September 2009 - Beginners

Help with CSV
by Hong Yang 07 Sep '09

07 Sep '09

I need to process csv files that have the characteristics as follows: 1) each file has thousands of columns which have String, Int, and Double types 2) the number of columns may change 3) for those columns whose name do not change, their location may change I want to process some columns in 3) using Haskell. In Perl, I can easily have the code like below: use Text::CSV; my $csv = Text::CSV->new( { allow_whitespace => 1 } ); open my $temp, "<", "temp.csv" or die "Cannot open temp.csv! ($!)"; my @fields = @{ $csv->getline($temp) }; $csv->column_names(@fields); while ( my $hr = $csv->getline_hr($temp) ) { my $sn = $hr->{"UNIT:unitSerialNumber"}; # processing goes here ... } close $temp; Can someone please give me an equivalent code in Haskell? Then I can digest and expand it. Thanks, Hong

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idiomatic haskell question
by Tom Doris 07 Sep '09

07 Sep '09

Hi all, I've recently started using Haskell and am looking for some feedback on code I've written; I sometimes feel that maybe I'm not doing things the best way possible that Haskell allows, and maybe missing out on obvious improvements in brevity or elegance. So here's a tic-tac-toe solver I wrote that does basic min-max search of the entire tree (not efficient algorithmically, but it's tictactoe so a blank board can be fully solved in a few seconds). Also, if people have suggestions on how to change the program to actually output the moves it would make, please let me know - right now it just responds with 1 for a win for X, 0 for a draw, and -1 for a win for O. And there are probably bugs! Thanks in advance import Data.List data Box = Blank | X | O deriving (Eq, Show) -- usage: call score with the board rows concatenated: score [Blank, Blank, Blank, Blank, Blank, Blank, Blank, Blank, Blank] -- or score [Blank, X, O, Blank, X, O, Blank, Blank, Blank] -- score is 1 if X wins, 0 for draw, -1 for lose score :: [Box] -> Int score g | haveline X g = 1 | haveline O g = -1 | gridfull g = 0 | isxmove g = maximum (map score (makeallmoves X g)) | otherwise = minimum (map score (makeallmoves O g)) tolines :: [Box] -> [[Box]] tolines [a1, a2, a3, b1, b2, b3, c1, c2, c3] = [ [a1,a2,a3], [b1,b2,b3], [c1,c2,c3], [a1,b1,c1], [a2,b2,c2], [a3,b3,c3], [a1,b2,c3], [a3,b2,c1] ] haveline :: Box->[Box] -> Bool haveline b g = any ([b,b,b]==) (tolines g) gridfull :: [Box] -> Bool gridfull g = not $ any (Blank==) g isxmove :: [Box] -> Bool isxmove g = let movecount = sum $ map (\b -> if b==Blank then 0 else 1) g in mod movecount 2 == 0 isomove = not . isxmove fl :: Box->[Box]->[Box]->[[Box]] fl b xs (Blank:ys) = [xs ++ b:ys] fl _ _ _ = [] makeallmoves :: Box->[Box]-> [[Box]] makeallmoves b g = concat $ zipWith (fl b) (inits g) (tails g)

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qtHaskell, ForeignPtr
by Michael Mossey 07 Sep '09

07 Sep '09

I'm trying to learn qtHaskell, an interface to the Qt GUI library. I am fairly new to Haskell, but have used Qt for a long time, so I thought I could probably reasonably attempt to grok qtHaskell at this point. My main question is: anyone recommended a good explanation of the foreign function or foreign pointer interface? My immediate question is that I was poking through the Qtc docs, and saw this: Documentation data Object a Constructors QObject !(ForeignPtr a) I am not sure how Haddock works. Is this telling me that an Object constructor takes one argument, which is of type QObject, where QObject is a type constructor that takes !(ForeignPtr a)? Or is this saying it takes two arguments, a QObject and a !(ForeignPtr a). That latter makes more sense I guess. What does the ! mean? Thanks, Mike

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oops
by Michael Mossey 07 Sep '09

07 Sep '09

I'm sorry, I just noticed one of my prior questions was pretty silly. The second Object is the name of the type constructor. I realized that now. I was just fooled by the layout of Haddock and got temporarily confused. However, I'm still interested in learning more about what the ! means, and where I can learn about ForeignPtr (maybe a short tutorial to start). Thanks, Mike

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graphics.ui.wxcore
by John Moore 06 Sep '09

06 Sep '09

Hi, Could someone please explain how to intall this. I have installed ghc and gtk2Hs and tried running small programs to get an idea of the gui system in Haskell. It keeps telling me cannot find graphics.ui.wxcore. Module failed etc. I just very recently started to learn Haskell. ( I need an idea for my final year project which at this moment is on functional programming and want to find advantages/disadvantages of the gui system in Haskell) Regards John

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Haskell data structures: cheap immutable manipulation and nested equality?
by Anand Patil 06 Sep '09

06 Sep '09

Hi all, I'm slowly shopping around for a functional language. The first one I seriously considered was Clojure, because of its agents; but I'm now realizing that most of my concurrency needs can be met using 'par' and 'seq', so I'm thinking about giving Haskell a closer look. However, Clojure's collections (maps, lists, vectors, sets) have a couple of features that I would really miss: - Cheap 'manipulation'. In the following: user=> (def m {:a 1 :b 2}) #'user/m1 user=> (assoc m :a 3) {:a 3, :b 2} the 'assoc' function produces a new map which actually shares structure with m, meaning it doesn't need to make a full copy in memory; but it also leaves m unchanged. This works efficiently even for nested, mixed collections. - Cheap equality by value: user=> (= m {:a 1 :b 2 :c {:d 3 :f 4}}) false user=> (= m {:a 1 :b 2}) true If I understand correctly, equality is computed based on some kind of hash rather than by comparing the two maps element by element, so it's efficient even for large and/or nested collections. Do Haskell's data structures have analogous properties? If not, are there libraries providing such data structures? Thanks, Anand

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Generalised data constructor matching
by Colin Campbell-McPherson 05 Sep '09

05 Sep '09

Hi Chaps, I've been trying to write a function that would get the name of most data that is an instance of class "Animal". This getName function could then be overwritten for data that doesn't follow the normal pattern for "Animal" data types. So the getName defined in the Animal class would be a generic, or default implementation. I've written a couple of examples that don't use classes, but that I hope expresses what I'm trying to accomplish. In the first example getName needs to be defined for Dogs and Birds, even though they're essentially identical. The tries to define a more general function that works for both Birds and Dogs, and anything else that might come along. The first examples works, the second gives a "Parse error in pattern" error. EXAMPLE 1 data Animal = Person String String | Dog String | Bird String deriving Show getName :: Animal -> String getName (Person firstName lastName) = firstName ++ " " ++ lastName getName (Dog name) = name getName (Bird name) = name logan = Person "Logan" "Campbell" ebony = Dog "Ebony" poly = Bird "Poly" main = do putStrLn $ show $ getName logan putStrLn $ show $ getName ebony putStrLn $ show $ getName poly EXAMPLE 2 data Animal = Person String String | Dog String | Bird String deriving Show getName :: Animal -> String getName (Person firstName lastName) = firstName ++ " " ++ lastName getName (_ name) = name logan = Person "Logan" "Campbell" ebony = Dog "Ebony" poly = Bird "Poly" main = do putStrLn $ show $ getName logan putStrLn $ show $ getName ebony putStrLn $ show $ getName poly I have a background with Ruby and Erlang, so if drawing from concepts in either of those languages would help explain please do so. I'm also quite new to haskell so if i've used the wrong terms, or I'm trying to apply concepts where they don't belong, sorry about that. TLDR: How can I write a generic function that will work on most instances of a class, but be over over-written for instances of that class that deviate from the normal structure (ie. a data constructor with two params, rather than one). Many thanks, Colin

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Multiplexing I/O in Haskell
by Sergey V. Mikhanov 03 Sep '09

03 Sep '09

Hello community, Is there a good/proper way to do multiplexing I/O in Haskell, similar to what we are getting by using select()/poll() in Unix kernel? It does not seem impossible to implement this on top of existing IO library (though I haven't tried this actually, so I may be wrong). What interest me most here is whether we could use Haskell's implicit multithreading here (select()/poll() combination is all about multithreading)? Thanks in advance, Sergey

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Identical function and variable names and type inference
by aditya siram 03 Sep '09

03 Sep '09

Hi all, Recently I wrote a function that takes a unique identifier that I called 'id'. I then tried to apply the 'id' function to it and GHC did not like that. But it should. For example in 'test' I have told the compiler that the id argument is an Int. So type inference should be able to determine the first 'id' in 'id id' couldn't possibly be an Int, but it complains. So I explicitly told the compiler the type of 'id' in test1 - this didn't work either. The final function 'test3' works as expected. Is there something I am not understand about the way type inference is supposed to work? test :: Int -> Int test id = id id test1 :: Int -> Int test1 id = idFunc id where idFunc :: a -> a idFunc = id test2 :: Int -> Int test2 myid@id = id myid test3 :: Int -> Int test3 id = Prelude.id id thanks ... -deech

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Figuring out errors
by Edward Z. Yang 02 Sep '09

02 Sep '09

Hello all, I've been looking at [1] and trying to make tops and bottoms (pardon the pun) of error handling in Haskell. I am still uncertain of what to do. I recognize that there are different areas of code that may have different requirements for errors: * Pure code that is simple enough can probably get away with returning Maybe a * Pure code that has multiple failure modes (the canonical example is parsing) should return a Either e a. The type of e is a little difficult: many standard libraries seem to use String, but coming from Python this seems analogous to the long deprecated "string exceptions", which are quick and easy but not long-term maintainable due to lack of an easy way to match for them. This leads naturally into Either MyError, which is supported using throwError and catchError. * However, [1] specifically warns against using this technique in the IO monad, and I need a way to short circuit execution when a crucial pure operation fails (in Python, this would have just been an uncaught exception). This suggests using ErrorT on IO, however, [1] also claims that this is generally not a good idea, and suggests to use throwDyn (which actually looks like it's been renamed to throw) * Unfortunately, when I've tried to use this technique, I've run up against the fact that throw is implemented using bottom, so if I do a throw in a pure function, the exception might not actually surface up until I'm, say, attempting to print its return value to IO. Denizens on #haskell have instructed me to treat bottom as the bane of existence and opt for stacking ErrorT on IO (it would be nice to know if this was a good idea or bad idea) At which point, I am most thoroughly confused. Pointers, please! Cheers, Edward [1] http://www.randomhacks.net/articles/2007/03/10/haskell-8-ways-to-report-err…

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