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Re: [Haskell-cafe] Re: Debugging partial functions by the rules
by John Hughes 16 Nov '06

16 Nov '06

> From: Robert Dockins <robdockins(a)fastmail.fm> > It seems to me that every possible use of a partial function has some > (possibly imagined) program invariant that prevents it from failing. > Otherwise it is downright wrong. 'head', 'fromJust' and friends > don't do anything to put that invariant in the program text. Well, not really. For example, I often write programs with command line arguments, that contain code of the form do ... [a,b] <- getArgs ... Of course the pattern match is partial, but if it fails, then the standard error message is good enough. This applies to "throw away" code, of course, and if I decide to keep the code then I sooner or later extend it to fix the partiality and give a more sensible error message. But it's still an advantage to be ABLE to write the more concise, but cruder version initially. This isn't a trivial point. We know that error handling code is a major part of software cost--it can even dominate the cost of the "correct case" code (by a large factor). Erlang's "program for the correct case" strategy, coupled with good fault tolerance mechanisms, is one reason for its commercial success--the cost of including error handling code *everywhere* is avoided. But this means accepting that code *may* very well fail--the failure is just going to be handled somewhere else. Haskell (or at least GHC) has good exception handling mechanisms too. We should be prepared to use them, and "let it fail" when things go wrong. The savings of doing so are too large to ignore. John

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submenu doesn't seem to work properly in wxhaskell
by Maarten 15 Nov '06

15 Nov '06

Dear all, The following code doesn't seem to work properly. Either the main entry (m1/mp1) or it's sub menu entry (ms1/mps1) do not seem to propagate the event when pressed. It is possible to make it working by uncomments the lines where the menu commands are registered in the frame. I have the following two questions: 1. Why doesn't the plain code work. Am I missing something or doing something wrong? 2. Doesn't registering eventhandlers in the frame introduce a memory/resource leak, especially in the case of popups? Any suggestions or comments appreciated. Thanks. Kind regards, Maarten Code: <imports> gui :: IO () gui = do f <- frame [ text := "Hello world!" ] m <- menuPane [ text := "&Menu" ] m1 <- menuItem m [ text := "Menu m1", on command := putStrLn "menu m1"] -- set f [ on (menu m1) := putStrLn "menu m1" ] menuLine m sub <- menuPane [text := "Sub menu"] ms1 <- menuItem sub [ text := "submenu ms1", on command := putStrLn "submenu ms1" ] -- set f [ on (menu ms1) := putStrLn "submenu ms1" ] menuSub m sub [ text := "Sub" ] menuItem m [text := "E&xit", on command := close f] set f [menuBar := [m], on mouse := mouseEvent f, clientSize := sz 200 200 ] return () mouseEvent f eventMouse = do case eventMouse of MouseRightDown mousePoint _ -> doPopup f mousePoint _ -> return () doPopup f mousePoint = do m <- makePopupMenu f "&Popup" "Doesnt' work..." menuPopup m mousePoint f objectDelete m makePopupMenu f c t = do mp <- menuPane [ text := c ] mp1 <- menuItem mp [ text := "Popup mp1", on command := putStrLn "popup mp1"] -- set f [ on (menu mp1) := putStrLn "popup mp1" ] menuLine mp sub <- menuPane [text := "more text"] mps1 <- menuItem sub [ text := "Popup mps1", on command := putStrLn "popup mps1"] menuSub mp sub [ text := "Sub" ] -- set f [ on (menu mps1) := putStrLn "popup mps1" ] return mp

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Haskell Debugging
by Valentin Gjorgjioski 15 Nov '06

15 Nov '06

I'm pretty new in Haskell, few days since I started learning it. I want to debu my programs. I'm currently using WinHugs, and I prefer debugger for this. I tried googling and I found Hugs.Observer. I like it how it works, but still I have one BIG problem with it. It doesn't work well with floats. Following example import Hugs.Observe ex8 :: [Float] ex8 = (observe "after reverse" ) reverse [10.0,7.0,3.0,0.0,4.0] gives me >ex8 [4.0,0.0,3.0,7.0,10.0] >>>>>>> Observations <<<<<< after reverse { \ ($-990871 : $-990888 : $-990905 : $-990922 : $-990939 : []) -> $-990939 : $-990922 : $-990905 : $-990888 : $-990871 : [] } Which is not useful. After that I try to install http://www.haskell.org/hat/ but for installing that I need hmake, and when I try to install hmake, I typed first configure -prefix, and then, make. cd src/hmake; make HC=ghc BUILDCOMP=ghc all config make[1]: Entering directory `/cygdrive/E/Haskell/hmake/src/hmake' Makefile:59: *** target pattern contains no `%'. Stop. make[1]: Leaving directory `/cygdrive/E/Haskell/hmake/src/hmake' make: *** [targets/ix86-CYGWIN_NT-5.1/hmake-ghc] Error 2 as you can see I'm using cygwin. I'm really stacked here, and I will kindly ask for help. Thanks, Valentin -- Valentin Gjorgjioski Bachelor of Computer Science Department of Knowledge Technologies, Jozef Stefan Institute Jamova 39, SI-1000 Ljubljana, Slovenia Phone: +386 1 477 3343 Fax: +386 1 477 3315 Web: http://kt.ijs.si/ValentinGjorgjioski/ Email: Valentin.Gjorgjioski(a)ijs.si

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Re: Haskell custom search engine
by Alberto G. Corona 15 Nov '06

15 Nov '06

By the way, I have developped the core of a search engine completely in Haskell. It can index arbitrary objects (including files, of course) has definitions for filters, with predefined filters for plain text, HTML/XML and a general filter for Haskell data types It has basic search capabilities: it returns a list of identifiers (URIs) of the objects that contains all the entered keywords. But it is fairly easy to accept arbitrary AND OR and NOT conditions. Just query parsing is necessary. More info: http://haskell-web.blogspot.com/2006/11/search-engine-written-in-haskell.ht… The source of Search.hs can be found here<http://www.box.net/public/fdkami7jbk>

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Re: [Haskell] I18N, external strings
by Bulat Ziganshin 15 Nov '06

15 Nov '06

Hello Johannes, Wednesday, November 15, 2006, 10:36:10 AM, you wrote: > What methods and tools are there for i18n of Haskell programs? > In Haskell, I see at least two problems: > a) reading the file is in IO if you need utf8 and other encodings support, you can use Streams lib: h <- openFile "test" ReadMode >>= withEncoding utf8 http://haskell.org/haskellwiki/Library/Streams > b) there are no "global variables". implicit parameters perhaps? there is a sort of global vars: ref_command = unsafePerformIO$ newIORef$ error "undefined ref_command" uiStartCommand command = do writeIORef ref_command command .. uiStartProcessing filelist = do command <- readIORef ref_command .. but implicit parameters, of course, are more intelligent approach -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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pleac, examples, etc
by brad clawsie 15 Nov '06

15 Nov '06

it would be great if some of the more informed posters here took a stab at filling in http://pleac.sourceforge.net/pleac_haskell/index.html a neat site for cookbook-style problem solving from looking over haskell documentation for a few months now, i would say what is missing is one central cookbook-style document (want to query a database? want to parse xml? etc etc) thanks

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Great language shootout: reloaded
by dons＠cse.unsw.edu.au 14 Nov '06

14 Nov '06

So back in January we had lots of fun tuning up Haskell code for the Great Language Shootout[1]. We did quite well at the time, at one point ranking overall first[2]. After doing all we could with ghc 6.4.2, the Haskell entries have been left for the last 10 months, while we worked on new libraries (bytestring, regex-*). 1. http://shootout.alioth.debian.org/ 2. http://www.cse.unsw.edu.au/~dons/data/haskell_1.html Now the time has come to reload the shootout for another round! GHC 6.6 is on the 'sandbox' debian machine, and will soon be on the other shootout boxes[3], which means we can use: * Data.ByteString * regex-* libraries 3. http://shootout.alioth.debian.org/sandbox/benchmark.php?test=all&lang=ghc&l… And thus greatly improve: fannkuch fasta k-nucleotide regex-dna reverse-complement sum-file While we're here we should fix: chameneos And anything else you want to take a look at. A community page has been set up to which you can submit improved entries: http://www.haskell.org/haskellwiki/Great_language_shootout So, install GHC 6.6, read up on Data.ByteString and the new regex libs, and submit faster code to the wiki! Our shootout-interface officer, musasabi, can then commit them to shootout cvs, once consensus is reached on the best code to submit. Let's take back first place! :) -- Don

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deepSeq vs rnf
by Chad Scherrer 14 Nov '06

14 Nov '06

Hi, I had posted this question a while back, but I think it was in the middle of another discussion, and I never did get a reply. Do we really need both Control.Parallel.Strategies.rnf and deepSeq? Should we not always have x `deepSeq` y == rnf x `seq` y ? Maybe there's a distinction I'm missing, but it seems to me they're basically the same. -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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best Linux for GHC?
by Bulat Ziganshin 14 Nov '06

14 Nov '06

Hello haskell-cafe, Now i'm consider installation of some Linux version at my box. My friend offered me 3 variants: SuSe, Fedora Core 5, free variant of RedHat (i can't remember its name, may be Ubuntu?) what may be best for GHC-based development? in particular, i want to compile Haskell itself i suspect that it is a really dumb question, and GHC will work great just with any linux i can find :) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Testing non-exported functions using ghci
by John Ky 13 Nov '06

13 Nov '06

Hello, I have modules that don't export some functions. Is there a way I can access them from ghci without exporting them? Thanks -John

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