April 2013 - Beginners

Multi-param type classes and type dependency
by Olivier Iffrig 12 Apr '13

12 Apr '13

Hello, I'm trying to write some code which boils down to this idea : I have a collection (Collection), and I want to play with some annotations (of any type) to the items of the collection. I will often have only one collection and many different annotation sets. What matters to me is to be able to have an arbitrary index type (or at least instances of Ix) for the collection, and use it with the annotations as well. I've come to this code so far : -------------------- {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-} -- Just for the example data Thing = Foo | Bar deriving Show -- A collection -- i is the index type -- c is the collection type class Collection i c where getThing :: c -> i -> Thing -- A set of annotations on collection objects -- a is an annotation type constructor (a y is an annotation of type y) class Collection i c => Annotation i c a where collection :: a y -> c -- access the underlying Collection select :: a y -> i -> y -- select the annotation of an item given its index -- My custom collection data Things = Things [Thing] deriving Show -- An annotation set based on a list data MyAnnotation c y = Ann c [y] instance Collection Int Things where getThing (Things ts) i = ts !! i instance Annotation Int Things (MyAnnotation Things) where collection (Ann c _) = c select (Ann _ as) i = as !! i main = print $ select ann (2 :: Int) where ts = Things [Foo, Bar, Foo, Foo] ann = Ann ts ["a", "b", "c", "d"] -------------------- This does not work (No instance for (Annotation Int c0 (MyAnnotation Things)) arising from a use of `select'). I do not know how I can tell the type system that c0 is Things here, if it is actually possible. I'll appreciate any help or comments on how to rewrite this. Cheers, -- Olivier Iffrig

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Haskell type definitions
by Angus Comber 11 Apr '13

11 Apr '13

I am learning Haskell using Learn You a Haskell. On page 54 is an implementation of take like so: take' :: (Num i, Ord i) => i -> [a] -> [a] take' n _ | n <= 0 = [] take' _ [] = [] take' n (x:xs) = x : take' (n-1) xs I understand all the code apart from the first line. The :: part I understand as meaning this is a type definition? (Num i, Ord i) is a tuple. The first element of the tuple has to be numeric, fair enough. the second param has to be able to be ordered. The parameter is the same - both are i. This means that the types have to be the same? Why is it not (Num i, Ord j)? Isn't the 2nd tuple element referring to the list? Which could be of any type? What does => signify? i -> [a] -> [a] means first parameter is numeric? 2nd param is any type list, 3rd param is any type list. So this is saying first param numeric, 2nd param a list of any type and it returns a list of any type. Well that is understandable I suppose.

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FFI, export a ByteString
by Kees Bleijenberg 11 Apr '13

11 Apr '13

My function in Haskell takes a CString and returns a ByteString (of Word8). I tried to put this function in a Haskell made Windows dll. I want to use this function in a program written in C. The most simple program that compiled was: Import qualified B.ByteString as B import Foreign.StablePtr import Foreign.C.String import Foreign.Ptr foreign export stdcall parseRuitType :: CString -> IO (Ptr ()) parseRuitType s = do p <- (newStablePtr ( B.pack [10,11,12])) return $ castStablePtrToPtr p For testing I ignore s. In the C-program : hs_init(NULL,NULL) res = parseRuitType ("5" ) No errors, but the returned value res (or the dereferenced res) makes no sense (not 10,11,12). My idea was: the result of the function(a ByteString of Word8) must be locked. No garbage collection. But newStablePtr does not return a pointer. So I have to convert the newStablePtr to a real pointer and then coerce this in the C program to a pointer to a array of word8.? I've tried to return the ByteString in the Haskell function. But now the compiler complains: Unacceptable result type in foreign declaration : B.ByteString I'am missing the big picture. Any ideas? Greetings Kees

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haskell testing framework - missing htfpp
by Ovidiu D 11 Apr '13

11 Apr '13

I'm trying to use HTF by following this tutorial: http://hackage.haskell.org/packages/archive/HTF/0.9.0.0/doc/html/Test-Frame… and when I build the project I get the error: ghc: could not execute: htfpp I'm using: $ dpkg -l | grep haskell-platform ii haskell-platform 2012.2.0.0ubuntu1 all Standard Haskell libraries and tools What am I doing wrong? Thanks, ovidiu

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More than one way to skin a cat question - repeat
by Angus Comber 11 Apr '13

11 Apr '13

LYAH beginner on page 55. What I try to do is see the heading of a section, eg repeat function and then try to come up with my own version before looking at books implementation. Here is my implementation: repeat' :: a -> [a] repeat' x = [x] ++ repeat' x Here is books: repeat' :: a -> [a] repeat' x = x:repeat' x Mine appears to work. Is mine just as good? Are there problems with my way? Is books way more idiomatic Haskell? I want to learn the best approaches hence my question.

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doesFileExist cannot get ~/blah_blah right
by Hong Yang 10 Apr '13

10 Apr '13

Hi, I got "not exists ..." from fileExists <- doesFileExist "~/blah_blah" if fileExists then print "exists ..." else print "not exists ..." using the directory-1.1.0.2 package, when ~/blah_blah does exist. I do not want to change "~" to a specific home directory such as "/home/a_userid" for the sake of portability. What else can I do before the bug (suppose this is a bug) is fixed? Thanks, Hong

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Re: [Haskell-beginners] Problems with parsing in attoparsec
by Michel Kuhlmann 10 Apr '13

10 Apr '13

Hi Mukesh, thanks for the hint. I will try to combine `take 6` with `number`, to avoid `read . T.unpack`. I post, if that succeeds. Greetings, Michel

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Conduit composition
by Ovidiu D 10 Apr '13

10 Apr '13

Given the following works as expected (i.e. prints the value twice): main = Conduit.sourceList [1..14] $= Conduit.map show $= Conduit.iterM putStrLn $= Conduit.iterM putStrLn $$ Conduit.sinkNull I would expect the following to work as well: main = Conduit.sourceList [1..14] $= Conduit.map show $= display $$ Conduit.sinkNull display = Conduit.iterM putStrLn $= Conduit.iterM putStrLn ...but I get the compilation error: Couldn't match expected type `String' with actual type `()' Expected type: Conduit.Conduit String m0 a0 Actual type: Conduit.Source IO () In the second argument of `($=)', namely `display' In the first argument of `($$)', namely `Conduit.sourceList [1 .. 14] $= Conduit.map show $= display' I don't understand why the type of display is inferred to a Conduit.Source. Can somebody please explain? What I want is to have readable names for certain segments in my pipe. Is that possible? Thanks, ovidiu

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Problems with parsing in attoparsec
by Michel Kuhlmann 09 Apr '13

09 Apr '13

Hi, I couldn't figure out, why I'm failing with attoparsec. I have the follwing code (https://github.com/michelk/devsurf/tree/readNet) import Data.Attoparsec.Text import qualified Data.Text as T import Language.DevSurf.Readers.SpringNet import Prelude hiding (takeWhile, take) data IndexedFace = IndexedFaceTriangle Int Int Int | IndexedFaceQuad Int Int Int Int deriving (Show) -- | block of faces-lines faces :: Parser [[(Int, IndexedFace)]] faces = faceLine `sepBy` endOfLine -- | Line containing up to two indexed faces faceLine :: Parser [(Int, IndexedFace)] faceLine = face `sepBy` (take 6) -- | single indexed-face face :: Parser (Int, IndexedFace) face = do i <- index v1 <- index v2 <- index v3 <- index v4 <- index case v4 of 0 -> return (i, IndexedFaceTriangle v1 v2 v3) _ -> return (i, IndexedFaceQuad v1 v2 v3 v4) -- | node or face index 6 wide index :: Parser Int index = do i <- take 6 return . read . T.unpack $ i When testing, the follwing is working: testEleL2 = do let ele = T.pack " 1 10 1 9 0 2 1 2 9 0\n" r = parse faceLine ele case r of Partial _ -> print $ feed r (T.pack "") _ -> print r testEles = do let eles = T.pack $ concat [" 1 10 1 9 0 2 1 2 9 0\n" --," 3 11 9 2 0\n" ," 4 12 8 1 0\n" ] r = parse faces eles case r of Partial _ -> print $ feed r (T.pack "") _ -> print r testEles = do let eles = T.pack $ concat [" 1 10 1 9 0 2 1 2 9 0\n" ," 3 11 9 2 0\n" --," 4 12 8 1 0\n" ] r = parse faces eles case r of Partial _ -> print $ feed r (T.pack "") _ -> print r But this is not working testEles = do let eles = T.pack $ concat [" 1 10 1 9 0 2 1 2 9 0\n" ," 3 11 9 2 0\n" ," 4 12 8 1 0\n" ] r = parse faces eles case r of Partial _ -> print $ feed r (T.pack "") _ -> print r testEles = do let eles = T.pack $ concat [" 1 10 1 9 0 2 1 2 9 0\n" ," 3 11 9 2 0 4 12 8 1 0\n" ] r = parse faces eles case r of Partial _ -> print $ feed r (T.pack "") _ -> print r Please help. Thanks, Michel

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wx lists on holidays? DnD Question..
by luc taesch 08 Apr '13

08 Apr '13

It looks like I cannot push any messges to the wx lists for 28/4. is there only moderator , that would be out in holidays ? Your mail to 'wxhaskell-users' with the subject ... Is being held until the list moderator can review it for approval. Is ther only one moderator possible ? (Otherwise, I may volonteer, as I may need it for the next month ;-)- are there any other channel in the meantime ? any IRC ? I had a Drag and Drop question I documented here : http://stackoverflow.com/questions/15867654/wx-haskell-drag-and-drop-example if anyone could help..

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