 to Hackage of Emping, a prototype tool using a new algorithm for data mining/machine learning. It won't build with the newer versions of GTk2Hs because the Gtk2Hs API changed since 0.9.13, and I used older versions of GHC and Gtk2Hs. For several reasons I don't have easy access to the newer versions, so I havn't fixed it yet. It should (that famous word) take no more than 15 minutes to change a Maybe Int into an Int in the main module. Apart from that it compiles warning free and (in my own opinion) the code is pretty clean now. See the user guide for some bugs I found and haven't fixed yet. The reason for this post is that I've been testing it, and have now built an iteractive demonstration of the result of a case study. This is a well known example of a table of 8124 rows and 23 columns, and the goal is to distinguish between poisonous and edible mushrooms. Emping found 3635 rules with antecedent length of 1 to 6, and the claim is that these rules fully describe all possibilities. So, I hope people will examine this case and, hopefully, will find no examples refuting this claim. As far as I know no one has ever analized this table in such detail, but that's another thing to be found out. With regard to Haskell, imo Haskell is very suitable and practical as a language for such a tool, because it is so modular. Changing something in one place is usually pretty safe. From a practical pov, amongst all the advantages and disadvantages, maintainability is surely a strength! Anyway, for anyone who's interested in the application, see http://muitovar.com/emp/emp_get.php and related links. Yes, unfortunately the demo is not in Haskell... Regards, Hans van Thiel

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language-dot usage
by minh thu 04 Dec '09

04 Dec '09

Hi, I'm using the language-dot package to generate some .dot file. I'm wondering how to make edges. I was expecting to generate something like 1 -> {2 ; 3} but get in fact 1 -> 2 -> 3 I used the (pseudo) statements [NodeStatement $ NodeID 1, EdgeStatement [NodeId 2, NodeId 3]] What is the proper way to do that ? Thanks, Thu

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Re: [Haskell-cafe] Wikipedia article
by Sean Leather 04 Dec '09

04 Dec '09

On Fri, Dec 4, 2009 at 13:11, Max Rabkin wrote: > On Fri, Dec 4, 2009 at 1:18 PM, Sean Leather <leather(a)cs.uu.nl> wrote: > > I'm confused. When I'm logged in to Wikipedia, I see a page that is > vastly > > different from when I'm not logged in. I haven't played with Wikipedia in > a > > long time, so perhaps there is some reason for this, of which I am not > > aware. > > Wikipedia uses a caching proxy for non-logged-in users. > Ah, thanks. I talked to somebody else who had the same problem, so now we know. Regards, Sean

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I miss OO
by Michael Mossey 04 Dec '09

04 Dec '09

I'm fairly new to Haskell, and starting to write some big projects. Previously I used OO exclusively, mostly Python. I really miss the "namespace" capabilities... a class can have a lot of generic method names which may be identical for several different classes because there is no ambiguity. In my musical application, many "objects" (or in Haskell, data) have a time associated with them. In Python I would have an accessor function called "time" in every class. So if I have objects/data note1, cursor1, and staff1, Python: note1.time() cursor1.time() staff1.time() Haskell needs something like note_time note1 cursor_time cursor1 staff_time staff1 which is a lot more visually disorganized. What's worse, I have a moderate case of RSI (repetitive strain injury) so I type slowly and depend on abbreviations a lot. I use the souped-up abbreviation capabilities of Emacs. Let's say I have a field/member-variable called orientedPcSet that is used across many classes. In Python, I can create an abbreviation for that and it is useful many times. In Haskell, I might need someType_orientedPcSet someOtherType_orientedPcSet thirdType_orientedPcSet which prevents me from using abbreviations effectively (especially the dynamic-completion feature). (It would help to make the underscore not part of word syntax, but that's not ideal.) So I'm thinking of moving to a scheme in Haskell using modules, most types being defined in their own modules, and doing qualified imports. Generic names like 'time' can be defined in each module w/o clashing. Then I have Note.time note1 Cursor.time cursor1 Staff.time staff1 This is very useful because I can define abbreviations for the type name and for oft-used accessor function names and these abbrevs are more organized, easier to remember, and easier to combine. I would be interested in comments... is this a good way to do things? Am I trying too hard to impose OO on Haskell and is there a better way? Thanks, Mike

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Implicit newtype unwrapping
by Martijn van Steenbergen 04 Dec '09

04 Dec '09

So here's a totally wild idea Sjoerd and I came up with. What if newtypes were unwrapped implicitly? What advantages and disadvantages would it have? In what cases would this lead to ambiguous code? Thanks, Martijn.

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happstack homepage
by Roel van Dijk 04 Dec '09

04 Dec '09

I noticed that happstack.com and tutorial.happstack.com are both equal to patch-tag.com. Google's cache has the original pages. Is this the result of some misconfiguration or something else? I want to play with happstack and this is a slight inconvenience. On the other hand, all happstack packages build without problems so I'll build the tutorial locally and work from there. Regards, Roel

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Second Call for Copy: Monad.Reader Issue 15
by Brent Yorgey 04 Dec '09

04 Dec '09

It's not too late to write something for Issue 15 of the Monad.Reader! Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 15 is **Friday, January 8, 2010** The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki-page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. I am also happy to provide feedback on draft versions before the submission deadline. Please submit articles for the next issue to me by e-mail (byorgey at cis.upenn.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll sort something out.

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Monomorphic local let bindings and GHCi
by Miran Lipovača 04 Dec '09

04 Dec '09

Hi! There have been proposals to abandon generalisation for local let bindings because they make specifying and implementing the type system very complicated when extensions like multi parameter type classes, type functions and GADTs are taken into consideration. I agree with what the paper says in general, I'm just interested/concerned as to how abandoning local let generealisation would affect GHCi. While we rarely rely on the polymorphism of functions defined in local let bindings, I think a lot of programmers rely on the polymorphism of functions that they define with let in GHCi. I have a feeling that GHCi would be a lot less useful if it didn't allow us to define functions that are polymorphic in it. Also, what would the behavior be anyway? If we did > let f x = x in GHCi and then inspected the type of f, what would GHCi report? Would it use defaulting and give us () -> ()? Or would it display a polymorphic type a -> a and then change that type to something monomorphic once we use f with some value? What if we defined several functions like this that called each other? So I think that if local let generalisation is abandoned, let bindings in GHCi would somehow have to be modified to remain polymorphic. Any thoughts?

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