August 2005 - Haskell-Cafe

XML Toolbox Question
by Dimitry Golubovsky 21 Aug '05

21 Aug '05

Hi, How can I obtain a list of all <a> tags from the result of parseHtmlContent? Unfortunately, the Haddock documentation coming with the package is not rich on examples. Any suggestions will be appreciated. Dimitry Golubovsky Middletown, CT

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map applications
by Brian McQueen 20 Aug '05

20 Aug '05

I have a simple list like this: qlist = ["ll", "kkkkkkk", "ddddd"] Why is it OK to do this: qwrap [] = putStrLn ":" qwrap (a:as) = do putStrLn a qwrap as qwap qlist but not this: map putStrLn qlist What is going on in the map function? McQueen

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Oracle + Haskell advice?
by Brian Strand 20 Aug '05

20 Aug '05

Hello, I'm thinking about (re)writing some perl code in Haskell (for performance and correctness reasons). Has anyone done much with Oracle and Haskell? So far I've gotten takusen (http://cvs.sf.net/viewcvs.py/haskell-libs/libs/takusen/) to compile and run on my Suse 9.3 x86-64 box against Oracle 10.1. If this pans out the production environment is a bunch of mid-size (4gb+ mem, 1tb+ disk) Oracle boxes typically processing millions of rows per day. Before I wade in too deep, I'm wondering if anyone has done much "business app" programming with Haskell (where "business app" is rather fuzzily defined as database access, web UIs, interfacing with various systems over ip, etc.). Does anyone have any advice? eg "don't go there" or "works great!" or "try OCaml" :) . Thanks, Brian

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RE: [Haskell-cafe] Re: Oracle + Haskell advice?
by Bayley, Alistair 19 Aug '05

19 Aug '05

> From: Brian Strand [mailto:bstrand@switchmanagement.com] > > So far Takusen is working great (once I figured out the > proper incantations > for getting "select * from dual" working, things went much > faster...). We > don't currently use LOBs, and very rarely call stored > procedures, so there's a > happy match between Takusen's missing features and our unused > features ;) One > thing we do use extensively is bind parameters; do you have > any hints on > getting doQueryBind working, or pointers on what's left to do > there? And is > the version in CVS on sf.net a good place to start? Hmm... I assume that you're using the experimental version from CVS, as you've mentioned doQueryBind, which is not in the Main branch. What problems are you having with bind parameters? Have you seen the tests in Database.Test.Enumerator? Specifically, the selectBindInt and selectBindDate functions demonstrate simple usage of bind parameters. There are a couple of implementation notes which might affect you: - I think you have to specify the type of the bind parameter list (i.e. help the type-checker) - user modules (like Database.Test.Enumerator) probably need to specify these pragmas: {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-overlapping-instances #-} - bind parameters are specified with ?'s, and we convert those to the Oracle :x syntax. I think you should be able to use the :x syntax too though; I believe it will be passed through unmolested. Reviewing at the bind variable code has highlighted a glaring design flaw: you can only pass homogenous lists of bind values! We originally had a design where you passed a list of bind actions (a monadic action which returned ()). I think that in my zeal to reduce programmer effort I have reduced this to a list of values (to which we apply the bind action in the library). Maybe that's what's biting you... I'd better get onto that sharpish. Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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RE: [Haskell-cafe] Oracle + Haskell advice?
by Bayley, Alistair 19 Aug '05

19 Aug '05

> From: Brian Strand [mailto:bstrand@switchmanagement.com] > > I've gotten takusen > (http://cvs.sf.net/viewcvs.py/haskell-libs/libs/takusen/) > to compile and run on my Suse 9.3 x86-64 box against Oracle > 10.1. Cool. I'm keen for feedback. I can also give general advice as far as interfacing to Oracle is concerned. To be fair, Takusen is usable, but far from feature-complete (no support for BLOBs, CLOBs, invoking procedures). You might want to compare it to HaskellDB over ODBC, if you can get HaskellDB running (I don't know how usable ODBC-on-Unix is). HaskellDB is a higher-level approach to database access, and might be better suited to "business apps". > Before I wade in too deep, I'm wondering if anyone has done > much "business > app" programming with Haskell (where "business app" is rather > fuzzily defined > as database access, web UIs, interfacing with various systems > over ip, etc.). As for the other stuff, here are a few projects which might interest you. I believe these represent the state-of-the-art in Haskell as far as this sort of stuff is concerned: http://www.informatik.uni-freiburg.de/~thiemann/haskell/WASH/ : WASH, for building browser-based apps http://www.haskell.org/haxr/ : HaXR, for XML-RPC (the "interfacing with various systems over ip" requirement) http://www.repton-world.org.uk/mediawiki/index.php/HAIFA_Wiki : HAIFA. Looks like it does SOAP. See also: http://haskell.org/~shae/haifa_tmr.pdf There are also a few XML libraries around; I can't comment on how well these work. It should be clear that there's not yet one clear bundle or approach that we can point to and say "here, use this". Like, say, Ruby-on-rails, Zope, Seaside, etc. Things are still a bit more DIY in Haskell land. I recommend starting with WASH and adding what you need from there. Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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static typing and interactivity
by Ketil Malde 19 Aug '05

19 Aug '05

Hi, One slight annoyance using Haskell is the inability to load modules with type problems in the interactive environment (i.e. GHCi). When I have a type error, it would be nice to have an interactive way to explore what the compiler thinks about the types involved -- as it is, I have to resort to adding type signatures more or less at random to narrow down the problem. I'm not sure if it is technically feasible to (partially) load a module with definitions that fail type checking, but if it were, I thing it would make developing Haskell programs even nicer :-) -k -- If I haven't seen further, it is by standing in the footprints of giants

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RE: [Haskell-cafe] Re: [Haskell] pros and cons of static typing andside effects ?
by Ralf Lammel 17 Aug '05

17 Aug '05

I don't even want this feature. :-) The point being that datatype declarations, *as such*, are explicit anyhow. Why bother about the explicit quantifiers then? Of course, in a language with inferred datatypes I would mind. That's an interesting question! I also wouldn't (yet?!) support this feature request because we will still confuse beginners, but that's just my feeling. Ralf > -----Original Message----- > From: Stefan Holdermans [mailto:stefan@cs.uu.nl] > Sent: Tuesday, August 16, 2005 11:48 PM > To: Ralf Lammel > Cc: haskell-cafe(a)haskell.org; Benjamin Franksen > Subject: Re: [Haskell-cafe] Re: [Haskell] pros and cons of static typing > andside effects ? > > Ralf, > > > Technically this is trivial it seems. I think that some people > > consider this proposal a problem because typos (misspelled type > > parameters) immediately lead to the accidental exploration of a more > > advanced type-system feature and correspondingly more involved error > > messages. Of course, the type checker could perhaps consider adding > > "Did you really mean to ...?". > > Well, okay, but as soon as the type checker starts to asking these > questions, I would immediately start adding the explicit quantifiers, > just to get rid of those annoying warning messages. ;) So one would we > also need to be able to control the behaviour of the type checker with > respect to these warnings by means of a compiler flag like > "fno-warn-on-implicit-existential-quantification". All of this is, of > course, still trivial. :) > > Have we thought about it enough to make it a feature request? > > Regards, > > Stefan

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RE: [Haskell-cafe] Re: [Haskell] pros and cons of static typing andside effects ?
by Ralf Lammel 17 Aug '05

17 Aug '05

Technically this is trivial it seems. I think that some people consider this proposal a problem because typos (misspelled type parameters) immediately lead to the accidental exploration of a more advanced type-system feature and correspondingly more involved error messages. Of course, the type checker could perhaps consider adding "Did you really mean to ...?". Ralf > I once read a paper about type classes and existentials (can't remember > exact title or author, was it Läufer?) where the proposal was to make > existential quantification implicit (just as the universal one is in > Haskell98). That is, any type variable that appears on the rhs of a > data type, but not on the lhs, is implicitly existentially quantified, > as in > > data XWrap = Show a => XWrap a > > I always thought this was a pretty nice idea.

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Seeking a small bug-fix for previously-posted code
by Andy Elvey 17 Aug '05

17 Aug '05

Hi all - I'm trying to run some code posted recently on this list ( the exact URI is as follows - http://comments.gmane.org/gmane.comp.lang.haskell.cafe/7860 ( The reason I'm doing this is because this code is quite close to what I'm trying to do. ) Anyway, I saved the code in a file called andy1.hs . The exact code is as follows - module Main where readVecs :: String -> IO [Vector3 GLfloat] readVecs f = do str <- readFile f let mkVec [x,y,z] = Vector3 x y z return $ map (mkVec . read . words) $ lines str ( I can see what the code seems to be doing : it's reading the file line-by-line, splitting it into "words", and filling the vectors with them. Not too bad a guess for a "Haskellista" of a few minutes standing ..... ;-) I created a small file with whitespace-separated floats so that I could run the above code. This data file looks like this - 23.596 38.201 79.903 32.647 65.559 15.438 21.648 34.978 90.089 55.587 76.763 96.634 When I run the above code (in GHCi , version 6.4) I get the message - Prelude> :load andy1 Compiling Main ( andy1.hs, interpreted ) andy1.hs:4:7: The last statement in a 'do' construct must be an expression Failed, modules loaded: none. So - I'm just wondering if anyone can see how the above code should be fixed in order to run. And just one last question - If the file looked like this (with column headings ) Name Ethnicity Age Mary_Smith NZ_European 27 Joe_Brown NZ_European 34 John_Watson Canadian 45 Mike_Tangaroa NZ_Maori 38 - what change would the above code need to run? Very many thanks in advance ... - Andy

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Re: [Haskell] pros and cons of static typing and side effects ?
by Mark Carroll 16 Aug '05

16 Aug '05

The previous comments make sense to me. The lots-of-unit-tests aspect of static typing I find really useful, far exceeding any BDSM cost. If I'm engaging in exploratory programming, the type inference combined with the ability to write 'error "armadillo"' in stubs for values I can't be bothered to generate right now really works conveniently for me. Although I agree that lots-of-lists is very handy in early prototyping, I don't feel at all constrained by using homogeneous lists, although very occasionally I may use existential types, and the way I write programmes is exactly to think in advance and then write the code: to do otherwise just wastes my time because then the code doesn't work in some confusing way and I have to do that thinking I postponed to figure out why - or, if it does work, I have to think about it to satisfy myself that appearances aren't deceiving. I'm not quite sure what macros would look like in Haskell, but I've not missed those either. In Lisp I would tend to use them for things that involved changing the values of variables, but that's not really a Haskellish thing to be doing anyway. Mind you, I learned Lisp after learning ML, so to some extent I was thinking in ML when writing in Lisp. Alas, dead-tree versions of "On Lisp" are hard to come by affordably, but I am now trying to learn more about what I might have missed about Lisp. I find monads useful because I find it a helpful debugging aid for functions to be quite clear about what side effects they may want to have. I posted this to Haskell-Cafe instead of the main Haskell list, because I'm rambling a bit. Puzzled Haskell-Cafe readers may like to check http://www.mail-archive.com/haskell@haskell.org/msg17009.html -- Mark

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