April 2013 - Beginners

code review
by Chris Bolton 08 Apr '13

08 Apr '13

Just a little code review if anyone has the time :) http://codereview.stackexchange.com/questions/24741/simple-in-memory-db

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operations on cyclic data structures
by Josh Stratton 07 Apr '13

07 Apr '13

I'm new to haskell and functional programming in general. One of the things I've tried to wrap my brain around is dealing with cyclic immutable data structures like the half-edge data structure. First, how would an experienced haskeller design their data structures to perform an operation on this structure? I've implemented this structure with mutable structures in c++ but when when I've asked about cyclic structures I'm pointed to research papers which overwhelm my experience in immutable data structures like those in haskell. Also how efficient could this structure be if only a small portion of the mesh is updated? Would the entire mesh need to be rebuilt? Thanks.

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showing user defined data types
by Dan Lior 05 Apr '13

05 Apr '13

Hello, I defined a type Pixel : data Pixel = Black | White deriving (Eq, Show) and would like to "show" it as show :: Pixel -> String show Black = "#" show White = "." Apparently, ghc finds this ambiguous. That seems strange to me since Pixel is my own custom type and I don't imagine that ghc has another definition for showing it. Isn't this what "overloading" is supposed to enable? I can get around the problem by : import Prelude hiding(show) but that seems to drastic since it hides all the various overloaded incarnations of "show". I only need it to hide one of them (the one for Pixel). I'm obviously misunderstanding something. Is there a simple way to define my own show function for my own user defined data type? Thanks in advance. dan

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Vertices and Indices
by Adrian May 05 Apr '13

05 Apr '13

Hi Folks, I'm looking at the OpenGL stuff in Haskell and one thing is worrying me. Doing OpenGL in the past I used to pass an array of vertices to the GPU and then a list of indices into that array together with a method of joining them up, e.g. TriangleStrip. In Haskell though, it seems like I'm just supposed to send a list of vertices and the method, meaning that lots of vertices get duplicated. Is Haskell doing something clever under the hood here or is it really that wasteful? Or can I persuade it to use these index lists? TIA, Adrian.

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Prelude.(!!): index too large
by Roger Mason 04 Apr '13

04 Apr '13

Hello, The following fragment compiles but causes a runtime error: Prelude.(!!): index too large If I comment out the line 'hPutStr outh ( show ncat)' the program runs fine (but there is no output, of course). Thanks for any help. Roger writeOutput infile outfile list fox mw nox ncat units = do inh <- openFile infile ReadMode -- the file of analyses, one per line outh <- openFile outfile WriteMode mainloop inh outh list fox mw nox ncat units hClose outh hClose inh -- the calculations are here mainloop inh outh list fox mw nox ncat units = do ineof <- hIsEOF inh if ineof then do print list -- For testing/debugging only return () else do inpStr <- hGetLine inh let list' = inpStr:list let dat = floatData list' -- the analytical input data let ta = zipWith (/) dat units -- convert all to wt% let moles = zipWith (/) ta mw let apo = zipWith (*) nox moles let factor = fox/(sum apo) let nanions= (map (* factor) apo) let catox = zipWith (/) ncat nox let ncat = zipWith (*) catox nanions hPutStr outh ( show ncat) mainloop inh outh list' fox mw nox ncat units This electronic communication is governed by the terms and conditions at http://www.mun.ca/cc/policies/electronic_communications_disclaimer_2012.php

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Web.Encodings replacement
by João Cristóvão 04 Apr '13

04 Apr '13

Hi, I have a specific question followed by more general ones... I would like to use the decodeHtml function from Web.Encodings, but cabal reports this package as 'deprecated', so my first question is: is there anything equivalent? A search in Hayoo returns decodeHtmlEntities from Text.XML, but if I load it in ghci: decodeHtmlEntities "abc&aaa" returns ContentEntity "abc&aaa" And not ContentEntity "abc&aaa" or something like that. Is there a direct equivalent, or did I found the right replacement, but I am missing something? The more general questions are: 1) A 'deprecated' package means, from my understanding, that it is abandoned or at least not maintained to compile on the latest Haskell Platform, correct? 2) However, sometimes the package may be deprecated mainly because there are better implementations in other packages, which I would assume is the case here. Is there any easy way to check those 'package evolutions'? Thanks for your help, João

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can't make distribution, requires rerunning ./configure
by Josh Stratton 04 Apr '13

04 Apr '13

I recently downloaded the ghc distribution for Linux (x86_64) and had difficulty installing to my machine. The configure step seems to have completed successfully, but when I run the followup "make install", it fails suggesting that I haven't run the configure command yet. atlas => ./configure --prefix=/opt/ghc checking for path to top of build tree... /tmp/ghc-7.6.2 Build platform inferred as: x86_64-unknown-linux Host platform inferred as: x86_64-unknown-linux Target platform inferred as: x86_64-unknown-linux GHC build : x86_64-unknown-linux GHC host : x86_64-unknown-linux GHC target : x86_64-unknown-linux checking for perl... /rel/map/generic-2012.18.last/bin/perl checking if your perl works in shell scripts... yes checking for a BSD-compatible install... /usr/bin/install -c checking whether ln -s works... yes checking for gsed... sed checking for gcc... /usr/bin/gcc checking for gcc... /usr/bin/gcc checking whether the C compiler works... yes checking for C compiler default output file name... a.out checking for suffix of executables... checking whether we are cross compiling... no checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether /usr/bin/gcc accepts -g... yes checking for /usr/bin/gcc option to accept ISO C89... none needed checking version of gcc... 4.4.6 checking how to run the C preprocessor... /usr/bin/gcc -E checking whether ld understands --hash-size=31... checking whether ld understands --reduce-memory-overheads... checking for extra options to pass gcc when compiling via C... -fwrapv checking Setting up CFLAGS, LDFLAGS, IGNORE_LINKER_LD_FLAGS and CPPFLAGS... done checking Setting up CONF_CC_OPTS_STAGE0, CONF_GCC_LINKER_OPTS_STAGE0, CONF_LD_LINKER_OPTS_STAGE0 and CONF_CPP_OPTS_STAGE0... done checking Setting up CONF_CC_OPTS_STAGE1, CONF_GCC_LINKER_OPTS_STAGE1, CONF_LD_LINKER_OPTS_STAGE1 and CONF_CPP_OPTS_STAGE1... done checking Setting up CONF_CC_OPTS_STAGE2, CONF_GCC_LINKER_OPTS_STAGE2, CONF_LD_LINKER_OPTS_STAGE2 and CONF_CPP_OPTS_STAGE2... done checking for .subsections_via_symbols... no checking whether your assembler supports .ident directive... yes checking for GNU non-executable stack support... yes checking for grep that handles long lines and -e... /bin/grep checking for egrep... /bin/grep -E checking for ANSI C header files... yes checking for sys/types.h... yes checking for sys/stat.h... yes checking for stdlib.h... yes checking for string.h... yes checking for memory.h... yes checking for strings.h... yes checking for inttypes.h... yes checking for stdint.h... yes checking for unistd.h... yes checking size of void *... 8 checking for ar... /usr/bin/ar checking whether /usr/bin/ar is GNU ar... yes checking for ar arguments... q checking whether ranlib is needed... no configure: creating ./config.status config.status: creating settings config.status: creating mk/config.mk config.status: creating mk/install.mk **************************************************** Configuration done, ready to 'make install' (see README and INSTALL files for more info.) **************************************************** atlas => make install make -r --no-print-directory -f ghc.mk install BINDIST=YES NO_INCLUDE_DEPS=YES ghc.mk:122: *** Please run ./configure first. Stop. make: *** [install] Error 2

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Cabal package with C module main entry point - Bad compile order
by Darren Grant 03 Apr '13

03 Apr '13

Hi all, I'm currently attempting to create a cabal Configure build-type package to create an executable whose main entry point comes from a C file. I've almost got it working where the Main.hs exports a foreign function that is called from within the C module's main(), except for the fact that cabal wants to compile the C file before the hs *_stub.h is generated. Is there a way to fix this dependency? Cheers, Darren PS- This is related to the old problem of making SDL apps work on OSX.

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EmptyDataDecls
by Philippe Sismondi 03 Apr '13

03 Apr '13

Can anyone point me in the direction of some explanatory material on the motivation behind empty data declarations? I have read the GHC manual on this, but am none the wiser for it. Googling does not seem to turn up much either, although I may not have tried hard enough. When and why might this language extension be useful? - P -

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GLFW-b and texture loading
by emacstheviking 03 Apr '13

03 Apr '13

In case anybody else nedds it, I found a "solution" here... https://github.com/dagit/nehe-tuts/blob/master/lesson06.hs So I guess I can continue my tinkering and understanding of how more talented Haskell hackers do their thing...

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