May 2007 - Haskell-Cafe

Parse error on input "|"
by Akijmo 31 May '07

31 May '07

Hi everyone. I am new to this Forum, Haskell and i am german, so i am sorry for "noob" failures or spelling mistakes. I am currently learning for an informatic exam (11th class) and i tried to code a function to sum a polynom with a pair of polynoms... (I actually want to to code a polynomdivision in which i need this) But I get the parse error mentioned in the headline. It is referring to the first line of the case differentiation. Hopefully you can help me, here's the code: polyplusd :: Polynom -> (Polynom, Polynom) -> Polynom polyplusd [] p = p polyplusd p [] = p polyplusd p@((g1,e1):p1) (n, (q@((g2,e2):p2)) | g1>g2 = (g1,e1):(polyplusd p1 (n,q)) | g2>g1 = (g2,e2):(polyplusd p (n,p2)) | g1==g2 && e1+e2 /=0 =(g1, e1+e2):(polyplusd p1 (n,p2)) | otherwise = polyplusd p1 (n,p2) -- View this message in context: http://www.nabble.com/Parse-error-on-input-%22%7C%22-tf3847082.html#a108958… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Network.HTTP+ByteStrings Interface--Or: How to shepherd handles and go with the flow at the same time?
by Thomas Schilling 31 May '07

31 May '07

Hello Cafe! I'd greatly appreciate any ideas/comments on the design of the interface to the Network.HTTP library with a LazyByteString (LBS) backend. As has been discussed previously on this list [1] lazy evaluation can complicate resource management, which is especially critical if resources are seriously limited--in this case network sockets and/or file handles. In [2] Oleg shows how we can use left folds as a general mechanism to traverse collections and shows that this is superior to cursors, generators or streams. I agree with his arguments, however, I still don't see that this can be a good interface for the http library. For a start, what should the collection type be? Word8, Char8, (strict) ByteString (BS)? Unless we get some kind of fusion for the first two we're pretty much stuck with the latter. The interface could look like this (untested): import Data.ByteString (ByteString) -- i.e., strict BSs import qualified Data.ByteString as BS handleRequest :: MonadIO m => Request -> a -> (ByteString -> a -> m (Either a a)) -> m a Ignoring error handling for now, this might be implemented as: handleRequest req seed processor = do a <- iter seed bodySize closeConnection req return a where iter seed 0 = return seed iter seed n = do let c = min chunkSize n bs <- liftIO $ readBlock req c -- calls BS.hGet handle c next <- processor bs seed case next of Left rslt -> return rslt Right seed' -> iter seed' (n-c) -- insert catchError stuff here (requires signature change, of course) This should work fine and if the 'processor' function does not hold on to the chunk would run in constant space. Unfortunately, this has a big disadvantage. Most operations on the returned data will probably be stream-like functions, such as parsing the data into some kind of tree. [2] shows a method how to convert enumerators to streams, but the used stream type data MyStream m a = MyNil (Maybe a) | MyCons a (m (MyStream m a)) is incompatible with [a] which is used by lazy ByteStrings due to the embedding of m. I also don't know if fusion can work on monads. My current suggestion would therefore be a less "save"[*] solution (again, untested and modulo error handling). -- | Execute the request and call @f@ with the returned response body. -- The socket will be closed immediately after @f@ terminates. You must therefore -- make sure that any data you might want to returned has to be forced, e.g. using -- (length . take) lbs withRequest :: MonadIO => Request -> a -> (LazyByteString -> a -> m a) -> m a The implementation would lazily read the contents (implemented as described in [3]) and forcing it would be left to the function parameter. E.g. getHTML :: String -> IO HTMLParseTree getHTML addr = do r <- mkRequest addr tree <- withRequest r emptyTree parseHTML seq tree $ return tree -- ! (I'm afraid this is necessary) tricky :: LazyByteString -> String -> IO LazyByteString -- result will not really be lazy trickyt str addr = do r <- mkRequest addr withRequest r L.empty dropNTake -- we might have to force the result here where dropNTake s _ = L.take 100000 . L.drop 100000 If these 'seq's are really necessary, then this would be a pretty hard to use interface. So, any ideas / suggestions ? / Thomas [*] .. "save" in the sense that it does not enforce certain behavior by means of the type signature or API design. [1] .. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/20528/focus=20635 [2] .. http://okmij.org/ftp/papers/LL3-collections-enumerators.txt [3] .. http://nominolo.blogspot.com/2007/05/networkhttp-bytestrings.html -- "Remember! Everytime you say 'Web 2.0' God kills a startup!" - userfriendly.org, Jul 31, 2006

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Re: [Haskell-cafe] Has anyone looked into adding subtyping to Haskell?
by Al Falloon 31 May '07

31 May '07

Mark T.B. Carroll wrote: > I don't know what the infamous "expression problem" is, nor am I > familiar with polymorphic variants or structural subtyping, but have you > looked at the Data.Generics.* stuff and Scrap Your Boilerplate papers? > They may be relevant. > The expression problem is "a new name for an old problem", basically being able to extend a data type and functions over the data type in seperate modules with no knowledge of each other. Here is a link to (I think) the original description: http://www.daimi.au.dk/~madst/tool/papers/expression.txt Structural subtyping is something like "duck typing" in dynamic languages, but checked at compile time. For records, it means that if you only access two labels then the function will work on any record that has those labels with those types, even if it may have more labels. For variants (or sum-types, or tagged unions, or whatever they are called in Haskell) it means that if your function can handle certain cases, then it can also handle values that range over less cases. So in pseudo-Haskell with imaginary subtyping: data Vertical a = U a | D a data Horizontal a = L a | R a data Direction a = #Vertical a | #Horizontal a -- borrowing ocaml syntax: this means that Direction shares constructors with Vertical and Horizontal getData :: Direction a -> a getData (U a) = a getData (D a) = a getData (L a) = a getData (R a) = a getLeftStick :: IO (Vertical Int) getRightStick :: IO (Horizontal Int) main = do { accel <- getLeftStick; print (getData accel) } So getData doesn't care that accel is Horizontal and not Direction because Horizontal is a sub-type of direction. Of course, in this syntax since you named the subtyping relationship its more technically nominal subtyping (like in traditional static OO languages) not structural subtyping (which figures out the subtype relationship from the structure automatically, so if we just reused the U,D,L, and R constructors in Direction). I have looked into SYB briefly. I will probably end up using it to keep the amount of code to a minimum, but it won't save me from having to define a different data type for each version of the AST if I want to preserve type safety.

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equations and patterns
by mingli yuan 31 May '07

31 May '07

Hi, buddies. I am a newbie on Haskell. Recently I want to implement a simple Lattice in Haskell, but I met some difficulties. Scrap of the code is as below, but I met syntax error: > class Lattice e where > join :: e -> e -> e > meet :: e -> e -> e > > -- associative law > join x (join y z) = join (join x y) z > join (join x y) z = join x (join y z) The main problem is that mathematic equations is bi-directed, but in Haskell the pattern matching is only in one direction - from left to right. And only variables and constructors could be occur in the right side of the pattern matching. Seems mathematic axioms and pattern matching are different things. So how could I rewrite the equations to pattern matching? Which technique should I learn? Thanks for your help. Regards, Mingli

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Building error Gtk2Hs under GHC 6.6.1 on Solaris 10 x86
by lebed 31 May '07

31 May '07

Hi, haskell-caffe! I'm trying to build Gtk2Hs 0.9.11 under GHC 6.6.1 on Solaris 10 x86: ./configure --with-hcflags=-O0 --disable-split-objs OK! gmake ........................ ........................ ........................ if test -f glib/libHSglib_a.deps; then touch glib/libHSglib_a.deps; else touch glib/libHSglib_a.deps; gmake glib/System/Glib/FFI.hs glib/System/Glib/UTFString.hs glib/System/Glib/Types.hs glib/System/Glib/GType.hs glib/System/Glib/GValue.hs glib/System/Glib/GValueTypes.hs glib/System/Glib/GObject.hs glib/System/Glib/Properties.hs glib/System/Glib/GError.hs glib/System/Glib/GList.hs glib/System/Glib/Signals.hs glib/System/Glib/MainLoop.hs glib/System/Glib/GTypeConstants.hs glib/System/Glib/GParameter.hs glib/System/Glib/StoreValue.hs glib/System/Glib.hs glib/System/Glib/Attributes.hs glib/System/Glib/Flags.hs; /usr/local/bin/ghc -M -optdep-f -optdepglib/libHSglib_a.deps -fglasgow-exts -O0 -fffi -iglib -package-conf package.conf.inplace -hide-all-packages -ignore-package glib -package base -package haskell98 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include glib/System/Glib/FFI.hs glib/System/Glib/UTFString.hs glib/System/Glib/Types.hs glib/System/Glib/GType.hs glib/System/Glib/GValue.hs glib/System/Glib/GValueTypes.hs glib/System/Glib/GObject.hs glib/System/Glib/Properties.hs glib/System/Glib/GError.hs glib/System/Glib/GList.hs glib/System/Glib/Signals.hs glib/System/Glib/MainLoop.hs glib/System/Glib/GTypeConstants.hs glib/System/Glib/GParameter.hs glib/System/Glib/StoreValue.hs glib/System/Glib.hs glib/System/Glib/Attributes.hs glib/System/Glib/Flags.hs; fi; gmake[1]: Entering directory `/usr/export/home/lebed/tmp/gtk2hs-0.9.11' ./mk/chsDepend -iglib:gtk:sourceview sourceview/Graphics/UI/Gtk/SourceView/Types.chs could not find {#import.chs on search path glib gtk sourceview gmake[1]: *** [sourceview/Graphics/UI/Gtk/SourceView/Types.dep] Error 1 gmake[1]: Leaving directory `/usr/export/home/lebed/tmp/gtk2hs-0.9.11' <no location info>: can't find file: glib/System/Glib/FFI.hs gmake: *** [glib/libHSglib_a.deps] Error 1 gmake: *** Deleting file `glib/libHSglib_a.deps' Where is my mistake? (I've got the same error under GHC 6.6 on Solaris 10 sparc.) -- View this message in context: http://www.nabble.com/Building-error-Gtk2Hs-under-GHC-6.6.1-on-Solaris-10-x… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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updating packages
by jeff p 31 May '07

31 May '07

Hello, I just moved to ghc-6.6.1and was wondering if there is an automatic way to update the various packages I had installed previously. thanks, jeff

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ANNOUNCE: xmonad 0.2
by Spencer Janssen 31 May '07

31 May '07

The xmonad dev team is pleased to announce the 0.2 release of: xmonad: a tiling window manager http://xmonad.org About: Xmonad is a tiling window manager for X. Windows are arranged automatically to tile the screen without gaps or overlap, maximising screen use. All features of the window manager are accessible from the keyboard: a mouse is strictly optional, greatly increasing productivity in X. Xmonad is written and extensible in Haskell, and custom layout algorithms, and other extesions, may be implemented by the user in config files. Layouts may be applied dynamically, and separate layouts can be used on each workspace. A guiding principle of the user interface is predictability: users should know in advance precisely the window arrangement that will result from any action, leading to an intuitive user interface. Features: * Automatic window tiling and management * First class keyboard support: a mouse is unnecessary * Full multihead/Xinerama support * XRandR support to rotate, add or remove monitors * Per-workspace layout algorithms * Per-screen non-built in status bars, with arbitrary geometry * Dynamic restart/reconfigure preserving workspace state * Tiny code base (~500 lines of Haskell) * Fast, small and simple. No interpreters, no heavy extension languages Since 0.1, the following notable features and bug fixes have appeared: New features: * XRandR support, for dynamically adding, removing or rotating monitors * State-preserving dynamic restart * Popup, customisable status bar support * Multiple clients may appear in the master pane * mod-shift-j/k, to swap windows with their neighbours * mod-n, to resize windows * User-specified layout algorithms may be written in config files * All layouts may be 'mirrored' (rotated) * configurable window border size and colour Design changes: * Reimplemented core of xmonad with a 'zipper' data type to track focus by construction. We believe this is a first. * Use of Neil Mitchell's 'catch' program to verify pattern match safety * Use of ReaderT and StateT to partition read-only and modifiable values * Custom layout messages handled with open data type simulation * More QuickCheck properties Bug fixes: * numlock handling is fixed More information, screenshots, documentation and community resources are available from: http://xmonad.org Xmonad is available from hackage, and via darcs. Happy hacking! The Xmonad Team: Spencer Janssen Don Stewart Jason Creighton Xmonad has also received patches from: Alec Berryman Chris Mears Daniel Wagner David Glasser David Lazar David Roundy Joe Thornber Miikka Koskinen Neil Mitchell Nick Burlett Robert Marlow Sam Hughes Shae Erisson

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Distributing a program with support files
by Neil Mitchell 30 May '07

30 May '07

Hi, I'm wanting to release a Haskell program, but am confused how I should distribute the associated files it needs. On Windows I would use the functions to find the path of the executable, and find the support files relative to that - in Haskell (because of Linux) that isn't the case. The basic setup I have is: foo.exe - main binary library.hs - required by the binary examples\*.hs - a massive tree of examples, which can be used by the binary foo.exe also needs to create temporary files relative to library.hs and the examples. Thanks Neil

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Re: [Haskell] ST vs State
by Bulat Ziganshin 30 May '07

30 May '07

Hello Federico, Wednesday, May 30, 2007, 12:54:35 PM, you wrote: > Control.Monad.ST > And > Control.Monad.State ST monad is just reduced IO monad which like IO organizes sequential (imperative) ordr of execution but unlike IO supports only a small closed set of operations - those working with STRef and STArray State monad provides just two operations - get and put, which provides access to some "monad state". unlike ST/IO monads operations in State monad (and all other monads) can be performed in any order as typical for lazy computations. of course, there are guarantees that put operation will be performed before get that reads appropriate state you may be interested looking at http://haskell.org/haskellwiki/IO_inside although it doesn't say about ST/State monads directly (btw, there is also Lazy.ST monad but it is not used too much :) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Re: [Haskell-cafe] The C Equiv of != in Haskell (Apologies)
by PR Stanley 30 May '07

30 May '07

>>However, I don't think these other individuals had any business >>poking their hooters into this. > >If you don't want other individuals "poking their hooters" in what you >write, I recommend you to avoid sending open questions to public >mailing lists. yes, I was indeed referring to you and your large beak over which you clearly have no control. Nobody invited you to this discussion and as you can see I have already apologized to Neil for the misunderstanding. Yet, you seem to be desperate to keep the thread alive. Uh, save me from the ...

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