- Beginners - Haskell.org

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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Pointfree expeiments
by Paul Sargent 26 Nov '10

26 Nov '10

I'd been playing with pointfree, looking at what certain patterns transformed into, and I came across this: pauls@Mudskipper> pointfree -v "\x -> x + x" ~ Transformed to pointfree style: ap (+) id Optimized expression: ap (+) id join (+) Basically I wanted to see what happened when you used a parameter multiple times within a function. Intrigued by this, I did the following: Prelude Control.Monad> :t ap (+) id ap (+) id :: (Num a, Monad ((->) a)) => a -> a I'm aware that 'ap' is related the liftM, but what is this monad, and why are we working in a monad at all? ...and surely this isn't the same as the original code as we now have a different type signature?

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Re: Problem "grouping" a list
by Hein Hundal 25 Nov '10

25 Nov '10

> From: Jo?o Paulo Pizani Flor <joaopizani(a)gmail.com> > OK, so my specific problem goes like this: I have a list of > tuples > :t myList > [ (Float, Integer) ] > > And I want to "group" this list into a nested list > groupAtoms :: [ (Float,Integer) ] -> [ > [(Float,Integer)] ] > > Of course, I want that the concatenation of the groups > yield me the original > list, i.e, (foldl (++) [] . groupAtoms == id), and > the criterion that > defines a group is that: > "The sum of the first elements in the tuples comprising the > list must be > greater than or equal to 1.0". Jo?o, I am a beginner also and I liked Daniel Fischer's solution. Here is another (less elegant) solution: -- Start Code Here -- foldfun takes an existing list of lists of pairs and decides -- whether to append p to the current list, pl, or start a new -- list of pairs. foldfun done p (pl:pls) = if done pl then [p]:pl:pls else (p:pl):pls foldfun done p [] = [[p]] -- mygroup2 folds list of pairs, pl, using the foldfun, and then -- puts all the lists in the correct order using reverse twice. mygroup2 pl = map reverse $ reverse $ foldr (foldfun done) [] (reverse pl) where done pl = sum (map fst pl) >= 1.0 -- End of Code Cheers, Hein

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Re: [Haskell-beginners] Problem "grouping" a list
by João Paulo Pizani Flor 24 Nov '10

24 Nov '10

Thank you all for the helpful responses! The solution from Daniel Fischer worked like a charm, the solution from Aai kinda worked, but I can't use because I don't want to modify the order of the elements in the list... And yes, my "criterion" was a bit ill-defined, so the very clever function from Ozgur satisfied it :P The real goal is to maximize the number of groups though. I'll test the solutions more hardly today and try to make the functions from Daniel even more clearer and elegant. I post the results back here! Thanks a lot! João Paulo Pizani Flor joaopizani(a)gmail.com Federal University of Santa Catarina 2010/11/23 Daniel Fischer <daniel.is.fischer(a)web.de> > On Tuesday 23 November 2010 18:23:28, João Paulo Pizani Flor wrote: > > Hello dear Haskellers! > > > > I've been a user and big fan of Haskell for a while, but only now I'm > > writing my first "big" project in Haskell (some thousands of lines of > > code perhaps). It's an interpreter for a programming language, the > > source code is music! Yes, sheet music! :D > > > > OK, so my specific problem goes like this: I have a list of tuples > > > > :t myList > > > > [ (Float, Integer) ] > > > > And I want to "group" this list into a nested list > > groupAtoms :: [ (Float,Integer) ] -> [ [(Float,Integer)] ] > > > > Of course, I want that the concatenation of the groups yield me the > > original list, i.e, (foldl (++) [] . groupAtoms == id), > > Here, foldr is the better choice. (++) can start delivering its result > before inspecting its second argument, hence > > foldr (++) [] (list1 : list2 : list3 : ...) > = list1 ++ (foldr (++) [] (list2 : list3 : ...)) > = list1 ++ (list2 ++ (foldr (++) [] (list3 : ...))) > > can be consumed as it is constructed, so it can run in constant space, and > it can stop early if not the entire result is needed. And it can deal with > infinite lists. > > The prelude function concat does this. > > foldl on the other hand can't deliver anything before the whole input list > has been traversed, in particular it doesn't terminate on infinite input, > it needs at least O(length result) space and it can't stop early. > Additionally, foldl (++) [] constructs left-nested (++)-applications, which > are bad for performance. > > > and the criterion that defines a group is that: > > "The sum of the first elements in the tuples comprising the list must be > > greater than or equal to 1.0". That is, given a list of tuples, the > > boolean predicate deciding whether this list is a PROPER group (True) or > > TOO SMALL (False) is: > > \g -> sum (map fst g) >= 1.0 > > > > > > Although the criterion is very clear, I've tried hard until now and > > couldn't come up with a function for producing the nested list based on > > this grouping criterion. > > Split the task into two parts. > > 1. collect the first group and the remainder of the list as a pair of lists > 2. cons the first group to what recursion on the remainder delivers > > Very much like the definition of `lines'. > > For your use-case, the condition when a group is complete doesn't only > depend on the list element but also on the previous, concretely the running > sum of the first components. > For a general such function, you need > - the stopping condition > - an accumulation parameter > - an update function for the accumulating parameter > as arguments. > For example (stupid names, sorry) > > > breakAcc :: (b -> Bool) -> (a -> b -> b) -> b -> [a] -> ([a],[a]) > breakAcc _ _ _ [] = ([],[]) > breakAcc cond comb acc xxs@(x:xs) > | cond acc = ([],xxs) > | otherwise = (x:ys,zs) > where > (ys, zs) = breakAcc cond comb (comb x acc) xs > > solves part 1, then > > > groupsAcc :: (b -> Bool) -> (a -> b -> b) -> b -> [a] -> [[a]] > groupsAcc _ _ _ [] = [] > groupsAcc cond comb acc xs = let (g,tl) = breakAcc cond comb acc xs > in g : groupsAcc cond comb acc tl > > gives you part 2, and your particular function is > > > groupAtoms :: [(Float,Integer)] -> [[(Float,Integer)]] > groupAtoms = groupsAcc (>= 1.0) ((+) . fst) 0 > > > Of course the last group need not be proper. > If there are long groups, the above can cause a space leak (as lines had in > GHC < 7), that can be avoided by sacrificing a little non-strictness and > replacing the let (g,tl) = ... in with a > > case ... of > (g,tl) -> g : groupsAcc ... > > or, if you need the non-strictness, with a slightly convoluted method as > used for lines in GHC 7. > > > I am sure that the Haskell Hierarchical > > Libraries have something to help me, but only now I see I'm still a big > > noob :P > > > > Could someone please help me writing this function? > > > > > > My best regards from Brazil, > > > > João Paulo Pizani Flor > > joaopizani(a)gmail.com > > Federal University of Santa Catarina > >

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