November 2010 - Beginners

dealing with chained functions
by Alec Benzer 21 Apr '11

21 Apr '11

Is there a generally accepted "better" way of dealing with the following: say I have three functions that all rely on output from each other. Ie, we get raw input, which one function processes. another function then requires the output from this function and produces some other output, which a third function then requires. would it be better to implement the functions like this: f1 :: a -> b f2 :: b -> c f3 :: c -> d final :: a -> d or like this: f1 :: a -> b f2 :: a -> c f3 :: a -> d Ie, in the latter set, f2 calls f1 on its input, and then proceeds to process it. f3 calls f2 on its input, etc. In the former case, final would just chain all three functions together itself (or I might just do it in main or somewhere else that's appropriate) Are one of these methods generally considered a better practice than the other, specifically in a functional context? My gut tells me that the former method looks more appropriate, but I'm not sure if it'd really be the accepted practice regardless of context, or if one might use either of these methods depending on the specifics of what they're doing. Or if it's just a matter of taste/opinion?

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Named Pipes Et Cetera
by matthew coolbeth 06 Dec '10

06 Dec '10

Hello, all. Three questions: 1> What, if anything, can I do in a Haskell program to do blocking reads from a Linux FIFO? For the curious, there is a small program below that doesn't block, but immediately terminates because of an EOF 2> How could I have found the answer to this question on my own? (The documentation that Google found for me was not completely adequate) 3> Is this sort of question generally regarded as too unixy or not haskelly enough to be appropriate for this mailing list? Thanks. --BEGIN PROGRAM-- module Main () where import Control.Monad import System.IO import System.Posix.Files pipeString = "/tmp/dld.fifo" main :: IO () main = do createNamedPipe pipeString $ unionFileModes ownerReadMode ownerWriteMode pipe <- openFile pipeString ReadMode forever (hGetLine pipe >>= putStrLn) -- mac

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Timer Events?
by matthew coolbeth 01 Dec '10

01 Dec '10

I'd like to write a Haskell program that executes an IO action every 50 milliseconds. It would be most helpful if someone could point me in the right direction. -- mac

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create a directory
by José Romildo Malaquias 30 Nov '10

30 Nov '10

Hello. I need a function to create a directory given its path. The function should return a "Maybe FilePath": "Nothing" for a failure, and "Just path" otherwise. As I am not yet good at exception handling, would someone help me with this function? Romildo

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hGetContents, unicode and linux
by Erik de Castro Lopo 28 Nov '10

28 Nov '10

Hi all, I've got a trivial test program: main :: IO () main = do text <- readFile "unicode.txt" putStr text which I compile with ghc-6.12.1 (from Debian) and when it runs I get: hGetContents: invalid argument (Invalid or incomplete multibyte or wide character) I've done some googling which seems to suggest that I need to set the LANG environment variable, but I already have that set to en_AU.UTF-8. Clues? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Re: [Haskell-cafe] Unable to install packages
by Tim Baumgartner 28 Nov '10

28 Nov '10

Hi Roland, I installed Leksah on Ubuntu 10.10 recently using the instructions found on http://rizwanbulbul.blogspot.com/2010/06/installing-leksah-gtk-gtk2hs-and-g… It was quite straight forward for me. Regards Tim 2010/11/27 Roland Senn <rsx(a)bluewin.ch>: > I learned a little bit Haskell with Hugs on Windows and now I want to > develop on Linux. > > I installed the Haskell Platform on Unbunto 10.10. There were no problems. > Then I tried to install Leksah with > sudo cabal install leksah --global > It failed with: > Resolving dependencies... > /tmp/gtksourceview2-0.11.12661/gtksourceview2-0.11.1/Gtk2HsSetup.hs:25:0: > 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 ( > /tmp/gtksourceview2-0.11.12661/gtksourceview2-0.11.1/Gtk2HsSetup.hs, > /tmp/gtksourceview2-0.11.12661/gtksourceview2-0.11.1/dist/setup/Gtk2HsSetup.o > ) > [2 of 2] Compiling Main ( > /tmp/gtksourceview2-0.11.12661/gtksourceview2-0.11.1/Setup.hs, > /tmp/gtksourceview2-0.11.12661/gtksourceview2-0.11.1/dist/setup/Main.o ) > Linking > /tmp/gtksourceview2-0.11.12661/gtksourceview2-0.11.1/dist/setup/setup ... > Configuring gtksourceview2-0.11.1... > Preprocessing library gtksourceview2-0.11.1... > Building gtksourceview2-0.11.1... > [ 1 of 13] Compiling Graphics.UI.Gtk.SourceView.Signals ( > dist/build/Graphics/UI/Gtk/SourceView/Signals.hs, > dist/build/Graphics/UI/Gtk/SourceView/Signals.o ) > Graphics/UI/Gtk/SourceView/Signals.chs:51:46: > Module `System.Glib.UTFString' does not export `maybePeekUTFString' > cabal: Error: some packages failed to install: > gtksourceview2-0.11.1 failed during the building phase. The exception was: > ExitFailure 1 > leksah-0.8.0.8 depends on gtksourceview2-0.11.1 which failed to install. > > I searched the Internet on "gtksourceview2 maybePeekUTFString" and found a > patch: > http://www.mail-archive.com/gtk2hs-devel@lists.sourceforge.net/msg00971/gtk… > > As a newby with the Haskell tools, I searched the Internet on how to install > a patch. > I found dpm and I tried to install it with: > > sudo cabal install dpm --global > > It failed with: > configure: error: in `/tmp/terminfo-0.3.1.31936/terminfo-0.3.1.3': > configure: error: curses headers could not be found, so this package cannot > be built > See `config.log' for more details. > cabal: Error: some packages failed to install: > DPM-0.2.1 depends on terminfo-0.3.1.3 which failed to install. > darcs-2.5 depends on terminfo-0.3.1.3 which failed to install. > haskeline-0.6.3.2 depends on terminfo-0.3.1.3 which failed to install. > terminfo-0.3.1.3 failed during the configure step. The exception was: > ExitFailure 1 > > So after installing Haskell Platform, I only found broken packages! > > What can I do to install Leksah? > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Learning StateT
by Tim Baumgartner 27 Nov '10

27 Nov '10

Hi Haskellers, in order to learn Monad Transformers, I'm trying to write a simple calculator that has an internal state of type Calc that is modified after each line the user enters. I started like this: main :: IO () handleLine :: StateT Calc IO Bool -- return's False in order to exit main uses runState in order to extract the action it needs from the transformer. Up to this point, it works fine. But in order to separate I/O and parsing, I'd prefer to have another function processInput :: String -> State Calc (Maybe String) that updates the internal state and returns either a String that should be printed or Nothing in order to exit. Trying to do that, I didn't see another possibility but to call runState again from inside handleLine. I guess there is a better solution. Can you help? Thanks in advance Tim

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Re: Beginners Digest, Vol 29, Issue 41
by Russ Abbott 27 Nov '10

27 Nov '10

Brent Yorgey wrote > Someone did respond to your question, with a link to the data-accessor > package on Hackage (you could also take a look at fclabels or > lenses). Do those help address your issue? It's not accessing the data that concerns me, it's updating other elements. Imagine a system that tracks student registration. data Student = Student { sName :: String {- , ... Other stuff -} , coursesTaking :: [Course] } * data Rank = Assist | Assoc | Full data Instructor = Student { iName :: String , rank :: Rank -- defined, say, as data Rank = Assist | Assoc | Full {- , ... Other stuff -} , coursesTeaching :: [Course] } data Course = Course { cName :: String , units :: Int {- , ... Other stuff -} , instructor :: Instructor , enrolledStudents :: [Student] } Suppose I want a function that drops a student from a class. drop :: (Course, Student) -> (Course, Student) I'm declaring the function as pair to pair because both change. If I run drop, I get a new Course record and a new Student record. Doesn't that mean I have to change all the Student, Instructor, and Course record s that refer to them, which also means I have to change all the Course record s that refer to them, etc.? Wouldn't I have to write something like this? data DataBase = DB { students :: [Student] , instructors :: [Instructor] , courses :: [Course] } drop :: DB -> Course -> Student -> DB But even if I do that, doesn't that require a lot of code to get everything updated? Is there a simple way to write that function? In a previous message I guessed that the best way to do it is with Maps, e.g. Map Student [Course] etc. I'd very much appreciate seeing the recommended code for the drop function. Thanks. * *-- Russ* P.S. I'll admit that I've spent virtually no time with monads and would prefer to avoid them if possible.

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Random computation with fixed seed value (Control.Monad.Random)
by Jan Snajder 26 Nov '10

26 Nov '10

Hi, I'd like to evaluate a random computation using Control.Monad.Random, but I'd like to be able to fix the seed value. I know I can use mkStdGen from System.Random to get an initial generator. I also know I can use evalRand :: RandomGen g => Rand g a -> g -> a from Control.Monad.Random to evaluate a random computation. But the problem is I don't see how I can turn a StdGen value that I get from mkStdGen into a Rand type. What am I missing here? Best, Jan

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Question about data structures
by Russ Abbott 26 Nov '10

26 Nov '10

I would appreciate some advice about the best way to manipulate data structures in Haskell. Let's assume I have what in an OO language would be a class with a number of instance variables. When I want to change one of the values of one of those instance variables in Haskell I must rebuild the entire structure. Even worse, if one of those instance variables is a reference to another data structure, then when I change that referenced data structure, I am forced to rebuild my top level variable. For example. data Struct1 = Struct1 {var1 :: Struct11, var2 :: Struct1, ... } data Struct11 = Struct11 {var11 :: ... } Let's assume I have x :: Struct1 and that I change the value of var11 . var1 $ x. Doesn't that require that I rebuild x? Is there a better way? Thanks. * -- Russ*

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