- Beginners - Haskell.org

growing trees
by Jeff Lasslett 03 Aug '12

03 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 03 Aug '12

03 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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Here's why functions should return functions
by Costello, Roger L. 30 Jul '12

30 Jul '12

Hi Folks, Recently I had a small epiphany: when creating functions, design them to return functions rather than non-function values (Integer, Bool, list, tuple, etc.). Here's why: Consider a function that returns, say, the integer four ( 4 ). The type of the value returned by the function is this: 4 :: Num a => a That is, the value returned is not a function, it is a number. However, there are advantages to returning a function rather than a number. Recall the composition operator ( . ) Its operands are functions, e.g. (+1) . (*3) In programming, one school of thought is that programs should be written as a chain of function compositions: a . b . c . d . e . f Composition is intimately connected to a branch of mathematics called Category Theory: Category theory is based on composition as a fundamental operation in much the same way that classical set theory is based on the 'element of' or membership relation. ["Category Theory for Computing Science" by Michael Barr and Charles Wells] By designing programs in this fashion--as a chain of compositions--you have Category Theory's vast body of knowledge to help you and give rigor to your programs. Let's revisit the function mentioned above, the one that returns the integer four ( 4 ). If the function were to return the four cloaked in a function then that returned value could be used in a composition. That would very useful. Here's a data type that lifts non-function values to functions: data Lift a = Function a deriving (Show) The constructor ( Function ) is a function, as its type signature shows: Function :: a -> Lift a So rather than returning 4, return Function 4. Here's a function that converts any value to a function: lift :: a -> Lift a lift = Function Thus, lift 4 returns Function 4 Given the Lift data type and the lift function we can now start chaining functions together. Here the value four is lifted and then composed with a successor function: (successor . lift) 4 returns Function 5 where successor is defined as: successor :: Num a => Lift a -> Lift a successor (Function a) = lift (a + 1) Notice that successor returns a function, not a non-function value. Consequently, the result of successor can also be used in a composition. For example, here the value four is lifted, composed with successor, and then square is applied: (square . successor . lift) 4 returns Function 25 where square is defined as: square :: Num a => Lift a -> Lift a square (Function a) = lift (a * a) Once again notice that square also returns a function, not a non-function value. Consequently, the result of square can be used in a composition. At some point we are finished manipulating the value four and want to just see the result, not the result wrapped in a constructor. So we can create a function to return the non-function value: value :: Lift a -> a value (Function a) = a Here's an example: (value . square . successor . lift) 4 returns 25 Comments welcome. /Roger

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