June 2008 - Haskell-Cafe

Parsing problem
by Eric 27 Jun '08

27 Jun '08

Hi all, I'm using the Parsec library to parse the following grammar expr = atom+ atom = integer | var | (expr) The input to the parser is a list of (Token, SourcePos). I have the following code for atom: atom = try variable <|> try integerr <|> do{sat(== Reserved "("); (e,pos) <- expression; sat(==Reserved ")"); return (e,pos)} sat p = do (t,pos) <- item if p t then return(t,pos) else pzero When I run the code on the input "(_ineg_ 0)" the parser fails, but removing sat(==Reserved ")") and it succeeds. Can any one see why? E.

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System.Console.Readline stifleHistory Question
by Daniel Kahlenberg 27 Jun '08

27 Jun '08

Hello all, I'm learning Haskell and so very likely will have advantages from the history persistence feature added to the ghci haskell interpreter. It drives very well in my setup (the history file is growing and used), but I wanted to increase the number of saved history entries and now my question concerning ghci: In the ticket (http://hackage.haskell.org/trac/ghc/ticket/2050) associated with the feature, there's written that one can call stifleHistory in its dotghci file. Have done it like: :m +System.Console.Readline stifleHistory 300 :m -System.Console.Readline but now my ghci.exe says: Could not find module `System.Console.Readline': Use -v to see a list of the files searched for. <interactive>:1:0: Not in scope: `stifleHistory' `uname -r`: MINGW32_NT-5.1 CLE10 1.0.11(0.46/3/2) 2007-01-12 12:05 i686 Msys Do you have an idea, where I'm wrong? I'm starting my ghci session from zsh shell, provided by cygwin, wrapped in a tty emulator: $Drive\puttycyg\putty.exe -cygterm $Drive\zsh\bin\zsh.exe -p -c 'PATH=/cygdrive/h/zsh/bin:$PATH HOME=/cygdrive/h/.homedir rlwrap $(which ghcii.sh) -package-conf $Drive/.homedir/ghc/i386-mingw32-6.8.3/package.conf -read-dot-ghci' So I did not expect Readline not to work, but maybe the ghc system only sees the result from `uname -r` above?? Bye, Daniel.

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Fwd: Stack Overflow with HaXml
by Samuel Silva 27 Jun '08

27 Jun '08

Hello I presents a small case studie, when I was using HaXml with DtdToHaskell. I write small DTD: > <!ELEMENT top (#PCDATA|child)* > > <!ELEMENT child (#PCDATA) > I run DtdToHaskell small.DTD > Extsubext.hs and test it with small XML > <top><child></child></top> Result it is presented below: >$ ghci Extsubset.hs >GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help >Loading package base ... linking ... done. >[1 of 1] Compiling Extsubset ( Extsubset.hs, interpreted ) >Ok, modules loaded: Extsubset. >*Extsubset> fReadXml "sample.xml" :: IO Top >Loading package array-0.1.0.0 ... linking ... done. >Loading package bytestring-0.9.0.1 ... linking ... done. >Loading package pretty-1.0.0.0 ... linking ... done. >Loading package old-locale-1.0.0.0 ... linking ... done. >Loading package old-time-1.0.0.0 ... linking ... done. >Loading package filepath-1.1.0.0 ... linking ... done. >Loading package directory-1.0.0.0 ... linking ... done. >Loading package random-1.0.0.0 ... linking ... done. >Loading package process-1.0.0.0 ... linking ... done. >Loading package haskell98 ... linking ... done. >Loading package polyparse-1.1 ... linking ... done. >Loading package containers-0.1.0.1 ... linking ... done. >Loading package HaXml-1.19.4 ... linking ... done. >*** Exception: stack overflow >*Extsubset> HaXml is able handle XML mixed elements? -- Don't hug that, Hugs ME! Samuel Silva

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Haskell, Microsoft, and interview questions
by Andrew Wagner 27 Jun '08

27 Jun '08

This post borders on being off-topic, but it seems like people on this list would be interested. First, some of you know that I had the distinct privilege of flying out to Microsoft this week to interview for a Software Development Engineer position on their Live Search team. So I want to first tell you about their reaction to my Haskell experience, and then give you some interesting coding problems that they asked me to solve, which could be good practice to tackle in Haskell. So the first task I was given to do, long before my in-person interview, was to complete a technical assessment. This consisted of two fairly simple algorithms to write. I chose to do so in Haskell. On the basis of this assessment, they decided I was worth a phone interview. When I got on that interview, one of the first things the interviewer said was "So, one thing that jumped out immediately was that you did the assessment in Haskell. Tell me a little bit about that." It led to a good discussion about why I like Haskell and functional programming. When I actually flew out there, several people made comments about me being "that guy who writes Haskell". The recruiter even said something along the lines of "anyone who knows haskell is certainly worth our time to talk to." Moral of the story: Haskell rocks, and even Microsoft knows it! That said, here are some interesting problems to work through in Haskell, if you'd like: 1.) A "rotated array" is an array of integers in ascending order, after which for every element i, it has been moved to element (i + n) mod sizeOfList. Write a function that takes a rotated array and, in less-than-linear time, returns n (the amount of rotation). 2.) You are given a list of Ball objects. Each Ball is either Red or Blue. Write a function that partitions these balls so that all of the balls of each color are contiguous. Return the index of the first ball of the second color (your result can be Red balls, then Blue balls, or the other way around). In haskell, you'll probably want to return a ([Ball],Int). 3.) Live Search is a search engine. Suppose it was to be tied into an online store. Now you're given two lists. One is a [(SessionId, NormalizedQuery)]. That is, when a particular user performs a query, it is turned into some consistent format, based on their apparent intent, and stored in this logfile. The second list is a [(SessionId, ProductId)]. This indicates the product bought by a particular user. Now, you want to take these two (potentially very long) lists, and return some structure that will make it easy to take a query and return a list of the most popular resulting purchases. That is, of people who have run this query in the past, what are the most common products they've wound up buying? The interviewer said that this is an instance of a well-known problem, but I didn't catch the name of it. 4.) You're given an array which contains the numbers from 1 to n, in random order, except there is one missing. Write a function to return the missing number. 5.) Write a function to reconstruct a binary tree from its preorder traversal and inorder traversal. Take into account that the traversals could be invalid. 6.) You have a [(WeatherStationId, Latitude, Longitude)]. Similar to #3, write a function which will, off-line, turn this into a data structure from which you can easily determine the nearest Weather Station, given an arbitrary Latitude and Longitude. 7.) Write a function for scoring a mastermind guess. That is, in a game of mastermind (http://en.wikipedia.org/wiki/Mastermind_(board_game)) given a guess, and the actual answer, determine the number correct, the number wrong, and the number out of place. 8.) Implement a trie (http://en.wikipedia.org/wiki/Trie) data structure. Write a function add, which takes a word, and a trie, and adds the word to the trie. Write another function lookup, which takes a prefix and a trie, and returns all the words in the trie that start with that prefix. 9.) Write an algorithm to shuffle a deck of cards. Now write a function to perform some kind of evaluation of "how shuffled" a deck of cards is. Hope you enjoy!

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haskell and dockapps
by Martin DeMello 26 Jun '08

26 Jun '08

Anyone written a windowmaker dockapp in Haskell? I thought it'd be a fun project, but I don't quite know where to start. martin

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Internet Communications Engine and Haskell
by Mitar 26 Jun '08

26 Jun '08

Hi! Has been any work on implementing Internet Communications Engine in Haskell already done? Any other suggestions how could I use ICE in Haskell? Through FFI calls to its C++ version? http://www.zeroc.com/ice.html Mitar

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hxt and pickler combinations
by Andrea Rossato 26 Jun '08

26 Jun '08

Hello, I'm using HXT for writing a Citation Style Language (http://xbiblio.sourceforge.net) implementation in Haskell and I'm trying to use the hxt pickler library to parse XML data contained in elements that can be interleaved, that is to say, elements that can appear in any order within other elements. For instance: <data> <string>ciao</string> <int>2</int> </data> or <data> <int>2</int> <string>ciao</string> </data> are both permitted. I'm not able to write picklers able to parse such kind of data. I indeed noticed that this is not possible with interleaved elements, but it is possible with attributes. To make myself hopefully clearer I included some code below. Suppose we have a data structure like: data Term = T Int String deriving ( Show ) If encoded in XML without respecting the ordering (first the Int and then the String), picklers seem to fail. But if I use attributes to store the values, this doesn't happen. In other word, the xp1 pickler (taken from the example below) will fail with such an xml doc: <data> <string>ciao</string> <int>2</int> </data> since it requires the 'int' element to appear before the 'string' element. To test this behaviour, run the code below and see that: - test doc1 xp1 will fail - test doc2 xp1 will succeed while: - test doc3 xp2 will succeed - test doc4 xp2 will succeed What am I getting wrong? It's just a matter of wrong combinator I'm choosing or I'm getting wrong something more fundamental? TIA. Andrea The code: import Text.XML.HXT.Arrow test :: String -> PU Term -> IO () test t xp = do p <- runX ( constA t >>> xread >>> xunpickleVal xp ) putStrLn (show p) data Term = T Int String deriving ( Show ) xp1, xp2 :: PU Term xp1 = xpElem "data" $ xpWrap (uncurry T, \(T i s) -> (i, s)) $ xpPair (xpElem "int" xpickle) (xpElem "string" xpText ) xp2 = xpElem "data" $ xpWrap (uncurry T, \(T i s) -> (i, s)) $ xpPair (xpAttr "int" xpickle) (xpAttr "string" xpText ) doc1, doc2, doc3, doc4 :: String doc1 = "<data><string>ciao</string><int>2</int></data>" doc2 = "<data><int>2</int><string>ciao</string></data>" doc3 = "<data int=\"2\" string=\"ciao\" />" doc4 = "<data string=\"ciao\" int=\"4\" />"

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ghc-HEAD: build succeeds, but install fails with linker errors?
by Reid Barton 26 Jun '08

26 Jun '08

I'm trying to install GHC from the darcs repository for the first time, so I'm hoping someone here can tell me if I'm doing something wrong before I bother cvs-ghc. I'm currently running ghc 6.8.2 on Debian x86 Linux. Following the instructions on the wiki, I put the following in a file and ran it: #!/bin/bash darcs get --partial http://darcs.haskell.org/ghc cd ghc chmod +x darcs-all ./darcs-all get sh boot ./configure --prefix=$HOME/ghc-HEAD make make install but during the "make install" step I get the error ../../compiler/stage1/ghc-inplace -no-user-package-conf -o ghc-pkg.bin -H32m -O -cpp -Wall -fno-warn-name-shadowing -fno-warn-unused-matches -package Cabal -Rghc-timing -package unix -package containers Main.o Version.o CRT_noglob.o Main.o: In function `s7AP_info': (.text+0x1d): undefined reference to `prettyzm1zi0zi0zi0_TextziPrettyPrintziHughesPJ_Str_con_info' {- hundreds of similar errors (not all from the pretty package, some from base and others as well) -} collect2: ld returned 1 exit status <<ghc: 50428432 bytes, 6 GCs, 131072/131072 avg/max bytes residency (1 samples), 31M in use, 0.00 INIT (0.00 elapsed), 0.07 MUT (0.55 elapsed), 0.02 GC (0.02 elapsed) :ghc>> make[2]: *** [ghc-pkg.bin] Error 1 Failed making install in ghc-pkg: 1 make[1]: *** [install] Error 1 make: *** [install] Error 1 Have I got some of the steps wrong? Regards, Reid Barton

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Access to Oracle database from Haskell
by Henning Thielemann 26 Jun '08

26 Jun '08

Is there a way of accessing a remote Oracle database by one of the common Haskell database interfaces (HaskellDB, Takusen, etc.) ? I tried to get unixODBC and Oracle's Instant Client running on a Linux machine, but I'm trapped in the notorious error: $ isql USER -v [IM004][unixODBC][Driver Manager]Driver's SQLAllocHandle on SQL_HANDLE_HENV failed [ISQL]ERROR: Could not SQLConnect This error message is discussed in various web forums, but there seems to be no systematic way to track down the problem. So I wonder whether there is another way to access the Oracle data base from Haskell.

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Stupid question about Cabal file
by Fernand 26 Jun '08

26 Jun '08

Hi, Excuse me in advance if this is a trivial question, but I have been unable to find (understand?) the answer in Cabal's documentation. My issue is simple : I build a Cabal package named, let's say, "foo". That package is a library (libHSfoo.a, something like that), which exposes its Foo.Bar and Foobar modules. I wrote the cabal file, register it, everything seems fine. The issue comes when I try to use that package when compiling a main program, like this : ghc -package foo MyMain.hs [other options] I get an error about the compiler not finding the interface files Foobar.hi and Bar.hi, as I have "import Bar" and "import Foo.Bar" in my "MyMain.hs" file. Of course, I could add "-l" flags to point to the ".hi" files that I used to compile my package, but I get the well-known issue about the fact that the package's names of the interface files (which were compiled with the "-package-name foo" option) are not the correct ones ("Bad interface file: ../foo/Foobar.hi, Something is amiss; requested module main:Foobar differs from name found in the interface file foo:Foobar") ; and that solution is ugly anyway. So far, I found some Cabal documentation talking about the import-dirs property, but I did not manage to "make it work", I mean, my package does not seems to "include" the interface files. I am sure, on the other hand, that it is possible :) Sincerely yours, Fernand

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