July 2008 - Haskell-Cafe

Trouble with non-exhaustive patterns
by Fernando Rodriguez 21 Jul '08

21 Jul '08

Hi, I defiend the following function to get the last element of a list: final [a] = a final (_:t) = final t and it works as expected. Since I didn't want to have a non exhaustive pattern, I added the following case: final [] = [] - I consider that the end of an empty list is the empty list final [a] = a final (_:t) = final t Suddenly, the function stoped working with a rather cryptic (for a newbie at least) error message: *Temp> final [4,5] <interactive>:1:9: No instance for (Num [a]) arising from the literal `5' at <interactive>:1:9 Possible fix: add an instance declaration for (Num [a]) In the expr*Temp> ession: 5 In the first argument of `final', namely `[4, 5]' In the expression: final [4, 5] What have I done so wrong? Thanks in advance, Fernando

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A good sign: languages used at Google Code Jam
by Don Stewart 21 Jul '08

21 Jul '08

http://www.go-hero.net/jam/lang Haskell as the highest ranked FP language in the Google Code Jam, with more submissions than Lisp, Scheme, SML and OCaml put together :) # C++ (used by 2875 people) # Java (used by 1747 people) # Python (used by 691 people) # C# (used by 609 people) # C (used by 453 people) # Perl (used by 193 people) # Ruby (used by 124 people) # PHP (used by 119 people) # Pascal (used by 90 people) # Haskell (used by 46 people) # Visual Basic (used by 43 people) # Objective-C (used by 24 people) # Lisp (used by 15 people) # Javascript (used by 13 people) # Scheme (used by 11 people) # Lua (used by 10 people) # Objective CAML (used by 10 people) # Basic (used by 9 people) # Bourne Shell (bash) (used by 5 people) # D (used by 4 people) # Groovy (used by 4 people) # J (used by 4 people) # Scala (used by 4 people) # ActionScript (used by 3 people) # Clojure (used by 2 people) # Eiffel (used by 2 people) # Erlang (used by 2 people) # SQL (used by 2 people) # Shell Script (sh) (used by 2 people) # Visual Basic Script (used by 2 people) # AWK (used by 1 people) # Arc (Lisp) (used by 1 people) # AutoIt Ver. 3 Script (used by 1 people) # Brainfuck (used by 1 people) # GNU Data Language (used by 1 people) # Informix 4GL (used by 1 people) # K (used by 1 people) # Maple (used by 1 people) # Standard ML (used by 1 people)

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Keeping a set of ForeignPtr's alive
by Felipe Lessa 21 Jul '08

21 Jul '08

Hi! -- Background: -- I have some foreign objects that are mapped to ForeignPtr's, say newtype Obj = O (ForeignPtr Obj) and a foreign "collection" where those objects are added. The adding is something on the lines of addObj, removeObj :: Collection -> Obj -> IO () id est, the C code of the collection maintains a reference to all objects added. That means I have to keep all the ForeignPtr's added alive and that I must not prevent garbage collection of those removed. Of course the 'Collection' isn't just a simple collection, otherwise I would be using normal Haskell counterparts. -- Question: -- What is the best way to keep the added objects alive? Using a list will keep the addition cheap and a low memory overhead, while a Data.Set will make removeObj work in sublinear time (on the Haskell side). Are there any other options? I'm currently heading towards data Collection = C !(ForeignPtr Collection) !(IORef [Obj]) like ForeignPtr itself (although in the case of ForeignPtr's there is no way to remove finalizers). Thanks! -- Felipe.

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Re: Help using Network.Curl
by Jim Burton 21 Jul '08

21 Jul '08

I tried to translate that using the Network.Curl docs and ended up with the following code, but it's not sending the post data correctly (an ebay api error re an "unsupported verb", which I am told means a malformed request). Any ideas? The code from a perl tutorial (which works for me, making me think that my problem is just about the right way to post xml to a web service) follows the haskell version, which is meant to be a straight translation of that. Thanks, Jim ---------------------- [..skipped definitions of tokens etc..] opts = [CurlEncoding "text/xml" , CurlHttpHeaders ["X-EBAY-API-COMPATIBILITY-LEVEL="++compatLevel , "X-EBAY-API-DEV-NAME="++devName , "X-EBAY-API-APP-NAME="++appName , "X-EBAY-API-CERT-NAME="++certName , "X-EBAY-API-CALL-NAME=GeteBayOfficialTime" , "X-EBAY-API-SITEID=0"]] body = "<?xml version='1.0' encoding='utf-8'?>" ++ "<GeteBayOfficialTimeRequest xmlns=\"urn:ebay:apis:eBLBaseComponents\">" ++ " <RequesterCredentials>" ++ " <eBayAuthToken>"++token++"</eBayAuthToken>" ++ " </RequesterCredentials>" ++ "</GeteBayOfficialTimeRequest>" url = "https://api.sandbox.ebay.com/ws/api.dll" postData = HttpPost {postName = "body" , contentType = Just "text/xml" , content = ContentString body , extraHeaders = [] , showName = Nothing} opts = [] --also tried using curlPost and setopts then perform... main = withCurlDo (do h <- initialize curlMultiPost url opts [postData] ) --------------------------- Original perl: use strict; use LWP::UserAgent; use HTTP::Request; use HTTP::Headers; # define the HTTP header my $objHeader = HTTP::Headers->new; $objHeader->push_header('X-EBAY-API-COMPATIBILITY-LEVEL' => '391'); $objHeader->push_header('X-EBAY-API-DEV-NAME' => 'yourdevname'); $objHeader->push_header('X-EBAY-API-APP-NAME' => 'yourappname'); $objHeader->push_header('X-EBAY-API-CERT-NAME' => 'yourcertname'); $objHeader->push_header('X-EBAY-API-CALL-NAME' => 'GeteBayOfficialTime'); $objHeader->push_header('X-EBAY-API-SITEID' => '0'); $objHeader->push_header('Content-Type' => 'text/xml'); # define the XML request my $request = "<?xml version='1.0' encoding='utf-8'?>" . "<GeteBayOfficialTimeRequest xmlns=\"urn:ebay:apis:eBLBaseComponents\">" . " <RequesterCredentials>" . " <eBayAuthToken>TOKENGoesHERE</eBayAuthToken>" . " </RequesterCredentials>" . "</GeteBayOfficialTimeRequest>"; # make the call my $objRequest = HTTP::Request->new( "POST", "https://api.sandbox.ebay.com/ws/api.dll", $objHeader, $request ); # deal with the response my $objUserAgent = LWP::UserAgent->new; my $objResponse = $objUserAgent->request($objRequest); if (!$objResponse->is_error) { print $objResponse->content; } else { print $objResponse->error_as_HTML; } ------------------------------ This is what I get when running my version: $ ./curlTest * About to connect() to api.sandbox.ebay.com port 443 (#0) * Trying 66.135.197.133... * connected * Connected to api.sandbox.ebay.com (66.135.197.133) port 443 (#0) * successfully set certificate verify locations: * CAfile: /etc/ssl/certs/ca-certificates.crt CApath: none * SSL connection using AES256-SHA * Server certificate: * subject: /C=US/ST=California/L=San Jose/O=eBay Inc./OU=Site Operations/CN=api.sandbox.ebay.com * start date: 2008-01-25 00:00:00 GMT * expire date: 2010-03-15 23:59:59 GMT * common name: api.sandbox.ebay.com (matched) * issuer: /C=US/O=VeriSign, Inc./OU=VeriSign Trust Network/OU=Terms of use at https://www.verisign.com/rpa (c)05/CN=VeriSign Class 3 Secure Server CA * SSL certificate verify ok. > POST /ws/api.dll HTTP/1.1 Host: api.sandbox.ebay.com Accept: */* Accept-Encoding: text/xml Content-Length: 1254 Expect: 100-continue Content-Type: multipart/form-data; boundary=----------------------------7c836e9408cd < HTTP/1.1 100 Continue < HTTP/1.1 200 OK < Date: Sat, 19 Jul 2008 20:04:12 GMT < Server: Microsoft-IIS/5.0 < X-EBAY-API-POOL-NAME: ___cDRidW90YmtiZWx9b2l3aQ== < X-EBAY-API-SERVER-NAME: ___dXUucnVlYnVvNzM3KTYzKzc3LTI0KTQxPD8zPTc= < Content-Type: text/xml < Content-Length: 330 < X-Cache: MISS from sjcsbagpigw02.sjc.ebay.com, MISS from sjcsbagpigw02.sjc.ebay.com < Connection: close < * Closing connection #0 <?xml version="1.0" encoding="UTF-8" ?><eBay><EBayTime>2008-07-19 20:04:55</EBayTime><Errors><Error><Code>2</Code><ErrorClass>RequestError</ErrorClass><SeverityCode>1</SeverityCode><Severity>SeriousError</Severity><Line>0</Line><Column>0</Column><ShortMessage><![CDATA[ Unsupported verb. ]]></ShortMessage></Error></Errors></eBay>

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GHC Error: "FATAL:Symbol _XJv_srt already defined."
by Ron Alford 21 Jul '08

21 Jul '08

Anyone know what this is about? Removing parts makes problem go away, but it's not clear what parts to remove! (that, and I wrote those parts for a reason!). bash-3.2$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.3 bash-3.2$ uname -a Darwin silverback-wired.home 9.4.0 Darwin Kernel Version 9.4.0: Mon Jun 9 19:30:53 PDT 2008; root:xnu-1228.5.20~1/RELEASE_I386 i386 $ rm -f DerivingError.o DerivingError.hi; ghc --make DerivingError.hs [1 of 1] Compiling DerivingError ( DerivingError.hs, DerivingError.o ) /var/folders/C0/C0SledGV2RaxbU+8ZLDnVU+++TI/-Tmp-//ghc27223_0/ghc27223_0.s:6080:0: FATAL:Symbol _XxG_srt already defined. -Ron Alford

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Haskell code contributions
by Galchin, Vasili 21 Jul '08

21 Jul '08

Hello, I have a password to upload Haskell code contributions but I can't find the email with the password and I have forgotten who is the "gatekeeper". ?? Regards, Vasili

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Data.Complex.magnitude slow?
by stefan kersten 20 Jul '08

20 Jul '08

hi, i've attached an example program which seems to indicate that the magnitude function from Data.Complex is very slow compared to a more naive implementation (for Complex Float). on my machine (intel core2 duo, osx 10.4) the CPU time using the library function is about 6-7 times as much as when using the other function. any ideas what might be going on? any flaws in my measurement code? thanks, <sk>

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Help with optimization
by Mitar 20 Jul '08

20 Jul '08

Hi! Profiling says that my program spends 18.4 % of time (that is around three seconds) and 18.3 % of allocations in this function which is saving the rendered image to a PPM file: saveImageList :: String -> Int -> Int -> [ViewportDotColor] -> IO () saveImageList filename width height image = do B.writeFile filename file where file = B.append header bytes header = C.pack $ "P6\n" ++ show width ++ " " ++ show height ++ "\n255\n" bytes = B.pack $ concatMap (color . dealpha . (\(ViewportDotColor _ c) -> c)) image where color (VoxelColor red green blue _) = [floor $ red * 255, floor $ green * 255, floor $ blue * 255] dealpha c = addColor c (VoxelColor 1.0 1.0 1.0 1.0) -- white background For a 921615 bytes large file to save this is too much in my opinion. And I think that it consumes to much allocations. Probably it should not store intermediate lists? Any suggestions? Best regards Mitar

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ghc 6.8.3 build error with __DISCARD__ linking problem, please help!
by Yu Di 19 Jul '08

19 Jul '08

Hi, I am trying to build ghc 6.8.3 from source release, my currently installed version is ghc 6.4.2 (x86 linux binary release version), and I got: /usr/local/ghc/bin/ghc -o ghc-pkg.bin -H16m -O -cpp -Wall -fno-warn-name-shadowing -fno-warn-unused-matches -DUSING_COMPAT -i../../compat -ignore-package Cabal -Rghc-timing -package unix -L../../compat -lghccompat Main.o Version.o CRT_noglob.o Main.o: In function `__stginit_ZCMain': ghc13533.hc:(.text+0x4): undefined reference to `__DISCARD__' ghc13533.hc:(.text+0x9): undefined reference to `__DISCARD__' Main.o: In function `__stginit_Main': ghc13533.hc:(.text+0x18): undefined reference to `__DISCARD__' ghc13533.hc:(.text+0x1d): undefined reference to `__DISCARD__' Main.o: In function `__stginit_Main_': ghc13533.hc:(.text+0x2c): undefined reference to `__DISCARD__' Main.o:ghc13533.hc:(.text+0x3d): more undefined references to `__DISCARD__' follow collect2: ld returned 1 exit status <<ghc: 13726704 bytes, 3 GCs, 105428/105428 avg/max bytes residency (1 samples), 15M in use, 0.00 INIT (0.00 elapsed), 0.01 MUT (1.98 elapsed), 0.01 GC (0.03 elapsed) :ghc>> Do you know how I can fix this problem? My ghc and gcc versions are: The Glorious Glasgow Haskell Compilation System, version 6.4.2 gcc (GCC) 4.1.1 20060724 (prerelease) (4.1.1-4pclos2007) Thanks! Di, Yu 7.19

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Trying to install cabal
by Eric 19 Jul '08

19 Jul '08

Dear all, I have downloaded cabal and am trying to install it but have gotten the following error message: C:\cabal\cabal-install-0.5.1>runghc Setup configure Configuring cabal-install-0.5.1... Setup: At least the following dependencies are missing Cabal >=1.4&&<1.5, HTTP >=3000&&<3002, zlib >=0.4 I'm not sure from this message what packages I need to install to get cabal up and running. Can anyone help? Eric M.

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