- Beginners - Haskell.org

Applicative functors and trees
by Darryn Reid 23 Mar '10

23 Mar '10

Hi, I'm playing around with Functors in Haskell with the aim of understanding more structured functors, particularly applicative functors and traversable and foldable. I'm using the standard binary tree type to play around with this, as follows: data Tree a = Empty | Leaf a | Node (Tree a) a (Tree a) I think I understand Functor fine: instance Functor Tree where fmap f Empty = Empty fmap f (Leaf x) = Leaf (f x) fmap f (Node l x r) = Branch (fmap f l) (f x) (fmap f r) (with (<$>) being a synonym for fmap). This satisfies the laws for Functor (I'll omit details). Likewise, I think I'm fine with Foldable, for Tree a over any monoid a: instance Foldable Tree foldMap f Empty = mempty foldMap f (Leaf x) = f x foldMap f (Node l x r) = foldMap f l `mappend` f x `mappend` foldMap f r Where I'm stuck is with Applicative: class (Functor f) => Applicative f where pure :: a -> f a (<*>) :: f (a -> b) -> f a -> f b I understand that pure lifts a value into the applicative functor, so this one is easy: pure = Leaf but I cannot see what <*> should look like. I know that <*> is essentially application ($) lifted into the functor, so if the functor were also a monad then <*> would be liftM2 ($), I suppose. But I want a "direct" implementation, without monads floating about, but I cannot see what to do. I think then I would be able to define an instance for traversable as something like this: instance Traversable Tree traverse f Empty = pure Empty traverse f (Leaf x) = Leaf `fmap` f x traverse f (Node l x r) = Node `fmap` traverse f (l <*> f x <*> traverse f r) (I think that <*> must be associative). Can anyone advise what <*> should look like, please? Thanks in advance for your help. Darryn.

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How to print the name of a variable
by John Velman 20 Mar '10

20 Mar '10

I have a number of variables (whose values are computed in my program) of the same type, and in ghci I want to be able to call a function to print something computed from the value of one or more of these to a file. I want the file name to reflect the name of the variable. This is probably not clear, but conceptually the function to do the output would look something like this: someFunction :: MyTypeVariable -> String writeFileFor :: MyTypeVariable -> IO () writeFileFor v = do nameOfV <- someFunction v outh <- openFile nameOfV ++ ".txt" WriteMode outputstring = calculationsFrom v hPutStr outh outputstring hClose outh Is there any such "someFunction", or is there a way to build one? Thanks, John Velman

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ghci parse error on input
by Rick Murphy 20 Mar '10

20 Mar '10

Hello All: Using GHCi, I am working from an existing example which surprisingly contains a parse error in an export statement. I have reproduced the parse error as follows: > module Export > ( > (^) > , (#) > ) > where > import Prelude hiding ((^)) GHCi reports: parse error on input (# I read 5.2 Export Lists in the Haskell Report, but the syntax above does not appear to contradict the Haskell Report. I have eliminated the following solutions: 1. Add (#) to the hiding list. 2. Swap the (#) and (^). 3. Load as non-literate file. Would anyone be able to provide advice on how to avoid this parse error? -- Rick blog: http://phaneron.rickmurphy.org

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Recursion/dynamic programming optimization
by ivovnenko 20 Mar '10

20 Mar '10

Hi. I tried to solve one of the google code jam's problems using haskel http://code.google.com/codejam/contest/dashboard?c=90101#s=p2: w :: Eq a => [a] -> [a] -> Int w text str = w' (reverse text) (reverse str) w' :: Eq a => [a] -> [a] -> Int -- Init w' _ [] = 0 w' [] _ = 0 -- For one symbol string w' text [a] = if head text == a then w' (tail text) [a] +1 else w' (tail text) [a] -- General case w' text str = if head str == head text then w' (tail text) (tail str) + w' (tail text) str else w' (tail text) str and it worked great on small inputs, not on large: w "viwiwhiuwwwgxzseebneesgzxiekzeeyivrbbhzrlllccxczyurxfhhccghkzvxusfbcucyvncpzpvfvccxcvbqnqogobzoiorxrbikyzzbobpmmmmpmvrzyivpfmmuzxixhqpeebyekkxffeyexeiukuebgbrukzefeu rkxy z h x vkq fsyp iy tfyzbptygtsrkhbytvruztzbttnoxuquokipyyqoik ib xu gxchcnccuccyfcccccsczccskifcprgovryxuxoozqoozsvudyfkdpkkxyhdvfsrnvpnpzvrhqhbduidkdpdysinyxnuxnxipddevqhgeufxeepyyezbekyviieeeryiexynevbzfeezryizvepynefffy y v zuzrbpzn sz y vzirkxjniuqvjjbjgubnjkujhjpiyanaabrqagzqqvspqpkvqyuhrgupbaiaasxybiygmmfimyvykkmrzum" "welcome to code jam" Can you please advice how to optimize this solution? Thanks. Regards, Ivan Vovnenko

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Simple lookahead with Parsec
by Derek Thurn 19 Mar '10

19 Mar '10

Hello. I'm just starting out with Parsec and Haskell, and I want to parse a very simple grammar with it. I think I may not understand how to use try correctly. Here's a simplified version of my grammar: myType = try (primitiveType) <|> arrayType primitiveType = do {reserved "int"; return "primitive"} arrayType = do { primitiveType; symbol "["; symbol "]"; return "array"} Basically, I want to use lookahead to produce the result "array" on inputs like "int[]" and the result "primitive" on inputs like "int". However, no matter what I do with the try function, I am not able to get what I expect. When I run this code using the runLex function described in the Parsec documentation, it chokes on "int[]", saying that the '[' character was unexpected. I've discovered that switching the order of primitiveType and arrayType solves this, which makes sense, but I still expected it to be possible to use try() in this manner to resolve the error with lookahead. Am I doing something wrong? (I've got some boilerplate code form the Parsec documentation defined as well, but I didn't include it for brevity's sake. Reserved and symbol are defined how you'd expect.)

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Getting into a Haskell project?
by Duncan Mortimer 19 Mar '10

19 Mar '10

Hi All, I'm very new to both Haskell, and open source development (well... really, development of any kind...). However, I'm really interested in getting good at both. I was wondering whether anyone has any suggestions about good open source Haskell projects I could cut my teeth on, and any advice in general in getting into this stuff more deeply? Thanks! Duncan

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how long it will take to install Haskell
by love_pku 18 Mar '10

18 Mar '10

Dear Sir or Madam: I met problems when install haskell. Following are the interface after do the command 'make',and it stay at the state 'Linking Setup'for a long time. So I am wondering how long it will take to finish 'linking setup'. #make scripts/build.sh Platform package mtl-1.1.0.2 is already installed. Skipping... Building "/usr/local/bin/ghc" "--make" "Setup" "-o" "Setup" "-package" "Cabal-1.6.0.3" Linking Setup ...

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about installation of Haskell
by love_pku 18 Mar '10

18 Mar '10

Dear Sir or Madam:: Sorry to trouble you! I am a beginner of Haskell and I just run into problems to install it. Things went smoothly when I do ./configure,but when I do the command 'make',errors appeared. The errors are as follows(red font): $make scripts/build.sh Building "/usr/bin/ghc" "--make" "Setup" "-o" "Setup" "-package" "Cabal" Linking Setup ... "./Setup" "configure" "--package-db=../../packages/package.conf.inplace" "--prefix=/usr/local" "--with-compiler=/usr/bin/ghc" "--with-hc-pkg=/usr/bin/ghc-pkg" "--with-hsc2hs=/usr/bin/hsc2hs" "--enable-library-profiling" Setup: Unrecognised flags: --package-db=../../packages/package.conf.inplace Error: Configuring the mtl-1.1.0.2 package failed make: *** [build.stamp] Error 2 So I do hope you can give me some suggestions. Thank you very much! Best wishes, Yours, juan

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[Announcement] Fedora 12 rpms for leksah 0.8.0.1 now available
by Lakshmi Narasimhan 18 Mar '10

18 Mar '10

Leksah - 0.8.0.1 rpms for Fedora 12 now available http://leksah.org/download.html About Leksah: *Leksah* is the Haskell IDE of choice. It is written in Haskell, uses Gtk, and runs on Linux, Windows and Mac OS X. Leksah is a practical tool to support the Haskell development process. It requires the Glasgow Haskell Compiler (GHC). *Installation Instructions * *Procedure for x86 platform* Before proceeding to install the leksah rpms, Uninstall leksah-0.6.1, ghc-binary-devel packages, incase you have installed them from the leksah download page. Use the following command yum remove leksah ghc-binary-devel Install these dependencies from the following location http://leksah.org/packages/dependencies-0.8.0.1.fc12.i386.tar.gz Extract the rpms from the tar.gz file. Select the rpms, right click and click Install. To install from command line, change into the extracted directory, become superuser and type in this command for x86 yum --nogpgcheck install ghc-binary-devel-0.5.0.2-1.fc12.i386.rpm ghc-deepseq-devel-1.1.0.0-1.fc12.i386.rpm ghc-ltk-devel-0.8-1.fc12.i386.rpm ghc-binary-shared-devel-0.8-1.fc12.i386.rpm ghc-hslogger-devel-1.0.10-1.fc12.i386.rpm haddock-leksah-2.5.0-1.fc12.i386.rpm Download the leksah rpms from this location http://leksah.org/packages/leksah-0.8.0.1-1.fc12.i386.rpm http://leksah.org/packages/leksah-server-0.8.0.1-1.fc12.i386.rpm Use the same procedure used for installing the dependencies. * Procedure for x86_64 platform* Before proceeding to install the leksah rpms, please install these dependencies from the following location http://leksah.org/packages/dependencies-0.8.0.1.fc12.x86_64.tar.gz Extract the rpms from the tar.gz file. Select the rpms, right click and click Install. To install from command line, change into the extracted directory, become superuser and type in this command yum --nogpgcheck install ghc-binary-devel-0.5.0.2-1.fc12.x86_64.rpm ghc-hslogger-devel-1.0.10-1.fc12.x86_64.rpm ghc-binary-shared-devel-0.8-1.fc12.x86_64.rpm ghc-ltk-devel-0.8-1.fc12.x86_64.rpm ghc-deepseq-devel-1.1.0.0-1.fc12.x86_64.rpm haddock-leksah-2.5.0-1.fc12.x86_64.rpm Download leksah rpms from this location http://leksah.org/packages/leksah-0.8.0.1-1.fc12.x86_64.rpm http://leksah.org/packages/leksah-server-0.8.0.1-1.fc12.x86_64.rpm Use the same procedure used for installing the dependencies *Running leksah *Once installation is completed, leksah menu item appears under Applications->Programming (in GNOME). -- Regards Lakshmi Narasimhan T V

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Strange error during compilation/linking
by Erik Quaeghebeur 18 Mar '10

18 Mar '10

Hi, I get the following error while compiling (I guess it's actually the linking step): $ghc -o cmimc cmimc.hs cmimc.o: In function `rP1_info': (.text+0x32a): undefined reference to `containerszm0zi2zi0zi1_DataziMap_zdf2_closure' cmimc.o: In function `rP3_info': (.text+0x396): undefined reference to `containerszm0zi2zi0zi1_DataziMap_zdf2_closure' cmimc.o: In function `rPh_info': (.text+0x666): undefined reference to `containerszm0zi2zi0zi1_DataziMap_fromList_closure' cmimc.o: In function `rPj_info': (.text+0x6ca): undefined reference to `containerszm0zi2zi0zi1_DataziMap_fromList_closure' cmimc.o: In function `sR5_info': (.text+0x1225): undefined reference to `__stginit_containerszm0zi2zi0zi1_DataziMap_' cmimc.o: In function `rP1_srt': (.data+0x90): undefined reference to `containerszm0zi2zi0zi1_DataziMap_zdf2_closure' cmimc.o: In function `rP3_srt': (.data+0xac): undefined reference to `containerszm0zi2zi0zi1_DataziMap_zdf2_closure' cmimc.o: In function `rPh_srt': (.data+0x15c): undefined reference to `containerszm0zi2zi0zi1_DataziMap_fromList_closure' cmimc.o: In function `rPj_srt': (.data+0x174): undefined reference to `containerszm0zi2zi0zi1_DataziMap_fromList_closure' collect2: ld returned 1 exit status However, running the program with ghci: $ghci cmimc.hs Prelude Main> :set args "testinput.txt" Prelude Main> :trace main gives the correct output... Where might there be a problem? In my installation (Kubuntu i386, ghc6.10.4-1ubunbtu2) perhaps? Please tell me if/which additional info I need to provide. TIA, Erik

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