- Beginners - Haskell.org

emacs mode?
by Rob Nikander 19 Mar '11

19 Mar '11

Hi all, Where can I get emacs mode for Haskell? I tried the following page but the links from it are hanging forever or dead. http://www.haskell.org/haskellwiki/Haskell_mode_for_Emacs Rob

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Lazy vs Strict ponderings...
by Sean Charles 19 Mar '11

19 Mar '11

I *think* I understand about lazy evaluation and its effects on I/O and that it can internally create thunks, promises, continuations etc. whatever you want to call them, and then at some point there will be a sudden spike in memory and CPU loads as something triggers a whole chain of promises to be fulfilled but what I don't have a feel for is just how serious a problem that might be today in say, a simple desktop application. For example, let's just say I write a simple application in 'C' and a comparable one in Haskell both running on the same hardware, but not concurrently! Would the Haskell one ever find itself running out of memory given the same input sequence as the 'C' program because of the way it works ? I know that the 'C' one could also run out of memory but I hope that the sentiment of my question is clear; what are the ramifications, gotchas and other considerations that using a purely functional lazy language like Haskell you need to be aware of when coming from an imperative / scripted background ? Notwithstanding the fact that there are strict flavours of functions that can be used.... but only if you are truly aware of the reasons for their existence in the first place, is this a real design issue to consider when coding an application in Haskell or is it only for certain 'groups' of applications. I guess the nature of the application would be the governing factor. Is it something one needs to worry about at all or should one just code away and write the application and then worry!? I think that having a clearer understanding of what 'types' of problems and their implementations have on CPU/RAM would be a good one to have! :) Thanks Sean Charles.

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Re: [Haskell-beginners] Unit conversion
by Tom Murphy 19 Mar '11

19 Mar '11

On Fri, Mar 18, 2011 at 5:02 AM, Magnus Therning <magnus(a)therning.org> wrote: > On Thu, Mar 17, 2011 at 21:17, Tom Murphy <amindfv(a)gmail.com> wrote: > > Hi All, > > Is there a good way to easily convert between units? > > For example, let's say I have a data type: > > data Volumes = Teaspoon | Tablespoon | Slice | FluidOunces > > ... and I want to define an infix function '<+>', which adds > together > > amounts of food: > > (1, Slice, cake) <+> (1, Volume Tablespoon, sugar) <+> (32, Volume > > FluidOunces, almondMilk) > > Which would return: > > (3200, Teaspoons) > > What is the most efficient way to define equivalency/conversion > between > > these measures? > > I remember an interesting method for celsius-farenheit conversion in > > Higher-Order Perl, using function composition, but that was between only > 2 > > units. I'd like something where I don't have to provide n^2 definitions. > > I wrote two blog posts on something that sounds related: > http://therning.org/magnus/archives/354 > http://therning.org/magnus/archives/355 > > Maybe they could help. > > /M > > -- > Magnus Therning OpenPGP: 0xAB4DFBA4 > email: magnus(a)therning.org jabber: magnus(a)therning.org > twitter: magthe http://therning.org/magnus So given data Measure = M Volume Float deriving (Show, Eq) data Volume = Slice | FluidOunce | Tablespoon | Teaspoon deriving (Show, Eq) Method one and two would be to convert all units to the smallest unit: toTsp :: Measure -> Measure [...] toTsp (M Slice a) = M Teaspoon (a * 350) toTsp (M FluidOunce a) = M Teaspoon (a * 34) [...] Perform the addition, then convert it to a more appropriate measure (back to Cake Slices...). It seems that there's a potential loss of precision, if I have to for example convert Megaliter measures to Teaspoons. Method three involves using typeclasses to define common "meeting points" for the units. So, we say that if we're adding Slices and FluidOunces, both measures should be converted to FluidOunces before the addition. The problem there is that, given n measurement units, we have to declare n^2 typeclass instances by hand. Another way that I can see to do this is to define a "chain" of conversions. I'd then "convert down" only to the smallest-common unit. So I could define (warning: extreme kludge ahead) a series of "step-down"s and "step-up"s: Given a largest-to-smallest ordering of Slice, FluidOunce, Tablespoon, Teaspoon, I can define: stepDown :: Measure -> Measure stepDown (M Slice a) = M FluidOunce (a * 120) stepDown (M FluidOunce a) = M Tablespoon (a * 2) stepUp (M FluidOunce a) = M Slice (a / 120) stepUp (M Tablespoon a) = M FluidOunce (a / 2) [...] and then a function to "step" as far down as needed: convertDown :: Measure -> Volume -> Measure convertDown (M unit measure) goalUnit | unit == goalUnit = M unit measure | otherwise = fun5 (stepDown (M unit measure)) goalUnit And then if I wanted to add a Slice of cake to a Tablespoon of almond milk, I'd have to find the smaller unit, convert down, perform the math and then do a "convert up" operation. An added benefit to this method is that defining a unit smaller than the current smallest wouldn't involve revising the "base unit" measures. This seems like a pretty common C.S. problem; I'm surprised I haven't been able to find more resources about it. Thanks for your time! Tom P.S. Thank you, Magnus, for those resources!

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haskell install on low memory server?
by Rob Nikander 19 Mar '11

19 Mar '11

Hi, I'd like to install Haskell on a linux virtual server with about 600 MB of RAM. (Maybe that is doomed to failure?) The goal is to try "happstack" for web apps, behind nginx. Compiling the whole platform is asking for OpenGL... so I thought I would get cabal and install pieces that I want. I downloaded the 'generic ghc binary' from here: http://hackage.haskell.org/platform/linux.html and that seems to work. But when I download the cabal-install tarball and try to run it's 'bootstrap.sh' I run out of memory. Is there a way to tell it to use less memory? ------- shell commands ------- $ cd cabal-install-0.10.2 $ sh bootstrap.sh Checking installed packages for ghc-7.0.2... Cabal is already installed and the version is ok. transformers-0.2.2.0 will be downloaded and installed. mtl-2.0.1.0 will be downloaded and installed. parsec-3.1.1 will be downloaded and installed. network-2.3.0.2 will be downloaded and installed. time is already installed and the version is ok. HTTP-4000.1.1 will be downloaded and installed. zlib-0.5.3.1 will be downloaded and installed. Downloading transformers-0.2.2.0... % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 100 17148 100 17148 0 0 27246 0 --:--:-- --:--:-- --:--:-- 39420 [1 of 1] Compiling Main ( Setup.hs, Setup.o ) Linking Setup ... collect2: ld terminated with signal 9 [Killed] Error during cabal-install bootstrap: Compiling the Setup script failed ------- end of shell commands ------- It seems strange that `ld` would need to use ~450 MB, which is what it's doing to get killed by the kernel. Any ideas? thanks, Rob

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Using Iteratees to write to a file
by Richard Wallace 19 Mar '11

19 Mar '11

Hello everyone, I've been reading "Iteratee: Teaching an Old Fold New Tricks" from the Monad Reader Issue 16 and am having a problem figuring out how to use the streamToFile function. Specifically, I can't figure out how to feed the Iteratee data to write to the file. If I run ghci> runIter $ streamToFile "/tmp/stream" Then the file is created but not populated with any data - which is natural since I didn't give it any data. I have no clue how to give it data though. I'd really appreciate it if someone could show me how to write "Hello world!" or something else equally mundane to get me going. I've been banging my head on this for hours now. Thanks, Rich

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Getting Gtk2Hs working in windows
by Kyle Murphy 19 Mar '11

19 Mar '11

I'm trying to follow the instructions provided on http://www.haskell.org/haskellwiki/Gtk2Hs to install Gtk2Hs in Windows 7 (64 bit) and running into problems. Originally I was trying to get wxHaskell installed, but it kept crashing trying to build the monolithic library due to an apparent bug in the latest port of GCC, so after talking with some of the people in #haskell I was advised that Gtk2Hs was better supported and might be easier to get working. For the most part, that appears to be true but I (as well as several others it seems) am running into a problem where cabal can't seem to find quite a few of the Gtk+ libraries (specifically z, cairo, gobject, glib, and intl). Near as I can tell everything should be functioning, and running pkg-config --modversion glib-2.0 outputs "2.20.3" so at least pkg-config seems to be able to find glib. Looking in the Gtk+\lib\pkgconfig folder I see pc files for everything but z and intl listed. Running cabal install gtk2hs-buildtools also succeeds, it's only when I get to the cabal install gtk step that things fail. cmd console output for the usual environment variables: INCLUDE: C:\libxml\include;C:\Program Files (x86)\Gtk+\include LIB: C:\libxml\lib;C:\Program Files (x86)\Gtk+\lib PKG_CONFIG_PATH: C:\libxml\lib\pkgconfig;C:\Program Files (x86)\Gtk+\lib\pkgconfig I trimmed the PATH output below for everyones sanity. The vast majority was stuff in the Windows folder or one of dozens of entries dumped in there by some version or other of Java. PATH: C:\Program Files (x86)\Haskell\bin; C:\Program Files (x86)\Haskell Platform\2011.2.0.0\lib\extralibs\bin; C:\Program Files (x86)\Haskell Platform\2011.2.0.0\bin; C:\MinGW\bin; C:\Users\Kyle\AppData\Roaming\cabal\bin; C:\Program Files (x86)\Haskell\bin; C:\Program Files (x86)\Gtk+\bin The exact error I'm getting when building is the following: C:\Users\Kyle>cabal install gtk Warning: The package list for 'hackage.haskell.org' is 23 days old. Run 'cabal update' to get the latest list of available packages. Resolving dependencies... C:\Users\Kyle\AppData\Local\Temp\cairo-0.12.05760\cairo-0.12.0\Gtk2HsSetup.hs:25:2: warning: #warning Setup.hs is guessing the version of Cabal. If compilationof Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when building (prefixed by --ghc-option= when using the 'cabal' command) [1 of 2] Compiling Gtk2HsSetup ( C:\Users\Kyle\AppData\Local\Temp\cairo-0.12.05760\cairo-0.12.0\Gtk2HsSetup.hs, C:\Users\Kyle\AppData\Local\Temp\cairo-0.12.05760\cairo-0.12.0\dist\setup\Gtk2HsSetup.o ) [2 of 2] Compiling Main ( C:\Users\Kyle\AppData\Local\Temp\cairo-0.12.05760\cairo-0.12.0\Setup.hs, C:\Users\Kyle\AppData\Local\Temp\cairo-0.12.05760\cairo-0.12.0\dist\setup\Main.o ) Linking C:\Users\Kyle\AppData\Local\Temp\cairo-0.12.05760\cairo-0.12.0\dist\setup\setup.exe ... Configuring cairo-0.12.0... setup.exe: Missing dependencies on foreign libraries: * Missing C libraries: z, cairo This problem can usually be solved by installing the system packages that provide these libraries (you may need the "-dev" versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. C:\Users\Kyle\AppData\Local\Temp\glib-0.12.05760\glib-0.12.0\Gtk2HsSetup.hs:25:2: warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when building (prefixed by --ghc-option= when using the 'cabal' command) [1 of 2] Compiling Gtk2HsSetup ( C:\Users\Kyle\AppData\Local\Temp\glib-0.12.05760\glib-0.12.0\Gtk2HsSetup.hs, C:\Users\Kyle\AppData\Local\Temp\glib-0.12.05760\glib-0.12.0\dist\setup\Gtk2HsSetup.o ) [2 of 2] Compiling Main ( C:\Users\Kyle\AppData\Local\Temp\glib-0.12.05760\glib-0.12.0\Setup.hs, C:\Users\Kyle\AppData\Local\Temp\glib-0.12.05760\glib-0.12.0\dist\setup\Main.o ) Linking C:\Users\Kyle\AppData\Local\Temp\glib-0.12.05760\glib-0.12.0\dist\setup\ setup.exe ... Configuring glib-0.12.0... setup.exe: Missing dependencies on foreign libraries: * Missing C libraries: gobject-2.0, glib-2.0, intl This problem can usually be solved by installing the system packages that provide these libraries (you may need the "-dev" versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. cabal: permission denied -R. Kyle Murphy -- Curiosity was framed, Ignorance killed the cat.

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Re: [Haskell-beginners] PHP vs Haskell... a challenge!
by Sean Charles 18 Mar '11

18 Mar '11

Thank you everybody for your responses, I am going to print them out and study them hard, particularly the use of left folds. I think I skipped that in RWH book... .I need to go back and re-cover the basic again I think... it's a tendency of mine to graze through stuff picking up what I need and sometimes you just can't beat learning the basics. As a still beginner after about eight months of (somewhat intermittent) usage I would say that in order to truly get to grips with Haskell as a concept let alone a language you need to know and understand ... * currying, which helped me to truly understand the signature notation used, a -> b -> c etc. I have used haXe, which is written in ocaml, and the notation is the same but now makes more sense! * function composition leading to point-less notation, this is better understood the currying penny has dropped * lazy versus strict evaluation and the concept of lazy I/O and why it can sometimes leave you scratching your head for a while!! LMAO! :) * monads: of course, for me the best video I've seen is this one. As a reluctant Drupal/PHP developer and wannabe FP developer for about six years now, each time I find myself coding some rubbish or other in PHP I have lots of 'ahah!' moments as I think that it would be nice to be able to express some idiom or form more succinctly in PHP and then I realise what X Y or Z is for in Haskell... that's the upside, the downside is I then have to continue the job in PHP! Thanks again, Sean.

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Unit conversion
by Tom Murphy 17 Mar '11

17 Mar '11

Hi All, Is there a good way to easily convert between units? For example, let's say I have a data type: data Volumes = Teaspoon | Tablespoon | Slice | FluidOunces ... and I want to define an infix function '<+>', which adds together amounts of food: (1, Slice, cake) <+> (1, Volume Tablespoon, sugar) <+> (32, Volume FluidOunces, almondMilk) Which would return: (3200, Teaspoons) What is the most efficient way to define equivalency/conversion between these measures? I remember an interesting method for celsius-farenheit conversion in Higher-Order Perl, using function composition, but that was between only 2 units. I'd like something where I don't have to provide n^2 definitions. Thank you! Tom

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Re: [Haskell-beginners] Expressing the idea of a group in Haskell
by . 17 Mar '11

17 Mar '11

Thanks to everyone for your explanations; and sorry about the group blunder ;) of course Brent is right, I should have said "R_+^*" plus multiplication. I will keep an eye on what the Haskell people do about Hierarchies of mathematical classes. Cheers Christian On Wed, 2011-03-16 at 16:03 +0100, beginners-request(a)haskell.org wrote: > Re: Expressing the idea of a group in Haskell

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PHP vs Haskell... a challenge!
by Sean Charles 17 Mar '11

17 Mar '11

I tried in vain using Text.CSV to write a small utility for myself that would *simply* map a CSV file across a template, specifically for mass-producing Drupal nodes from a CSV file... Here's what the Haskell code has to compete with :- TESTDATA.CSV eric,42,"Hacker" bert,20,"Janitor" harry,15,"Web nerd" TEST.SQL UPDATE foo SET rating = @1, role = '@2' where name = '@0'; And the PHP code that is used to produce ANY output from the template and CSV data (forget injection attacks and all that for now, this is proof-of-concept remember)... <?php $template = file_get_contents($argv[1]); if ($fh=fopen($argv[2],"r")) { while($line = fgetcsv($fh)) { foreach ($line as $k => $v ) $line['@'.$k] = $v; echo strtr($template,$line)."\n"; } fclose($fh); } ?> Note that I have had to remap the index keys to @n in order not to accidentally hit any numerical data that may be in the template. This is my test, the *real* template creates a Drupal node dynamically and has lots of [0]['value'] bits in it so I had to do this, I will allow any shortcuts to be used though as I would have liked to have coded the utility in Haskell to get better at it but tonight it defeats me! Here then is my unsatisfactory code and no, it doesn't compile or work very well as I am still trying hard with it! I want to learn! Still my biggest headache is type matching and deciphering the sometimes complete gibberish output from the type inference system when I screw up... I am finding it very hard to work out what I did wrong at times. Sitting here with Real World Haskell is proving fruitless tonight. :( Here's the rubbish code... import Text.CSV import System.Environment import System.IO import Text.ParserCombinators.Parsec.Error main = do args <- getArgs case length args of 2 -> do csvdata <- parseCSVFromFile (last args) either (\error -> print error) (\csvdata -> processData (head args) csvdata) csvdata _ -> print "Usage: csvsql template datafile.csv (options later!)" {- csvdata is [CSV] a CSV is [Record] a Record is [Field] a Field is String -} processData :: String -> CSV -> IO () processData filename csvdata = do tin <- openFile filename ReadMode template <- hGetContents tin processRow template csvdata hClose tin return () processRow :: String -> [Record] -> String processRow template row = "eric" ++ template

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