- Beginners - Haskell.org

GHC / system libraries
by Christopher Howard 04 Aug '12

04 Aug '12

I know that GHC allows you to create shared libraries and create binaries that use shared libraries, because I asked that question some time ago on this list. However, I was wondering: can the GHC "standard libraries" be decoupled from the compiler? I.e., so you can install the libraries and the binaries on a system without the compiler being installed? I use a source-based distro so it is of little relevance to me. However, I was discussing Haskell apps online with some friends who use binary-based distros and had to admit that I wasn't clear on this point. -- frigidcode.com indicium.us

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growing trees
by Jeff Lasslett 04 Aug '12

04 Aug '12

Greetings, For my own edification I have written a few functions for creating a tree from a list of 'associations'. An association is defined as data Assoc = Assoc { label :: String , assocId :: Int , assocPid :: Int } deriving ( Show ) where assocId is an int that uniquely identifies a node, and assocPid is the id of that node's parent. The idea is to feed an array of these associations into a function and get a Data.Tree Assoc out. So I have code that does the job ... ... but it's not very good. I thought the problem might be solved with a fold, but that proved difficult due to the Assocs not being in a suitable insertion order. Perhaps the routines can be improved such that the array order doesn't matter. Any advice, or better versions would be appreciated. thanks, Jeff PS: AssocTree.hs should be attached.

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Why is the the transpose function in Data.List more complicated?
by KC 04 Aug '12

04 Aug '12

Transpose in Data.List is the following: transpose :: [[a]] -> [[a]] transpose [] = [] transpose ([] : xss) = transpose xss transpose ((x:xs) : xss) = (x : [h | (h:_) <- xss]) : transpose (xs : [ t | (_:t) <- xss]) Yet this seems to work. transp :: [[b]] -> [[b]] transp ([]:_) = [] transp rows = map head rows : transp (map tail rows) Why is the the transpose function in Data.List more complicated? -- -- Regards, KC

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vector indexing time
by Ivan Vyalov 03 Aug '12

03 Aug '12

Hi everyone! I have a question about time complexity of vector indexing. Obviously, it should be constant, but if I do the following naive tests it looks linear. What do I do wrong? Ivan $ for i in 10000000 20000000 40000000 80000000 ; do cat vec_in_"$i".hs; ghc -fforce-recomp -O2 --make vec_in_"$i".hs && time ./vec_in_"$i"; done import Data.Vector.Unboxed main = do let a = enumFromN 1 100000001 :: Vector Int print $ a ! 10000000 [1 of 1] Compiling Main ( vec_in_10000000.hs, vec_in_10000000.o ) Linking vec_in_10000000 ... 10000001 real 0m0.033s user 0m0.032s sys 0m0.000s import Data.Vector.Unboxed main = do let a = enumFromN 1 100000001 :: Vector Int print $ a ! 20000000 [1 of 1] Compiling Main ( vec_in_20000000.hs, vec_in_20000000.o ) Linking vec_in_20000000 ... 20000001 real 0m0.062s user 0m0.060s sys 0m0.000s import Data.Vector.Unboxed main = do let a = enumFromN 1 100000001 :: Vector Int print $ a ! 40000000 [1 of 1] Compiling Main ( vec_in_40000000.hs, vec_in_40000000.o ) Linking vec_in_40000000 ... 40000001 real 0m0.125s user 0m0.120s sys 0m0.004s import Data.Vector.Unboxed main = do let a = enumFromN 1 100000001 :: Vector Int print $ a ! 80000000 [1 of 1] Compiling Main ( vec_in_80000000.hs, vec_in_80000000.o ) Linking vec_in_80000000 ... 80000001 real 0m0.244s user 0m0.240s sys 0m0.000s

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Resources to learn functional programming
by Homero Cardoso de Almeida 02 Aug '12

02 Aug '12

Hi everyone, I'm fairly new to Haskell. I'm trying to learn it, but got stuck when i reached high-order functions. I understand what they are, but I cannot wrap my head on how they work, how to use them, and such. It seems so abstract to me. I tried to move on past it, but it only got harder and harder. Looks like I have problems learning functional programming. Do you have any good resources to learn functional programming? I am a decent C++ programmer. Thanks. Homero Cardoso de Almeida

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Is there any money in Haskell programming?
by Christopher Howard 01 Aug '12

01 Aug '12

Wait! Don't delete this e-mail yet! I don't mean to imply that Haskell is only worth learning if you can make money with it, or to say that it is impossible to make a profitable program with it. I was just wondering what (if any) commercial, educational, or government sectors use Haskell, and if there is anyone on this list who makes a living in some way writing Haskell code. -- frigidcode.com indicium.us

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Re: [Haskell-beginners] [Haskell-cafe] The following is supposed to be initializing a 2D array but it doesn't seem to work.
by KC 31 Jul '12

31 Jul '12

That was it! On Tue, Jul 31, 2012 at 3:34 PM, Jan-Willem Maessen <jmaessen(a)alum.mit.edu>wrote: > > > On Tue, Jul 31, 2012 at 5:52 PM, KC <kc1956(a)gmail.com> wrote: > >> All I am getting is this: >> >> array ((1,5),(1,5)) [((1,5),1.0)] >> >> Maybe the behaviour of ghc was changed since the article was written. >> > > I think you've made a common mistake here. Array bounds are (lower in all > dimensions, upper in all dimensions). So perhaps you intended to use the > bounds ((1,1), (5,5)) in your code? > > I make this mistake frequently. > > -Jan-Willem Maessen > -- -- Regards, KC

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The following is supposed to be initializing a 2D array but it doesn't seem to work.
by KC 31 Jul '12

31 Jul '12

newArr :: (Ix i) => i -> i -> e -> Array i e newArr n m x = listArray (n,m) (repeat <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:repeat> x) ----- Prelude <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html> Data.Array> newArr 0 10 0 array (0,10) [(0,0),(1,0),(2,0),(3,0),(4,0),(5,0),(6,0),(7,0),(8,0),(9,0),(10,0)] Prelude <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html> Data.Array> newArr (0,0) (5,5) 0 array ((0,0),(5,5)) [((0,0),0),((0,1),0),((0,2),0),((0,3),0),((0,4),0),((0,5),0),((1,0),0),((1,1),0),((1,2),0),((1,3),0),((1,4),0),((1,5),0),((2,0),0),((2,1),0),((2,2),0),((2,3),0),((2,4),0),((2,5),0),((3,0),0),((3,1),0),((3,2),0),((3,3),0),((3,4),0),((3,5),0),((4,0),0),((4,1),0),((4,2),0),((4,3),0),((4,4),0),((4,5),0),((5,0),0),((5,1),0),((5,2),0),((5,3),0),((5,4),0),((5,5),0)] All I am getting is this: array ((1,5),(1,5)) [((1,5),1.0)] Maybe the behaviour of ghc was changed since the article was written. -- -- Regards, KC

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XmlPickler on ADT
by Tim Perry 31 Jul '12

31 Jul '12

Hi everyone, I'm trying to learn to write XmlPicklers. I've been trying to follow the tutorial on the HaskelWiki, but it doesn't quite cover an issue I've run into. http://www.haskell.org/haskellwiki/HXT/Conversion_of_Haskell_data_from/to_X… I have a ADTs like so: data Plant = Plant { renewables :: [Renewable] , rewewableMonthly :: [RenewableMonthly] , waterHeating :: [WaterHeating] , hvac :: [HVAC] } deriving (Read, Show, Eq) data Renewable = Renewable { dcRating :: Double -- kW , annualTDV :: Double -- CA Energy Comm. TDV } deriving (Read, Show, Eq) data RenewableMonthly = RenewableMonthly { month :: Int , cost :: Double , demand :: Double , energy :: Double } deriving (Read, Show, Eq) data WaterHeating = WaterHeating { whName :: String -- name for reference purposes. , whQuantity :: Int ... data HVAC = HVAC { slaStatus :: String -- Specific leakage area construction type. , newCFM50 :: Double -- New building leakage in cubic feet per minute. , existCFM50 :: Double I want to use an XmlPickler to read and write this. The XML looks like this: <Plant> <Renewables DCRating="8.4" AnnualTDV="0" /> <RenewableMonthly Month="1" Cost="0" Demand="0" Energy="0" /> <RenewableMonthly Month="2" Cost="0" Demand="0" Energy="0" /> <RenewableMonthly Month="3" Cost="0" Demand="0" Energy="0" /> <RenewableMonthly Month="4" Cost="0" Demand="0" Energy="0" /> <RenewableMonthly Month="5" Cost="0" Demand="0" Energy="0" /> <RenewableMonthly Month="6" Cost="0" Demand="0" Energy="0" /> <RenewableMonthly Month="7" Cost="0" Demand="0" Energy="0" /> <RenewableMonthly Month="8" Cost="0" Demand="0" Energy="0" /> <RenewableMonthly Month="9" Cost="0" Demand="0" Energy="0" /> <RenewableMonthly Month="10" Cost="0" Demand="0" Energy="0" /> <RenewableMonthly Month="11" Cost="0" Demand="0" Energy="0" /> <RenewableMonthly Month="12" Cost="0" Demand="0" Energy="0" /> <WaterHeating Name="Daikin Altherma HP" Quantity="1" ConstructionStatus="New" EnergyFactor="2.344" SolarFraction="0" .... <HVAC SLAStatus="New" NewCFM50="0" ExistCFM50="0" NewSLA="4.9000000953674316" ExistSLA="4.9" DuctStatus="New" .... <Zone ... Running my code I get an error: -- (1) readDocument: start processing document "1101_11-001 Walsh.xml" -- (1) getXmlContents: content read and decoded for "file:///fairoaks1/all_proj/1101%20PG&E%20CAHP%2011-12/CRM/EProDataExtractPlanning/XML%20samples/1101_11-001%20Walsh.xml" -- (1) getXmlContents' -- (1) readDocument: "1101_11-001 Walsh.xml" (mime type: "text/xml" ) will be processed -- (1) readDocument: "1101_11-001 Walsh.xml" processed fatal error: document unpickling failed xpCheckEmptyContents: unprocessed XML content detected context: element "Plant" contents: Is it possible to write an XmlPickler for this? Do I need to make Renewable, RenewableMonthly, WaterHeating, and HVAC all the same type with 4 constructors and use xpAlt? Ideas? Thanks, Tim

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Is there a way to hide an existing data type so that I can use my own version?
by Costello, Roger L. 31 Jul '12

31 Jul '12

Hi Folks, For learning purposes, I created an "Either" data type: data Either a b = Left a | Right b deriving (Show) However, when I try to use Left (or Right), I get an error message saying that it is ambiguous whether I am using Prelude.Left or Main.Left So I tried hiding Prelude.Either like so: import Prelude hiding Either But that produced an error. Is there a way to hide the existing Either data type so that I can use my own version? /Roger

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