- Beginners - Haskell.org

Problem with an if statement
by Karol Samborski 09 Nov '11

09 Nov '11

Hi, With a function defined like this (colors is type of Array Int [Int] and edges is [Int]): saturation (_,edges,_) = foldr (\ed ac ->if null (colors!ed) then ac else ac+1) 0 edges I'm getting an infinite loop. But when I remove the if statement like this: saturation (_,edges,_) = foldr (\ed ac -> ac+1) 0 edges everything's working. How should I define my function with the if statement to not getting an infinite loop? Best Regards, Karol Samborski

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Laziness and XML parsing
by Sean Hess 09 Nov '11

09 Nov '11

I want to parse a large xml file (2GB), without putting the whole thing into memory. It's pretty simple with a sax parser in most languages, you just stream bytes to the sax parser, and wait for sax events. Here's what I think the equivalent is in Haskell - https://gist.github.com/1346854 Is the xml file being read lazily? It seems lazy, but it also seems like all the sax events would be loaded into memory. If not, how is that possible? In order to be lazy, it seems like parse would have to be an impure function, so that it could back to the disk to get more stuff. ~sean

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Noisy Linking
by Zhi-Qiang Lei 08 Nov '11

08 Nov '11

Hi, The console comes out lots of warning message and performs slowly when I compile my code. Google does not help. Does anybody know what it refers to? I'm using Mac OS X 10.7.2. bogon% make test ghc --make -isrc -fhpc Test.hs [1 of 2] Compiling Miner ( src/Miner.hs, src/Miner.o ) [2 of 2] Compiling Main ( Test.hs, Test.o ) Linking Test ... ld: warning: could not create compact unwind for .LFB3: non-standard register 5 being saved in prolog ld: warning: text reloc in _r15W_info to _base_GHCziWord_zdfIntegralWord32_closure ld: warning: text reloc in _r15W_info to _Cryptozm4zi2zi4_DataziDigestziSHA2_zdfShowHash8_closure ld: warning: text reloc in _r15W_info to non_lazy_ptr ld: warning: text reloc in _r15W_info to non_lazy_ptr ld: warning: text reloc in _r15W_info to non_lazy_ptr ld: warning: text reloc in _r15Y_info to _base_GHCziWord_zdfEqWord32_closure ld: warning: text reloc in _r15Y_info to _Cryptozm4zi2zi4_DataziDigestziSHA2_zdfEqHash8_closure ld: warning: text reloc in _r15Y_info to non_lazy_ptr ld: warning: text reloc in _r15Y_info to non_lazy_ptr ld: warning: text reloc in _r15Y_info to non_lazy_ptr ld: warning: text reloc in _r160_info to _r15Y_closure ld: warning: text reloc in _r160_info to _base_GHCziWord_zdfOrdWord32_closure ld: warning: text reloc in _r160_info to _Cryptozm4zi2zi4_DataziDigestziSHA2_zdfOrdHash8_closure ld: warning: text reloc in _r160_info to non_lazy_ptr ld: warning: text reloc in _r160_info to non_lazy_ptr ld: warning: text reloc in _r160_info to non_lazy_ptr ld: warning: text reloc in _r162_info to _base_GHCziWord_zdfEqWord8_closure ld: warning: text reloc in _r162_info to _base_GHCziClasses_zdfEqZMZN_closure ld: warning: text reloc in _r162_info to non_lazy_ptr ld: warning: text reloc in _r162_info to non_lazy_ptr ld: warning: text reloc in _r162_info to non_lazy_ptr ld: warning: text reloc in _r164_info to _r162_closure ld: warning: text reloc in _r164_info to _base_GHCziWord_zdfOrdWord8_closure ld: warning: text reloc in _r164_info to _base_GHCziClasses_zdfOrdZMZN_closure ld: warning: text reloc in _r164_info to non_lazy_ptr ld: warning: text reloc in _r164_info to non_lazy_ptr ld: warning: text reloc in _r164_info to non_lazy_ptr …... Best regards, Zhi-Qiang Lei zhiqiang.lei(a)gmail.com

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Issue with lazy eval + trace
by Hugo Ferreira 08 Nov '11

08 Nov '11

Hello, I am using "import Debug.Trace" to debug some code but it is not working as expected. I believe this is due to lazy evaluation. But I cannot see how I can circumvent it. To exemplify I have the following function: tagFun :: (POSTags -> Tag) -> POSTags -> POSTags tagFun f corpus = Z.foldlz' tag Z.empty corpus where tag acc z = Z.insert newTag acc where (token, correctTag, _proposdTag) = trace "cursor " Z.cursor z proposed = trace "proposed tag" (f z) newTag = ( token, correctTag, proposed ) If I place tracing at: tag acc z = trace ("tag "++token++" = "++ show newTag) Z.insert newTag acc then it works ok. However the lines (token, correctTag, _proposdTag) = trace "cursor " Z.cursor z proposed = trace "proposed tag" (f z) don't work. They only appear once. I assume they should appear with every element in the zipper. Can anyone explain why? TIA, Hugo F.

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multi-haskell management
by Rustom Mody 08 Nov '11

08 Nov '11

Ive just finished building haskell from source. [Thanks to all the folks who've made this so easy] 1. What is the official way for ensuring that some command like $ cabal install vector goes into my new haskell and not the old? 2. If I modify PATH so that the new ghc is found before the system one, will not system-ly things break (when for example I mess around in the haskell sources and break it?) [BTW I am on debian testing/gnome and I gather that an increasing amt of sysad stuff is being done in haskell. So I certainly do not want that I play around inside haskell sources and synaptic for example stops working :-) ]

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Re: [Haskell-beginners] Yesod umlaut issue
by Chaddaï Fouché 05 Nov '11

05 Nov '11

On Sat, Nov 5, 2011 at 3:59 PM, Obscaenvs <obscaenvs(a)gmail.com> wrote: > > As for a related issue, it isn't immediately obvious from the Yesod book how > I should go about producing "Γ" for instance in the html output -- if > I do > > gammaString = "Γ" > ...later... > #{gammaString} > > ...then this will result in "&Gamma;" being output in the html source. > Should I in this case also produce Html directly? It's pretty good having > these as shortcuts, as long as I do not have a keyboard that can easily > switch to, say, Greek encoding... > Well, I guess those shortcuts are only present in your own code and strings, so using something like > #{preEscapedText myStringWithEntities} is probably not too much of a security risk, just be more cautious if you're getting outside content... -- Jedaï

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Yesod umlaut issue
by Obscaenvs 05 Nov '11

05 Nov '11

Hello, friends. I come in peace. I would like there to be an "umlaut-translator"-function for Yesod (i.e. "ö" -> "ö"), but I have yet to find it. Furthermore the "#{}"-syntax does escaping of stuff like '<', '>' and, a more troubling '&' which is used in the umlaut-syntax. So any umlaut-translator would have to do this *after* the application of toHtml by the "#{}"-syntax which complicates writing ones own umlaut-converter. I may be totally wrong here; if so, feel free to correct me. I am also rather new to Yesod, but I am intent on giving it a good evaluation. I am going to write several web applications in the near future for different clients, and Yesod seems like the promising tool. However, I need this conversion tool -- I *think*; maybe I have the wrong angle altogether. /Fredrik

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Cabal: Upgrading to the latest version of library
by Hugo Ferreira 04 Nov '11

04 Nov '11

Hello, Recently I had to update and install a newer version of a library. I executed the "cabal update", which downloads the latest package list. I had assumed that this command would also detect, build and install those libraries that have been upgraded. However this does not seem to be the case. I had to manually build the library and then use GHC's package manager do unregister the old version. My question is: how does one check for and upgrade the cabal libraries automatically? TIA, Hugo F.

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Selecting single result of function application to list
by Hugo Ferreira 04 Nov '11

04 Nov '11

Hello, Apologies the simpleton question but I would like to know how it is done in Haskell. I have a list of values, and I am applying a function to each of these elements. The result is a Maybe. I would like to return the first occurrence which is not a Nothing. I am considering something like: selectOne f = take 1 . filter (\e -> case e of Just _ -> True _ -> False ) . map f I this how it is done? TIA, Hugo F.

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Stack space overflow: using strict accumulator still fails
by Hugo Ferreira 01 Nov '11

01 Nov '11

Hello, Have a stack overflow but cannot see why (read up on [1], may be missing something trivial). Once again using the http://nlpwp.org/ book code. If I call the following function, it blows its top: scoreRule :: TransformationRule -> Z.Zipper (Tag, Tag) -> Int scoreRule r z = nCorrect - nIncorrect where (nCorrect, nIncorrect) = scoreRule_ r z scoreRule_ :: TransformationRule -> Z.Zipper (Tag, Tag) -> (Int, Int) scoreRule_ r = Z.foldlz' (scoreElem r) (0, 0) where scoreElem r s@(nCorrect, nIncorrect) z = case ruleApplication r z of Just tag -> if tag == correct then (nCorrect + 1, nIncorrect) else (nCorrect, nIncorrect + 1) Nothing -> s where (correct, _) = Z.cursor z however I see that the eager version of foldlz is being used. I also though that maybe ruleApplication my not be executing immediately. But I cannot see why (added definition below for reference). Can anyone point out why this is not strict? TIA, Hugo F. [1] http://www.haskell.org/haskellwiki/Stack_overflow ruleApplication :: TransformationRule -> Z.Zipper (Tag, Tag) -> Maybe Tag ruleApplication (NextTagRule (Replacement old new) next) z = do (_, proposed) <- Z.safeCursor z (_, nextProposed) <- rightCursor z if proposed == old && nextProposed == next then Just new else Nothing ruleApplication (PrevTagRule (Replacement old new) prev) z = do (_, proposed) <- Z.safeCursor z (_, prevProposed) <- leftCursor z if proposed == old && prevProposed == prev then Just new else Nothing ruleApplication (SurroundTagRule (Replacement old new) prev next) z = do (_, proposed) <- Z.safeCursor z (_, nextProposed) <- rightCursor z (_, prevProposed) <- leftCursor z if proposed == old && prevProposed == prev && nextProposed == next then Just new else Nothing

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