January 2015 - Beginners

Show and showList
by Animesh Saxena 24 Jan '15

24 Jan '15

I am trying to understand the concept of overlapping instances. In the book "Real World Haskell" there is an example of showList which avoids overlapping instances. As per my understanding showList is taking the same problem that method "show" would have for overlapping instances. Here's the snippet which explains how it works... "The Show class defines both a show method, which renders one value, and a showList method, which renders a list of values. The default implementation of showList renders a list using square brackets and commas. The instance of Show for [a] is implemented using showList. The instance of Show for Char provides a special implementation of showList that uses double quotes and escapes non-ASCII-printable characters. As a result, if someone applies show to a [Char] value, the implementation of showList will be chosen, and it will correctly render the string using quotes. At least sometimes, then, we can avoid the need for the OverlappingInstances extension with a little bit of lateral thinking." GHC Code class Show a where showsPrec :: Int -> a -> ShowS show :: a -> String showList :: [a] -> ShowS *Main> show [1,2,3] "[1,2,3]" *Main> show ['a','b','c'] "\"abc\"" *Main> It's the same problem now transfered to showList which will be declared differently for a list of int's or a list of chars, but still it outputs them differently as can be seen from the above code. How it is able to differentiate without throwing up the overlap error? -A

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Help in reading XML into a data type with HXT
by Alexis Praga 24 Jan '15

24 Jan '15

Hi, I am trying to read a very simple xml node containing a list of other xml nodes into a data structure. Running the code yields : fatal error: document unpickling failed xpCheckEmptyContents: unprocessed XML content detected context: element "animelist" contents: <title>Bleach</title> <title>Naruto</title> Can you help me debug this ? The code and xml are below : 8<---Code--------------------------------------------------------------------- module Main where import System.Environment import Text.XML.HXT.Core type Anime = String type AnimeList = [Anime] xpAnime :: PU Anime xpAnime = xpElem "title" $ xpText xpAnimeList :: PU AnimeList xpAnimeList = xpElem "animelist" $ xpList $ xpAnime main :: IO () main = do [src] <- getArgs a <- runX ( xunpickleDocument xpAnimeList [] src) mapM_ putStrLn a return () 8<---------------------------------------------------------------------------- 8<---XML--------------------------------------------------------------------- <animelist> <title>Bleach</title> <title>Naruto</title> </animelist> 8<---------------------------------------------------------------------------- Thanks, -- Alexis Praga, PhD Student (CERFACS) GPG key : AD4A AF6D BB5C 042F 9422 1223 06E1 C1BF E287 65D0

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Defining an instance: Syntax that works exactly sometimes
by Jeffrey Brown 23 Jan '15

23 Jan '15

Dear Haskellers, The following compiles. (Rev stands for Reversible, and Dirn for Direction.) class Rev a where rev :: a -> a data Dirn = Succ | Pred deriving (Eq, Show, Ord) -- implement Ord (<=) Succ Pred = False (<=) _ _ = True -- implement Rev instance Rev Dirn where rev Succ = Pred rev Pred = Succ But if I try to define the Rev instance the same way the Ord instance is being defined, it does not compile: class Rev a where rev :: a -> a data Dirn = Succ | Pred deriving (Eq, Show, Ord, Rev) -- implement Ord, because Dirn is used as a key in a Map (<=) Succ Pred = False (<=) _ _ = True -- implement Rev rev Succ = Pred rev Pred = Succ What's going on? Many thanks, Jeff

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help with IO guards
by Miro Karpis 23 Jan '15

23 Jan '15

Hi, please is there a way to have guards with 'where' that communicates with IO? Or is there some other more elegant way? I can do this with classic if/else,...but I just find it nicer with guards. I have something like this (just an example): f :: Int -> IO String f x | null dbOutput = return "no db record" | otherwise = return "we got some db records" where dbOutput = getDBRecord x getDBRecord :: Int -> IO [Int] getDBRecord recordId = do putStrLn $ "checking dbRecord" ++ show recordId --getting data from DB return [1,2] problem is that db dbOutput is IO and the guard check does not like it: Couldn't match expected type ‘[a0]’ with actual type ‘IO [Int]’ In the first argument of ‘null’, namely ‘dbOutput’ In the expression: null dbOutput Cheers, Miro

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Let the compiler do the work
by Elias Diem 19 Jan '15

19 Jan '15

Hi there I'm referring to a post from Stefan Höck and will quote from there: http://article.gmane.org/gmane.comp.lang.haskell.beginners/14198 <quote> Now, load this into GHCi or compile with GHC. If it compiles, you're on the right track. Now, you want to implement it using a fold (try both, foldl and foldr): last5 :: [a] -> Maybe a last5 xs = foldr _ _ xs The underscores are 'type holes'. This tells the compiler to give you some information about what is supposed to be placed at the two positions. For the moment, we are only interested in the types of the things that go there. The compiler will tell you, that the hole at the first position is of type a -> Maybe a -> Maybe a </quote> How does the compiler tell me this? I didn't find any flags for GHC to turn this on. What do I miss? -- Greetings Elias

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Error while trying to install ByteString Package
by Abhinav Kalawatia 17 Jan '15

17 Jan '15

Hi, I get the following error while I tried installing the ByteString package. Could you please help me resolve this >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> C:\Users> cabal install bytestring Resolving dependencies... Configuring bytestring-0.10.4.1... Failed to install bytestring-0.10.4.1 Last 10 lines of the build log ( C:\Users\AppData\Roaming\caba l\logs\bytestring-0.10.4.1.log ): setup-Cabal-1.18.1.3-i386-windows-ghc-7.8.3.exe: Missing dependency on a foreign library: * Missing (or bad) header file: fpstring.h This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. If the header file does exist, it may contain errors that are caught by the C compiler at the preprocessing stage. In this case you can re-run configure with the verbosity flag -v3 to see the error messages. cabal: Error: some packages failed to install: bytestring-0.10.4.1 failed during the configure step. The exception was: ExitFailure 1>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Regards,Abhinav

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garbage collected object
by Elise Huard 16 Jan '15

16 Jan '15

Hi, Maybe a stupid question: is there a way to check whether a particular data structure (or set of data structures) has been garbage collected? Or indirectly: is there a way to check what's still alive in the heap, so that you can potentially diff from one moment to another? Thanks, Elise Huard

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bitmap image with transparent background
by Letizia Galli 13 Jan '15

13 Jan '15

Hi :-) I have just joined and I don't know if this is a beginner question, anyway I have been given an Haskell code which uses images, but when I convert a tiff with transparent background into a bitmap, their background becomes white and I need it to be transparent. Is there a way in Haskel to convert a bitmap with solid background into a bitmap with transparent background? Is there another format that can be used instead of bitmap in Haskell? Best, Letizia -- Letizia Galli 0039 328 6917393 www.letiziagalli.it

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Associated data type confusion
by Alex Hammel 12 Jan '15

12 Jan '15

Hello list, I've got a collection of types which come from generated code (it's protocol buffer stuff). I'd like to write a polymorphic function which maps the protocol buffer generated data types to native data types. I've tried something like this: class FromProto a where type InputDataType fromProto :: InputDataType -> a instance ToProto SomeNativeType where type OutputDataType = SomeGeneratedType toProto = {- etc -} instance FromProto SomeOtherNativeType where type InputDataType = SomeOtherGeneratedType fromProto = {- etc -} Which works fine for mapping one native data type to *one* generated data type, but breaks when I want to define different *InputDataType*s for different instances of *FromProto*. I feel like I'm making this more complicated than I need to. Is there an easy way to get the data type to data type mapping that I'm looking for? Cheers, Alex

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help understanding error from attempt at CIS 194 : Homework 7 - indexJ
by Dustin Lee 08 Jan '15

08 Jan '15

Here's what I have so far for JoinList.hs: ( http://www.seas.upenn.edu/~cis194/spring13/hw/07-folds-monoids.pdf) ==== module JoinList where import Data.Monoid import Sized data JoinList m a = Empty | Single m a | Append m (JoinList m a) (JoinList m a) deriving (Eq, Show) tag :: Monoid m => JoinList m a -> m tag Empty = mempty tag (Single m a) = m tag (Append m _ _) = m (+++) :: Monoid m => JoinList m a -> JoinList m a -> JoinList m a (+++) x y = Append (tag x <> tag y) x y indexJ :: (Sized b, Monoid b) => Int -> JoinList b a -> Maybe a indexJ _ Empty = Nothing indexJ i (Single m a) | i == 0 = Just a | otherwise = Nothing indexJ i (Append m x y) | (getSize (tag x)) >= i = indexJ i x | otherwise = indexJ (i - (getSize (tag x))) y ===== Here is the error I'm getting. Haven't been able to make sense of it yet. *Sized> :load "JoinList.hs" [1 of 2] Compiling Sized ( Sized.hs, interpreted ) [2 of 2] Compiling JoinList ( JoinList.hs, interpreted ) JoinList.hs:50:21: Could not deduce (b ~ Size) from the context (Sized b, Monoid b) bound by the type signature for indexJ :: (Sized b, Monoid b) => Int -> JoinList b a -> Maybe a at JoinList.hs:44:11-63 `b' is a rigid type variable bound by the type signature for indexJ :: (Sized b, Monoid b) => Int -> JoinList b a -> Maybe a at JoinList.hs:44:11 Expected type: JoinList Size a Actual type: JoinList b a In the first argument of `tag', namely `x' In the first argument of `getSize', namely `(tag x)' In the first argument of `(>=)', namely `(getSize (tag x))' Failed, modules loaded: Sized. thanks! -- Dustin Lee qhfgva=rot13(dustin)

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