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Re: [Haskell-cafe] ANNOUNCE: enumerator, an alternative iteratee package
by John Lato 23 Aug '10

23 Aug '10

> > From: John Millikin <jmillikin(a)gmail.com> > > On Sat, Aug 21, 2010 at 15:35, Paulo Tanimoto <ptanimoto(a)gmail.com> wrote: > > Apologies if I'm asking you to repeat yourself, but I couldn't find > > the explanation. Â What was the reason why you went with IterateeM > > instead of IterateeMCPS? Â Simplicity? > > Iteratees are difficult enough to understand already -- requiring > prospective users to learn and understand CPS would just be another > roadblock. The CPS implementation is also slower -- I performed some > basic benchmarking of IterateeM.hs and IterateeMCPS.hs, and CPS is > only faster without optimizations. At -O, they are equal, and at -O2, > IterateeM is faster. Apologies if this discussion has already moved on; I'm just catching up on weekend email but wanted to respond to this directly. It's not necessary to understand CPS to use CPS-based iteratees. The CPS implementation generally simplifies the types and removes the necessity for special combinators like ($$) and (>>==), so I strongly suspect newcomers will find it easier to use than other variants (although unfortunately I can no longer say this from personal experience). It incorporates the best features of Oleg's two implementations in IterateeM.hs. The only drawback is the added thought overhead of CPS, but users need not be aware of this for the most part. For those who do want to have a thorough understanding of the implementation, I think that the CPS variant is usually more understandable than alternatives. The "take" family and stream converters (maps, convStream) are all simplified compared to alternative definitions. This isn't always true; enumPair is a counterexample. But I think it's helpful in many common cases, and enumPair is tricky in any implementation. It might be true that many programmers will find CPS difficult in the abstract (I certainly do), but when it occurs in a specific implementation the concepts are usually much more tractable. At least for iteratees, there's a very direct correspondence between the CPS-style and IterateeM-style, which greatly eases understanding. Also, while the IterateeM implementations may be faster for certain operations than CPS, they are also slower for others, sometimes significantly so. My tests (all with -O2, and various other compiler options tried) prior to switching to the CPS implementation showed that it is competitive with, if not being, the fastest implementation in all cases. Most importantly, I didn't find any comparatively slow operations, which wasn't true for either of the IterateeM implementations. I think that a CPS implementation of iteratees is the best of all current alternatives for ease of use, and possibly the best-performing implementation depending on exactly what operations are being performed. Even if it's not the absolute fastest, it should be close enough that the other benefits outweigh a performance gain. Cheers, John

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Fast Integer Input
by 200901104＠daiict.ac.in 23 Aug '10

23 Aug '10

I need a function which has fast conversion from Bytestring to Integer. I am currently using this: import qualified Data.ByteString.Lazy.Char8 as BS readInteger :: BS.ByteString -> Integer readInteger x = case BS.readInteger x of Just (i,_) -> i Are there faster implementations? This function takes 1.8 seconds to convert 2000 integers of length 10^13000. I need it to be smaller that 0.5 sec. Is it possible?

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Second Call for Copy: Monad.Reader Issue 17
by Brent Yorgey 23 Aug '10

23 Aug '10

Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 17 is **Friday, October 1, 2010** That's little more than a month away! Get started on your article now! =) The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki- page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (byorgey at cis.upenn.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll work something out.

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empty .prof file
by Vo Minh Thu 23 Aug '10

23 Aug '10

Hi, I'd like to profile a GLUT application. After a run of the application with the +RTS -p flag, the .prof file is actually created but is always empty. Does someone have a clue of what's going on? Would it be related to my application calling GLUT's leaveMainLoop to exit? Thanks, Thu

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Re: [Haskell-cafe] Fast Integer Input
by 200901104＠daiict.ac.in 23 Aug '10

23 Aug '10

>>Does the length of those numbers happen to be fixed? It they are all >>exactly 13000 decimals then it should be possible to create a more >>optimised parser. Well actually they can have any number of digits less than 13000. But the only post processing of the numbers is calculating the binary logarithm of the number. Does that help? >>It would also help if you could post a working example of your code >>and some test data somewhere so people can run it and test if for >>themselves. I have a slow Internet connection. So I will attach the script I used to generate the cases instead.(Note: It will take a few minutes to complete)

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[Haskell-cafe] ANNOUNCE: enumerator, an alternative iteratee package
by John Millikin 23 Aug '10

23 Aug '10

Most of you have probably read Oleg's essays on using left-fold enumerators for incremental IO. In short, by encapsulating monadic left-folds in an "Iteratee" type, incremental pure processing is possible without using lazy IO. Sources to read: Oleg: Streams and Iteratees < http://okmij.org/ftp/Streams.html > Magnus Therning: Trying to work out iteratees < http://therning.org/magnus/archives/735 > cdsmith: Iteratees Step By Step (Part 1) < http://cdsmith.wordpress.com/2010/05/23/iteratees-step-by-step-part-1/ > John Millikin (me): Understanding Iteratees < http://ianen.org/articles/understanding-iteratees/ > Currently, the primary package for left-fold enumerators is John Lato's "iteratee". It is based on Oleg's original code, extended to support various forms of containers, platform-specific IO, and codecs for the WAV and TIFF formats. While I appreciate Mr. Lato's development of the package, I find it far too large, and its documentation too sparse, to effectively use. To correct this, I've written the "enumerator" package. It is also derived from Oleg's IterateeM.hs , but with a simplified API and significantly reduced dependency list. Hackage entry: http://hackage.haskell.org/package/enumerator Haddock docs: http://ianen.org/haskell/enumerator/api-docs/ Source code (literate PDF): http://ianen.org/haskell/enumerator/enumerator.pdf darcs get http://ianen.org/haskell/enumerator/ Additionally, I've included examples of using enumerators to implement simplified versions of the "cat" and "wc" utilities. These should serve as a useful starting point for anybody who wants to use enumerators in their own code: http://patch-tag.com/r/jmillikin/enumerator/snapshot/current/content/pretty… http://patch-tag.com/r/jmillikin/enumerator/snapshot/current/content/pretty… There are already a few libraries using the existing "iteratee" package (snap, attoparsec-iteratee, hexpat-iteratee); I am very interested in advice from the authors of these libraries. In particular, are any of the removed features (ListLike, WrappedByteString, seeking) something your libraries depend on? Are there any useful combinators you'd like to see included?

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Re: [Haskell-cafe] ANNOUNCE: Sifflet visual programming language, release 1.0!
by Andrew Coppin 23 Aug '10

23 Aug '10

gdweber(a)iue.edu wrote: > I hope it does work on Windows! I don't know of any reason why not, > but I haven't tested it on Windows. Please let me know if there's > any problem on that platform. > Unfortunately, yes: Indirectly it requires the "curl" package. Since that's a binding to a C library, it won't build on Windows. (I have every reason to believe that everything else will work though...)

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Compiling UHC
by Henk-Jan van Tuyl 23 Aug '10

23 Aug '10

L.S., I am trying to compile the current release of UHC (1.0.1); I started with the installation of the most recent version of uulib and uuagc. When running make uhc I received the following message: EH\Util\CompileRun.hs:46:7: Could not find module `UU.DData.Scc': Use -v to see a list of the files searched for. This module is not in the most recent version of uulib, so I installed uulib-0.9.10, but then I received the message: EH\Util\CompileRun.hs:46:7: Could not find module `UU.DData.Scc': It is a member of the hidden package `uulib-0.9.10'. Use -v to see a list of the files searched for. What can I do to get UHC compiled? (I am using Haskell Platform 2010.2.0.0) Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

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Simple Sudoku solver using Control.Monad.Logic
by azwhaley 23 Aug '10

23 Aug '10

Hello All, Apologies if some have you have got this twice but I posted this once via fa.haskell on Goggle but I don't think it goes anywhere outside Google. In an attempt to learn how to use monads, I've tried to write a simple sudoku solver using the LogicT monad. I think it works but it is extremely slow, in fact it won't finish at all if I attempt to enforce the grid constraints. Just using row and column constraints, it will finish for some problems. Am I doing something dreadfully wrong here or is this just a hard problem to solve ? Thanks Andrew here's the listing :- module Main where import Control.Monad.Logic import Data.List (delete, (\\)) board :: [[Int]] board = [ [7, 9, 0, 0, 0, 0, 3, 0, 0], [0, 2, 0, 0, 0, 6, 9, 0, 0], [8, 0, 0, 0, 3, 0, 0, 7, 6], [0, 0, 0, 0, 0, 5, 0, 0, 2], [0, 0, 5, 4, 1, 8, 7, 0, 0], [4, 0, 0, 7, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0]] -- accessors for row, column and grid row b = (b!!) col b c = [x!!c | x <- b] grid b g = (t 0) ++ (t 1) ++ (t 2) where t i = take 3 $ drop x $ b !! (y + i) x = 3 * (g `mod` 3) y = 3 * (g `div` 3) -- Ensures all numbers in the list are unique unique :: [Int] -> Bool unique r = null (foldl (\a x -> delete x a) [x | x <- r, x /= 0] [1..9]) choose choices = msum [return x | x <- choices] -- Test a cell (0 = unknown value) test :: Int -> Logic [Int] -> Logic Int test 0 c = do choices <- c choose choices test x c = return x -- helper to produce a diff list from a wrapped monadic list mdiff :: [Logic Int] -> [Int] -> Logic [Int] mdiff a c = do i <- sequence a return ([1..9]\\(i++c)) -- the actual solver - attempts to limit choices early on by using diff list of remaining values sudoku :: Logic [[Int]] sudoku = do solution <- foldl (\b r -> do m <- b row <- sequence $ foldr (\(n,x) a -> (test x (mdiff a $ col m n)):a) [] [(n,x) |x <- r | n <- [0..8]] guard $ unique row sequence [guard $ unique $ col m i | i <- [0..8]] return (m ++ [row]) ) (return []) board sequence $ [guard $ unique $ grid solution i | i <- [0..8]] return solution -- solve and print main = do let solution = observe sudoku sequence [print s | s <- solution]

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Trouble with winGHCi
by David Webster 23 Aug '10

23 Aug '10

I just installed Haskell 2010.2.0.0 on Windows XP Pro SP3. When I run winGHCI, I briefly get a displayed window, immediately followed by a "black screen". I have to restart my computer to get rid of it. I have been advised that this is a bug and wonder if it can be fixed. David Webster

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ICFP '10: Final call for participation
by Wouter Swierstra 23 Aug '10

23 Aug '10

===================================================================== Final Call for Participation The 15th ACM SIGPLAN International Conference on Functional Programming (ICFP 2010) http://www.icfpconference.org/icfp2010/ Baltimore, Maryland September 25 – October 2 ===================================================================== ICFP 2010 provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. The conference covers the entire spectrum of work, from practice to theory, including its peripheries. ** Not that the early registration deadline and discount hotel rates expire next week. ** * Program: http://www.icfpconference.org/icfp2010/program.html * Invited speakers: - Mike Gordon ML: Metalanguage or Object Language? - Matthias Felleisen TeachScheme!: A Checkpoint - Guy Blelloch Functional Parallel Algorithms Schedule including related events: * September 25: Workshop on Mechanizing Metatheory (WMM) Workshop on Mathematically Structured Functional Programming (MSFP) Workshop on High-Level Parallel Programming and Applications (HLPP) * September 26: Workshop on ML Workshop on Generic Programming (WGP) * September 27-29: ICFP 2010 * September 30: Haskell Symposium Erlang Workshop * October 1: Commercial Users of Functional Programming – Day 1 (CUFP Tutorials) Haskell Implementors' Workshop * October 2: Commercial Users of Functional Programming – Day 2 (CUFP Talks) This year there will also be a special series of Birds-of-a-Feather sessions associated with CUFP. More information can be found at: http://cufp.org/bofs-2010 Registration information: * Registration link: https://regmaster3.com/2010conf/ICFP10/register.php Local arrangements (including travel and accommodation): * http://www.icfpconference.org/icfp2010/local.html * Conference reservation/rate deadline: September 1st Conference organizers: * General Chair: Paul Hudak, Yale University * Program Chair: Stephanie Weirich, University of Pennsylvania * Local Arrangements Chair: Michael Hicks, University of Maryland * Workshop Co-Chairs: Derek Dreyer, Max Planck Institute for Software Systems Christopher Stone, Harvey Mudd College * Programming Contest Chair: Johannes Waldmann, Hochschule für Technik, Wirtschaft und Kultur, Leipzig * Video Chair: Scott Smith, Johns Hopkins University * Publicity Chair: Wouter Swierstra, Vector Fabrics =====================================================================

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A cabal odyssey
by Andrew Coppin 23 Aug '10

23 Aug '10

And so today, just for giggles, I tried to get Sifflet to work. Along the way, I encountered a number of... "glitches", if you will. First of all, I tried to get it to work on Windows. I fired up a new Windows VM and installed Haskell Platform 2010.1.0.0. It seems that (finally) this includes the cabal-install tool, which is nice. It seems it includes cabal-install 0.8.0, so as soon as I tried "cabal update", it tells me a new version is available. It claims I just need to "cabal install cabal-install" (which is amusingly riddle-like). Unfortunately, although doing this *does work*, the new cabal.exe is installed lower down the search path than the existing one, so you still get the old version (unless you manually fiddle with the search path). In fact, it seems that the HP install folder is higher up the search path than the "global" binary target, which is higher than the "local" binary target. It seems to me like this ought to be the other way around. The next problem is that "cabal install sifflet" gets mighty confused and outright fails. Basically it can't figure out how to resolve all the dependencies. It seems that Sifflet demands GTK 0.11.0 (i.e., gtk-0.11.0, pango-0.11.0, glib-0.11.0, etc.) However, Cabal looks at gtk-0.11.0, sees it depends on (say) "glib-0.11.*", and wants to use glib-0.11.1 (the latest one). But then Sifflet wants glib-0.11.0, not glib-0.11.1, and it seems Cabal just can't figure out what the heck to do. Which is slightly surprising, really. The solution (of sorts) is to painstakingly resolve the dependencies by hand, by asking Cabal to install the correct packages one at a time in the correct order. (I still love the way Gtk2hs *actually compiles* on Windows now. That's pretty sweet!) That reminds me. What the heck is actually *in* file 126? I don't know why, but compiling file 126 (Graphics.UI.Gtk.Gdk.Cursor) takes up 75% of the entire Gtk2hs build time! What's that all about? Anyway, having finally built Gtk2hs version 0.11.0 successfully, I continued trying to get Sifflet working... only to discover it wants the "curl" package. And when I ask Cabal to build it, it just retorts that it has a configure script. *sigh* So that's the end of that. I have absolutely no idea why a tool like Sifflet would need access to the Curl library. Presumably this is just another one of those obscure dependency-chasing artifacts that happen from time to time? (After dealing with Linux, I'm used to this kind of weirdness.) OK, so it's only possible to run Sifflet under Linux. Let's give that a try... So I fire up a new OpenSUSE VM. I quickly discovered that gtk seems to want Alex and Happy, but cabal-install is defaulting to doing a "local" rather than "global" install, and thereafter it can't "find" Alex or Happy, even though they're installed. (Yeah, great, thanks for that...) So I rewind the VM, build the latest version of cabal-install, and edit the configuration to do global installs instead of local. The amusing part is, if you "sudo cabal install" so it has permission to put the installed files into place, it then uses root's configuration file instead. *sigh* Well anyway, I managed to work around that. But... Cabal *still* fails to find Alex or Happy, even though they're now in the search path. Oh, wait. They're in *my* search path. They're not in root's search path. (As per good security practise, root's search path is rather short.) I can pass some CLI switches to tell it where these are, but then gtk2hs-buildtools makes a whole bunch of stuff which Cabal also can't find. Eventually, the easiest thing I could come up with was to do "cabal unpack" to get a source tree, configure and build as me, and sudo for the install. Except that then it tries to reconfigure...? Wuh? So instead of "sudo cabal install", I tried "sudo runhaskell Setup install", which works just fine (although obviously it's rather wordy!) So, I manually hold Cabal's hand through the process of building all the 0.11.0 packages one at a time, in a way that it can find all the stuff at configure-time. Gosh this is a faff! Well anyway, it worked. Trips over when I reach Curl, but that's because I need to ask YaST to install the curl-devel package. And, finally, I can build and install Sifflet itself. After 4 hours or so of typing commands, it was nice to do some stuff with my mouse. ;-) I spent about 20 minutes trying to figure out how to build high-order functions before I discovered that you can't. Still, the stated design goal of Sifflet is "to teach beginners about recursion". Clearly it's design achieves this goal quite well. Equally clearly, Haskell has damaged my mind. I actually cannot think of *any* interesting low-order functions! 5 years of programming in Pascal with low-order *monomorphic* functions, and today I can't think of a single interesting use for such a thing. ;-) Heh, well anyway... I don't know how many of the things I've mentioned are known or likely to be fixed. Just thought I'd share.

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Re: [Haskell-cafe] A cabal odyssey
by Mathew de Detrich 23 Aug '10

23 Aug '10

What haskell really needs is a big ' hit' where someone shows how powerful haskell is in some environment. Ruby had RoR, which is what splurged the language into the open On 22/08/2010 3:46 PM, "wren ng thornton" <wren(a)freegeek.org> wrote: Daniel Fischer wrote: > > On Saturday 21 August 2010 15:35:08, Ivan Lazar Miljenovic wrote: >> >> A ... Three! -- Live well, ~wren _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org ...

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GADT and problems with rigid type variables
by Markus Barenhoff 23 Aug '10

23 Aug '10

Hello, playing with GADTs I ran into a problem with rigid type variables which is ilustrated by the following example. I think it should be pretty clear what I'am trying to express... Any suggestions? ---- snip ---- {-# LANGUAGE GADTs #-} data Foo where Foo :: (Eq t) => t -> Foo instance Eq Foo where (Foo a) == (Foo b) = a == b {- Scratch.hs:7:28: Couldn't match expected type `t' against inferred type `t1' `t' is a rigid type variable bound by the constructor `Foo' at /home/alios/src/lab/Scratch.hs:7:3 `t1' is a rigid type variable bound by the constructor `Foo' at /home/alios/src/lab/Scratch.hs:7:14 In the second argument of `(==)', namely `b' In the expression: a == b In the definition of `==': (Foo a) == (Foo b) = a == b Failed, modules loaded: none. -} ---- snip ---- thnx Markus

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Error in enumerator when using interpreter instead of compiler
by Tobias Brandt 23 Aug '10

23 Aug '10

Hi all, I was trying out the enumerator package. I wanted to copy the contents of one file to another: module Main where import Data.Enumerator import Data.Enumerator.IO main = run (enumFile "foo" $$ iterFile "bar") If I compile this code with GHC, it works as expected. But if I run it with runhaskell I get the following error: Left bar: hPutBuf: illegal operation (handle is closed) If I replace the above code with this: module Main where import Data.Enumerator import Data.Enumerator.IO import System.IO main = withBinaryFile "bar" WriteMode $ \h -> run (enumFile "foo" $$ iterHandle h) It works both compiled and interpreted. Is this a bug in enumerator (probably in the iterFile function) or a deficiency of the interpreter?

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Perform a research a la Unix 'find'
by Yves Parès 22 Aug '10

22 Aug '10

Hello, I would like to recode in Haskell a piece of bash program that uses find. The advantage of find is that it is quite easy to use and fast. Is there a package that let us access to find-like functionnalities, with similar performances? Or at least some C functions that could be used through the FFI? (I would like to avoid to recur through directories and match files myself, I tried it and the search lasts for ages).

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Re: [Haskell-cafe] Code that writes code
by Andrew Coppin 22 Aug '10

22 Aug '10

Graham Klyne wrote: > Maybe not helpful to you at this stage, but... > > An alternative to generating source code is to factor out the common > "boilerplate" elements into separate functions, suitably > parameterized, and to use higher order functions to stitch these > together. Well, yeah, if you've got so much boiler plate that you have to automate generating the boilerplate, you're probably doing it wrong. ;-) All I'm actually using it to do is generate a set of fixed-size containers (each of which has a bazillion class instances). I've got a variable-sized container, but sometimes it's useful to statically guarantee that a container is a specific size. In addition, by being fixed-size you can get a few small performance gains. That's really all I'm using autogeneration for. I suppose instead of building an ADT for each container size, I could just write a newtype over the variable-size container and put a phantom type on it representing the size... That would give me the static guarantees but not the efficiency.

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Re: [Haskell] Re: [Haskell-cafe] ANNOUNCE: enumerator, an alternative iteratee package
by John Millikin 22 Aug '10

22 Aug '10

On Sat, Aug 21, 2010 at 14:41, Michael Snoyman <michael(a)snoyman.com> wrote: > Hey John, > As I mentioned, I'm considering having persistent depend upon enumerator. Do > you think it's too early in enumerator's life to do so and I should wait > till the API stabilizes a bit more? Also, two other packages I would think > to bring into the enumerator family would be: > * yaml > * wai-extra, providing an enumerator layer for more easily dealing with the > Source and Enumerator datatypes in wai. I might just release a > wai-enumerator package instead. > Thanks again for your work on this, > Michael I think the API is pretty stable. Most of the significant research into iteratee-based APIs has already been performed by users of the "iteratee" library, and by Oleg. There might be a few backwards-compatible changes (new modules, new exports, etc). I'm not planning to make any large changes, such as Mr. Lato's transition to CPS-based iteratees. As long as you import the enumerator modules with "qualified" (to avoid Prelude name clashes), it should be safe to start porting libraries.

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Hugs & Haskore loading
by Hans Aberg 22 Aug '10

22 Aug '10

When trying to load Haskore in Hugs Sept 2006 (say by :l), it does not work because it is in a subdirectory Haskore/Src/, despite that the Hugs searchpath includes /usr/local/lib/hugs/packages/* I could of course change this searchpath, but I'm curios about how to fix it without it - it seems to work for a lot of other packages.

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Equality constraints and RankNTypes - how do I assist type inference
by DavidA 21 Aug '10

21 Aug '10

Hi, I have the following code, using equality constraints and (I believe) RankNTypes: {-# LANGUAGE MultiParamTypeClasses, TypeFamilies, RankNTypes, ExistentialQuantification #-} {-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-} -- import Math.Algebra.Group.PermutationGroup -- Vector space over field k with basis b data Vect k b = V [(b,k)] deriving (Eq,Show) data TensorBasis a b = T a b deriving (Eq, Ord, Show) -- Tensor product of two vector spaces type Tensor u v = (u ~ Vect k a, v ~ Vect k b) => Vect k (TensorBasis a b) -- ** class Algebra k v where -- "v is a k-algebra" unit :: k -> v mult :: Tensor v v -> v type GroupAlgebra k = Vect k Int -- (Permutation Int) instance Num k => Algebra k (GroupAlgebra k) where unit 0 = V [] unit x = V [(1,x)] mult (V ts) = V [(g*h,x) | (T g h, x) <- ts] Everything is fine except for the last line, which causes the following error message: Couldn't match expected type `Tensor (GroupAlgebra k) (GroupAlgebra k)' against inferred type `Vect k1 b' In the pattern: V ts In the definition of `mult': mult (V ts) = V [(g * h, x) | (T g h, x) <- ts] In the instance declaration for `Algebra k (GroupAlgebra k)' But according to me, I've told it that these two types are the same at the line marked -- ** How do I help it out with type inference? (It, in this case, is GHCi 6.12.1) Any ideas?

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System.Posix, forked processes and psuedo terminals on Linux
by Erik de Castro Lopo 21 Aug '10

21 Aug '10

Hi all, I've got a but it code below thats not quite working as I expect it to. The basic idea is that it opens a master/slave pair of pseudo terminals, forks a child process and then perform bi-directional communication between the parent and the child via the master/slave psuedo terminal. The thing does really need to forkProcess because once the comms is working I want to exec another process in the child. I also intend to dup the file descriptors so that stdin, stdout and stderr all point to the slave's end of the pty. The code below *almost* works. Its currently printing out: parent : Forked child was here! parent : Message from parent. Read 21 bytes while I think it should print: parent : Forked child was here! parent : Read 21 bytes Any clues on why 'Message from parent.' is also ending up on stdout? This is ghc-6.12.1 on Debian Linux. Cheers, Erik import System.Posix.IO import System.Posix.Process import System.Posix.Terminal import System.Posix.Types main :: IO () main = do (master, slave) <- openPseudoTerminal _childId <- forkProcess $ forkedChild (master, slave) closeFd slave runParent master runParent :: Fd -> IO () runParent fd = do (str, _) <- fdRead fd 1024 putStr $ "parent : " ++ str _ <- fdWrite fd "Message from parent.\n" (str2, _) <- fdRead fd 1024 putStr $ "parent : " ++ str2 forkedChild :: (Fd, Fd) -> IO () forkedChild (master, fd) = do closeFd master _ <- fdWrite fd "Forked child was here!\n" (_, count) <- fdRead fd 1024 _ <- fdWrite fd $ "Read " ++ show count ++ " bytes\n" closeFd fd -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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cabal-install profiling libraries
by Johannes Waldmann 21 Aug '10

21 Aug '10

Here's another instance of the machine (*) telling me what to do, instead of doing it (or am I missing something): I have a large set of cabal packages installed with ghc. Then suddenly I need some package Foo with profiling. So I switch to library-profiling: True in my .cabal/config, and then "cabal install Foo" - failing with the message: Perhaps you haven't installed the profiling libraries for package `Bar' for some package Bar that Foo depends upon. - Dear Cabal: Yes! I know that I haven't installed them! I want you to install them for me! But it isn't listening ... (*) "machine" = everything in that metal box that was so expensive and has a lot of cables coming out, and ventilators running. Of course you know that I have the highest respect for the work of the cabal authors. I'm just suggesting that the above feature (auto-re-install dependencies) would be helpful. Perhaps it's already there? If not - would it be hard to specify? To build? Or would it have bad consequences? Is it "cabal upgrade --reinstall"? But that was deprecated? Here I really want "reinstall with exactly the same versions". Is it the problem that their sources may have vanished, meanwhile? Could it be solved by having "cabal install" storing a copy of the source package that it used? Thanks - J.W.

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ANNOUNCE: text-json-qq 0.2.0 (now with haskell-src-meta goodness)
by Oscar Finnsson 21 Aug '10

21 Aug '10

Hi, I've just uploaded a new version of text-json-qq, the json quasiquoter. Now it's possible (thanks to haskell-src-meta) to insert Haskell code inside the qq-code: > myCode = [$jsonQQ| {age: <| age + 34 :: Integer |>, name: <| map toUpper name |>} |] > where age = 34 :: Integer > name = "Pelle" For further info read the documentation at http://hackage.haskell.org/package/text-json-qq or read/fork the source code at http://github.com/finnsson/text-json-qq -- Oscar

1 0

ANNOUNCE: binary-generic-0.2, generic binary serialisation using binary and syb.
by Lars Petersen 21 Aug '10

21 Aug '10

Hello cafe, although there was no announcement for version 0.1 there is one for the current 0.2: binary-generic allows to perform binary serialisation without explicitly defining every type specific case. If an algebraic type instantiates the 'Data' class the library is able to serialize it in a canonical way. Unfortunately version 0.2 is not binary compatible with 0.1. I decided to break this for the sake of simplicity: In 0.2 all multibyte values are encoded big-endian. For further version I'll try to supply compatibility functions even if something changes, but since 0.1 is not even a week old, I think it's not necessary this time. Okay, features: Common primitive types are supported out of the box: * Char, Word, Int are serialised as big-endian, taken from Data.Binary * Float and Double are serialised big-endian according to binary-ieee754 * Integer is serialized as in Data.Binary, but consistently big-endian * Data.ByteString as it is * Data.Text as Utf8 For types that are not supported yet, there is described an easy way of extension in 'Data.Binary.Extensions'. You are also free to ovveride certain choices and supply your own. If you think there are more types that should be supported without explicit extension, drop me a line. Cheers, Lars

2 1

Creating binary distributions with Cabal
by Christopher Done 20 Aug '10

20 Aug '10

Does Cabal have a way to produce binary distributions from a package? I need to create a binary distribution of my project which does not depend on GHC or any development tools. The package should include all required data files and configuration files. I've got the latter covered with Data-Files and getDataFileName, but not sure about what to do regarding configuration files -- read/write to $HOME/.myproject/config or $HOME/.myprojectrc, etc., or what? I'm specifically targeting Redhat because that's the production server, but I'm wondering if there is or will be a way to agnostically access data files and configuration files without having to think about what OS it will be running on, in the same way I can use sockets or file access without worrying about the particular OS. Something like cabal sdist --binary --rpm/deb/arch/win/etc? How does everyone else package up their Haskell programs for binary distribution? Cheers!

3 4

GtkImageView release!
by Andy Stewart 20 Aug '10

20 Aug '10

Hi all, I have release gtkimageview, it's a Gtk APIs for build image viewer. Here is screenshot : http://www.flickr.com/photos/48809572@N02/4909785139/ You can click right-bottom to popup navigate window to drag area in image. Here is demo : https://patch-tag.com/r/AndyStewart/gtkimageview/snapshot/current/content/p… Enjoy! -- Andy

1 0

Academic Haskell Course
by Eyal Lotem 20 Aug '10

20 Aug '10

I was invited to give a course on Haskell (More in the lines of "Advanced FP in Haskell") for graduate students who already know a bit of ML, in an academic institution. I could lay out my own syllabus and course, but finding academic sources for all of the ideas is difficult - and I am probably duplicating work. Can anyone point me towards existing work I could use? Open course material and syllabuses I could use, with the necessary references? Thanks! Eyal

4 3

philosophy of Haskell
by Michael Mossey 20 Aug '10

20 Aug '10

When I started to study Haskell, I was surprised that so much emphasis was placed on simple things. Monads were introduced to me as basically a wrapper, and a bind function that unwrapped something and wrapped something else back up again. I didn't understand what the fuss was about. Later I saw the amazing feats of expressiveness that were possible. I scratched my head in confusion---"Wait, say that again?" Here's a quote from Bertrand Russell about philosophy (read: Haskell). He's actually being humorous, but it applies, in a way: "The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical no one will believe it."

10 18

Having a connection between kind * and kind * -> *
by Ivan Lazar Miljenovic 20 Aug '10

20 Aug '10

I'm trying to update container-classes to duplicate the pre-existing classes defined in the Prelude (Functor, etc.) and am trying to get my approach on how to have functions/classes that work on types of kind * (e.g. Bytestring) as well as kind * -> * (e.g. lists), as my previous approach didn't work. To define the connection, I've copied the style set out by Ganesh's rmonad ( http://hackage.haskell.org/package/rmonad ) package: ,---- | -- | Indicates what kind of value may be stored within a type. Once | -- superclass constraints are available, the @v@ parameter will | -- become an associated type. | class Stores c v | c -> v | | data family Constraints :: (* -> *) -> * -> * | | class (Stores (c v) v) => Suitable c v where | constraints :: Constraints c v `---- This works. However, what doesn't work is when I try to use these classes to re-define Functor: ,---- | class (Stores c v) => Mappable c v where | rigidMap :: (v -> v) -> c -> c | | class (Mappable (c v) v) => Functor c where | fmap :: (Suitable c a, Suitable c b) => (a -> b) -> c a -> c b `---- GHC doesn't like this definition of Functor; the only way to get it to work is to make `v' a parameter of the Functor class as well, even though it doesn't actually get used. Why is this? Is there any way around it? (Limitations like this are making me more and more consider dumping this whole concept as it's just becoming a pain, but more importantly rather boring :s) -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

4 6

Sifflet dependencies
by Don Stewart 19 Aug '10

19 Aug '10

$ cabal update Downloading the latest package list from hackage.haskell.org $ cabal install sifflet Resolving dependencies... cabal: cannot configure pango-0.11.1. It requires cairo >=0.11.1 && <0.12 and glib >=0.11.1 && <0.12 For the dependency on cairo >=0.11.1 && <0.12 there are these packages: cairo-0.11.1. However none of them are available. cairo-0.11.1 was excluded because sifflet-lib-1.0 requires cairo ==0.11.0 cairo-0.11.1 was excluded because cairo-0.11.0 was selected instead cairo-0.11.1 was excluded because sifflet-1.0 requires cairo ==0.11.0 For the dependency on glib >=0.11.1 && <0.12 there are these packages: glib-0.11.1. However none of them are available. glib-0.11.1 was excluded because sifflet-lib-1.0 requires glib ==0.11.0 glib-0.11.1 was excluded because glib-0.11.0 was selected instead glib-0.11.1 was excluded because sifflet-1.0 requires glib ==0.11.0 So that's pretty simple. 'sifflet' requires cairo ==0.11.0 and pango. But since pango doesn't work with cairo 0.11.0, the package can't be built. The solution is to ask the sifflet author to adjust the dependencies to be more flexible. http://hackage.haskell.org/package/sifflet Those specific versions of packages are overly constrained. They should follow the PVP, and be thus, 0.11.* -- Don

6 6

Re: Specific to General to Specific
by oleg＠okmij.org 19 Aug '10

19 Aug '10

Given a widely parameterized type class > class Monad (m g n) => GenericNetworkMonad g m n where > ret :: a -> m g n a > ret = return the question seems to be about defining a specialized alias of it, instantiating the parameters g m n in some way. The hope is that the alias has fewer parameters and so is more convenient to use. Would not the standard approach of defining aliases work then? For example, > class NetworkMonad mg n where > instance GenericNetworkMonad g m n => NetworkMonad (m g) n where Granted, we would have to write boilerplate as we have to re-direct specialized methods to the general ones. Here is the complete code > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE FlexibleContexts, FlexibleInstances #-} > {-# LANGUAGE UndecidableInstances #-} > > data family GNT g :: * -> * -> * > data family GNQ g :: * -> * -> * > > class Monad (m g n) => GenericNetworkMonad g m n where > ret :: a -> m g n a > ret = return > > -- A few instances > data instance GNT Int n a = GNT_Int a -- n is phantom > data instance GNQ Int n a = GNQ_Int a > > instance Monad (GNT Int n) where > return = GNT_Int > GNT_Int x >>= f = f x > > instance Monad (GNQ Int n) where > return = GNQ_Int > GNQ_Int x >>= f = f x > > type NT n a = GNT Int n a > type NQ n a = GNQ Int n a > > instance GenericNetworkMonad Int GNT n > instance GenericNetworkMonad Int GNQ n > > genericMonadicValue :: GenericNetworkMonad g m n => m g n () > genericMonadicValue = ret () > > monadicValue :: GenericNetworkMonad Int m n => m Int n () > monadicValue = ret () > > class NetworkMonad mg n where > mv'' :: mg n () > > instance GenericNetworkMonad g m n => NetworkMonad (m g) n where > mv'' = genericMonadicValue > > monadicValue'' :: NetworkMonad m n => m n () > monadicValue'' = mv'' In general, the Apply class trick should work for defining arbitrary aliases to class constraints. One Apply constraint is all one ever needs in Haskell: http://okmij.org/ftp/ftp/Haskell/types.html#Haskell1 Here is the instance of this trick, adopted to use type families rather than functional dependencies (no UndecidableInstances is required now): > class Apply label where > type Typ label :: * > apply :: label -> Typ label > > data NM (mg :: * -> * -> *) n = NM > instance GenericNetworkMonad g m n => Apply (NM (m g) n) where > type Typ (NM (m g) n) = m g n () > apply _ = genericMonadicValue > > monadicValue''' :: > forall mg n m. (Apply (NM mg n), > Typ (NM mg n) ~ m n ()) => > m n () > monadicValue''' = apply (NM :: NM mg n)

2 1

Maintainer wanted for pappy
by Christopher Done 19 Aug '10

19 Aug '10

Hi, I thought I'd go through my uploaded Hackage packages and decide which ones I am going to maintain, which are worth others maintaining, and which are probably not worth maintaining (spoiler, most aren't). 1. Interested in and will continue maintaining: gd, higherorder, cgi-utils, fastcgi, ircbouncer For gd, cgi-utils, fastcgi, ircbouncer I'd like to write test cases and do some profiling and general proper quality assurance. For the rest... 2. Not really interested in maintaining, but in a good state and probably worth maintaining: pappy (Bryan Ford gave me permission to upload and for someone to maintain this) 3. Not interested in maintaining, but in a good state, not worth maintaining: kibro, blogination, simplesmtpclient (this has been usurped by http://hackage.haskell.org/package/hsmtpclient (same author)) 4. Not interested in maintaining, in a bad state (i.e. build failure): validate, lojban, goa, wordcloud The build failure ones and ones not worth maintaining are probably best being taken out back. What do we do with packages we're not interested in maintaining anymore? Gracenotes recommended something like changing the category to Unmaintained (and on Hackage 2.0, a tag). Good idea for now? Chars!

4 5

Re: [Haskell-cafe] ANNOUNCE: enumerator, an alternative iteratee package
by John Lato 19 Aug '10

19 Aug '10

Congratulations on this release. I notice that enumerator doesn't use any extensions, and thus should be easier to use with jhc and other Haskell compilers. This is something I would have liked to support with iteratee, but couldn't due to conflicting design goals. I'd also like to point out that the Tiff and Wave modules of iteratee are primarily meant as examples. The Wave module at least is far too slow to actually use for anything. John > From: John Millikin <jmillikin(a)gmail.com> > > Most of you have probably read Oleg's essays on using left-fold > enumerators for incremental IO. In short, by encapsulating monadic > left-folds in an "Iteratee" type, incremental pure processing is > possible without using lazy IO. Sources to read: > > Oleg: Streams and Iteratees < http://okmij.org/ftp/Streams.html > > Magnus Therning: Trying to work out iteratees < > http://therning.org/magnus/archives/735 > > cdsmith: Iteratees Step By Step (Part 1) < > http://cdsmith.wordpress.com/2010/05/23/iteratees-step-by-step-part-1/ > > > John Millikin (me): Understanding Iteratees < > http://ianen.org/articles/understanding-iteratees/ > > > Currently, the primary package for left-fold enumerators is John > Lato's "iteratee". It is based on Oleg's original code, extended to > support various forms of containers, platform-specific IO, and codecs > for the WAV and TIFF formats. > > While I appreciate Mr. Lato's development of the package, I find it > far too large, and its documentation too sparse, to effectively use. > To correct this, I've written the "enumerator" package. It is also > derived from Oleg's IterateeM.hs , but with a simplified API and > significantly reduced dependency list. > > Hackage entry: http://hackage.haskell.org/package/enumerator > Haddock docs: http://ianen.org/haskell/enumerator/api-docs/ > Source code (literate PDF): > http://ianen.org/haskell/enumerator/enumerator.pdf > > darcs get http://ianen.org/haskell/enumerator/ > > Additionally, I've included examples of using enumerators to implement > simplified versions of the "cat" and "wc" utilities. These should > serve as a useful starting point for anybody who wants to use > enumerators in their own code: > > > http://patch-tag.com/r/jmillikin/enumerator/snapshot/current/content/pretty… > > http://patch-tag.com/r/jmillikin/enumerator/snapshot/current/content/pretty… > > There are already a few libraries using the existing "iteratee" > package (snap, attoparsec-iteratee, hexpat-iteratee); I am very > interested in advice from the authors of these libraries. In > particular, are any of the removed features (ListLike, > WrappedByteString, seeking) something your libraries depend on? Are > there any useful combinators you'd like to see included? > > >

1 0

cabal, haddock, hscolour
by Johannes Waldmann 19 Aug '10

19 Aug '10

How is the "cabal" magic that would run hscolour? I am using "cabal haddock --executables --hyperlink-source" and this generates the API docs for all modules, and all have source links, but the actual html-ized source is only generated for the main module, and missing for all others. Thanks - J.W. cabal --version cabal-install version 0.8.2 using version 1.8.0.6 of the Cabal library haddock --version Haddock version 2.7.2, (c) Simon Marlow 2006 Ported to use the GHC API by David Waern 2006-2008 HsColour --version HsColour 1.17 ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.3

4 8

ANNOUNCE: darcs 2.5 beta 3
by Reinier Lamers 19 Aug '10

19 Aug '10

Hi all, The darcs team would like to announce the immediate availability of darcs 2.5 beta 3 (also known as darcs 2.4.98.3 due to Cabal restrictions). Important changes since darcs 2.4.4 are: * trackdown can now do binary search with the --bisect option * darcs always stores patch metadata encoded with UTF-8 * diff now supports the --index option * amend-record now supports the --ask-deps option * apply now supports the --match option * amend-record has a new --keep-date option * inventory-changing commands (like record and pull) now operate in constant time with respect to the number of patches in the repository * the push, pull, send and fetch commands no longer set the default repository by default * the --edit-description option is now on by default for the send command You may notice that a line "obliterate has a -o flag to save obliterated patch" was removed from this list of changes since the darcs 2.5 beta 2 announcement. We have postponed this feature because we discovered some problems with it, and we want to take the time to fix them adequately. On the other hand, a line "diff now supports the --index option" was added since the last beta :) If you have installed the Haskell Platform or cabal-install, you can install this beta release by doing: $ cabal update $ cabal install darcs-beta Alternatively, you can download the tarball from http://darcs.net/releases/darcs-2.4.98.3.tar.gz and build it by hand as explained in the README file. (You may notice that the Unix permissions in this tarball are weird - all files are 0600. That is because of Cabal bug #627: http://hackage.haskell.org/trac/hackage/ticket/627. It will be fixed in the final release.) Kind Regards, the darcs release manager, Reinier Lamers

5 5

gtk2hs with old gtk2
by Johannes Waldmann 19 Aug '10

19 Aug '10

I'm trying to build gtk2hs on debian/lenny, that is, with gtk2 v. 2.12 if I understand this correctly. I got the gtk2hs sources from darcs (I guess it's 0.11.0) and http://www.haskell.org/gtk2hs/ says "You need to specify -f-gtk_2_20 for Gtk+ 2.18 etc." I would love to, but where exactly? (When bootstrap.sh) Thanks - J.W.

3 6

Handling absent maintainers
by Mark Wotton 19 Aug '10

19 Aug '10

Hello all, I've recently had problems with haskell-src-meta. While it's a great package, it doesn't currently compile on GHC 6.12, and Matt Morrow doesn't seem to be around to push the version that does to Hackage. Our "one-world" approach with cabal seems to discourage forking as a casual act, so when a package that others rely on goes AWOL, it's very awkward to fix it. I can think of a few ways to get around my current problems: 1. upload haskell-src-meta-placeholder, or haskell-src-meta-mwotton, or something similar - this could be said to pollute the namespace, but would solve my immediate problem. I'd have to similarly specialise the chain of packages up to the one I actually want to use as well, though. 2. run my own hackage server and tell my users to use that instead. 3. ... profit? Ideally, I'd like to be able to say something like "cabal install my-hacked-package --as original-package" - are there fundamental reasons that wouldn't be possible, or a bad idea? mark

10 23

ANNOUNCE: haskell-src-meta 0.1.0
by Ben Millwood 19 Aug '10

19 Aug '10

Hello cafe, The haskell-src-meta package was originally written by Matt Morrow, to provide a translation from the syntax tree parsed from String by the haskell-src-exts package to Template Haskell's representation of Haskell source code, making possible a variety of interesting quasiquoters and metaprogramming. However, while the package was still fairly embryonic and incomplete, Matt disappeared from the haskell community, and his work no longer compiles with the latest versions of template haskell and GHC. Many people still had use for it, so there was some discussion about its future in a haskell-cafe thread [1] resulting in a fork [2]. After I and at least one other person duplicated the work of updating haskell-src-meta, ignorant of the fork, it was decided that maintaining forks was a less-than-ideal solution, so I volunteered to take up maintainership of a package that had by now been untouched for some ten months. Hence, haskell-src-meta-0.1.0, from a new maintainer (hello!). The functionality changes in this release are minimal, but nonzero: its primary purpose is to let the original work be used with updated TH. However, there have been some changes and additions since 0.0.6: * The previously somewhat lengthy Language.Haskell.TH.Instances.Lift is now mostly generated with template haskell using the th-lift package. * Contexts, kinds, bang patterns, unboxed word literals, newly supported in template-haskell-2.4.0.0, are used where necessary. * Use of haskell-src-exts is brought in line with newer versions. * Thanks to a patch by Jonas Duregard, a new ToDecs class has been added to handle cases where more than one Dec must be returned. I don't have any big plans for future releases: there is a lot that could do with tidying up, and some small aspects of the syntax conversion that aren't covered, but I mostly just intend to ensure it keeps building and that anyone with patches to contribute has a prompt response. How You Can Help: the easiest way is to tell me that I've put the version constraints too tightly on some dependency or other -- I have erred on the conservative side. Any other comments on the package in general are appreciated too, since this is the first time I've ever uploaded anything to Hackage. There is a github repository at http://github.com/benmachine/haskell-src-meta if you want to submit patches. Yours sincerely, Ben Millwood [1] http://www.haskell.org/pipermail/haskell-cafe/2010-July/080390.html [2] http://hackage.haskell.org/package/haskell-src-meta-mwotton

2 1

String vs ByteString
by Erik de Castro Lopo 19 Aug '10

19 Aug '10

Hi all, I'm using Tagsoup to strip data out of some rather large XML files. Since the files are large I'm using ByteString, but that leads me to wonder what is the best way to handle clashes between Prelude functions like putStrLn and the ByteString versions? Anyone have any suggestions for doing this as neatly as possible? Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

38 160

building ghc on opensolaris
by Günther Schmidt 19 Aug '10

19 Aug '10

Hi, does anybody know why --with-gcc= ... has no effect when passed to ./configure? This is under OpenSolaris only, under Mac OS X for instances it takes. Günther

2 2

Specific to General to Specific
by Edward Z. Yang 19 Aug '10

19 Aug '10

Imagine that I have the following type-class: class (Monad (m n)) => NetworkMonad m n data NT n a instance NetworkMonad NT n data NQ n a instance NetworkMonad NQ n monadicValue :: NetworkMonad m n => m n () It defines some family of phantom monads, in this case NT n and NQ n, and permits some values to inhabit either monad (resolved by the surrounding code). Suppose, however, that there are multiple possible implementations for the pair (NT, NQ). Then we might like to generalize this typeclass with an associated type. (Suppose that our old implementation was g = Int.) {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleContexts #-} class Network g where data GNT g :: * -> * -> * data GNQ g :: * -> * -> * class (Monad (m g n)) => GenericNetworkMonad g m n type NT n a = GNT Int n a type NQ n a = GNQ Int n a genericMonadicValue :: GenericNetworkMonad g m n => m g n () genericMonadicValue = undefined monadicValue :: GenericNetworkMonad Int m n => m Int n () monadicValue = undefined My question is, how can I write a specialized typeclass NetworkMonad that recaptures the original simple interface? monadicValue' :: NetworkMonad m n => m n () monadicValue' = undefined I've tried something like this: type family GetM (x :: * -> * -> *) :: (* -> * -> * -> *) type instance GetM (m g) = m type family GetG (x :: * -> * -> *) :: * type instance GetG (m g) = g class (GenericNetworkMonad (GetG mg) (GetM mg) n) => NetworkMonad mg n where But this doesn't seem to be enough information to do inference with: monadicValue'' :: NetworkMonad m n => m n () monadicValue'' = genericMonadicValue gives the type error: Test.hs:28:17: Could not deduce (GenericNetworkMonad g m n) from the context (NetworkMonad m1 n) arising from a use of `genericMonadicValue' at Test.hs:28:17-35 Possible fix: add (GenericNetworkMonad g m n) to the context of the type signature for `monadicValue''' In the expression: genericMonadicValue In the definition of `monadicValue''': monadicValue'' = genericMonadicValue Test.hs:28:17: Couldn't match expected type `m1' against inferred type `m g' `m1' is a rigid type variable bound by the type signature for `monadicValue''' at Test.hs:27:31 In the expression: genericMonadicValue In the definition of `monadicValue''': monadicValue'' = genericMonadicValu Am I out of luck without superclass equality constraints? Cheers, Edward

2 1

Failed inference: maybe bug?
by Edward Z. Yang 19 Aug '10

19 Aug '10

On the prompting of napping, I humbly submit the following code to haskell-cafe: ezyang@javelin:~/Dev/haskell/generic-typeclass$ cat > Bar.hs {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleContexts #-} data Foo b = Foo class FooTwo m b where type family Ctor a :: (* -> *) type instance Ctor (f a) = f type family CtorVal a :: * type instance CtorVal (f a) = a instance FooTwo Foo Int where class FooTwo (Ctor mb) (CtorVal mb) => FooOne mb where foo :: FooOne (m Int) => m Int foo = fooGeneric fooGeneric :: FooTwo m b => m b fooGeneric = undefined ezyang@javelin:~/Dev/haskell/generic-typeclass$ ghci Bar.hs GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( Bar.hs, interpreted ) Bar.hs:19:6: Could not deduce (FooTwo m Int) from the context (FooOne (m Int)) arising from a use of `fooGeneric' at Bar.hs:19:6-15 Possible fix: add (FooTwo m Int) to the context of the type signature for `foo' or add an instance declaration for (FooTwo m Int) In the expression: fooGeneric In the definition of `foo': foo = fooGeneric Failed, modules loaded: none. Prelude> Leaving GHCi. It seems that GHC is unable to unify the two instances. So, bug or expected behavior? Cheers, Edward

1 0

regex-compat on latest Haskell Platform
by Kevin Jardine 19 Aug '10

19 Aug '10

I'm running Haskel Platform 2010.2.0.0 and attempting to compile with regex-compat under Windows XP. The link stage fails with a number of lines of the form: D:\Program Files\Haskell Platform\2010.2.0.0\lib\extralibs\regex- posix-0.94.2\ghc-6.12.3/libHSregex-posix-0.94.2.a(Wrap.o):fake:(.text +0x493): undefined reference to `regerror' I have googled for a solution but cannot see a fix anywhere. Any suggestions? Kevin

5 15

Re: [Haskell-cafe] Embedded scripting Language for haskell app
by John Lato 18 Aug '10

18 Aug '10

> > From: Hemanth Kapila <saihemanth(a)gmail.com> > Subject: [Haskell-cafe] Embedded scripting Language for haskell app > > Hi, > > Can some one please give me a suggestion on the best choice for an embedded > scripting Language for a haskell application? > I mean, something like guile/lua for c/c++ and groovy/jruby for java. > I'm sure that the best option depends on the features you want from your embedded scripting language. With no prior experience, it took me about 15 minutes to install hint and add scripting support to an application. This involves interpreting an EDSL which uses associated types, so at least that much appears to work without problems. The example file on the hint project website is terse but sufficient. One question I have regards distributing applications using hint. Does the target computer need ghc/libs installed? I think so, but would like to have this confirmed. If so, this might be a good reason to use Lua. John > For quite some time, I've been using a lisp-like interpreter that I > implemented myself. But this is not going too well - going by this road, I > suspect I will end up with a mule. I am looking for a pony (a declarative > programming language). I am okay with a donkey too. > > baskell[1] seems interesting. And there's hslua[2]. > Can one use hint[3] like this ? > > Thanks > Hemanth K > [1] baskell - http://hackage.haskell.org/package/baskell > [2] hslua - http://hackage.haskell.org/package/hslua > [3] hint - http://hackage.haskell.org/package/hint > >

4 3

Re: [Haskell-cafe] Heavy lift-ing
by aditya siram 18 Aug '10

18 Aug '10

I wouldn't-it was a bad example. My only point was that because of the way (>>=) is implemented for lists the order of the arguments 'a' and 'b' in 'liftM2 f a b' matters. -deech On Sat, Jul 24, 2010 at 1:37 AM, Lennart Augustsson <lennart.augustsson(a)gmail.com> wrote: > Why would you expect swapped operands to (-) ? > > > Sent from my iPad > > On Jul 23, 2010, at 20:12, aditya siram <aditya.siram(a)gmail.com> wrote: > >> Lists are non-deterministic, but the function taken by liftM2 does not >> necessarily generate all possible outcomes. In the case of (+) it >> does, not in the case of (-): >> liftM2 (-) [0,1] [2,3] => [0-1,0-2,1-2,1-3] => [-2,-3,-1,-2] >> if all possible cases were generated between the two lists we have to >> include also: >> [2-0,2-1,3-0,3-1] >> >> -deech >> >> On Fri, Jul 23, 2010 at 3:44 PM, Alex Stangl <alex(a)stangl.us> wrote: >>> On Fri, Jul 23, 2010 at 11:43:08AM -0700, michael rice wrote: >>>> What does it mean to "promote a function to a monad?" >>>> >>>> It would seem that the monad values must understand the function that's being promoted, like Ints understand (+). >>>> >>>> Prelude Control.Monad> liftM2 (+) (Just 1) (Just 1) >>>> Just 2 >>>> >>>> But how does one add [0,1] and [0,2] to get [0,2,1,3]? >>> >>> It depends upon the semantics of the particular monad. List monads >>> represent nondeterminism. So, for example, [0,1] represents a 0 or >>> 1, and [0,2] represents a 0 or 2. >>> >>> When you add 0 or 1 to 0 or 2, your possible answers are [0,2,1,3]. >>> >>> Alex >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Different ByteString types
by Henk-Jan van Tuyl 18 Aug '10

18 Aug '10

L.S., I hope someone can help me with the following: I am trying to check the SHA1 hash of a file, using the package cryptohash [0]; the following error message is displayed when I try to compile my program: SHA1check.lhs:35:32: Couldn't match expected type `B.ByteString' against inferred type `Data.ByteString.Internal.ByteString' In the second argument of `(.)', namely `hashlazy' In the second argument of `(.)', namely `B.unpack . hashlazy' In the first argument of `(<$>)', namely `show . B.unpack . hashlazy' The relevant lines of code are: > import qualified Data.ByteString.Lazy as B > import Data.CryptoHash.SHA1 ( hashlazy ) > hash <- show . B.unpack . hashlazy <$> B.hGetContents h There is only one version of the bytestring package installed. Details: - Windows XP - bytestring-0.9.1.7 - cryptohash-0.5.1 - Haskell Platform 2010.2.0.0 Regards, Henk-Jan van Tuyl P.S. A patch is needed if you want to install cryptohash with MinGW [0] http://hackage.haskell.org/package/cryptohash -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

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Anything like Mythryx in Haskell?
by Martin DeMello 18 Aug '10

18 Aug '10

Mythryl [http://mythryl.org/] is an ML dialect that mostly puts an Algolish syntax and some good posix interoperability atop SML/NJ. More than the language (which doesn't seem to be as tastefully designed as Haskell, particularly in terms of syntax - what do people have against MLish syntax anyway?) I like the goals of the associated Mythryx project [http://mythryl.org/my-Mythryx.html] namely, to develop a well-engineered unixy ecosystem in a strongly typed functional language. Has anyone attempted something like this in Haskell? martin

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Re: [Haskell-cafe] Question about memory usage
by Sebastian Fischer 18 Aug '10

18 Aug '10

[CC-ing café again] On Aug 16, 2010, at 5:52 PM, Daniel Fischer wrote: >> I am a bit concerned about the memory usage. > > Of your implementation of the matrix power algorithm? Yes. > Making the fields of Matrix strict should help: > > data Matrix a = Matrix !a !a !a Making all fields strict makes memory usage worse. When making only the first field strict, memory usage hardly differs (from the current implementation without strictness annotations). Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)

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Re: [Haskell-cafe] Re: Handling absent maintainers
by Antoine Latter 18 Aug '10

18 Aug '10

It's a bad call to put any version restriction on the template haskell package in your package description - the cabal solver will still go for the latest version of your package and then try to install the listed version of template haskell from Hackage, which leads to confusing errors. Even if you try to use the version of template haskell to set a flag in your package description, cabal-install will always form an install plan based on the default value of your flag (leaving someone out). Antoine On Aug 18, 2010 3:59 AM, "Jonas Almström Duregård" <jonas.duregard(a)gmail.com> wrote: > I had been in contact with the maintainer of the > package who has prepared a version 0.5 and was ... I can upload a working .4 version of th-lift if needed, but I guess it might be undesirable to depend on it if 0.5 changes the API. I'll have a look at your version and see if I can merge my additions into it. For now, perhaps somebody should re-upload the current version of src-meta with the correct dependencies (TH<2.4)? 2010/8/17 Ben Millwood <haskell(a)benmachine.co.uk>: > 2010/8/17 Jonas Almström Duregård <jonas.duregard(a)gmail.com>: >> Hi, >> >> Has there been any prog...

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Embedded scripting Language for haskell app
by Hemanth Kapila 18 Aug '10

18 Aug '10

Hi, Can some one please give me a suggestion on the best choice for an embedded scripting Language for a haskell application? I mean, something like guile/lua for c/c++ and groovy/jruby for java. For quite some time, I've been using a lisp-like interpreter that I implemented myself. But this is not going too well - going by this road, I suspect I will end up with a mule. I am looking for a pony (a declarative programming language). I am okay with a donkey too. baskell[1] seems interesting. And there's hslua[2]. Can one use hint[3] like this ? Thanks Hemanth K [1] baskell - http://hackage.haskell.org/package/baskell [2] hslua - http://hackage.haskell.org/package/hslua [3] hint - http://hackage.haskell.org/package/hint

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