- Haskell-Cafe - Haskell.org

Could someone teach me why we use Data.Monoid?
by Magicloud Magiclouds 16 Nov '09

16 Nov '09

Hi, I have looked the concept of monoid and something related, but still, I do not know why we use it? -- 竹密岂妨流水过山高哪阻野云飞

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Coercing numeric and string constants
by Mark Lentczner 16 Nov '09

16 Nov '09

I'm looking for a good way to handle a library interface that accepts both strings and numbers in particular argument positions: Start with the following definitions. I've defined ResourceTree as a class here since the details don't matter. > data Segment = Key String | Index Int > deriving (Eq, Show) > > type Path = [Segment] > > class ResourceTree a where > lookupPath :: Path -> a -> Maybe String What I'm after is to make the following code, representative of intended client usage to work: > examples :: (ResourceTree a) => a -> [Maybe String] > examples r = [ > r `at` "status", > r `at` 7, > r `at` "part" ./ "sku", > r `at` "items" ./ 2, > r `at` 7 ./ "name", > r `at` 7 ./ 9 > ] The first way I thought to do this was with type classes: > class Segmentable a where { toSegment :: a -> Segment } > instance Segmentable Segment where { toSegment = id } > instance Segmentable String where { toSegment = Key } > instance Segmentable Int where { toSegment = Index } > > class Pathable a where { toPath :: a -> Path } > instance Pathable Path where { toPath = id } > instance Pathable String where { toPath s = [ Key s ] } > instance Pathable Int where { toPath i = [ Index i ] } > > (./) :: (Segmentable a, Pathable b) => a -> b -> Path > a ./ b = toSegment a : toPath b > infixr 4 ./ > > at :: (ResourceTree a, Pathable b) => a -> b -> Maybe String > a `at` b = lookupPath (toPath b) a > infix 2 `at` This works great for all uses in the client code where the type of the numeric arguments are known or otherwise forced to be Int. However, when used with numeric constants (as in the function example above), it fails due to the way that numeric constants are defined in Haskell. For example, the constant 9 in example results in this error: Ambiguous type variable `t4' in the constraints: `Pathable t4' arising from a use of `./' at Test.hs:48:15-20 `Num t4' arising from the literal `9' at Test.hs:48:20 Probable fix: add a type signature that fixes these type variable(s) I suppose that even though there is only one type that is both an instance of Num and of Pathable (Int), that can't be deduced with certainty. In the client code, one could fix this by typing the constants thus: > r `at` (7::Int) ./ (9::Int) But to me that makes a hash out of the concise syntax I was trying to achieve. Also, this code requires both FlexibleInstances and TypeSynonymInstances pragmas (though the later requirement could be worked around.), though I'm lead to understand that those are common enough. I think also that, these are only needed in the library, not the client code. The other way I thought to do this is by making Path and Segment instances of Num and IsString: > instance Num Segment where { fromInteger = Index . fromInteger } > instance IsString Segment where { fromString = Key . fromString } > instance Num Path where { fromInteger i = [ Index $ fromInteger i ] } > instance IsString Path where { fromString s = [ Key $ fromString s ] } > > (./) :: Segment -> Path -> Path > a ./ b = a : b > infixr 4 ./ > > at :: (ResourceTree a) => a -> Path -> Maybe String > a `at` b = lookupPath b a > infix 2 `at` This works but has two downsides: 1) Segment and Path are poor instances of Num, eliciting errors for missing methods and resulting in run-time errors should any client code accidentally use them as such. 2) It requires the OverloadedStrings pragma in every client module. Any comments on these two approaches would be appreciated, How to improve them? Which is the lesser of two evils? On the other hand, I realize that many may object that intended interface isn't very Haskell like. The data object I need to represent (ResourceTree) comes from external input and really does have the strange "paths of strings or integers" construction, I can't change that. And it is expected that much client code will use constant paths to access and manipulate various parts of such objects, hence the desire for a concise operator set that works with constants. Given that there are actually several operations on ResourceTree involving paths (where the operation requires the whole Path as a single value), any thoughts on a more Haskell like construction? Thanks, - MtnViewMark Mark Lentczner http://www.ozonehouse.com/mark/ mark(a)glyphic.com

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[Haskell-cafe] unboxed arrays restricted to simple types (Int, Float, ..)
by Tillmann Vogt 16 Nov '09

16 Nov '09

Hi, I tried to use unboxed arrays for generating an antialiased texture. To make it easier to understand, here is the stripped down code that produces an error: >import Control.Monad.ST >import Data.Array.ST >import Data.Array.Unboxed >import Data.Word >type BitMask = UArray Int Word16 -- for determining the grey value of a pixel >type Pixels = (Int, Int, T) >data T = N | B BitMask -- this does not work >-- type T = Int -- this works if int the next line N is replaced by ..lets say 0 >f = newArray (0,10) N :: (ST s (STUArray s Int T)) http://hackage.haskell.org/packages/archive/array/0.2.0.0/doc/html/Data-Arr… shows that mutable/unboxed arrays only allow simple types: i.e. MArray (STUArray s) Int32 (ST s) Isn't this ugly? Imagine this would be the case in C: struct stupidArrayElement{ int a; int b; // not allowed! } stupidArrayElement s[10]; Wouldn't it be nice to have something like: MArray (STUArray s) e (ST s) with e being a non-recursive data type (like data T = N | B Bitmask). My understanding of Haskell isn't deep enough to know if I have overlooked something or if the problem is solvable without a language extension. With a language extension I guess that it is not hard to find out if an abstract data type is non-recursive. Then this type should be serializable automatically. What do you think?

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ANNOUNCE: New Industrial Haskell Group membership options
by Duncan Coutts 16 Nov '09

16 Nov '09

The Industrial Haskell Group (IHG) is an organisation to support the needs of commercial users of the Haskell programming language. The second round of the IHG's collaborative development scheme will be running for 6 months from January. We are now inviting additional companies to take part. This scheme is ideal for companies that make significant use of Haskell and wish to fund specific projects. Additionally, there are new associate and academic membership options. These enable companies and academic groups to support the general health of the Haskell development platform, but with a lower financial commitment. For more details on all of the above, please see http://industry.haskell.org/ If your company or group is interested in joining then please e-mail info(a)industry.haskell.org -- Duncan Coutts, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

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who wrote the Relational_Algebra page
by Günther Schmidt 16 Nov '09

16 Nov '09

on the Haskell wiki? http://www.haskell.org/haskellwiki/Relational_algebra Günther

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Trouble installing HDBC 2.1.1 with ghc 6.10.4
by Colin Paul Adams 16 Nov '09

16 Nov '09

I'm seeing messages such as: Database/HDBC/SqlValue.hs:610:32: No instance for (Typeable Day) .... Is this connected with the in-and-out status of the time library in the GHC 6.10.x series? Is there a work-around? -- Colin Adams Preston Lancashire

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haskell-src-exts Question
by Dominic Steinitz 16 Nov '09

16 Nov '09

I've been generating Haskell using haskell-src-exts but the prettyprinter isn't producing what I would expect. I would expect parse . prettyPrint == id i.e. the AST should be unchanged if you prettyprint it then parse it. Here's an example generated expression: App (App (Var (UnQual (Ident "pay"))) (Var (UnQual (Ident "tPD")))) (App (Var (UnQual (Ident "a"))) (InfixApp (App (Var (UnQual (Ident "length"))) (Var (UnQual (Ident "tOD")))) (QVarOp (UnQual (Symbol "+"))) (Lit (Int (-1))))) Here's what prettyPrint produces: pay tPD a length tOD + -1 Parsing it gives this (i.e. not the expression I first thought of): InfixApp (App (App (App (App (Var (UnQual (Ident "pay"))) (Var (UnQual (Ident "tPD")))) (Var (UnQual (Ident "a")))) (Var (UnQual (Ident "length")))) (Var (UnQual (Ident "tOD")))) (QVarOp (UnQual (Symbol "+"))) (NegApp (Lit (Int 1))) I would have expected the prettyprinter to produce this: pay tPD (a (length tOD + -1)) Do I have to write my own prettyprinter? Do I have to put in explicit parentheses? The latter seems unsatisfactory as my generated AST is unambiguous and bracketing ought to be part of the prettyprinter. The former would be quite a lot of code as there are many cases to consider. Dominic.

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poor perfomance of indexU in uvector package
by Alexey Khudyakov 16 Nov '09

16 Nov '09

Hello This post meant to be literate haskell. I found that perfomace of indexU is very poor and it is not fast O(1) operation which is very surprising. Here is some benchmarcking I've done. Everything compiled with -O2 Code below converts square 2D array to list of 1D arrays. Summation of array contents is done in force evaluation > import Control.Monad > import Control.Monad.ST > import Data.Array.Vector > import System > > arr :: Int -> UArr Double > arr n = toU $ map fromIntegral [1 .. n*n] > This is fastest function. It slice arrays along another direction and used mainly as upper bound of speed > sliceY :: Int -> UArr Double -> [UArr Double] > sliceY n a = map (\i -> sliceU a (i*n) n) [0 .. n-1] > Naive implementation using lists and index lookup. 2.15 second for 200*200 array > sliceXlist :: Int -> UArr Double -> [UArr Double] > sliceXlist n a = map mkSlice [0 .. n-1] > where > mkSlice x = toU $ map (\y -> indexU a (x + y*n)) [0 .. n-1] Similar implementation in ST monad and it uses indexU too. 2.14 seconds for 200*200 array > sliceXst :: Int -> UArr Double -> [UArr Double] > sliceXst n a = map mkSlice [0 .. n-1] > where > mkSlice x = runST $ do arr <- newMU n > forM_ [0 .. n-1] $ \y -> writeMU arr y (indexU a (y*n + x)) > unsafeFreezeAllMU arr This implementation avoids use of indexU by copying entire 2D array into mutable array and using it for lookup. Surprisingly it outperform previsious implementations for sufficiently big n 1.19 seconds for 200*200 array > sliceXcopy :: Int -> UArr Double -> [UArr Double] > sliceXcopy n a = map mkSlice [0 .. n-1] > where > mkSlice x = runST $ do arr <- newMU n > cp <- newMU (n*n) > copyMU cp 0 a > forM_ [0 .. n-1] $ \y -> writeMU arr y =<< readMU cp (y*n + x) > unsafeFreezeAllMU arr This is another implementation with lists which convert whole array to list and picks appropriate element it. It is fastest implementation so far. 0.039 seconds for 200*200 array > sliceXlistfast :: Int -> UArr Double -> [UArr Double] > sliceXlistfast n a = map mkSlice [0 .. n-1] > where > takeEvery n [] = [] > takeEvery n (x:xs) = x : takeEvery n (drop (n-1) xs) > mkSlice x = toU $ takeEvery n . drop x $ fromU a > > > ---------------------------------------------------------------- > main :: IO () > main = do > [str,a] <- getArgs > let n = read str > case a of > "y" -> print $ sum $ map sumU (sliceY n (arr n)) > "list" -> print $ sum $ map sumU (sliceXlist n (arr n)) > "lf" -> print $ sum $ map sumU (sliceXlistfast n (arr n)) > "st" -> print $ sum $ map sumU (sliceXst n (arr n)) > "copy" -> print $ sum $ map sumU (sliceXcopy n (arr n))

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Opinion about JHC
by Philippos Apolinarius 16 Nov '09

16 Nov '09

I discovered a Haskell compiler that generates very small and fast code. In fact, it beats Clean. It has the following properties: 1 --- One can cross-compile programs easily. For instance, here is how I generated code for Windows: jhc --cross -mwin32 genetic.hs -o genetic 2 -- It seems to be quite complete. 3 -- However, it often compiles a file, but the program fails to run. I have the following questions about it: 1 -- How active is the team who is writing the JHC compiler? 2 -- Is it complete Haskell? The author claims that it is; it compiled all programs that I wrote, but that does not mean much, because my programs are quite simple. 3 -- Why the Haskell community almost never talks about JHC? __________________________________________________________________ The new Internet Explorer® 8 - Faster, safer, easier. Optimized for Yahoo! Get it Now for Free! at http://downloads.yahoo.com/ca/internetexplorer/

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About xmonad
by zaxis 16 Nov '09

16 Nov '09

%uname -a Linux myarch 2.6.31-ARCH #1 SMP PREEMPT Tue Nov 10 19:48:17 CET 2009 i686 AMD Athlon(tm) 64 X2 Dual Core Processor 3600+ AuthenticAMD GNU/Linux %xmonad --version xmonad 0.9 In firefox, the `save as` dialog doesnot appear when i want to choose picture to save by right clicking the mouse. %cat ~/.xmonad/xmonad.hs import XMonad import XMonad.Hooks.ManageDocks import XMonad.Hooks.EwmhDesktops import XMonad.Hooks.DynamicLog import XMonad.Hooks.ManageHelpers import XMonad.Util.Run(spawnPipe) import XMonad.Layout.TwoPane import XMonad.Layout.WindowNavigation import qualified XMonad.StackSet as W import qualified Data.Map as M main = do xmonad $ defaultConfig { borderWidth = 1 , focusedBorderColor = "#ff6666" , normalBorderColor = "#2222aa" , manageHook = manageHook defaultConfig <+> myManageHook , workspaces = map show [1 .. 10 :: Int] , terminal = "roxterm" , modMask = mod4Mask , focusFollowsMouse = True , startupHook = myStartupHook , logHook = myLogHook , layoutHook = windowNavigation $ avoidStruts $ (Mirror tall ||| tall ||| Full) --, layoutHook = ewmhDesktopsLayout $ windowNavigation $ avoidStruts $ (Mirror tall ||| tall ||| Full) , keys = \c -> myKeys c `M.union` keys defaultConfig c --, mouseBindings = \c -> myMouse c `M.union` mouseBindings defaultConfig c } where tall = Tall 1 (3/100) (1/2) myStartupHook :: X () myStartupHook = do { spawn "fcitx"; spawn "roxterm"; spawn "lxpanel"; spawn "/home/sw2wolf/bin/kvm.sh"; } myLogHook :: X () myLogHook = ewmhDesktopsLogHook myManageHook :: ManageHook myManageHook = composeAll . concat $ [ [ className =? c --> doFloat | c <- myCFloats] ,[ resource =? r --> doFloat | r <- myRFloats] ,[ title =? t --> doFloat | t <- myTFloats] ,[ className =? c --> doIgnore | c <- ignores] ,[ className =? "Audacious" --> doShift "3" ] ,[ className =? "Firefox" --> doF W.swapDown] ,[(role =? "gimp-toolbox" <||> role =? "gimp-image-window") --> (ask >>= doF . W.sink)]] where myCFloats = ["Thunderbird-bin", "GQview", "MPlayer", "Gimp","Vncviewer","Xmessage"] myRFloats = ["Dialog", "Download", "Places"] myTFloats = ["Firefox Preferences", "Element Properties"] ignores = ["trayer"] role = stringProperty "WM_WINDOW_ROLE" myKeys (XConfig {modMask = modm}) = M.fromList $ -- Apps and tools [ ((modm, xK_F2), spawn "gmrun") , ((modm, xK_f), spawn "/home/firefox/firefox") , ((modm, xK_t), spawn "thunderbird") --, ((modm, xK_p), spawn "exe=`dmenu_path | dmenu -b` && eval \"exec $exe\"") , ((modm, xK_F11), spawn "sudo shutdown -r now") , ((modm, xK_F12), spawn "sudo shutdown -h now") , ((modm .|. controlMask, xK_Print), spawn "sleep 0.2; scrot -s") , ((modm, xK_Print), spawn "scrot '/tmp/%Y-%m-%d_%H:%M:%S_$wx$h_scrot.png' -e 'mv $f ~'") , ((modm, xK_c), kill) -- Window Navigation , ((modm, xK_Right), sendMessage $ Go R) , ((modm, xK_Left ), sendMessage $ Go L) , ((modm, xK_Up ), sendMessage $ Go U) , ((modm, xK_Down ), sendMessage $ Go D) -- swap... , ((modm .|. controlMask, xK_Right), sendMessage $ Swap R) , ((modm .|. controlMask, xK_Left ), sendMessage $ Swap L) , ((modm .|. controlMask, xK_Up ), sendMessage $ Swap U) , ((modm .|. controlMask, xK_Down ), sendMessage $ Swap D) ] ----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/About-xmonad-tp26367498p26367498.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Strange parallel behaviour with Ubuntu Karmic / GHC 6.10.4
by Michael Lesniak 16 Nov '09

16 Nov '09

Hello, I'm currently developing some applications with explicit threading using forkIO and have strange behaviour on my freshly installed Ubuntu Karmic 9.10 (Kernel 2.6.31-14 SMP). Setup: Machine A: Quadcore, Ubuntu 9.04, Kernel 2.6.28-13 SMP Machine B: AMD Opteron 875, 8 cores, 2.6.18-164 SMP- (some redhat) Machine C: Dual-Core, Ubuntu 9.10, Kernel 2.6.31-14 SMP Compiler on all machines: ghc 6.10.4 (downloaded from GHCs official website) Program, Compilation, Execution A simple taskqueue with independent tasks and explicit parallelization (hence should deliver more or less perfect speedup). For one core wall-times around 16 are ok, for 2 a bit more than 8 seconds. Since I used the same sources and Makefiles on all machines all files were compiled with -threaded and started with +RTS -N2 -RTS. Testing: Machine A: Ok (meaning works and delivers the expected speedup) Machine B: Ok Machine C: Not ok (with -N2 wall times around 14-15 seconds) Looking at the core usage, for example with htop, I see that the second core is not really used on C. Executing OpenMP programs shows the expected speedup and usage of both cores, hence I do not think its a kind of general linux configuration problem. So, after all the testing I think its either the Linux Kernel or some other component of Ubuntu 9.10. But: Ubuntu is often used and I did not found any information regarding this problem. The simple solution of installing the old version of Ubuntu would probably help but should not be the way to go, should it? I'd be glad for any hints or comments, Michael -- Dipl.-Inf. Michael C. Lesniak University of Kassel Programming Languages / Methodologies Research Group Department of Computer Science and Electrical Engineering Wilhelmshöher Allee 73 34121 Kassel Phone: +49-(0)561-804-6269

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How to update SSH key in code.haskell.org
by Andy Stewart 16 Nov '09

16 Nov '09

Hi all. I'm a developer of gtk2hs project at code.haskell.org But i found i lost my SSH key when i update new system, and i can't push patch to gtk2hs. I have send mail to support(a)community.haskell.org for this problem, but i'm not sure that's the right place. So how to update SSH key with my account (AndyStewart) ? Thanks, -- Andy

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Logistic regression
by Colin Paul Adams 16 Nov '09

16 Nov '09

Is there any free code anywhere for performing logistic regression? I can't see anything on hackage. -- Colin Adams Preston Lancashire

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What does the `forall` mean ?
by zaxis 16 Nov '09

16 Nov '09

import Text.ParserCombinators.Parsec data PermParser tok st a = Perm (Maybe a) [Branch tok st a] data Branch tok st a = forall b. Branch (PermParser tok st (b -> a)) (GenParser tok st b) I have hoogled the `forall` but i cannot find any appropriate answer! thanks! ----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/What-does-the-%60forall%60-mean---tp26311291p26311291… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Why can `env` be assigned value two times ?
by zaxis 16 Nov '09

16 Nov '09

defineVar :: Env -> (Id, Val) -> IOThrowsError Val defineVar envRef (id, val) = do { env <- liftIO $ readIORef envRef; env <- return $ filter (\(_id, _) -> _id/=id) env; -- clear the current scope valRef <- liftIO $ newIORef val; liftIO $ writeIORef envRef $ ((id, valRef):env); return val; } In haskell, the variable canot change its value , right? If so, why can the `env` be assigned value twice? Sincerely! ----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/Why-can-%60env%60-be-assigned-value-two-times---tp263… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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attoparsec and parsec
by Manlio Perillo 16 Nov '09

16 Nov '09

Hi. I'm writing a generic log parsing package, and I'm serching a parser combinators library. Parsing log files is a simple task, so I would like to use the more efficient solution. I have looked at attoparsec source code, and it seems very specialized for lazy bytestrings parsing, so I assume it is very efficient. How stable is the attoparsec package? How much more efficient is attoparsec than standard packages like parsec? By the way: there seem to be problems with generated documentation in http://hackage.haskell.org/package/attoparsec. Moreover, there is a similar package: http://hackage.haskell.org/package/bytestringparser what is the status of this package? It has the same API of attoparsec, but its older. However there is no indication that this package is deprecated in favor of attoparsec. Thanks Manlio Perillo

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slightly off-topic
by Günther Schmidt 16 Nov '09

16 Nov '09

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Some thoughts about Hackage
by Vasyl Pasternak 15 Nov '09

15 Nov '09

Hi, Yesterday Max complained about documentation for many Haskell modules. But I found another similar problem with Hackage. Before coding some Haskell program I try to find most appropriate libraries, which help me to do task more efficiently. But the problem, that there are to many libraries with similar functionality (for example - networking, web servers etc.). And to find the best solution is not so obvious. My idea is to improve Hackage to help everyone with package selecting. I propose the following: - add download counter for each package, it could show how popular the package is - allow registered users set the quality mark of the package (from 1 to 5) and show the average mark of each packet - add counter which shows how many packages depend on this package (direct/indirect) - create an aggregate package rank (a function on previous three values), similar to Google's PageRank, i. e. rank of the package is proportional to the package mark and weighted rank of dependent packages - allow comments on the package page, so anyone could tell its opinion or other useful info for this package. All messages should be delivered to the maintainer. This is useful, because it could speedup the feedback on the packages, and also could form large knowledge base on each package. Now everyone, who wants to read more about some package should use Google to extract info from HaskellWiki, Haskell-Cafe or hundreds of blog posts from different authors. What do you think ? Best regards, Vasyl Pasternak

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I'm also missing examples of library function and Hackage usage. Haskell Wiki?
by Casey Hawthorne 15 Nov '09

15 Nov '09

I'm also missing examples of library function and Hackage usage. Where does one put up code on the Haskell Wiki? :) -- Regards, Casey

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Type-indexed expressions with fixpoint
by oleg＠okmij.org 14 Nov '09

14 Nov '09

Brent Yorgey wrote: > This email is literate Haskell. I'm struggling to come up with the > right way to add a fixpoint constructor to an expression language > described by a type-indexed GADT (details below). > > but since Haskell doesn't have type-level lambdas, I don't see how > to make that work. John Reynolds showed long ago that any higher-order language can be encoded in first-order. We witness this every day: higher-order language like Haskell is encoded in first-order language (machine code). The trick is just to add a layer of interpretive overhead -- I mean, a layer of interpretation. The closure conversion on type level was shown in http://okmij.org/ftp/Computation/lambda-calc.html#haskell-type-level Here's how a similar technique applies to the problem at hand. There is an alternative solution: encode higher-order functors by means of SKI combinators. Somehow I prefer the pointed solution. The complete code follows. {-# LANGUAGE TypeFamilies, KindSignatures, GADTs, FlexibleInstances #-} data U :: (* -> *) -> * where Unit :: U None Var :: U Id (:+:) :: U f -> U g -> U (Sum f g) (:*:) :: U f -> U g -> U (Prod f g) Mu :: HOFunctor f => f -> U (MU f) data None a = None deriving Show data Id a = Id a deriving Show data Sum f g a = Inl (f a) | Inr (g a) deriving Show data Prod f g a = Prod (f a) (g a) deriving Show newtype MU f a = MU (Res f (MU f) a) type family Res f self :: * -> * type instance Res List self = Sum None (Prod Id self) data List = List -- the code for the HO functor class HOFunctor f where fn :: f -> U g -> U (Res f g) instance HOFunctor List where fn _ self = Unit :+: (Var :*: self) enumShapes :: Int -> U f -> [f ()] enumShapes 0 Unit = [None] enumShapes _ Unit = [] enumShapes 1 Var = [Id ()] enumShapes _ Var = [] enumShapes n (f :+: g) = map Inl (enumShapes n f) ++ map Inr (enumShapes n g) enumShapes n (f :*: g) = [ Prod x y | x <- enumShapes n f, y <- enumShapes n g] enumShapes n self@(Mu f) = map MU $ enumShapes (n-1) (fn f self) test0 = enumShapes 0 (Unit :+: (Var :*: Var)) -- [Inl None] test1 = enumShapes 1 (Unit :+: (Var :*: Var)) -- [Inr (Prod (Id ()) (Id ()))] test2 = enumShapes 1 (Mu List) -- [Inl None] test3 = enumShapes 2 (Mu List) -- [Inr (Prod (Id ()) Inl None)] instance Show (MU List ()) where show (MU x) = show x

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Where is a good place to place code like this, so if I may be so bold, people can learn from it?
by Casey Hawthorne 14 Nov '09

14 Nov '09

Where is a good place to place code like this, so if I may be so bold, people can learn from it? {- Author Modifications: Casey Hawthorne Author Original: Jeff Newbern Maintainer: Casey Hawthorne <caseyh(a)istar.ca> Maintainer?: Jeff Newbern <jnewbern(a)nomaware.com> Time-stamp: Jeff <Tue Aug 19 09:31:32 2003> Time-stamp: Casey <Sat Nov 14 10:10 2009> License: GPL The N-queens puzzle is to place N queens on an N by N chess board so that no queen can attack another one. Compiler Flags: ghc -O2 -fvia-c --make N-Queens.hs Description http://www.haskell.org/all_about_monads/html/stacking.html#example Original Code http://www.haskell.org/all_about_monads/examples/example25.hs -} {- Description Example 25 - Using the StateT monad transformer with the List monad to achieve non-deterministic stateful computations, and the Writer monad to do logging Usage: Compile the code and run it with an argument between 1 and 8. It will print a solution to the N-queens puzzle along with a log of the number of choices it had at each step. The N-queens puzzle is to place N queens on a chess board so that no queen can attack another one. The original version always used an 8x8 board. Try: ./ex25 8 ./ex25 1 ./ex25 7 Added by Casey: - different board sizes -- up to a maximum 26x26 square board - updated imports list -} import IO import System import Monad import Data.Maybe import Data.List import Data.Char (toLower) import Control.Monad.State import Control.Monad.Writer -- describe Chess Units and positions type Rank = Int data File = A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | X | Y | Z deriving (Eq, Show, Ord, Enum) data Position = Pos {file::File, rank::Rank} deriving Eq instance Show Position where show (Pos f r) = (map toLower (show f)) ++ (show r) instance Ord Position where compare p1 p2 = case (rank p1) `compare` (rank p2) of LT -> GT GT -> LT _ -> (file p1) `compare` (file p2) data Kind = Pawn | Knight | Bishop | Rook | Queen | King deriving (Eq, Ord, Enum) instance Show Kind where show Pawn = "P" show Knight = "N" show Bishop = "B" show Rook = "R" show Queen = "Q" show King = "K" data Color = Black | White deriving (Eq, Ord, Enum) instance Show Color where show Black = "b" show White = "w" data Unit = Unit {color::Color, kind::Kind} deriving (Eq, Ord) instance Show Unit where show (Unit c k) = ((show c) ++ (show k)) data Board = Board {size::Int, psns::[(Unit,Position)]} -- newtype Board = Board [(Unit,Position)] -- newtype BoardMax = BoardMax Int instance Show Board where show (Board n ps) = let ordered = (sort . swap) ps ranks = map (showRank ordered) [n,(n-1)..1] board = intersperse (concat (take n (repeat "--+"))) ranks rlabels = intersperse " " (map (\n->(twoSpaces n)++" ") [n,(n-1)..1]) flabels = take (n*3) " a b c d e f g h i j k l m n o p q r s t u v w x y z" twoSpaces n | length (show n) == 2 = show n | otherwise = " " ++ (show n) in unlines $ zipWith (++) rlabels board ++ [flabels] where swap = map (\(a,b)->(b,a)) showRank ps r = let rnk = filter (\(p,_)->(rank p)==r) ps cs = map (showUnit rnk) (take n [A .. Z]) in concat (intersperse "|" cs) showUnit ps f = maybe " " (show . snd) (find (\(p,_)->(file p)==f) ps) data Diagonal = Ascending Position | Descending Position deriving (Eq, Show) -- define the diagonal according to its interesction with rank 1 or size of board) -- or with file a normalize :: Int -> Diagonal -> Diagonal normalize n d@(Ascending psn) | (rank psn) == 1 = d | (file psn) == A = d | otherwise = normalize n (Ascending (Pos (pred (file psn)) ((rank psn)-1))) normalize n d@(Descending psn) | (rank psn) == n = d | (file psn) == A = d | otherwise = normalize n (Descending (Pos (pred (file psn)) ((rank psn)+1))) -- get the diagonals corresponding to a location on the board getDiags :: Int -> Position -> (Diagonal,Diagonal) getDiags n p = (normalize n (Ascending p), normalize n (Descending p)) -- this is the type of our problem description data NQueensProblem = NQP {board::Board, ranks::[Rank], files::[File], asc::[Diagonal], desc::[Diagonal]} -- initial state = empty board, all ranks, files, and diagonals free initialState n = let fileA = map (\r->Pos A r) [1..n] rankMax = map (\f->Pos f n) (take n [A .. Z]) rank1 = map (\f->Pos f 1) (take n [A .. Z]) asc = map Ascending (nub (fileA ++ rank1)) desc = map Descending (nub (fileA ++ rankMax)) in NQP (Board n []) [1..n] (take n [A .. Z]) asc desc -- this is our combined monad type for this problem type NDS a = WriterT [String] (StateT NQueensProblem []) a -- Get the first solution to the problem, by evaluating the solver computation with -- an initial problem state and then returning the first solution in the result list, -- or Nothing if there was no solution. getSolution :: NDS a -> NQueensProblem -> Maybe (a,[String]) getSolution c i = listToMaybe (evalStateT (runWriterT c) i) -- add a Queen to the board in a specific position addQueen :: Position -> NDS () addQueen p = do (Board n b) <- gets board rs <- gets ranks fs <- gets files as <- gets asc ds <- gets desc let b' = (Unit Black Queen, p):b rs' = delete (rank p) rs fs' = delete (file p) fs (a,d) = getDiags n p as' = delete a as ds' = delete d ds tell ["Added Queen at " ++ (show p)] put (NQP (Board n b') rs' fs' as' ds') -- test if a position is in the set of allowed diagonals inDiags :: Int -> Position -> NDS Bool inDiags n p = do let (a,d) = getDiags n p as <- gets asc ds <- gets desc return $ (elem a as) && (elem d ds) -- add a Queen to the board in all allowed positions addQueens :: NDS () addQueens = do rs <- gets ranks fs <- gets files (Board n b) <- gets board allowed <- filterM (inDiags n) [Pos f r | f <- fs, r <- rs] tell [show (length allowed) ++ " possible choices"] msum (map addQueen allowed) -- Start with an empty chess board and add the requested number of queens, -- then get the board and print the solution along with the log main :: IO () main = do args <- getArgs let n = read (args!!0) cmds = replicate n addQueens sol = (`getSolution` (initialState n)) $ do sequence_ cmds gets board case sol of Just (b,l) -> do putStr $ show b -- show the solution putStr $ unlines l -- show the log Nothing -> putStrLn "No solution" -- END OF FILE -- Regards, Casey

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ANN: hesql
by Christoph Bauer 14 Nov '09

14 Nov '09

Hello, sure, your program could use a database with HDBC. But I'll guess (since you love static typing so much) you dislike formulating queries in strings and to check the positions of your ?-placeholders and to convert your values with fromSql/toSql. Maybe you would prefer for your select query a list of tuples instead of a list of lists, because your beloved pattern matching needs always a dummy case. And here [1] is hesql... it's a preprocessor, which rewrites SQL-Queries as haskell functions. You put all your SQL-Queries (for example verifyLogin login' password' = select1' Accountid, Login, RealName, Email from Account where (password is null or password = md5(password')) and Lower(Login) = login' and not Locked; ) in a module (example/Account.hesql) and run $ hesql example/Account.hesql dbname=example which will write example/Account.hs, the real haskell module. It will generate code for a haskell function with the type: verifyLogin :: Stmts -> String -> String -> IO (Maybe (Int, String, String, String)) (Ok, this is not quite correct. Check the README.) And Of course my SQL-Parser is very incomplete. Hesql works only with postgresql, etc, etc. So, please send me patches :-) Don't expect too much. Christoph Bauer [1] on hackageDB

3 2

Collection of sets containing no sets which are a subset of another in the collection
by Mark Wassell 14 Nov '09

14 Nov '09

Hi, I am looking for a data structure that will represent a collection of sets such that no element in the collection is a subset of another set. In other words, inserting an element that is already a subset of another element will return the original collection, and inserting an element that is a superset of any elements will result in a collection with the superset added and the subsets removed. What I have so far is the below but I am wondering if there has been any prior work on this (particularly using Haskell but also conceptual work). Inserting a set that is a subset is easy to handle, inserting a superset and remove subsets of it is a little tricker. Cheers Mark module SetTrie where -- -- A set of sets which does not contain elements which are subsets of any other element. -- ie insert a element which is a proper subset of another set returns the same set of sets -- insert a element which is a proper superset of one or more elements causes those elements to be removed -- (and the first element to be added) -- import Data.Set hiding (toList,singleton,map,insert) import Data.Map hiding (fromList,showTreeWith,toAscList,toList,singleton,map,insert) import qualified Data.Map as M (toList,fromList,lookup,insert) import qualified Data.Set as S (toList,fromList) -- Normally we would have a flag at a node to indicate a subset is there, but we -- don't store subsets. data SetTrie a = Leaf [a] | Node (Map a (SetTrie a)) deriving (Show,Eq) singleton :: Ord a => Set a -> SetTrie a singleton = Leaf . S.toList toList' :: Ord a => SetTrie a -> [[a]] toList' (Leaf xs) = [xs] toList' (Node m) = concatMap (\(x,y) -> map (x:) (toList' y)) $ M.toList m toList :: Ord a => SetTrie a -> [Set a] toList x = map S.fromList $ toList' x insert :: Ord a => SetTrie a -> Set a -> SetTrie a insert t s = insert' t $ toAscList s insert':: Ord a => SetTrie a -> [a] -> SetTrie a insert' (Leaf (y:ys)) (x:xs) = Node (M.fromList [(y,Leaf ys),(x,Leaf xs)]) insert' (Node m) (x:xs) = case M.lookup x m of Nothing -> case xs of [] -> Node $ M.insert x (Leaf xs) m _ -> Node $ M.insert x (Leaf xs) m Just t' -> case xs of [] -> Node m _ -> Node $ M.insert x (insert' t' xs) m -- removeSubsets :: -- terminate (Node m) = Node mTrue m -- terminate (Leaf (x:xs)) = Node True (M.fromList [(x,Leaf xs)]) s1 = fromList [1,2,3,5,2] s2 = fromList [2,3,5] t1 = Node (M.fromList [(1,Leaf [2]),(3,Leaf [5]),(2,Node (M.fromList [(4,Leaf [6])]))]) t2 = insert (singleton (S.fromList [1])) $ S.fromList [1,2,3] t3 = insert t1 $ S.fromList [2,4,7] t4 = insert t2 $ S.fromList [1] t5 = insert t3 $ S.fromList [2,5] t6 = insert t5 $ S.fromList [2,4] -- Now add a superset of everything t7 = insert (singleton (S.fromList [8])) $ S.fromList [1,2,3,4,5,6,7,8,9] Mark

4 3

Writing great documentation
by Max Rabkin 14 Nov '09

14 Nov '09

Haskellers, I have heard many complaints about the average quality on documentation. Therefore, I'd like to encourage you all to read Jacob Kaplan-Moss's series on writing great documentation: http://jacobian.org/writing/great-documentation/. The articles are themselves well-written and contain excellent advice (though I disagree somewhat with the comments on automatically-generated documentation: I find many libraries are excellently haddocumented). Jacob Kaplan-Moss is a developer on the Django project, which is well known for the quality of its documentation. One issue he brings up is having different types of documentation. My impression of many Haskell libraries (my own included) is that, while they may have good reference documentation, they lack tutorials and topic guides. Perhaps we could bring up some examples of Haskell projects with particularly good documentation, as examples to look up to. XMonad has very good overviews and guides for developers, and I like how each user-facing xmonad-contrib module gives a small snippet showing how to use it in ones own config. One area where I think it could be improved (and I plan to do some work on this when I have more free time) is in topical guides on things like "how to write your own layout". --Max

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Lexical Syntax and Unicode
by Manlio Perillo 14 Nov '09

14 Nov '09

Hi. Reading the Haskell 98 Report (section 9.2), I have found a possible problem. The lexical syntax supports Unicode, however this is not true for the newline: newline -> return linefeed | return | linefeed | formfeed The Unicode standard adds two additional characters: U+2028 LINE SEPARATOR U+2029 PARAGRAPH SEPARATOR The Unicode Character Database, also defines two general categories: Zl = Separator, line Zp = Separator, paragraph The Zl category only contains the LINE SEPARATOR character and the Zp category only contains the PARAGRAPH SEPARATOR character. So, IMHO, the lexical syntax should be changed in : newline -> return linefeed | return | linefeed | formfeed | uniLine | uniPara uniLine -> any Unicode character defined as line separator uniPara -> any Unicode character defined as paragraph separator or, alternatively: uniLine -> LINE SEPARATOR uniPara -> PARAGRAPH SEPARATOR Manlio Perillo

1 0

Package Woes
by Daniel Fischer 14 Nov '09

14 Nov '09

Sorry, slightly off-topic. I wanted to install LHC to compare that to GHC and JHC, but alas: dafis@linux-mkk1:~/Haskell/LHC/lhc-0.8> cabal install -fwith-libs -flhc-regress Resolving dependencies... Configuring libffi-0.1... cabal: The pkg-config package libffi is required but it could not be found. cabal: Error: some packages failed to install: lhc-0.8 depends on libffi-0.1 which failed to install. libffi-0.1 failed during the configure step. The exception was: exit: ExitFailure 1 Okay, so I installed libffi, but to no avail, still can't find libffi. The problem is apparently that libffi doesn't come with a .pc file, so pkg-config doesn't know about it. Does anybody know how to fix that? (openSuse 11.1, if that matters)

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Haskell Weekly News: Issue 139 - November 14, 2009
by jfredett＠gmail.com 14 Nov '09

14 Nov '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20091114 Issue 139 - November 14, 2009 --------------------------------------------------------------------------- Welcome to issue 139 of HWN, a newsletter covering developments in the [1]Haskell community. Lots of good discussion this week about everything from Monoids to Memory Leaks, Parsers (for Token Streams) and pushing Haskell onto Medical devices! I could go on some long rant rife with really righteous alliteration or a touch of timely consonance, but instead I'll leave you all, my fellow Haskellers, to read your Haskell Weekly News! Announcements Two Open PhD positions at the Technical University Munich. Axel Simon [2]announced two PhD positions are open in Low-level and High-level analysis, see the post for details. (ED: Apologies for being so late in this announcment, it slipped under my radar! ) Final CFP: WFLP 2010. Deadlines extended: Abstract due Nov 18; Full paper due Nov 25 (LNCS). Pablo Nogueira [3]announced a deadline extension for the WLFP 2010 conference, abstracts are now due November 18, and full papers by the 25th. hesql. Christoph Bauer [4]announced hesql, a preprocessor for writing SQL statements in pure haskell. dbus-core 0.6 and dbus-client 0.2. John Millikin [5]announced the second release of his dbus libraries. Changes include performance improvments, better support for byte arrays, and TCP/IP transport (though this remains untested). simple-observer-0.0.1, a simple implementation of the observer design pattern. Andy Gimblett [6]announced an implementation of the Observer pattern in Haskell ICFP 2010: Call for papers. Wouter Swierstra [7]announced a call for papers for ICFP 2010. Calling all Haskellers in Huntsville, Alabama, or surrounding areas! Jake McArthur [8]announced the formation of a new Haskell User Group in Alabama. (ED: Apparently, Shae is the Johnny Appleseed for the Haskell Community, #haskell, BAHUG, now AHUG... when will it end? ) acme-dont. Gracjan Polak [9]announced the acme-dont package, providing a vital missed feature to our language -- a don't monad. See the post for all the revolutionary details. Discussion Could someone teach me why we use Data.Monoid? Magicloud Magiclouds [10]1fa7 gmane.comp.lang.haskell.cafe/66223 asked why we use monoids so much in Haskell. Least common supertype. Sean Leather [11]posed an interesting question about 'antiunification' -- finding a common supertype of two types which is 'most-constrained'. Long running Haskell program. David Leimbach [12]asked about managing memory leaks in long running programs. Parsec - separating Parsing from Lexing. Fernando Henrique Sanches [13]asked about using Parsec to parse a Token Stream. Help Haskell driving Medical Instruments. Philippos Apolinarius [14]talked about using Haskell to drive medical instruments. Blog noise [15]Haskell news from the [16]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Neil Brown: [17]The Observer Pattern using a Broadcast Channel. * Martijn van Steenbergen: [18]GADTs in Haskell 98. * Mikael Vejdemo Johansson (Syzygy-): [19][MATH198] Multiple lectures posted. * Andy Gill: [20]LAMBDA this week â ChalkBoard. * London Haskell Users Group: [21]Functional Programming Exchange. * Neil Brown: [22]The Observer Pattern using a Message-Passing Process. * Luke Plant: [23]Is static type checking a redundant testing mechanism?. * Twan van Laarhoven: [24]Four ways to fold an array. * Martin Sulzmann: [25]Tag-free Combinators for Binding-Time Polymorphic Program Generation. * Luke Palmer: [26]Collision Detection with Enneatrees. * Dan Piponi (sigfpe): [27]Memoizing Polymorphic Functions with High School Algebra and Quantifiers. * Luke Plant: [28]Haskell blog software. Quotes of the Week * c2.com: If you can program anything in HappS you actually already learned Haskell * Cale: Chew new Lasty ST Gum! It lasts until _|_! * danderson: [using unsafeFreeze for an ST action] that sounds like a way to shoot myself in the foot with high efficiency, given my knowledge of haskell. * BONUS: C++ is saner than something? imo C++ is like the guy that goes around shouting "I am napoleon!!!" * kmc:: (): worst monoid ever * DanWeston: Bottom has only one value, not two. Otherwise bottom would have been called buttocks. About the Haskell Weekly News New editions are posted to [29]the Haskell mailing list as well as to [30]the Haskell Sequence and [31]Planet Haskell. [32]RSS is also available, and headlines appear on [33]haskell.org. To help create new editions of this newsletter, please see the information on [34]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [35]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://www.mail-archive.com/haskell@haskell.org/msg22432.html 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/66215 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/66184 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/66110 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/66103 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/66021 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/66018 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/65958 10. http://thread.gmane.org/ 11. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/66080 12. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/66056 13. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/66012 14. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/65996 15. http://planet.haskell.org/ 16. http://haskell.org/haskellwiki/Blog_articles 17. http://chplib.wordpress.com/2009/11/12/the-observer-pattern-using-a-broadca… 18. http://martijn.van.steenbergen.nl/journal/2009/11/12/gadts-in-haskell-98/ 19. http://blog.mikael.johanssons.org/archive/2009/11/math198-multiple-lectures… 20. http://www.ittc.ku.edu/csdlblog/?p=16 21. http://www.londonhug.net/2009/11/10/functional-programming-exchange/ 22. http://chplib.wordpress.com/2009/11/10/the-observer-pattern-using-a-message… 23. http://lukeplant.me.uk/blog/posts/is-static-type-checking-a-redundant-testi… 24. http://twan.home.fmf.nl/blog/haskell/four-ways-to-fold.details 25. http://sulzmann.blogspot.com/2009/11/tag-free-combinators-for-binding-time.… 26. http://lukepalmer.wordpress.com/2009/11/08/collision-detection-with-enneatr… 27. http://blog.sigfpe.com/2009/11/memoizing-polymorphic-functions-with.html 28. http://lukeplant.me.uk/blog/posts/haskell-blog-software/ 29. http://www.haskell.org/mailman/listinfo/haskell 30. http://sequence.complete.org/ 31. http://planet.haskell.org/ 32. http://sequence.complete.org/node/feed 33. http://haskell.org/ 34. http://haskell.org/haskellwiki/HWN 35. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

1 0

FFI binding -- different behaviour under compilation and interpretation.
by Jason Dusek 14 Nov '09

14 Nov '09

I'm binding to `wcwidth` to determine the column widths of various Unicode characters. I noticed a lot of -- in fact all -- Chinese characters were being given widths of `-1` when of course they should have width `2`. This only showed up when I compiled my program though -- within GHCi, it never happened. Below my signature is a parred down example that demoes the bug. It tries to get the width of only one Chinese character. You can see it like this: :; ghc --make DemoFailure.hs -o demo && demo [1 of 1] Compiling Main ( DemoFailure.hs, DemoFailure.o ) Linking demo ... 0x00005cff -1 峿 :; chmod ug+x DemoFailure.hs && DemoFailure.hs 0x00005cff 2 峿 Switching between safe/unsafe does not make any difference. This was run on a Macintosh. -- Jason Dusek #!/usr/bin/env runhaskell {- DemoFailure.hs -} {-# LANGUAGE ForeignFunctionInterface #-} import Foreign.C import Data.Char import Text.Printf import qualified System.IO.UTF8 as UTF8 main = do (sequence_ . fmap (UTF8.putStrLn . uncurry fmt)) widths where widths = [ (c, wcwidth c) | c <- ['\x5cff'] ] --widths = [ (c, wcwidth c) | c <- [minBound..maxBound] ] fmt c cols = printf "0x%08x %2d %s" (fromEnum c) cols rep where rep | ' ' == c = "\\SP" | isSpace c = '\\' : show (fromEnum c) | isPrint c = [c] | otherwise = (reverse . drop 1 . reverse . drop 1 . show) c wcwidth :: Char -> Int wcwidth = fromEnum . native . toEnum . fromEnum foreign import ccall unsafe "wchar.h wcwidth" native :: CWchar -> CInt

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looking for a good algorithm
by Hong Yang 14 Nov '09

14 Nov '09

The question is more about algorithm than Haskell. But I am going to code in Haskell which I am still learning. Suppose I have a large table, with hundreds of columns and thousands of rows. But not every cell has a value (of String, or Int, or Double type). I want to shuffle the rows to maximize the number of columns whose first 100 rows have at least one number, given a list of preferred column names since there is no guarantee that every number column will have at least one number in its first 100 rows after shuffling. Can someone provide a good algorithm for this problem? (I do not have any background in algorithms.) You can assume I already know which columns are of Int or Double type. This is not a homework. Thanks, Hong

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Re: [Haskell-cafe] Could someone teach me why we use Data.Monoid?
by David Place 14 Nov '09

14 Nov '09

Magicloud Magiclouds <magicloud.magiclouds(a)gmail.com> wrote: > Message: 26 > Date: Sat, 14 Nov 2009 01:11:27 +0800 > From: Magicloud Magiclouds <magicloud.magiclouds(a)gmail.com> > Subject: Re: [Haskell-cafe] Could someone teach me why we use > Data.Monoid? > To: Stephen Tetley <stephen.tetley(a)gmail.com> > Cc: haskell-cafe <haskell-cafe(a)haskell.org> > Message-ID: > <3bd412d40911130911v4f3ac0b9laebca79f5921491d(a)mail.gmail.com> > Content-Type: text/plain; charset=UTF-8 > > That is OK. Since understand the basic concept of monoid (I mean the > thing in actual math), the idea here is totally not hard for me. But > the sample here does not show why (or how) we use it in programming, > right? For an example of it's use, you might enjoy reading my article in the Monad.Reader "How to Refold a Map." > http://www.haskell.org/sitewiki/images/6/6a/TMR-Issue11.pdf Cheers, David

5 5

A small (?) problem with type families
by Andy Gimblett 14 Nov '09

14 Nov '09

Hi all, This email is literate Haskell. I'm trying to use type families to express some dependencies between type classes, and I'm running into trouble, I think because I'm producing chains of dependencies which the checker can't resolve... Here's a minimised version of the state I've got myself into. :-) > {-# LANGUAGE FlexibleContexts #-} > {-# LANGUAGE TypeFamilies #-} > module Families where First a type family where the type Y is functionally dependent on the type X, and we have a function from Y to (). > class X a where > type Y a > enact :: Y a -> () Now another type family, where the type Q is functionally dependent on the type P, _and_ it must also be an instance of the X class. > class (X (Q s)) => P s where > type Q s (Perhaps there's a better way to express that dependency?) Now a function which takes a value whose type is an instance of the Y depending on the Q depending on the P. (Phew!) The function just tries to call enact on that value. > bar :: P s => Y (Q s) -> () > bar w = enact w The error we get is: src/Families.lhs:35:16: Couldn't match expected type `Y a' against inferred type `Y (Q s)' In the first argument of `enact', namely `w' In the expression: enact w In the definition of `bar': bar w = enact w Presumably this way I'm chaining type dependencies is flawed. Any suggestions on how to improve it, and/or what to read to understand what I'm dealing with better? (So far I've read "Fun with type functions V2", but that's about it, and I admit I didn't grok it all.) Thanks! -Andy

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ANN: wxHaskell 0.12.1.2
by Jeremy O'Donoghue 13 Nov '09

13 Nov '09

Hi all, On behalf of the wxHaskell maintainers, I am very pleased to announce the release of wxHaskell 0.12.1.2. The key feature of this release is that it is now possible to install wxHaskell entirely using cabal on all targets (with a minor proviso on Windows). The credit for this achievement is almost all down to new contributor Brian Lewis, who has worked very hard to make this work 'just right', and given me (at least) an object lesson in how to use Cabal. The small proviso for Windows machines is that wxWidgets should be built using MSys - none of the other compilers supported by wxWidgets is currently working, but we consider this a small price to pay for the convenience of getting a GUI straight from cabal. For Unix machines which have wxWidgets 2.8.x installed, you should now be able to install wxHaskell with a simple: cabal install wx For Windows machines, there are a few prerequisites: 1) You will need the Windows port of wx-config in your path. This can be downloaded from http://wxconfig.googlepages.com/ 2) You will need to compile wxWidgets using MSys. We have tested against MinGW 5.1.6 with g++ compiler, MinGW Make, MSYS-1.0.11, wxMSW-2.8.10, which are the latest versions, and can be downloaded from their respective websites. I used a completely clean install of the latest Haskell Platform to validate the procedure. Please note: all commands to be entered in an MSys shell. cd /c/path/to/wxWidgets-2.8.10/build/msw mingw32-make -f makefile.gcc BUILD=release MONOLITHIC=1 SHARED=1 UNICODE=1 set PATH=$PATH:/c/path/to/wx-config set WXWIN=/c/path/to/wxWidgets-2.8.10 set WXCFG=gcc_dll/mswu cabal install wx I will be updating the wxHaskell wiki to reflect these changes over the next day or so. Best Regards Jeremy O'Donoghue on behalf of the wxHaskell maintainers.

1 0

Pattern Matching
by Casey Hawthorne 13 Nov '09

13 Nov '09

Why in a pattern match like score (1 3) = 7 can I not have sizeMax = 3 score (1 sizeMax) = 7 -- Regards, Casey

7 8

Weird dependency failure log
by Maurício CA 13 Nov '09

13 Nov '09

Hi, all, Hackage shows a log failure for 'bindings-gsl': Configuring bindings-gsl-0.1.1... cabal-setup: At least the following dependencies are missing: bindings-DSL ==1.0.* But here is, at version 1.0.1, no building problems: http://hackage.haskell.org/package/bindings-DSL There's one thing special about bindings-DSL. It's a package with a set of macros for hsc2hs, and contains no Haskell code. Maybe this revealed some hidden error in package dependency checking. Thanks, Maurício

3 6

Cabal and autogenerated files
by Jeremy O'Donoghue 13 Nov '09

13 Nov '09

Hi all, Another, probably simple, question regarding cabalization. Part of wxcore, the low level abstraction in wxHaskell, consists of haskell modules which are generated automatically by parsing C headers using another tool, wxdirect. When trying to create an sdist package, we run into the problem that because we export modules which are automatically generated, after a 'clean', they do not exist, so the sdist package generation fails. We have tried using 'extra-tmp-files' to list the modules which are autogenerated, but this isn't working. Is this because we are generating the autogen modules in an autogen directory, or is this approach likely to fail wherever we put the autogenerated files? Our use case seems a reasonable one, as it could reasonably exist for any project which relies on automatically generated code. Thanks for any suggestions. Jeremy

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Strange parallel behaviour with Ubuntu Karmic / GHC 6.10.4
by Michael Lesniak 13 Nov '09

13 Nov '09

Hello, I'm currently developing some applications with explicit threading using forkIO and have strange behaviour on my freshly installed Ubuntu Karmic 9.10 (Kernel 2.6.31-14 SMP). Setup: Machine A: Quadcore, Ubuntu 9.04, Kernel 2.6.28-13 SMP Machine B: AMD Opteron 875, 8 cores, 2.6.18-164 SMP- (some redhat) Machine C: Dual-Core, Ubuntu 9.10, Kernel 2.6.31-14 SMP Compiler on all machines: ghc 6.10.4 (downloaded from GHCs official website) Program, Compilation, Execution A simple taskqueue with independent tasks and explicit parallelization (hence should deliver more or less perfect speedup). For one core wall-times around 16 are ok, for 2 a bit more than 8 seconds. Since I used the same sources and Makefiles on all machines all files were compiled with -threaded and started with +RTS -N2 -RTS. Testing: Machine A: Ok (meaning works and delivers the expected speedup) Machine B: Ok Machine C: Not ok (with -N2 wall times around 14-15 seconds) Looking at the core usage, for example with htop, I see that the second core is not really used on C. Executing OpenMP programs shows the expected speedup and usage of both cores, hence I do not think its a kind of general linux configuration problem. So, after all the testing I think its either the Linux Kernel or some other component of Ubuntu 9.10. But: Ubuntu is often used and I did not found any information regarding this problem. The simple solution of installing the old version of Ubuntu would probably help but should not be the way to go, should it? I'd be glad for any hints or comments, Michael -- Dipl.-Inf. Michael C. Lesniak University of Kassel Programming Languages / Methodologies Research Group Department of Computer Science and Electrical Engineering Wilhelmshöher Allee 73 34121 Kassel Phone: +49-(0)561-804-6269

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Final CFP: WFLP 2010. Deadlines extended: Abstract due Nov 18; Full paper due Nov 25 (LNCS)
by Pablo Nogueira 13 Nov '09

13 Nov '09

[Deadlines extended: Abstract due Nov 18; Full paper due Nov 25] ******************************************************************** Final Call For Papers 19th International Workshop on Functional and (Constraint) Logic Programming Madrid, Spain, January 17, 2010 http://babel.ls.fi.upm.es/events/wflp2010/ ********* colocated with Principles of Programming Languages POPL 2010 http://www.cse.psu.edu/popl/10/ ******************************************************************** IMPORTANT DATES Abstract Submission: November 18, 2009 **extended** Full Paper Submission: November 25, 2009 **extended** Acceptance Notification: December 20, 2009 Preliminary Proceedings: January 7, 2010 Workshop: January 17, 2010 SCOPE The aim of the Workshop on Functional and (Constraint) Logic Programming is to bring together researchers interested in functional programming and (constraint) logic programming with special emphasis on the integration of both paradigms and of other declarative programming extensions. It promotes the cross-fertilizing exchange of ideas and experiences among researchers and students from the different communities interested in the foundations, applications, and combinations of high-level declarative programming languages and related areas. The previous WFLP editions are: WFLP 2009 (Brasilia, Brazil), WFLP 2008 (Siena, Italy), WFLP 2007 (Paris, France), WFLP 2006 (Madrid, Spain), WCFLP 2005 (Tallinn, Estonia), WFLP 2004 (Aachen, Germany), WFLP 2003 (Valencia, Spain), WFLP 2002 (Grado, Italy), WFLP 2001 (Kiel, Germany), WFLP 2000 (Benicassim, Spain), WFLP'99 (Grenoble, France), WFLP'98 (Bad Honnef, Germany), WFLP'97 (Schwarzenberg, Germany), WFLP'96 (Marburg, Germany), WFLP'95 (Schwarzenberg, Germany), WFLP'94 (Schwarzenberg, Germany), WFLP'93 (Rattenberg, Germany), and WFLP'92 (Karlsruhe, Germany). LOCATION WFLP 2010 will be held on January 17, 2010 in Madrid, Spain, colocated with the 37th ACM SIGACT-SIGPLAN Symposium on Principles of Programming Languages (POPL 2010). WFLP 2010 solicits papers in all areas of functional and (constraint) logic programming, including but not limited to: * Foundations: formal semantics, logic variables, binding and abstract syntax, rewriting and narrowing, unification, constraint solving, dynamics, type theory, meta-theory, effects, etc. * Language Design: security, services, modules, type systems, multi-paradigm languages, concurrency and distribution, objects, libraries, generic programming, interoperability, etc. * Implementation: abstract machines, parallelism, compile-time and run-time optimizations, foreign-language interfaces, memory management, multi-threading, exploiting parallel hardware, etc. * Transformation and Analysis: abstract interpretation, specialization, partial evaluation, program transformation, program calculation, program proof, meta-programming, generative programming, etc. * Software Development: algorithms, data structures, design patterns, components and composition, specification, proof assistants, verification and validation, model checking, debugging, testing, profiling, tracing, etc. * Paradigm Integration: integration of declarative programming with other paradigms or features such as imperative, object-oriented, aspect-oriented, concurrent, real-time programming, event-driven architectures, etc. * Applications: education, industry, commercial uses, domain-specific languages, visual/graphical user interfaces, embedded systems, WWW applications, XML processing, artificial intelligence, knowledge representation and machine learning, deductive databases, advanced programming environments and tools, etc. SUBMISSIONS and PROCEEDINGS Authors are invited to submit papers presenting original unpublished work. Papers must be at most 15 pages long. Exceptionally, authors may surpass the page limit by providing well-marked appendices intended as reviewing aids. Appendices will not appear in the final publication. Submission categories include regular research papers, system descriptions, and short papers describing on-going work (at most 8 pages). Submissions must be formatted in Lecture Notes in Computer Science (LNCS) style. This requirement needs not apply to appendices. Papers should be submitted in pdf format electronically via the web-based submission site http://www.easychair.org/conferences/?conf=wflp2010 Preliminary proceedings will be available at the workshop. Selected authors will be invited to submit a full version of their papers after the workshop. Contributions accepted for the post-workshop proceedings will be published in Springer's Lecture Notes in Computer Science series. INVITED SPEAKER Mariangiola Dezani (University of Torino, Italy) PROGRAM CHAIR Julio Marino (Universidad Politecnica de Madrid, Spain) PROGRAM COMMITTEE Maria Alpuente (Universidad Politecnica de Valencia, Spain) Sergio Antoy (Portland State University, USA) Bernd Brassel (CAU Kiel, Germany) Olaf Chitil (Univ. of Kent, UK) Rachid Echahed (CNRS-IMAG, France) Santiago Escobar (Universidad Politecnica de Valencia, Spain) Moreno Falaschi (Universita di Siena, Italy) Murdoch Gabbay (Heriot-Watt University, UK) Maria Garcia de la Banda (Monash University, Australia) Victor Gulias (Lambdastream SL, Spain) Michael Hanus (CAU Kiel, Germany) Herbert Kuchen (Univ. of Muenster, Germany) Francisco Lopez-Fraguas (Universidad Complutense de Madrid, Spain) James Lipton (Wesleyan University, USA) Mircea Marin (Univ. of Tsukuba, Japan) Juan Jose Moreno-Navarro (Ministry of Science & Innovation, Spain) Brigitte Pientka (McGill University, Canada)

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Sometimes pinned memory?
by Gregory Crosswhite 13 Nov '09

13 Nov '09

Hey everyone! Do you have any suggestions for how I might allocate an aligned block of memory that I can pin while making foreign calls, but leave unpinned the rest of the time to potentially improve allocation and garbage collector performance? Or is this even a good idea? Thanks, Greg

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Resource compilation in GHC
by Konstantin Vladimirov 13 Nov '09

13 Nov '09

Hello. I'm writing an wxHaskell application. Everything is ok, but now I need a separate folder for icons, bitmaps, and so on, from where they are loaded at runtime. How can I compile resources, and link them into my executable to provide for users single .exe file with resource section inside it? -- With best regards, Konstantin

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C structures
by Vasiliy G. Stavenko 13 Nov '09

13 Nov '09

Hello everyone. What about passing complex c-types (structures) to c-functions. More detailed: I have an application in production which was written in Delphi. IT has ability to create pluggable modules to it. Interface realized by sending Win32Api messages to application. function in haskell Win32 Graphics.Win32.sendMessage :: HWND -> WyidowMessage-> WPARAM -> LPARAM -> IO RESULT Application wants to get different data in WPARAM and LPARAM. Such as window descriptor (HWND), string pointers and datastructer pointers. The latter i don't know how to create. I need anyones experience.

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The weirdest error I've ever seen...
by Joe Fredette 13 Nov '09

13 Nov '09

Hiya Haskellers, So there I was, punching away at the keys, working on the Haskell Weekly News tools when the solution to one of my problems fell on me like a ton of lambdas. The solution and problem it solved are immaterial, but suffice to say it involved the combination of associated types and monad transformers, as well as some fancy deriving to end up with this code: ##### type Context = ReaderT Email type Match t = StateT t IO type ContextMatch t a = Context (Match t) a newtype FilterState t => Filter t a = Filter (ContextMatch t a) deriving (Functor, Monad, MonadReader Email, MonadState Bool, MonadIO) class FilterState t where data FState t deliver :: FState t -> IO () ##### Again, the fine details are unimportant, but the punchline is `Filter` is a Monad which houses not only results, but also an internal state which will be used in the delivery of emails in some yet-to-be- determined way. Naturally, I want to use `deriving` to turn this puppy into a monad over it's second argument. In fact, the whole thing kind- checks alright, but presents me with this, the titular 'weirdest error I've ever seen...' ##### [1 of 3] Compiling Network.HackMail.Email.ParseEmail ( Network/ HackMail/Email/ParseEmail.hs, interpreted ) [2 of 3] Compiling Network.HackMail.Email.Email ( Network/HackMail/ Email/Email.hs, interpreted ) [3 of 3] Compiling Network.HackMail.Filter.Filter ( Network/HackMail/ Filter/Filter.hs, interpreted ) *** Exception: No match in record selector Var.tcTyVarDetails ##### Now, there are three tickets open on the GHC trac, found for me by the ever-helpful `copumpkin` on #haskell -- because I didn't think to look -- they are numbers 3621, 3422 and 2714. But none of them are sufficiently close to my case for them to make sense to me, nor are the solutions presented suitable for entry into my feeble noggin. (Thats just a purty way of saying I'm not smart enough to understand what any of it means...) So I beseech my fellow Haskellers[1], What the heck did I do to anger the Var.tcTyVarDetail gods? My guess (given what I can glean from the Trac entries) is that the `deriving ... MonadState ...` needs changing in some specific-yet- cryptic way, but I've only got my gut to go on... For the Record, and in the event it matters... [jfredett@Erdos]$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.10.4 [jfredett@Erdos]$ uname -a Linux Erdos 2.6.31-ARCH #1 SMP PREEMPT Fri Oct 23 11:12:58 CEST 2009 i686 Intel(R) Celeron(R) CPU 3.06GHz GenuineIntel GNU/Linux Thanks in advance for any help offered. /Joe [1] Bet you've never been beseeched before...

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flow2dot
by Anakreon Mendis 12 Nov '09

12 Nov '09

I've installed the flow2dot utility. It fails to produce a dot file from the sample provided by it's author. The output of the program is: .cabal/bin/flow2dot sample.flow flow2dot: Input: order a b c d a -> b: let's play "catch a ball"! b -> c: i'll pass it along c: what to do next? c -> a: lets throw it back again a: "not fair!" a -> c: find someone else! c -> d: ok, here you go: catch! d: zZzZzZzZzzz.... Error: "sample.flow" (line 1, column 1): unexpected "o" expecting "->", ":" or end of input

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Why can I run (runghc) some Haskell scripts but I cannot seem to compile them?
by Casey Hawthorne 12 Nov '09

12 Nov '09

Why can I run (runghc) some Haskell scripts but I cannot seem to compile them? e.g. http://www.haskell.org/all_about_monads/examples/example25.hs I've changed the import listing to the following: import IO import System import Monad import Data.Maybe import Data.List import Data.Char (toLower) import Control.Monad.State import Control.Monad.Writer The compiler errors are like the following: N-Queens.o:fake:(.text+0x2fe): undefined reference to `mtlzm1zi1zi0zi2_Controlzi MonadziStateziLazzy_zdf9_closure' N-Queens.o:fake:(.text+0x422): undefined reference to `mtlzm1zi1zi0zi2_Controlzi MonadziStateziLazzy_zdf7_closure' N-Queens.o:fake:(.text+0x66f6): undefined reference to `mtlzm1zi1zi0zi2_Controlz iMonadziWriterziLazzy_zdf1_closure' -- Regards, Casey

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Why can I run (runghc) some Haskell scripts but I cannot seem to compile them?
by Casey Hawthorne 12 Nov '09

12 Nov '09

Why can I run (runghc) some Haskell scripts but I cannot seem to compile them? e.g. http://www.haskell.org/all_about_monads/examples/example25.hs I've changed the import listing to the following: import IO import System import Monad import Data.Maybe import Data.List import Data.Char (toLower) import Control.Monad.State import Control.Monad.Writer -- Regards, Casey

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Linker?
by Gregory Crosswhite 12 Nov '09

12 Nov '09

Does anyone know here how GHC links in object files from other languages? I am getting a strange issue where it seems to be getting the calling convention on Fortran calls wrong. Specifically, on one computer (Gentoo Linux) I have with gcc and gfortran v4.4 and ghc compiled using gcc v4.3, I have no problems at all. On another computer (a Mac) I have with gcc 4.0 and gfortran 4.4 and a binary of ghc installed (as downloaded from haskell.org) I run into problems. Specifically, I noticed that the first parameter seems to get eaten when I declared a fortran routine as a function but not when I declare it as a subroutine. So I went ahead and converted the fortran routines into subroutines, but then I get another crash elsewhere in the code where a module calls a LAPACK routine for reasons unknown; I do not get this crash when running the same code path but called using a Python wrapper that uses gcc + gfortran + f2py rather than from my Haskell wrapper. I thought that this might be a problem with mixing gcc 4.0 and gfortran 4.4 (and ultimately I think this might still be the case), but when I tried writing small test cases using *only* gcc and gfortran and not ghc, everything worked just fine. Any thoughts? - Greg

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Re: [Haskell-cafe] looking for a good algorithm
by Casey Hawthorne 12 Nov '09

12 Nov '09

To: Casey Hawthorne <caseyh(a)istar.ca> Subject: Re: [Haskell-cafe] looking for a good algorithm From: Casey Hawthorne <caseyh(a)istar.ca> Date: Thu, 12 Nov 2009 11:14:02 -0800 On third thought, convert the table to a 2D array of bits (or a 1D array of bits mapped to a 2D coordinate system). The bit is true for either an Int or Double. If space is a concern and one can tolerate a low rate of false positives, than a 2D Bloom Filter might be appropriate, or a 1D Bloom Filter with mapping to 2D coordinates. http://en.wikipedia.org/wiki/Bloom_filter Then on to simmulated annealing, possibly. Note: Donald Knuth's new fascicle is out: Volume 4 Fascicle 1, Bitwise Tricks & Techniques; Binary Decision Diagrams (2009), xiii+261pp. ISBN 0-321-58050-8 On Wed, 11 Nov 2009 15:32:35 -0800, you wrote: >So, as I understand it, you have a very large sparse table, thousands >of rows and hundreds of columns, of which each cell within a column of >type String, Int, or Double can contain one of those types or nothing. > >Then you to want to shuffle the rows to maximize the number of columns >whose first 100 rows have at least one number (Int or Double), given a >list of preferred column names since there is no guarantee that every >number column will have at least one number in its first 100 rows >after shuffling. > > >I'm wondering about hashing on the rows and hashing on the columns, >then the column hash has the number of Int's or Double's (don't need >the String's) in that column and the rows they are in. > >The row hash would have the number of Int's and Double's in that row >and what column's they are in. > >Then; > >Then scan the row hash and sort into descending order, and by tagging >those rows, not by actually moving them. > >Then I think your ready for simmulated annealing. -- Regards, Casey

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Cabal upload issue
by Jeremy O'Donoghue 12 Nov '09

12 Nov '09

Hi all, I'm in the process of trying update the revisions of wx (part of wxHaskell) on hackage. I'm getting an error I find slightly surprising: "400 Error in upload The dependency 'build-depends: base' does not specify an upper bound on the version number. Each major release of the 'base' package changes the API in various ways and most packages will need some changes to compile with it. The recommended practise is to specify an upper bound on the version of the 'base' package. This ensures your package will continue to build when a new major version of the 'base' package is released. If you are not sure what upper bound to use then use the next major version. For example if you have tested your package with 'base' version 2 and 3 then use 'build-depends: base >= 2 && < 4'." In this case, we have the following in wx.cabal Library if flag(splitBase) build-depends: base >= 3, wxcore >= 0.12.1.1, stm else build-depends: base < 3, wxcore >= 0.12.1.1, stm Is this a bug, or am I doing something stupid? I should point out that this builds perfectly on my machine. Regards Jeremy

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Fwd: (Solved) cabal install with external gcc tool chain not the ghc-bundled one
by Daniel Kahlenberg 12 Nov '09

12 Nov '09

to answer this question myself how the use of another gcc is specified with effect, I used the following options with the 'cabal install' call: --ghc-options="-pgmc e:/programme/ghc/mingw-gcc4/bin/gcc.exe -pgml e:/programme/ghc/mingw-gcc4/bin/gcc.exe" See http://www.haskell.org/ghc/docs/latest/html/users_guide/options-phases.html… (searched in the wrong direction, too many trees...). Slightly tuned, this should be the way to go in all similar cases. One thing I haven't considered yet is if the '--with-ld' and '--with-gcc' options (if curious too, see logs in my previous mail - Subject "[Haskell-cafe] caba install with external gcc toolchain not the ghc-bundled one") only effect what gets written into the setup-config/package.conf file or what other effects these have. Greets Daniel

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socket error
by Alberto G. Corona 12 Nov '09

12 Nov '09

<socket: 1796>: hPutBuf: illegal operation (handle is finalized) I´m a bit lost trying to find the source of this error. I´m running an hack application (package hack). Basically it is a handler of web requests. hack can be used with different web servers: Hyena, simpleserver, HappStack.... all of them produce this error after a few interactions., Supposedly, it happens within the socket module since neither my module, nor hack, nor the hack simpleserver (package hack-handler-simpleserver) call explicitly hPutBuf I tried to reproduce the error under linux, but my ubuntu installation is too old and I´m in the process of reinstalling everything again. In the meantime, Any of you can give me any hint about the error?

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Long running Haskell program
by David Leimbach 12 Nov '09

12 Nov '09

As some of you may know, I've been writing commercial Haskell code for a little bit here (about a year and a half) and I've just recently had to write some code that was going to run have to run for a really long time before being restarted, possibly months or years if all other parts of the system cooperate as it's part of a server infrastructure management system. I recently ran into some serious space leak difficulties that would ultimately cause this program to crash some time after startup (my simulator is also written in Haskell, and runs a LOT faster than the real application ever could, this has enabled me to fast forward a bit the data growth issues and crash in minutes instead of days!) Anyway, rather than try to paste it all here with images and such I thought I'd stick it up on my blog so others could maybe benefit from the anecdote. It's difficult to disclose enough useful information as it is commercial code not under an open source license, but there's neat diagrams and stuff there so hopefully the colors are at least amusing :-) http://leimy9.blogspot.com/2009/11/long-running-haskell-applications.html Dave

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