- Haskell-Cafe - Haskell.org

LIPL: a tiny functional language implemented
by sam lee 27 May '08

27 May '08

Dear Haskell-Cafe, This is a sort of spam. I've been working on a tiny functional language as a term project to learn Haskell. And I submitted it today. I wrote the code in literate Haskell. So it could be used as Haskell tutorial (minus the fact that it could be error prone). Source code and documentation can be viewed at: http://www.lipl.googlepages.com/index.html#source Thank you. Sam.

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Haskell ghc log?
by Galchin, Vasili 27 May '08

27 May '08

Hi Duncan, You mentioned a log in a posting vis-a-vis "configure". Where are all ghc-related logs? Kind regards, Vasili

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ghc on mac
by Vikrant 27 May '08

27 May '08

Hi, I am trying to install ghc-6.8.2 on mac leopard. I get following error if I try to run ghci dyld: Library not loaded: /opt/local/lib/libreadline.5.2.dylib Referenced from: /Users/vikrant/Applications/ghc/lib/ghc-6.8.2/ghc-6.8.2 Reason: image not found Trace/BPT trap Vikrant

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Examples of using Haskell for mathematics
by Galchin, Vasili 27 May '08

27 May '08

Hi Henning, I have often wondered if the notion of lazy list evaluation isn't an example of constructive mathematics. ? Vasili

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status of dynamic loading
by Evan Laforge 27 May '08

27 May '08

I have an application where I'd like to load and run code at runtime. As far as I know, the possibilities are: Use hs-plugins. I'm not sure if this still works on modern ghc, or if it works on my platform (darwin intel), but I managed to get this compiled after "fixing" a use of STArray whose arity apparently changed, but I probably broke it because now I get "Error in array index" all the time. I gather Austin Seipp has been doing some work on this. Is there a newer version that should work? An easy fix for the error? Then there's the ghc api, which I gather is seen as the way forward from hs-plugins, saving it from hi-parsing brittleness and platform specific breakage. I think Yi uses this, apparently in in the Shim package. I poked around some, but wasn't too clear to me what was going on, if I was looking in the right place, and if this is a reasonable way forward. lambdabot also appears to have its own dynamic loading mechanism, I haven't looked at the source enough yet to understand what it's doing. Maybe there's some documentation out there. The low-tech solution, used by xmonad, would be to pass the "dynamic" code to main and just recompile the app for each instance, passing in an instance specific set of config arguments. Of course, this isn't really dynamic at all, and compiling all the various instances could be clunky and awkward, but I know it works and could possibly be reasonable if I also write my own little language for the stuff that really has to be dynamic (anyone got one of these? lisplike would be easy and ok, an haskell subset might be nicer... I would like to avoid having *two* languages to write extensions in). The last solution I can think of is to export an RPC API instead of dynamically loading code. The difficulty here is coming up with a vocabulary that can be flattened and shipped to another program without becoming an excessive maintenance issue or being too inflexible. All in all, the dynamic loading approach of hs-plugins seems like the easiest and most flexible, provided I can get its staged type checking. Before I go trying to hack hs-plugins into working, or poking around inside yi or lambdabot, can anyone point me at some documentation, resources, advice, etc.? Thanks!

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Re: Haskell idioms for hierarchical memory modeling ?
by Dmitri O.Kondratiev 27 May '08

27 May '08

On Mon May 26 09:02:01 EDT 2008 Marc Weber [marco-oweber at gmx.de] wrote: >Searching haskell.org only gives one match: > http://jpmoresmau.blogspot.com/2007/06/very-dumb-neural-network-in-haskell.… >(I haven't read it) >http://hackage.haskell.org/packages/archive/pkg-list.html >is the other most commonly used source of finding aready existing code >(There is package about nn) > >Should your nn also support kind of learning by giving feedback? Marc, thanks for explaining how to use State and the links! As for your question about my NN learning, I need to tell more how it all works. My classifier NN (Memory Tree - MT - as I call it), that I previously described may work in two modes: 1) Usupervised learning. In this mode each node learns new categories when it encounters new input vectors that classifier has not seen before. These input vectors become new categories and get stored in node category memory (CM). When MT starts in this mode all nodes have empty CMs. Size of CM defines the number of categories that node may learn. 2) Supervised learning. In this mode MT also has a category node. Category node feeds in category numbers to a top level node. Learning is done with prepared in advance training data set. Traing data consits of pairs (input vector, category number). Bottom nodes read input vector from this pair while category node simultaneously sends corresponding category number to the top level node. Thus when input reaches top node it already knows what category this input belongs to. After MT consumes all training data it is belived to be fully learned and ready to infer categories from 'work' data set. Hope this answers your question about learning. -- Dmitri O. Kondratiev dokondr(a)gmail.com http://www.geocities.com/dkondr

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developer docs with haddock: cabal or make?
by Misha Aizatulin 27 May '08

27 May '08

hello, for the project I am doing, I would like to be able to create two documentation sets: - the user documentation, done with normal Setup.hs --haddock and - the developer documentation, where all modules are exposed and haddock is run with --ignore-all-exports What is the right way to do this? I see two options, but both are somewhat clumsy: 1. Write a custom Setup.hs, using the Cabal library. Ideally, it would accept a new command, like "Setup.hs --haddock-devel". However Cabal doesn't seem to be designed for adding new commands or options to them - the interface is hardcoded. For instance, I can't add a command in such a way that it would be listed alongside others when calling "Setup.hs --help". 2. Call haddock from a makefile. This is a one-liner with haddock 0.8, but not with haddock 2.0 and above. The problem is that haddock 2.0 only supports --read-interface and doesn't support --use-package any more. Thus I have to find all the haddock interfaces by hand, for instance by calling ghc-pkg for each library I depend on. This is of course straightforward, but I feel that it shouldn't be my job. Any comments or ideas? Cheers, Misha

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Has anyone worked on checking SQL-queries at compile time?
by Marc Weber 27 May '08

27 May '08

Yes, I know about haskelldb.. But some more advanced optimizations can't be expressed.. so I'd like to ask if someone has done some work in the other direction not ensuring type safety by using the haskell type system to derive SQL queries but to use template haskell to derive functions from given SQL queries thereby checking them for syntax error ? It be a little bit like SQLJ (Java tool for db connectivity) Eg $(q "SELECT '2', 4") should result in [(String, Int)] and $(q "INSERT INTO foo (2, ?1, ?2)" ) should result in Int -> <whatever type ?1 is> -> <whatever type ?2 is> -> IO () Ideally this would not only result in a query but in a prepared statement. I have no clue how much RDBMS such as Postgresql provide some help determining type of parameters or results without acutally doing a query (but bcause Postgresql provides transactions this should not be a problem) I see the following benefits: You can use your existing SQL- Knowledge and don't have to dive into deep type hackery yet gettitng as much type safety as possible Marc Weber

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Announce: ghc-core, command line pager for reading GHC Core
by Don Stewart 26 May '08

26 May '08

Just a quick announcement, I've uploaded to hackage 'ghc-core' , a wrapper over ghc for displaying the optimised core and assembly language ghc produces from your programs. The code is colourised by hscolour, and displayed in a pager, git-log style. This will be useful for those who like looking at optimised Haskell. You can quickly get an idea of what kind of core and assembly your code is turning into, and the effect of various flags on the result. Usage: $ ghc-core Foo.hs $ ghc-core Foo.hs -optc-O2 -fvia-C Get it here, http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ghc-core Screenshot, http://galois.com/~dons/images/ghc-core.png Cheers, Don

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Haskell idioms for hierarchical memory modeling ?
by Dmitri O.Kondratiev 26 May '08

26 May '08

*** How FP in general and Haskell in particular can help in modeling memory with neuron-like structures? *** The particular model discussed here - latter in this text I call it Memory Tree - is a type of neural network (NN) where the network consists of a collection of data processing nodes arranged in a tree-shaped hierarchy. Memory Tree (MT) is a hierarchical classifier, that infers recurring data patterns observed in its input during some period of time T. Time period is a set of discreet times (or moments) T = [t1, t2, ...tN]. Each node reads inputs from all its children and then builds from these inputs a single input vector as a concatenation of children outputs. Node then classifies this newly built input vector assigning it to one of the category groups that node creates. Next node outputs group number that current input belongs to. This output goes to the node parent. All nodes at all all levels do these steps in turn, inputs flowing from bottom level nodes up to a top node through all nodes at all intermidiate levels. Next I illustrate main ideas of MT with a simple example. *** Simple MT Example Primitive MT classifier, used in this example, will (I hope :) discern, for example, these images: 1) ######## ######## ######## ######## 2) ######## #......# #......# ######## 3) ###..### #.####.# #.####.# ###..### ('#' - black dot, '.' - white dot) In my case at each moment MT uses bottom-level nodes to read input from a data file. Input data is a collection of strings. Each string is a binary vector representing data read by some sensor at time tj. For example: v1(t1) : 11111111111111111111111111111111 v2(t2) : 11111111100000011000000111111111 v3(t3) : 11100111101111011011110111100111 ... ... vN(tN) : ... In this example input is 32 bit vector read at consecutive times t1 < t2 < t3 ... tN This vector represents a rectangular view of a camera - 8 x 4 matrix of black (1) and white (0) dots. Shown above images are encoded in 32 bit vectors, one image - one vector, where black dot '#' becomes 1 and white dot '.' becomes 0. In this case MT may have the following topology: -- One Top Level Node: T -- Two Middle Level Nodes: M1, M2 -- Four Bottom Level Nodes: B1,B2,B3,B4 -- Input Vectors: v1(t1), v2(t2), v3(t3), ... vN(tN) To show how nodes in this tree are connected I will use brackets. In my 'bracket notation' parent node name is followed by its child node names in brackets: (T (M1 (B1, B2), M2(B3, B4))) This describes topology where: -- T node has two children: M1 and M2 -- M1 node has two children: B1 and B2 -- M2 node has two children: B3 and B4 (Sorry for poor illustrations, unfortunately I can't use graphics are supported in this post, including ascii art :) For this topology each input vector, for example v1(t2), may be split in four equal parts of 8 bits. Chunks will then represent four regions of the camera image. Each 8 bit chunk is sent as input to the corresponding Bottom Level nodes: B1,B2,B3,B4 v2(t2)_b31-b24 : 11111111 --> B1 v2(t2)_b23-b16 : 10000001 --> B2 v2(t2)_b15-b8 : 10000001 --> B3 v1(t1)_b7-b0 : 11111111 --> B4 To work with these vectors each Bottom node will have dimension of input vector BK = 8 so it can classify 2 ** 8 = 256 input vectors. Let's say Bottom node can classify these 256 input vectors into 2 ** 3 = 8 output groups, so it will have dimension of output vector BL = 3. In turn, each M1 and M2 node will have input dimension equal to the sum of all its children (B nodes) output dimensions: M1K = B1L + B2L = 3 + 3 = 6 M2K = B3L + B4L = 3 + 3 = 6 Our M nodes can classify 2 ** 6 = 64 vectors, and we assume that they can categorize these 64 vectors in 2 ** 2 = 4 groups, so M node output vector is 2 bit long: ML = 2 Connecting two M nodes to a single top node T will requre T input dimension equal to the sum of two M node output dimensions: TK = M1L + M2L = 2 + 2= 4 T node can classify 2 ** 4 = 16 vectors, and we assume T node can group these vectors in 2 groups, so T node output vector is 1 bit long: TL = 1 To summarize: *** Node Attributes Each node in this tree has the following attributes: 1) K-bit input vector. 2) L-bit output vector, where K >= L 3) Node Category Memory (CM) - stores unique category vectors. Each vector is stored as a tuple (V, C) where V is a vector value and C is a category number. Both V and C values, as well as (V,C) pairs are unique for the node. Node can store a limited amount of (V,C) pairs. This amount (CMSize) is equal to the number of categories that node can differentiate and also equal to the node output vector dimension. *** Node Data Processing algorithm: Every Node runs the following endless loop: 1) Read inputs from children 2) Build input vector X 3) Calculate distance between X and every vector in CM and determine if X belongs to any existing node category. If X belongs to existing category Then do step 4) Else If CM is not full create new node category pair (X, C_new) Else return 4) Output vector group number to a higher level node in the tree. *** Main Questions 1) How to model node vector processing and node state? Node receives input and produces output and can change its state by adding new category to Node CM. Will State monad help to 'thread' node state between inputs? I don't see how. State monad encapsulates state transition function: State s a = State (\s -> a, s) that inputs some state value and outputs a pair (value, newState). In my case node category memory (CM) could be such a changing state of node , but what then will be node input vector in State monad? 2) How to traverse classifier tree (MT) so inputs from bottom level nodes could flow up to a top node through all nodes at all intermediate levels? In contrast to a 'normal' tree where all variations of tree traversals start at root node, classifier tree must be traversed starting from bottom level nodes. 3) How to build tools that will allow to construct classifier trees with different topologies? In this case one topology differers from another by a) number of levels and b) numer of children at each level. Ideally I would like to have a simple DSL that will allow specify tree topology declaratively, easy to understand for non-programmer. 4) Most important: In what direction should I look as a Haskell newbie to find answers to these and similar questions? Should I build my own monad to traverse this tree, what other standard monads may help? What combinator libraries should I learn? Will I need Arrows (that at the moment I know only name and nothing more about)? Thanks for reading all this and any help! -- Dmitri O. Kondratiev dokondr(a)gmail.com http://www.geocities.com/dkondr

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what appears to be a persistent problem for me ..
by Galchin, Vasili 26 May '08

26 May '08

Hello, I seem to have recovered from my Ubuntu problems by installing the ghc binary from http://www.haskell.org. However, if I modify a .cabal file, e.g. unix.cabal, then "runhaskell Setup.hs build" is always in the "re-configure" state even if I do a "runhaskell Setup.hs configure" ... It seems the re-configure state is determined by: -- |Check that localBuildInfoFile is up-to-date with respect to the -- .cabal file. checkPersistBuildConfig :: FilePath -> IO () checkPersistBuildConfig pkg_descr_file = do t0 <- getModificationTime pkg_descr_file t1 <- getModificationTime localBuildInfoFile when (t0 > t1) $ die (pkg_descr_file ++ " has been changed, please re-configure.") in /ghc-6.8.2/libraries/Cabal/Distribution/Simple. ?? Kind regards, Vasili

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HaXml and the XHTML 1.0 Strict DTD
by Peter Gammie 26 May '08

26 May '08

Is anyone using HaXml to validate XHTML Strict? The old 1.13.2 version has some bugs in how it handles attributes that stop me from using it. It handled the DTD parsing fine. The most-recent darcs version relies on a newer ByteString than I have, so it is not easy for me to test it. A recent (this year) darcs version failed to parse the DTD, yielding this error: validate: In a sequence: in content spec of ELEMENT decl: head When looking for a non-empty sequence with separators: In a sequence: Expected % but found | in file xhtml1 at line 252 col 50 when looking for a content particle when looking for a content particle This is the context:  <!ENTITY % head.misc "(script|style|meta|link|object)*">  <!ELEMENT head (%head.misc;, ((title, %head.misc;, (base, %head.misc;)?) | (base, %head.misc;, (title, %head.misc;))))> I appreciate any advice on this. cheers peter

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Announce: glome-hs-0.51 (Haskell raytracer, now with type classes)
by Jim Snow 26 May '08

26 May '08

A new version of my raytracer has been posted: http://syn.cs.pdx.edu/~jsnow/glome/ http://hackage.haskell.org/cgi-bin/hackage-scripts/package/glome-hs-0.51 (This should really be named 0.5.1, but I didn't think of that until after I uploaded it to hackage.) There's not much new functionality, but it now uses type classes for the supported primitives, and has been optimized a bit more. Much of the tutorial I hastily wrote for 0.4.x (http://www.haskell.org/haskellwiki/Glome_tutorial) is now quite out of date. Most of the primitives have been moved to their own module, with the exception of SolidItem (an existential type used to make composite primitives), [SolidItem] (allowing me to treat lists of Solids like single solids), Void (a non-object, equivalent to []::[SolidItem]), and Instance (used for transformations). It might be possible to move those to their own modules as well, but it would require mutual recursion between modules, and that's probably more trouble than it's worth. (I made an attempt at that, but I quickly gave up.) I also gave up on trying to use a global mutable variable to count the number of bounding hierarchy nodes a particular ray hits; instead, I added rayint_debug, which behaves just like rayint (the standard ray-object intersection routine), except that it returns an integer (that I can use to count whatever I like) along with the ray intersection. Using a global counter in this instance would have been much simpler, but I don't think I understand "seq" well enough to be able to force the increment to actually happen. http://syn.cs.pdx.edu/~jsnow/glome/Screenshot-glome-hs-bih.png The resulting renders can be very useful to determine where Glome is spending most of its time, and to verify that the the bounding interval hierarchy is really doing the right thing. I also added packet tracing, which makes it possible to trace four rays at a time, using a specialized ray intersection method "packetint". (This is a common technique to amortize the acceleration structure's memory lookup cost over multiple rays.) It seemed to be a big win when I first implemented it before converting over to type classes, but now it seems to be faster without it, so I probably made a mistake somewhere. A cosmetic change is that Glome now renders into a drawlist instead of directly to the screen, so the whole image doesn't get laboriously re-traced whenever there's window damage. Unfortunately, that means you can't watch as it draws anymore, which was a useful way of knowing which parts of the image were slow to render. I've started looking more seriously into optimization (suggestions welcome). Don Stewart's blog post (http://cgi.cse.unsw.edu.au/~dons/blog/2008/05/16#fast) was quite useful, but it seems like there's a lot of arcane knowledge required to understand what's really happening in "core" code. Is there any better reference than Andrew Tolmach's paper "An External Representation for the GHC Core Language (2001)" http://citeseer.ist.psu.edu/tolmach01external.html? -jim

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newbie: maintaining relationships
by geoffrey 25 May '08

25 May '08

I am learning haskell and i need some advice about writing functional-style code. Give the following code below, my question is - how do i setup a dependency of User on Common? Obviously what i have there is wrong, because a change in common (i know it's actually immutable) is not reflected in the version captured by User. I understand why my current approach has not a chance a of working, I am using it to, hopefully, illustrate my goal. Thanks for any suggestions > data Common = Common { > cName :: String, > cValue :: Int > } deriving (Show) > data User = User { > uCommon :: Common > } deriving (Show) > makeWorld = (commons, users) > where > commons = [Common "A" 1, Common "B" 2] > users = [User (head commons)] > > main = do > let world = makeWorld > putStrLn (show world)

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strange cabal error
by John Lato 25 May '08

25 May '08

Hello, I recently ran into a strange cabal error. When trying to configure a package (leksah 0.1.1) I get the following error: MacBook-Pro:leksah-0.1.1 jwl$ runhaskell Setup.lhs configure Configuring leksah-0.1.1... Setup.lhs: ghc version >=6.2 is required but the version of /usr/local/bin/ghc could not be determined. MacBook-Pro:leksah-0.1.1 jwl$ /usr/local/bin/ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 This is on Mac OS 10.5.2, with GHC 6.8.2 (installed from Manuel C.'s binary package). Thus far several other packages have built and installed just fine (bytestring, binary, parsec, gtk2hs (from source, not darwinports)). Looking through leksah.cabal and Setup.lhs, I didn't see anything unusual (although I'm far from a cabal expert). I only found one relevant result from google, a Nov. 20th message on cabal-devel about another user getting this message on Debian with ghc 6.8.1. He reported getting this message when running install from a directory without write permissions. It doesn't seem to be a permissions problem for me; I do have write permissions for the directory in which I'm running cabal and all subdirectories. Can anyone tell me what's going on, or give me any advice to try? Would a different version of cabal possibly work? Thanks, John Lato

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Re: [Haskell-cafe] Newbie: State monad example questions
by Olivier Boudry 24 May '08

24 May '08

On Fri, May 23, 2008 at 5:17 PM, Thomas Hartman <tphyahoo(a)gmail.com> wrote: > > Maybe I don't like to read backwards. ;-) > > >>> (from Control.arrow) is like flip (.) > > That's something I considered. I like the `|>` operator found in F# (I hope it's not a curse word on this list) which is equivalent to `flip (.)`. It's easy to define in Haskell (|>) :: (a -> b) -> (b -> c) -> a -> c (|>) = flip (.) I even started to use it in my code and then stopped. It may be a stupid concern but as many optimizations performed by GHC are made through rewrite rules and I was worried that those rules may not fire when using this new operator. I never thought of using `>>>` for this, but again I wonder how it will work with rewrite rules. Olivier.

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Data.Tree.Zipper in the standard libraries
by Krasimir Angelov 24 May '08

24 May '08

Hello Guys, We have Data.Tree in the standard libraries for a long time but for some reason we still don't have standard implementation for Zipper. I wrote recently one implementation for Yi but there are many other versions hanging around. At least I know for some. I propose to add one in the standard libraries i.e. the "containers" package. The version that I use currently is here: http://code.haskell.org/yi/Data/Tree/Zipper.hs If you would like to do code review I will be happy to hear comments. After the API is settled down I will write test cases also. One thing that is worying me is that the current version uses State monad which is in the "mtl" package while the natural place for Data.Tree.Zipper is in "containers". This will create an extra dependency. Is this acceptable? Regards, Krasimir

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gtk2hs linking problems
by Nabil Alsharif 24 May '08

24 May '08

I downloaded the gtk2hs 0.9.12.1 tarball and did the standard configure, make, make install but when I go to compile a test program I get a bunch of errors that look like: compilation IS NOT required [..snip..] (.data+0x38): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziTypes_zdf548_closure' gui.o: In function `r2JC_srt': (.data+0x60): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziButtonsziButton_onClicked_closure' gui.o: In function `r2JC_srt': (.data+0x68): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziTypes_zdf662_closure' gui.o: In function `r2JE_srt': (.data+0x90): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziButtonsziButton_buttonLabel_closure' gui.o: In function `r2JE_srt': (.data+0x98): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziTypes_zdf662_closure' gui.o: In function `r2JG_srt': (.data+0xc0): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziAbstractziContainer_containerChild_closure' gui.o: In function `r2JG_srt': (.data+0xc8): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziTypes_zdf662_closure' gui.o: In function `r2JG_srt': (.data+0xd0): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziTypes_zdf548_closure' gui.o: In function `r2JG_closure': (.data+0x108): undefined reference to `glibzm0zi9zi12zi1_SystemziGlibziAttributes_set_closure' gui.o: In function `r2JG_closure': (.data+0x110): undefined reference to `gtkzm0zi9zi12zi1_GraphicsziUIziGtkziGeneralziGenera This was done on a Ubuntu system. It looks like it can't find a reference to a library but it doesn't say which one. If any one has ran into this problem before or knows what wrong help would be appreciated.

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Re: [Haskell-cafe] Announce: ghc-core, command line pager for reading GHC Core
by Henning Thielemann 24 May '08

24 May '08

On Fri, 23 May 2008, Ketil Malde wrote: > Henning Thielemann <lemming(a)henning-thielemann.de> writes: > >>> An even more advanced tool could show differences between two Core >>> listings [..] >> In this special example, actually simple 'diff' spotted the critical >> difference > > I would suggest Emacs's ediff for this, it lets you refine differing > chunks and highlights the actual differences. If you are an Emacs > user, if not, there are probably other nice diff tools out there.. :-) tkdiff, kompare

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Re: [Haskell-cafe] Newbie: State monad example questions
by Olivier Boudry 24 May '08

24 May '08

On Fri, May 23, 2008 at 2:20 PM, Thomas Hartman <tphyahoo(a)gmail.com> wrote: > > The big benefit I got from using the State Monad was that I was able to > reorder the functions > by just copy/pasting the function name from one > place to another. > > I don't understand... why do you need state to do this? Couldn't you > have a function pipeline using dots for composition like > > .... ( > ... > parseAttn . > parsePoBox . > ... > ) address ... > > and have the functions be equally switchable? (well, the first and > last can't quite be copy pasted, but close enough.) > > Introducing state seems like a lot of trouble to me if the only one is > easier reorderability of lines. > Agreed, in fact I started with a function pipeline and then switched to using the State Monad. As the program was written months ago I don't remember exactly why. Maybe I don't like to read backwards. ;-) Funtions running in the state monad can call other functions with the same `State s a` signature (and so on as deep as you want). You never have to care about passing parameters and restarting a new pipeline. But of course you can easily do without the State Monad. I don't think the State Monad allows to do thing that you can't do with basic Haskell. It just makes code more readable (in my opinion at least). Olivier.

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RE: [Haskell-cafe] force inlining in GHC (fwd)
by Henning Thielemann 24 May '08

24 May '08

maybe also of interest in the ghc-core thread ---------- Forwarded message ---------- Date: Fri, 23 May 2008 21:21:03 +0200 (CEST) From: Henning Thielemann <lemming(a)henning-thielemann.de> To: Simon Peyton-Jones <simonpj(a)microsoft.com> Cc: glasgow-haskell-users(a)haskell.org Subject: RE: [Haskell-cafe] force inlining in GHC On Wed, 30 Apr 2008, Henning Thielemann wrote: > On Tue, 29 Apr 2008, Simon Peyton-Jones wrote: > > > As luck would have it, I'm working on INLINE pragmas for Roman right at > this moment. > > > > Could you spare a moment to give me a concrete test case, to make sure I > > hit your case too? If you can give me a program that doesn't optimise as > > you expect, I'm much more likely to get it right. I examined some more examples and found out that the behaviours of GHC-6.4.1 and GHC-6.8.2 are quite consistent, that is, accelerations I achieved for GHC-6.8.2 also worked for GHC-6.4.1. Main problem in both GHC-6.4.1 and GHC-6.8.2 remains that sometimes GHC decides to SPECIALISE a function that is tagged with INLINE. I think that SPECIALISE should either copy the INLINE tag to the specialised function (confluent rewriting, right?) or should not SPECIALISE an INLINE function at all. I can now present a simple example, however it needs a (self-contained) package: http://darcs.haskell.org/storablevector/ module Main where import qualified Data.StorableVector.Lazy as SV zipData :: SV.Vector Double zipData = SV.take 500000 $ SV.zipWith (+) (SV.iterate SV.defaultChunkSize ((1-1e-6)*) 0.5) (SV.iterate SV.defaultChunkSize (1e-6 +) 0) main :: IO () main = do SV.writeFile "test-data" zipData ghc-core -o dist/build/fusiontest/fusiontest -O -Wall -fexcess-precision -package storablevector speedtest/SpecialiseTest.hs I get quite at the beginning of the Core: a1 :: Data.StorableVector.Lazy.ChunkSize -> (Double -> Data.Maybe.Maybe (Double, Double)) -> Double -> [Data.StorableVector.Base.Vector Double] This looks much like specialised SV.unfoldr, the expansion of SV.iterate. It seems to be that GHC detects that SV.unfoldr is called twice with the same type and thus specialises it to Double - while forgetting that SV.unfoldr should be inlined anyway.

1 0

Re: throwDynIO ?
by Aleksey Nogin 24 May '08

24 May '08

Back in October'06 Carlos Pita sent a message to this list: > Currently I'm trying to grasp haskell exception handling and although > I find it mostly clear I'm a bit confused regarding the lack of a > throwDynIO action "paralelling" throwIO, as there are dynamic variants > for the other throw functions: > > throwDyn - throw > throwDynTo - throwTo > > I understand that the same effect could be conjured up by > > throwIO (DynException (...))) > > magic. But the lack of symmetry still disturbes me. Am I missing something? As far as I can tell, his question was left unanswered. As another Haskell newcomer (crossing over from the OCaml world), I found the absence of throwDynIO weird as well - as far as I understand things, throwing an error in the IO monad seems like a safer thing to do (as I'd know precisely when to expect it to be raised) than throwing it from a pure code and then relying on the "imprecise exceptions" mechanism to get it right. Looking at the ghc-6.8.2/libraries/base/Control/Exception.hs in the GHC source code, I see the following definitions: throw = unsafePerformIO . throwIO throwDyn exception = throw (DynException (toDyn exception)) so I guess I can get define throwDynIO myself by "taking back" the unsafePerformIO: throwDynIO exception = throwIO (DynException (toDyn exception)) but I am still wondering whether the library is simply incomplete here, or whether there is some reason for not defining/using throwDynIO. Thanks in advance for any clarifications! Aleksey

1 0

Trying to write a very simple HTTP Client
by Eric 24 May '08

24 May '08

Hi all, I've written the following program to connect to a submit an HTTP GET request to a server and print the response: module Main where import Network import System.IO main = withSocketsDo go go = do putStrLn "Connecting..." out <- connectTo "haskell.org" (PortNumber 80) hPutStrLn out "GET /\r" hPutStrLn out "Host: haskell.org\r" hPutStrLn out "\r" cs <- hGetLine out hClose out print cs When I run the program, however, I get the following error: HTTPClient: <socket: 1872>: hGetLine: end of file Can anyone see what's wrong? E.

4 4

Newbie: State monad example questions
by Dmitri O.Kondratiev 23 May '08

23 May '08

I am trying to understand State monad example15 at: http://www.haskell.org/all_about_monads/html/statemonad.html Example 15 uses getAny that I don't understand at all how it works: getAny :: (Random a) => State StdGen a getAny = do g <- get (x,g') <- return $ random g put g' return x Questions: 1) random has type: random :: (Random a, RandomGen g) => g -> (a, g) and for State monad: return a = State (\s -> (a, s)) then: return (random g) = State (\s -> ((a,g), s)) Is it correct? 2) What x and g' will match to in: do ... (x,g') <- return $ random g which, as I understand equals to: do ... (x,g') <- State (\s -> ((a,g), s)) What x and g' will match to in the last expression? 3) In general, in do expression (pseudo): do { x <- State (\s -> (a, s)); ...} What x will refer to? Will x stand for a whole lambda function: \s -> (a, s) ? 4) How 'g <- get' works in this function (getAny) ? 5) Why we need 'put g'? Thanks!

8 16

What to do when GHC works, RUNGHC fails
by Peter Verswyvelen 23 May '08

23 May '08

I'm experimenting creating a Cabal script on Windows that installs a module that refers to some DLLs (freetype, png, ...) via C wrappers When configuring, building, and installing using cabal, a test program that uses this installed module runs fine when compiled with GHC --make, but fails miserably when run with RUNGHC (or GHCi) The first thing I noticed is that RUNGHC looks for a DLL called "png.dll" (which it doesn't find and then bails out with an error), while the executable build with GHC uses the correct "libpng3.dll". When I rename the libpng3.dll into png.dll, RUNGHC goes a bit further but then fails with an "unknown symbol". Obviously I did something wrong in my Cabal script, but I have no idea what it is. There seems to be a big difference when it comes to linking between of GHCi and GHC . Any guidelines on what to do in cases like the one above? I really like to use the interpreter because it provides much shorter turnaround times, and things like this stop me from using it. Thanks, Peter

2 3

London Haskell Get Together
by Dominic Steinitz 23 May '08

23 May '08

For information, not an "official" user group get together but I believe the evening was enjoyed by all participants. Dominic. http://enhyper.blogspot.com/2008/05/haskell-get-together-22nd-may-2008.html

1 0

whhaskell on osx 10.5
by Kirk Peterson 23 May '08

23 May '08

I had a difficult time getting wxhaskell installed and working on a mac running os x 10.5. I did a quick write up of the process I got working here: http://necrobious.blogspot.com/2008/05/wxhaskell-go-go.html Im pretty new to haskell, so if if you see anything that needs correcting please let me know, hope this helps cheers, -kirk

4 4

MD5 performance optimizations, and GHC -via-C producing segfaulting binary
by Salvatore Insalaco 23 May '08

23 May '08

2008/5/17 Andrew Coppin <andrewcoppin(a)btinternet.com>: > Hi folks. > > OK, try this: > > darcs get http://darcs.orphi.me.uk/MyMD5 > cd MyMD5 > ghc -O2 --make md5sum > md5sum <some large filename> I've got some time to take a look at the code. It's very nice, readable and declarative, but obviously not optimized for "raw speed". There're a few things to do to have a speed-up of 2x, without going "low-level". 1) There's a lazyness leak in Pad.hs. The sum in: else make_block_bs bs0 : work (n + block_size_bytes) bs1 is not strict. With a very large file (e.g. 100 mb) it means stack overflow. [roxas:~/Desktop/test2/MyMD5] kirby% ghc -O2 --make md5sum.hs [roxas:~/Desktop/test2/MyMD5] kirby% time ./md5sum ../../jboss-4.2.2.GA.zip Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. To solve it just add a bang (!) before the n parameter: work !n bs = You've got to add {-# LANGUAGE BangPatterns #-} at the top of the file too. There're solutions that don't imply BangPatterns, and use only seq, but I like bang patterns! Now it works: [roxas:~/Desktop/test2/MyMD5] kirby% time ./md5sum ../../jboss-4.2.2.GA.zip E6542028D538BAEC180A96A5D1E6EC3A 11.958u 0.210s 0:12.19 99.7% 0+0k 0+2io 0pf+0w 2) make_block_bs is sub-optimal, and very critical to performance. I decided to use Data.Binary for it (it's also more readable, and you get rid of the unsafeWrite): import Data.Binary import Data.Binary.Get import Control.Monad // ... instance Binary Block where put _ = undefined get = do xs <- replicateM 16 getWord32le return $ Block $ listArray (0, 15) xs make_block_bs :: B.ByteString -> Block make_block_bs = decode Now we are getting faster: [roxas:~/Desktop/test2/MyMD5] kirby% time ./md5sum ../../jboss-4.2.2.GA.zip E6542028D538BAEC180A96A5D1E6EC3A 8.063u 0.174s 0:08.23 100.0% 0+0k 0+0io 0pf+0w 3) You are doing a lot of access to fields of a strict data type (State). You can at least ask the compiler to help you a bit with -funbox-strict-fields. Now we have: [roxas:~/Desktop/test2/MyMD5] kirby% time ./md5sum ../../jboss-4.2.2.GA.zip E6542028D538BAEC180A96A5D1E6EC3A 6.780u 0.133s 0:06.91 100.0% 0+0k 0+1io 0pf+0w We have got a good, nearly 2x, speed-up with very few optimizations, and we run in a very small constant amount of memory. Probably compiling with -fvia-C could help even more, but strangely: [roxas:~/Desktop/test2/MyMD5] kirby% ghc -fvia-C -funbox-strict-fields -O2 --make md5sum.hs [roxas:~/Desktop/test2/MyMD5] kirby% time ./md5sum ../../jboss-4.2.2.GA.zip Segmentation fault Is this supposed to happen? There're no "unsafe" functions or imports used in the program. Maybe a bug in GHC? Salvatore

7 20

Network.CGI and error handling
by Ketil Malde 23 May '08

23 May '08

Hi, I'm trying to implement a CGI, but I have encountered some problems with handling program errors properly. I think it boils down to this: The first program from the documentation at http://hackage.haskell.org/packages/archive/cgi/3001.1.5.2/doc/html/Network… import Network.CGI cgiMain = output "Hello World!" main = runCGI (handleErrors cgiMain) works fine, and ouputs a page with the expected test. Replacing line 2 with: cgiMain = output $ error "Hello World!" returns a blank page, although according to the documentation, 'handleErrors' should produce an error page (500) instead. The error is recorded in the log, where one can read: [Thu May 22 14:35:27 2008] [error] [client 10.1.9.227] test.cgi: Hello World! I've toyed with various approaches, including using catchCGI and replacing 'error' with other run-time failures, but only getting the same result. Any hints? (ghc 6.8.1, cgi-3001.1.5.1, apache 2.0, linux.) -k -- If I haven't seen further, it is by standing in the footprints of giants

1 2

relational data representation in memory using haskell?
by Marc Weber 22 May '08

22 May '08

I'm kind of stuck that's why I'm posting here to ask wether this makes sense at all, maybe someone else has already done it? What I'd like to have: Some way representing relational data which is typically stored in databases such as Postgresql.. Rewriting something like Postgresql in haskell would take ages.. So I'd be satisfied with having in memory representation only (this would fit the HAppS state system very well .. :) Why ? * type safety * less conversions compared to SQL data * no need to switch processes, parse SQL etc so maybe it's even faster? (a small benchmark showed that inserting 20000 Ints into a list was 8 times faster than using MySQL parsing 20000 INSERT INTO x (1) statements ) I'd like to illustrate two different ideas using a small example: (A) data CD = CD { title :: String, tracks :: [ Track ] } data Track = Track { track :: String, cd :: CD } data PDB = PDB { cds :: Set CD, tracks :: Set Track } because it's not using foreign ids but kind of pointers I'll call this the pointer method using uniq ids it would look like this: (B) data CD = CD { id : Int, title :: String, tracks :: [Int ] } data Track = Track { trackId :: Int, track :: String, cd :: Int } data IDB = IDB { cds :: Map Int CD, tracks :: Map Int Track } I will call it I DB (I = using ids) PDB: pro : * less work when doing joins (no need to look foreign rows up) con : * you need uniq ids or such when serializing to disk * When updating a track you'll also have to update the pointer stored in cds. and if you had another table shelfs.. this had to be updated as well.. IDB: the other way round I find the idea not using any lookups when using joins appealing. Of course having a simple data Table = Table Map UniqId Rec isn't enough, sometimes you need more than one index or even a multi index: data Table = Table { byId :: Map Int Rec , byNameAndAge :: Map String (Map Int (Set Rec)) } Note that I've used Set here as well because this index does'nt have to be uniq! starting to write an insertTable :: Table -> Rec -> Table more than twice is getting tedious.. Of course you can start using some type hackery to insert a rec into all maps automatically.. but you'll get into trouble making the type system apply the best index not the first matching one. (I bet this could be done using HList etc somehow as well.. ) So my current attempt is defining the database using some data types and make template haskell derive those insertIntoTable and update functions. I've added the draft below. But before continuing spending much time on it I'd like to get your advice: Is there a chance that it will pay off? Some general considerations: haskell solution con: haskell can get close to C but in general it may be >10 times slower when not caring too much about design or writing low level (see recent thread about md5 or one where David Roundy has said something about a matrix thread: only 10 times slower?) Using a garbage collector on database data (some hundred MB) might not be the optimal way because I feel you can tell exactly when you no longer need a piece of allocated memory here? So some time might be wasted. projects tend to run longer as expected.. And if data no longer fits into memory .. :(... -> bad performance I think systems such as postgresql do scale much better if you have some gbs of data and only use the most recent X records frequently.. So maybe you'll have to spend time later which you've won by using a haskell relational data representation in memory only.. Another solution: use clusters - I don't have any experience. pro: much more safety (STM, type system ..) there are less possibilities making compared to C / PHP etc Do you also think (A) is more interesting because some load (looking up foreign keys) is moved on insert / delete and update operations taking less time in but are called more frequently thus maybe reducing peak load on queries? Of course some time would have to be spend on queries wich might look like this: let queryresult = $(query ( tables + constraints + relations ) ) db automatically generating the query function taking into account expected index cardinality etc.. Any comments, suggestions, links to existing solutions (except coddfish, haskelldb) ? Marc Weber draft ============= types represeting tables and db ======================== module RDMH.Types where import Language.Haskell.TH import Language.Haskell.TH.Syntax data Uniqueness = Uniq | NotUniq deriving (Show, Eq) data ModifyMode = InsertOnly | UpdateInsert | UpdateInsertDelete deriving (Show, Eq) type TypeS = String -- a name of a data type (data A = ..) data Index = I { uniqueness :: Uniqueness , key :: Exp -- a fuction rec -> key , subkeys :: [ Index ] , keyType :: TypeS -- the key type (this *should* be determined by the function.. But I don't know yet how to do it.. Probably using some GHC API magic -- , expectedCardinality :: Int -- how should this look like exactly? This will be used to select the best indexes } deriving (Show, Eq) -- Int is negative weight / cardinality. -- If you issue a query having two contstraints on types beeing indexed the one having biggest cardinality is taken first -- first type:` -- convinient function to get the type of a name - see TestCase.hs t = ConT . mkName -- type (either key or row) k = VarE . mkName -- gets the key -- app (VarE $ mkName "fst" ) -- a table representation data Table = Table { tableName :: String , row :: TypeS -- this row type must be an instance of Eq , indexes :: [ Index ] -- [ ( [ mkName "Int", mkName "String" ]] is represented as Map Int ( Map String rec ) -- , history :: Bool -- to be implemented. Each time a record is updated the original row should be saved somehow somewhere , modify :: ModifyMode -- , insertTimeStamp :: Bool -- , updateTimeStamp :: Bool } deriving (Eq, Show) data DB = DB { dbName :: String , tables :: [ Table ] , oneToN :: [ (Table, Table, String) ] -- String must be set if you want two relations to the same table (eg one ticket has foreign inbound and outbound flight) -- , nToM :: [ ( Table, Table ) ] } ============= example use case ======================================= {-# OPTIONS_GHC -XTemplateHaskell #-} module TestCase where import Language.Haskell.TH import RDMH.Types import RDMH.TH import Data.Time -- the data data RCD = CD { title :: String , artist :: String , year :: Int } data RTrack = Track { trackTitle :: String , recordingDate :: UTCTime } -- a track table with two indexes title and recordingDate trackT = Table "track" (t "RTrack") -- row type -- indexes: [ I NotUniq (k "trackTitle" ) [] (t "String") , I NotUniq (k "recordingDate" ) [] (t "UTCTime") ] UpdateInsertDelete -- a cd table with two indexes title and year cdT = Table "cd" (t "RCD" ) -- row type -- indexes: [ I Uniq (k "title" ) [] (t "String") , I NotUniq (k "year" ) [] (t "Int") ] UpdateInsertDelete -- the db: both tables and a simple relation db = DB "cdDB" [ cdT, trackT ] [ OneToN cdT trackT "" ) ] $(createDB db)

6 16

A chance to share your experience
by Simon Peyton-Jones 22 May '08

22 May '08

Friends I know for a fact that many of you are using Haskell successfully for real applications. Yet none of you have sent me an offer of a presentation at the Commercial Users of Functional Programming workshop, which happens in late Sept. I'm the program co-chair, and I confidently expecting several Haskell offers... but not one so far. I am sad. I am very sad. Make me happy! I'm sure you are very busy making your business work. But think what fun it'd be to share your experience with others, meet other people with similar experiences, recruit bright students to your company... Happily, there is still time. The deadline for offers is 2nd June. All we need by then is a short summary of what you think you might say. You don't have to write a paper (not now, not ever). Your presentation does not have to be highly technical. On the contrary, the emphasis is on the process issues that surround functional programming: convincing management, changes to software development, testing, benefits, costs, successes, concerns. I'm going to the Lake District for a week's holiday next week (it's half term in the UK). Please send me a lovely full mailbox to come back to. The CFP is below, and at http://cufp.galois.com/. best wishes Simon Commercial Users of Functional Programming Workshop (CUFP) 2008 26 September 2008, Victoria, British Columbia ============== CALL FOR PRESENTATIONS ====================== Presentation proposals due 2 June 2008 http://cufp.functionalprogramming.com Functional Programming As a Means, Not an End Sponsored by SIGPLAN Co-located with ICFP 2008 _________________________________________________________________ Functional languages have been under academic development for over 25 years, and remain fertile ground for programming language research. Recently, however, developers in industrial, governmental, and open source projects have begun to use functional programming successfully in practical applications. In these settings, functional programming has often provided dramatic leverage, including whole new ways of thinking about the original problem. The goal of the CUFP workshop is to act as a voice for these users of functional programming. The workshop supports the increasing viability of functional programming in the commercial, governmental, and open-source space by providing a forum for professionals to share their experiences and ideas, whether those ideas are related to business, management, or engineering. The workshop is also designed to enable the formation and reinforcement of relationships that further the commercial use of functional programming. Speaking at CUFP If you use functional programming as a means, rather than as an end, we invite you to offer to give a talk at the workshop. Alternatively, if you know someone who would give a good talk, please nominate them! Talks are typically 30-45 minutes long, but can be shorter. They aim to inform participants about how functional programming played out in real-world applications, focusing especially on the re-usable lessons learned, or insights gained. Your talk does not need to be highly technical; for this audience, reflections on the commercial, management, or software engineering aspects are, if anything, more important. You do not need to submit a paper! If you are interested in offering a talk, or nominating someone to do so, send an e-mail to jim (dot) d (dot) grundy (at) intel (dot) com or simonpj (at) microsoft (dot) com by 2 June 2008 with a short description of what you'd like to talk about or what you think your nominee should give a talk about. Such descriptions should be about one page long. Program Plans CUFP 2008 will last a full day and feature an invited presentation from Michael Hopcroft, the product unit manager for the forthcoming release of Microsoft Visual Studio F#. Additionally, the program will include a mix of presentations and discussion sessions. Topics will range over a wide area, including: * Case studies of successful and unsuccessful uses of functional programming; * Business opportunities and risks from using functional languages; * Enablers for functional language use in a commercial setting; * Barriers to the adoption of functional languages, and * Mitigation strategies for overcoming limitations of functional programming. There will be no published proceedings, as the meeting is intended to be more a discussion forum than a technical interchange. Program Committee * Lennart Augustsson <lennart (dot) augustsson (at) gmail (dot) com> * Matthias Blume <blume (at) tti-c (dot) org> * Adam Granicz <granicz (dot) adam (at) intellifactory (dot) com> * Jim Grundy (co-chair) <jim (dot) d (dot) grundy (at) intel (dot) com> * John Lalonde <lalonde (at) abstrax (dot) com> * Andy Martin <akmartin (at) us (dot) ibm (dot) com> * Yaron Minsky <yminsky (at) janestcapital (dot) com> * Simon Peyton Jones (co-chair) <simonpj (at) microsoft (dot) com> * Ulf Wiger <ulf (dot) wiger (at) ericsson (dot) com> This will be the fifth CUFP, for more information - including reports from attendees of previous events - see the workshop web site: http://cufp.functionalprogramming.com

1 0

Re: [Haskell-cafe] one-way monads
by Dan Doel 22 May '08

22 May '08

On Tuesday 20 May 2008, ajb(a)spamcop.net wrote: > Actually, it's true less than 50% of the time. In particular, it's > not true of any monad transformer. Sure it is. Any particular transformer t typically comes with some particular way of writing a function of type t m a -> m a (you may have to throw away some t-related stuff, of course). Since a specific transformed monad is built from a specific monad, and a specific transformer, and specific transformers are likely to have a function of type t m a -> m a, and specific monads are likely to have functions of type m a -> a, you can compose them to get a function of type t m a -> a for the specific monad t m. And so on for transformed-transformed monads. :) That only fails if either of the specific pieces fails to have the right function, which happens well under 50% of the time, I think (IO and STM are the ones that immediately occur to me (barring a certain evil function), although you could make a case for ST by technicality; no failing transformers come to mind (except CCT if we're counting ST), but I haven't wracked my brain very hard). -- Dan

7 6

Return user state in Parsec
by Maciej Podgurski 22 May '08

22 May '08

Hi, I'm currently writing a parser using the Parsec library. What I want is to store the order of each subparser called in a user state. So every single parser will be marked with a label that is stored in a special treelike structure when the parser is run. My problem is to return the last state of this structure when a parse error occurred. This information shall be used to display a kind of stack trace showing the order of the parser calls for debug purposes. Any ideas how to achieve this? Thanks, Maciej

2 1

Installing Cabal-Install
by Aditya Siram 22 May '08

22 May '08

Hi all, I am trying to install cabal-install so I can install xmonad-contrib. I have all the dependancies in place but when I do: runhaskell Setup.lhs build I get: Hackage/Types.hs:19:29: Module `Distribution.Version' does not export `Dependency' Any ideas? Thanks ... Deech _________________________________________________________________ Make every e-mail and IM count. Join the i’m Initiative from Microsoft. http://im.live.com/Messenger/IM/Join/Default.aspx?source=EML_WL_ MakeCount

2 2

ghc-pkg package.conf files?
by Galchin, Vasili 21 May '08

21 May '08

Hello, I fairly innocuous question ;^). How does ghc-pkg know where are the *package.conf files are located? Regards, Vasili

2 3

RealFloat constraint on Complex type
by Conal Elliott 21 May '08

21 May '08

GHC's (maybe Haskell98's?) Complex type is defined with a RealFloat constraint on type type itself, rather than on some of the instances and functions: data (RealFloat a) => Complex a = !a :+ !a I think the practice of constraint in type definitions is generally discouraged, and I'm wondering if there are reasons other than history for having the constraint here. Is removing it on the table for Haskell'? I just got bit by what I think is a typical problem. I added a VectorSpace instance for 'Complex a' and discovered that my 'a' must be in RealFloat, even though I use only zero, addition, subtraction, and scaling. I suspect this gripe has been raised before ... Thanks, - Conal

5 4

one-way monads
by Zsolt SZALAI 21 May '08

21 May '08

Hi! Now, i'm getting familiar with monads, but there is a little discredit about one-way monads. For example, in monads presented in Monads for Functional Programming by Philip Wadler, they are all two-way monads, internal data can be extracted from the monad, and these data, call them state, are in deed _local_ to a function using the monadic computation, so the state there is just a little magic, and they are really just functions(as in a pure language). An other example is Random. I've figured out, that when initializing a generator, it uses the actual timestamp and cputime to initialize. If it wouldnt happen, two distinct uses of Random in a program would result the same random number sequence. Here comes IO and one-way monads, where the internal state can not be extacted, and seems, that the internal data is global to the program. Hows that could be? Is it just because main::IO() or because the implementation of IO uses external C functions and there are no internal state in the monad itself at all? And why one can not write a function that makes an IO computation and the return type does not include and IO contructor? It is a feature of the language and specific to IO, or anybody could write a monad with this property(one-way), assuming the implementation does not use external languages? Or the one-way property is just that, there is no such functions, that allow extracting internal data? Also, is there any other monad, that has the one-way property? Thanks, -- Zsolt

7 6

are there any good c parsers + ast builders available for haskell?
by Carter Tazio Schonwald 21 May '08

21 May '08

Hello, I'm thinking about having a go at writing some simple static analysis tools for C, and I think using Haskell would make it a happier time coding wise, the question being, are there preexisting parsing tools for C available for haskell? many thanks -Carter

2 2

Performance: MD5
by Andrew Coppin 21 May '08

21 May '08

Hi folks. OK, try this: darcs get http://darcs.orphi.me.uk/MyMD5 cd MyMD5 ghc -O2 --make md5sum md5sum <some large filename> On my PC, it takes 3.5 seconds to compute the MD5 hash of a 10 MB file. For comparison, the md5.exe I downloaded from the Internet does it instantaneously. It's literally so fast I can't even time it. As far as I know, my program always produces the *correct* answer, it just takes far too long to do it. If anybody has any interesting insights as to why my version is so damned slow, I'd like to hear them. (Indeed, a description of the process for tracking the problem down would also be useful!) [Much to my bemusement, when I had a look at the code, I discovered that tucked away in a corner somewhere, it reads a ByteString from disk and instantly converts it to [Word8]. Uh... oops? Anyway, eliminating this changed the numbers I get from the profiler, but it's still far too slow.]

14 47

Link to Hierarchical-libraries documentation generated with haddock 2.0?
by Alfonso Acosta 21 May '08

21 May '08

Hi all, GHC's documentation page [1] only points to hierarchical-libraries documentation generated by haddock-0.8 [2]. I need the 2.0 version in order to get links in the documentation of the project I'm working on (which needs haddock 2.0 due to the use of quite a few GHC extensions). Before I proceed to generate them myself, has anyone done it already? (I guess that must be the case since HackageDB users haddock 2.0 and gets all the links right) I would be more than happy to just click a link instead :) Either way, I think it would be a good idea to provide an alternative link to the 2.0 version. Thanks, Fons [1] http://www.haskell.org/ghc/documentation.html [2] http://www.haskell.org/ghc/docs/latest/libraries.html.tar.gz

1 1

FD Int Constraint Solving Library
by Alexander Fuchs 21 May '08

21 May '08

Hi, I was wondering if someone could point me to a Haskell library for constraint solving over finite domains with integer arithmetic, i.e. something like FaCiLe ( http://www.recherche.enac.fr/log/facile/ ). A Haskell FF interface to such a library would probably be fine as well. It seems to me that such a library/wrapper should exist, but I couldn't find one. Thanks, Alexander

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ByteString.pack behavior
by Dušan Kolář 21 May '08

21 May '08

Hello all, Maybe there is something obvious I can't see, but I have this behavior for 6.8.2 ghci: $ghci ttest1p.hs GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. [1 of 1] Compiling Main ( ttest1p.hs, interpreted ) Ok, modules loaded: Main. *Main> encode' [1..100] Loading package array-0.1.0.0 ... linking ... done. Loading package bytestring-0.9.0.1 ... linking ... done. [1,0,2,0,3,0,4,0,5,0,6,0,7,0,8,0,9,0,10,0,11,0,12,0,13,0,... // deleted *Main> B.pack [0..100] "\NUL\SOH\STX\ETX\EOT\ENQ\ACK\a\b\t\n\v\f\r\SO\SI\DLE\DC1\DC2\DC3\DC4\NAK\SYN\ETB\CAN\EM\SUB\ESC\FS\GS\RS\US !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcd" *Main> B.pack $ encode' [1..100] "*** Exception: divide by zero where ttest1p.hs: import qualified Data.ByteString as B encode' [] = [] encode' (x:xs) = if x==0 then 0:0:encode' xs else (x `mod` 256) : (x `div` 256) : encode' xs What is the difference, except list length and value structure? Where is my error? Thanks for any hint, Dusan

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How does GHC read UNICODE.
by L.Guo 20 May '08

20 May '08

Hi Haskellers: I am a Chinese. Mostly, it is needed to read/write UNICODE charactors. Currently, I can only use the ByteString module in GHC 6, 2007. But I feel it is not an easy method. Does GHC support it now ? or, is there any other way to do this ? Regards -------------- L.Guo 2008-05-20

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question on ghc-pkg database
by Galchin, Vasili 20 May '08

20 May '08

Hello, What is the difference between the following two ghc-pkg database instances: /usr/lib/ghc-6.8.2/package.conf /usr/local/lib/ghc-6.8.2/package.conf ?? Thanks, Vasili

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Experimental compilation of Haskell to Erlang
by Dimitry Golubovsky 20 May '08

20 May '08

Hi, Some time ago I became interested in compilation of Haskell programs (via Yhc Core) to Erlang to be able to run Haskell code in Erlang environment. This experiment seems to have been successful, so I'd like to publish its results for everyone to read and criticize. Results of the experiment are described in this Haskell Wiki article: http://www.haskell.org/haskellwiki/Yhc/Erlang/Proof_of_concept Any feedback is appreciated. Thanks. -- Dimitry Golubovsky Anywhere on the Web

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[tagsoup] is it the expected behaviour ?
by Fernand 20 May '08

20 May '08

Hi, Experimenting with tagsoup (I'm using GHC 6.8.2 and tagsoup-0.6), I found something which appears to me as strange behaviour : when parsing tag's attributes that have spaces enclosing the "=" sign, tagsoup seems to interpret these as empty attributes' names and values. For instance (notice the spaces enclosing the equal sign) : $ ghcii.sh -package tagsoup [...] Loading package tagsoup-0.6 ... linking ... done. Prelude> :m +Text.HTML.TagSoup Prelude Text.HTML.TagSoup> parseTags "<a href = \"what\">uh ?</a>" [TagOpen "a" [("href",""),("","what")],TagText "uh ?",TagClose "a"] Here, am I wrong when expecting [TagOpen "a" [("href","what")],TagText "uh ?",TagClose "a"], or is there some HTML interpretation I don't know on the parsing of attributes ? Sincerely yours, Fernand

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lambdabot on GHC 6.8.2
by Tony Morris 20 May '08

20 May '08

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 So after some effort, I managed to get lambdabot installed on a ubuntu machine using GHC 6.8.2, however, it fails when evaluating an expression: > 1 + 1 setResourceLimit: invalid argument (Invalid argument) Googling around brings up a bug (#2038) against GHC 6.8.2. Am I hitting this bug? If so, can I get around it to get a working lambdabot? Thanks for any tips. - -- Tony Morris http://tmorris.net/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFIMnFnmnpgrYe6r60RAp7DAJ9JhFsWwL05eyrcECf6C9Z/ML+h0gCgt5/I 3E7eOjVMm+RSXumbYiCsgR8= =bpjW -----END PGP SIGNATURE-----

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Newbie: State monad example questions
by Luke Palmer 20 May '08

20 May '08

Hi Dmitri. I'm just going to ramble on about what I know and how I think of things, and maybe you'll pick something up :-) On 5/19/08, Dmitri O.Kondratiev <dokondr(a)gmail.com> wrote: > getAny :: (Random a) => State StdGen a > getAny = do g <- get > (x,g') <- return $ random g > put g' > return x > > Questions: > return (random g) = State (\s -> ((a,g), s)) > > Is it correct? Yes, where a is a random value and g is a new generator. > 2) What x and g' will match to in: > do ... > (x,g') <- return $ random g > > which, as I understand equals to: > do ... > (x,g') <- State (\s -> ((a,g), s)) > > What x and g' will match to in the last expression? Well, I also suggest working through the desugared version of >>= as the other poster suggested: State (\s -> ((a,g), s)) >>= (\(x,g') -> ...) The question is, what values do x and g' have in '...'. Expand out the definition and see. State is kind of complicated, but you will get an answer. But I can provide some intuition aside from the formality. x <- action "Does" the action and names the return value "x". Your action is "return $ random g", namely it's an action that doesn't have any effects and just returns a value. So: (x,g') <- return $ random g Is equivalent to: let (x,g') = random g Nothing particular to State is happening here. > 3) In general, in do expression (pseudo): > do { x <- State (\s -> (a, s)); ...} > > What x will refer to? Will x stand for a whole lambda function: \s -> (a, s) > ? Here x will be equal to a, and the implicit state will be unchanged after the binding. > 4) How 'g <- get' works in this function (getAny) ? The representation of a state action is "State f", where f takes the current state as an argument and returns the result of the computation along with the new state. "get" is defined as: get = State $ \s -> (s,s) So its function takes a state, and returns the current state as the return value as well as the next state; that is to say, it does not update the state and it also returns it. So: x <- get ... Gives the name "x" to the implicit state at the time "get" was called. > 5) Why we need 'put g'? Otherwise this function would not modify the implicit state, and you'd always get the same random number back when you called it. Something to keep in mind: If you have an "action :: m a" for some monad m, then in: x <- action x will name a value of type Foo. Luke

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My experience setting up Haskell up for GUI development
by Ben Arnold 20 May '08

20 May '08

I'm running Windows Vista and I've been trying to set up an environment for writing GUI applications. A Google search pointed me at two major GUI toolkits for Haskell: gtk2hs and wxHaskell. I started with gtk2hs. The installation instructions were straightfoward and easy to follow, but they just didn't work. The installer did nothing. It opened a command window and immediately terminated. When I ran it from a command window it terminated immediately, apparently doing nothing at all. Running as Administrator didn't help. There was no error message, the log file option didn't generate a log file and Googling for a solution didn't come up with anything. So I gave up on gtk2hs. WxHaskell was a little trickier but more successful. There didn't appear to be any installation instructions on the website, but it did imply that I needed to install wxWidgets first. I did that, and made sure the paths didn't have spaces in them (yawn). And after restarting the PC eventually I got a working Hello World dialogue box. Which is nice, except that when I call "main" twice in succession from GHCi, ghc.exe crashes. So I have no confidence in the wxHaskell libraries either. I don't want to invest time in programming with libraries that perform illegal operations on my operating system. I like Haskell; I really do. I've toyed around with it before a few times and I want to learn more about it, but I can't confine myself to command-line applications forever and for better or worse I run a Windows box. So, in practice, do other people write GUI apps with Haskell on Windows? And if they do, how do they do it? I feel I've got to the stage where I need a concrete recommendation from the experts. My installation: - Windows Vista Home Premium 32-bit 6.0.6001 First attempt - GHC 6.6.1 - gtk2hs-09.12.exe Second attempt - GHC 6.8.2 - wxMSW 2.6.3 - wxHaskell 0.10.3 Ben

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cabal Setup.hs configure - cabal doesnt produce a setup-config
by geoffrey 19 May '08

19 May '08

Hi, I am trying to install nanocurses, but cabal isn't creating the setup-config file in dist/ I have run (and re-run) runghc Setup.hs configure The first time it complained about mpg123 (whatever that is) so i installed the other dependencies - unix & bytestring Then i tried to re-run setup configure for nano again. Configure creates the dist directory but no files. I have deleted and untarred the files and retried,but to no avail. Any clues? Thanks. (OS: Ubuntu hardy, GHC: 6.8.2)

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