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Hierarchical modules issue
by Jonathan Geddes 19 Jul '10

19 Jul '10

When I'm developing Haskell code I like to frequently compile the file I'm currently working on so that the compiler will tell me if and where I've made mistakes. In my text editor I have a key mapped to do just that. I have another key mapped to fire up ghci with the current file so I can play around with the functions I've defined. I find this workflow to be very productive. I recently reorganized a project to take advantage of hierarchical modules and it broke my workflow. Now when I'm working in a subdirectory, say Foo where the modules are named Foo.Bar, Foo.Baz, Foo.etc., and I try to compile Bar.hs which imports Foo.Baz, the compiler looks for the file Foo/Baz.hs. The problem is that I'm already in the directory Foo and so the compiler fails to find the file for the module Foo.Baz. If I compile the whole project from the root project directory where the main file is, the compiler finds every module just fine. It seams very strange to me that when the compiler is compiling a module named Foo.Bar and there is an import for Foo.Baz, the compiler doesn't look in the directory where Foo.Bar is located (the current dir)! My temporary soluation has been to create symlinks to help the compiler find what it's looking for. For example in the Foo directory I have a symlink to that same Foo directory so that when compiling Bar.hs, the compiler finds Foo/Baz.hs when it looks for it. However, this seems like a dirty hack and it's not portable besides. Is this a known issue? Is there a better solution than mine? Is there something I'm missing? --Jargv

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Refactoring type-class madness
by Andrew Webb 19 Jul '10

19 Jul '10

Hi there haskellers, I have thoroughly confused myself with type-classes in a haskell system I am writing, and I was wondering if anyone had some useful suggestions to get me out of my mess. I apologise if this is all long and rambling, but that maybe why I can't solve it... The system itself performs analyses on data from human eye-tracking experiments, where the tracking data is simplified from raw positional data into "events" which are either fixations or saccades. This is easy to do: data Event = Fixation {...} | Saccade {...} Because, at the basic level all of the experiments share this type of data, it seems that I should be able to write analysis functions that work for any experiment. However, the experiments differ in the stimuli used, and associated with each stimulus set is a set of "milestones" that give times at which important things happen in the stimuli, and "regions of interest" that give areas of the visual scene that are considered important. For a single experiment I would have: data Experiment = Exp [Trial] data Trial = Trial [Event] (Map MileStone Time) data MileStone = M1 | M2 | ... and the analysis functions can take an experiment, use the events and the milestones, and return whatever. The problem arises when I want to represent a second experiment. I assumed the way to solve this was with a type class: class ExperimentClass a data ExperimentOne = Exp [Trial] instance ExperimentClass ExperimentOne and have the analysis functions take (ExperimentClass a) => a -> ... This, however, necessitates a change to Trial as well, because the MileStones for each experiment are different: class MileStoneClass a data (MileStoneClass a) => Trial a = Trial [Event] (Map a Time) which means that: data (MileStoneClass a) => ExperimentOne a = Exp [Trial a] and maybe even: class (MileStoneClass b) => ExperimentClass a b instance ExperimentClass ExperimentOne ExOneMileStones which all seems fine. But then I need to add regions of interest, and a corresponding type parameter, and a couple of more things with type parameters, and I get something like: class (MileStoneClass b, RegionClass c, ... d, ... e) => ExperimentClass a b c d e which is a lot of type parameters that are all dependent on a, since the experiment defines the milestones etc. I can use fundeps to enforce that in the type system, but it is still quite messy. So, I was wondering whether there was something wrong with my basic model which leads to this ugly type class, or whether this is the proper way forward. Either is fine, really, it would just be nice to know for certain. Thanks in advance, Andrew

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Docs on the current and future constraint solver?
by Corey O'Connor 18 Jul '10

18 Jul '10

I believe I have run headlong into issue #3064 in ghc (http://hackage.haskell.org/trac/ghc/ticket/3064) All I think I know is this: * this is a performance issue with the system used to solve type constraints. * the solver is undergoing an overhaul to resolve performance issues in addition to other issues. * An efficient constraint solver is difficult. NP-Complete in the general case? Beyond that I'm at a loss. What can I read to understand the constraint satisfaction problem as it stands in GHC? Is there a paper on the implementation of the current solver? Anything on how the new solver will differ from the current? I think I located one page on the new solver: http://hackage.haskell.org/trac/ghc/wiki/TypeFunctionsSolving Cheers, Corey O'Connor

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Fwd: [Haskell-cafe] Bytestrings and [Char]
by Alberto G. Corona 18 Jul '10

18 Jul '10

The question can be generalized via type classes: Is there any common set of primitives encapsulated into a single type class that has instances for Strings (Data.List) ByteStrings, Data.Text, Lazy bytestrings, Arrays, vectors and wathever container that can store an boxed, unboxed, packed unpacked sequence of wathever including chars? All of them have folds, heads, tails and a lot of common functions with the same name, but since there is not a single type class, the library programmer can not abstract his code when it is possible, so the library user can chose the particular instance for his particular problem. 2010/3/23 Johann Höchtl <johann.hoechtl(a)gmail.com> On 22.03.2010 14:15, Ivan Miljenovic wrote: > >> On 23 March 2010 00:10, Johan Tibell<johan.tibell(a)gmail.com> wrote: >> >> >>> A sequence of bytes is not the same thing as a sequence of Unicode >>> code points. If you want to replace String by something more efficient >>> have a look at Data.Text. >>> >>> >> Though Data.Text still has the disadvantage of not being as nice to >> deal with as String, since you can't pattern match on it, etc. >> >> Whilst it may degrade performance, treating String as a list of >> characters rather than an array provides you with greater flexibility >> of how to deal with it. >> >> >> > But doesn't this basically mean sacrifice performance / applicability for > algorithmic beauty? Sure, pattern match comes in handy, but in the case of > strings at a high price > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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ANNOUNCE: container-classes-0.0.0.0
by Ivan Lazar Miljenovic 18 Jul '10

18 Jul '10

I'm pleased to announce the fruits of my labours during AusHac [1]: version 0.0.0.0 of the container-classes library [2] I was talking about in my last blog post [3]. [1]: http://www.haskell.org/haskellwiki/AusHac2010 [2]: http://hackage.haskell.org/package/container-classes [3]: http://ivanmiljenovic.wordpress.com/2010/07/14/data-oriented-hierarchies/ The purposes of this library is two-fold: * As a library writer, let your users choose which data structure they want to use rather than whichever one you decided upon when writing the library (e.g. you use lists predominantly, but your users may want to use Sets; by outputting a generic Container value yoru users don't need to keep using Set.fromList and Set.toList). * When writing your own code and you realise that you're using the wrong data structure for your purposes, if you used this library (correctly) then the amount of work you need to do to swap data structures is minimised. This current version is only a first draft. As such, I would appreciate feedback on the API. Currently, it provides its own definitions of all list-oriented functions in the Prelude plus a few more (e.g. partition), and list instances of all the classes. In terms of definitions, most class methods have default definitions in terms of fold. Furthermore, as much as possible I've defined them using a build/fold setup outlined in the paper "A Short Cut to Deforestation" by Andrew Gill, John Launchbury and Simon Peyton Jones (however I haven't yet defined the rules to get this used yet). However, the current list instances use the pre-defined versions in the Prelude and Data.List; I'm not sure if I should keep it this way or - where equivalent - use the pre-defined ones so that hopefully conversions from lists to other data types would also be removed without needing intermediary data structures. I also plan on adding more instances and a Lookup class for Maps, etc. However, we have to start packing up the room now and so my hacking is at an end for this weekend :( -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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DpH/repa cache locality
by Gregory Crosswhite 18 Jul '10

18 Jul '10

Hey everyone, Just out of curiosity, what work is being done in the data parallel haskell / repa projects regarding cache locality? The reason I am asking is because, as I understand it, the biggest bottleneck on today's processors are cache misses, and the reason why optimized platform-specific linear algebra libraries perform well is because they divide the data into chunks that are optimally sized for the cache in order to maximize the number of operations performed per memory access. I wouldn't expect data parallel haskell/repa to automatically know what the perfect chunking strategy should be on each platform, but are there any plans being made at all to do something like this? (To be explicit, this isn't meant as a criticism; I'm just curious and am interested in seeing discussion on this topic by those more knowledgeable than I. :-) ) Thanks! Greg

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Best links for Haskell Platform distro packages?
by Don Stewart 18 Jul '10

18 Jul '10

Can distro maintainers confirm these are the best links for each distro package? Debian http://packages.debian.org/squeeze/haskell-platform (or should it be sid?) Fedora: https://admin.fedoraproject.org/pkgdb/acls/name/haskell-platform Gentoo: http://www.haskell.org/haskellwiki/Gentoo/HaskellPlatform NixOS: http://hydra.nixos.org/job/nixpkgs/trunk/haskellPackages_ghc6121.haskellPla… OpenSUSE: https://build.opensuse.org/project/show?project=devel:languages:haskell ?? For Ubuntu, I find a few confusing things: Ubuntu: https://launchpad.net/ubuntu/+source/haskell-platform But nothing at: http://packages.ubuntu.com/lucid/haskell-platform But there's instructions for the src build: http://davidsiegel.org/haskell-platform-in-karmic-koala/ -- Don

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RE: Design for 2010.2.x series Haskell Platform site (Don Stewart)
by Niemeijer, R.A. 18 Jul '10

18 Jul '10

Here's my take on the new design: Screenshot: http://imgur.com/9LHvk.jpg Live version: http://dl.dropbox.com/u/623671/haskell_platform_redesign/index.htm

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unusual behavior from cabal
by Michael Litchard 18 Jul '10

18 Jul '10

Not sure what the correct list is for this observation. I was trying to install gitit, and here is what happened. michael@michael:~/haskell/blog-example$ cabal install gitit Resolving dependencies... cabal: dependencies conflict: happstack-server-0.5.1 requires time ==1.1.4 however time-1.1.4 was excluded because happstack-server-0.5.1 requires time ==1.2.0.3 huh? Cabal seems to think happstack-server-0.5.1 has two conflicting requirements. How do I suss this out?

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ANN: hledger 0.11
by Simon Michael 18 Jul '10

18 Jul '10

hledger 0.11 is released! Thanks to all testers and to Michael Snoyman for much help keeping up with Yesod. Best, -Simon home: http://hledger.org Release notes: 2010/07/17 hledger 0.11 ........................ * split --help, adding --help-options and --help-all/-H, and make it the default command * use "journal" instead of "ledger file"; default suffix is .journal, default file is ~/.journal * auto-create missing journal files rather than giving an error * new format-detecting file reader (mixed journal transactions and timelog entries are no longer supported) * work around for first real-world rounding issue (test zero to 8 decimal places instead of 10) * when reporting a balancing error, convert the error amount to cost * parsing: support double-quoted commodity symbols, containing anything but a newline or double quote * parsing: allow minus sign before commodity symbol as well as after (also fixes a convert bug) * parsing: fix wrong parse error locations within postings * parsing: don't let trailing whitespace in a timelog description mess up account names * add: allow blank descriptions * balance: --flat provides a simple non-hierarchical format * balance: --drop removes leading account name components from a -- flat report * print, register, balance: fix layout issues with mixed-commodity amounts * print: display non-simple commodity names with double-quotes * stats: layout tweaks, add payee/description count * stats: don't break on an empty file * stats: -p/--period support; a reporting interval generates multiple reports * test: drop verbose test runner and testpack dependency * web: a new web ui based on yesod, requires ghc 6.12; old ghc 6.10- compatible version remains as -fweb610 * web: allow wiki-like journal editing * web: warn and keep running if reloading the journal gives an error * web: --port and --base-url options set the webserver's tcp port and base url * web: slightly better browser opening on microsoft windows, should find a standard firefox install now * web: in a web-enabled build on microsoft windows, run the web ui by default Stats: 55 days and 136 commits since last release. Now at 5552 lines of code with 132 tests and 54% unit test coverage.

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Cabal lib (Setup.hs), --package-db, --enable-shared, haskell platform
by Albert Y.C.Lai 17 Jul '10

17 Jul '10

I accidentally found a rarely encountered omission in Cabal (the lib, because via Setup.hs) by building Haskell Platform as shared libs from source. It is rare because you have to use both --enabled-shared and --package-db=blah together to run into it. --package-db=blah is already rare enough (probably only Haskell Platform's build script uses it); --enable-shared is my addition to make it even rarer. (Also I have only tried this on linux x86 32-bit.) Following the essence of scripts/build.sh plus my own --enable-shared: Create an empty lookaside package database with: echo '[]' > /tmp/lookaside Install a package X that depends only on base (say), ask for shared libs, register in /tmp/lookaside only: ./Setup configure --package-db=/tmp/lookaside \ --ghc-pkg-option=--package-conf=/tmp/lookaside \ --enable-shared ./Setup build ./Setup register --inplace Install another package Y that depends on base and X only (say), ask for shared libs. There is a problem now related to X known to /tmp/lookaside only, but not known to the global or the user database: ./Setup configure --package-db=/tmp/lookaside \ --ghc-pkg-option=--package-conf=/tmp/lookaside \ --enable-shared ./Setup build -v2 Now it bombs. As the verbose build messages show, there is no problem creating the static *.a version; it bombs when creating the shared *.so version, and it is because it can't find X, and it is because -package-conf=/tmp/lookaside is not given to ghc at the *.so stage. (Though, the *.dyn_o stage works correctly.) My current fix is: ./Setup configure --package-db=/tmp/lookaside \ --ghc-pkg-option=--package-conf=/tmp/lookaside \ --ghc-option=-package-conf=/tmp/lookaside \ --enable-shared I surely hope we no longer need --ghc-pkg-option and --ghc-option in a week! :)

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Re: [Haskell-cafe] is this a bug ?
by gate03＠landcroft.co.uk 17 Jul '10

17 Jul '10

On Sat 17/07/10 04:17 , Alexander Solla ajs(a)2piix.com sent: > Why are you performing unsafe IO actions? They don't play nice > with laziness. OK, fair cop, but without the unsafe IO action, it still misbehaves. http://hpaste.org/fastcgi/hpaste.fcgi/view?id=27650 Michael.

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Re: in-equality type constraint?
by oleg＠okmij.org 17 Jul '10

17 Jul '10

Ryan Ingram wrote: > But it doesn't generalize; you need to create a witness of inequality > for every pair of types you care about. One can do better, avoiding the quadratic explosion. One merely needs to establish a map from a type to a suitable, comparable representation -- for example, to a type level list of type level numerals. Comparing types for equality, inequality and even order is a simple matter of comparing their representations. The fact that types become totally ordered lets us even implement type-level maps: Data.Map on types. (We may need a Type.* module hierarchy.) That idea was described in the HList paper http://homepages.cwi.nl/~ralf/HList/paper.pdf Section 9. The code is still available, http://code.haskell.org/HList/examples/TTypeable.hs see also TypeEqExplosive.hs, TypeEqTTypeable.hs. > But given the use of UndecidableInstances and OverlappingInstances, I > was hoping that type families could come a little cleaner. Normally if the use of functional dependencies requires UndecidableInstances, type families would ask for them too. As to OverlappingInstances -- that is the key to generic inequality, isn't it? Given the set of pairs of types, the set of the elements representing non-equal types is the complement of the set of elements representing equal types (by equal I mean identical). Overlapping instances is the way to express set complementation. The most general instance is chosen when none of the more specific apply. The set of types chosen by that general instance is the complement for the set of types chosen by the specific instances. > Does "TypeEq a c HFalse" imply proof of inequality, or unprovability > of equality? We are all constructivists here... If 'a' and 'c' are type variables, (TypeEq a c HFalse) is the constraint -- proof obligation if you will -- making sure the variables will never be instantiated to identical types. Strictly speaking, the constraint is discharged if 'a' and 'c' are two ground, and syntactically distinct (non-identical) types. In reality, I think GHC is able to discharge the constraint if 'a' and 'c' are grounded ``sufficiently enough'' for the difference to become apparent (e.g., the head constructors are different).

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ghc api printing of types
by Phyx 17 Jul '10

17 Jul '10

I was wondering how given a Type I can get a pretty printed type out of it. I'm currently using showSDocUnqual . pprType . snd . tidyOpenType emptyTidyEnv But this has the problem that predicates don't get printed, anyone know how GHCi does this? Thanks, Phyx

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Plotting Vectors with GnuPlot Wrapper
by Maryam Moghadas 17 Jul '10

17 Jul '10

Hi When I use Vectors as a PlotStyle in Graphics.Gnuplot.Simple, the output curve.gp and curve0.csv is not generated correctly. For example when I write in ghci: plotPathStyle [] (PlotStyle Vectors (DefaultStyle 1)) [(1,1),(2,7)] the output files are: curve.gp: plot "curve0.csv" using 1:2 with vectors linestyle 1 curve0.csv: 1, 1 2, 7 But they must be: curve.gp: plot "curve0.csv" using 1:2:3:4 with vectors linestyle 1 curve0.csv: 1, 1, 2, 7 Is there any option that should I use to correct this behavior? Regards Mary

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Hot-Swap with Haskell
by Andy Stewart 17 Jul '10

17 Jul '10

Hi all, I'm research to build a hot-swap Haskell program to developing itself in Runtime, like Emacs. Essentially, Yi/Xmonad/dyre solution is "replace currently executing" technology: re-compile new code with new binary entry when re-compile success $ do save state before re-launch new entry replace current entry with new binary entry (executeFile) store state after re-launch new entry There are some problems with re-compile solution: 1) You can't save *all* state with some FFI code, such as gtk2hs, you can't save state of GTK+ widget. You will lost some state after re-launch new entry. 2) Sometimes re-execute is un-acceptable, example, you running some command in temrinal before you re-compile, you need re-execute command to restore state after re-launch, in this situation re-execute command is un-acceptable. I wonder have a better way that hot-swapping new code without re-compile/reboot. Thanks, -- Andy

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in-equality type constraint?
by Paul L 17 Jul '10

17 Jul '10

Does anybody know why the type families only supports equality test like a ~ b, but not its negation? -- Regards, Paul Liu Yale Haskell Group http://www.haskell.org/yale

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ANNOUNCE: feldspar 0.3
by Emil Axelsson 17 Jul '10

17 Jul '10

On behalf of the Feldspar team, I'm happy to announce a new release of the embedded language Feldspar and its C code generator: http://hackage.haskell.org/package/feldspar-language http://hackage.haskell.org/package/feldspar-compiler The main changes in 0.3 are: * Signed/unsigned integers of different bit widths (8, 16 and 32). * Fixed-point numbers. * Slight change of the vector library interface: All intermediate vectors are guaranteed to be removed. * A new Stream data type. * Support for user-defined types. * Tracing functions. * Limited support for code generations for the TMS320C64x chip family. * Bugfixes. For more information, see: http://feldspar.inf.elte.hu/feldspar/ / Emil

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ANNOUNCE: smt-lib-0.0.1
by Tom Hawkins 17 Jul '10

17 Jul '10

smt-lib [1] is a library for reading and writing SMT-LIB [2] files via Haskell. SMT-LIB is a common language and benchmark suite used by most SMT solvers. Currently the library supports the full SMT-LIB version 2 syntax. However at this time, only command scripts -- not responses -- can be parsed. Comments, bug reports, or suggestions for improvements welcome. -Tom [1] http://hackage.haskell.org/package/smt-lib [2] http://goedel.cs.uiowa.edu/smtlib/

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lambda calculus and equational logic
by Patrick Browne 17 Jul '10

17 Jul '10

Hi, In Haskell what roles are played by 1)lambda calculus and 2) equational logic? Are these roles related? Hopefully this question can be answered at a level suitable for this forum. Thanks, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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is this a bug ?
by gate03＠landcroft.co.uk 17 Jul '10

17 Jul '10

http://hpaste.org/fastcgi/hpaste.fcgi/view?id=27631 The comments in the code explain the problem. If ".Lazy" be removed from the code (occurs three times), i.e., the code is changed to strict byte strings, it works as expected. Michael Mounteney.

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passing a polymorphic function as a parameter vs using it directly in a function definition
by Pasqualino "Titto" Assini 17 Jul '10

17 Jul '10

Hi, can anyone please explain why in the following code evalAST compiles while evalAST2 doesn't?: Is that because the polymorphic function k is specialised in two different ways in evalAST while in evalAST2 it is constrained to be the same function? {-# LANGUAGE GADTs #-} test = evalAST (TxtA "abc") -- This is OK evalAST :: AST -> IO () evalAST (IntA i) = k $ Lit i evalAST (TxtA i) = k $ Lit i k :: (Show a) => Expr a -> IO () k e = print $ eval e -- This is the same thing, only the k function is passed as a parameter. -- But it won't compile. -- I would expect its type to be: -- evalAST2 :: (Expr a -> IO()) -> AST -> IO () -- But is actually: -- evalAST2 :: (Expr Int -> IO ()) -> AST -> IO () evalAST2 k (IntA i) = k $ Lit i -- evalAST2 k (TxtA i) = k $ Lit i -- The untyped expression data AST = IntA Int | TxtA String -- A typed expression. data Expr a where Lit :: a -> Expr a eval :: Expr a -> a eval (Lit i) = i Thanks titto

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lists of arbitrary depth
by vadali 16 Jul '10

16 Jul '10

hello, iam really new to haskell, i want to define a function which takes as a parameter a list which can contain other lists, eg. [1,[2,3],[4,[5,6]]] how would i define a function that can iterate through the items so (in this example) iter1 = 1 iter2 = [2,3] iter3 = [4,[5,6]] ? ( can i do that without using the Tree data type? ) THANKS! -- View this message in context: http://old.nabble.com/lists-of-arbitrary-depth-tp29139641p29139641.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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trees and pointers
by Sergey Mironov 16 Jul '10

16 Jul '10

Hi cafe! I have a question of C-to-Haskell type:) Imagine web application wich allows users to browse some shared filesystem located at the server. Application stores every users's position within that filesystem (current directory or file). In C this can be implemented with the help of following data types: struct tree_node { union item { // some file data struct file *file; // struct dir has link to another list of tree_node struct dir *dir; }; int type; // List of tree_nodes struct tree_node *next; struct tree_node *prev; }; struct user { struct tree_node *position; // List of users struct user *next; struct user *prev; }; This implementation will give us 1) O(1) time to insert to shared tree 2) O(1) time to access user's current position Is it possible to reach this requirements in haskell? For example, managing distinct tree type like data TreeNode = File | Dir [TreeNode] will lead to failure of req. 2 (have to traverse this tree to find each user's position). Also one could manage several zipper types (one for every user): data TreeNodeCtx = Top | TreeNodeCtx { left :: [TreeNode], right :: [TreeNode], up :: TreeNodeCtx } data TreeNodeZ = TreeNodeZ { ctx :: [TreeNodeCtx] pos :: TreeNode } It works for one user but not for many because of req. 1 (have to insert new item into several zippers). Any ideas? -- Thanks, Sergey

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Re: [Haskell-cafe] Hot-Swap with Haskell
by austin seipp 16 Jul '10

16 Jul '10

Sorry Andy! CC'ing to the rest of -cafe in case anybody notices (I need to stop haskelling so early in the morning...) On Fri, Jul 16, 2010 at 8:59 AM, austin seipp <as(a)0xff.ath.cx> wrote: > You also may like one project I wrote, an IRC bot that used hs-plugins > to do hot code reloading (only works on GHC 6.8.) Inspired by > lambdabot, it could swap the entire running program as well as its own > plugins, on the fly and maintain state pretty successfully. I haven't > touched the source code in a long time - it was one of my first bigger > Haskell projects after reading Dons paper about yi. I tried a bunch of > things with various versions of it. > > If I wrote the code again today (providing plugins worked) it would > likely be 1/10th the size and more manageable as I have become much > better at Haskell, but oh well. Looking over it now though, most of > the code is pretty simple. :) > > http://code.haskell.org/infinity/src/ > > On Fri, Jul 16, 2010 at 8:04 AM, Andy Stewart <lazycat.manatee(a)gmail.com> wrote: >> Bartek Ćwikłowski <paczesiowa(a)gmail.com> writes: >> >>> Hello Andy, >>> >>> 2010/7/16 Andy Stewart <lazycat.manatee(a)gmail.com>: >>> >>>> There are some problems with re-compile solution: >>>> >>>> 1) You can't save *all* state with some FFI code, such as gtk2hs, you >>>> can't save state of GTK+ widget. You will lost some state after >>>> re-launch new entry. >>> >>> For my 2008 GSOC application I wrote a demo app that used hot code >>> reloading and it maintained gtk state just fine. The code (and video) >>> is available at http://paczesiowa.dw.pl/ , it worked in ghc 6.8 and >>> needed hs-plugins, so it won't work now, but the approach should work. >> Wow, thanks for your hint, i will wait dons fix hs-plugins then try this >> cool feature.. >> >> Thanks, >> >> -- Andy >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > > > -- > - Austin > -- - Austin

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Haskell Platform dependency hell
by Kevin Jardine 16 Jul '10

16 Jul '10

I have Haskell Platform 2010.1.0.0 installed under Windows and I often find that Haskell breaks if I try upgrading some of the current modules. For example, after using cabal to upgrade to the latest version of Network.CGI, I can no longer compile any code and instead get this message: cabal.exe: dependencies conflict: ghc-6.12.1 requires array ==0.3.0.1 however array-0.3.0.1 was excluded because ghc-6.12.1 requires array ==0.3.0.0 The only way I have found to fix this problem in the past has been to uninstall the Haskell Platform and start over. Can anyone tell me: a. why does this occur, and b. what can I do to stop it from occurring again? Is it the case that the Haskell Platform is simply not upgradable and that I need to wait for the next release? Kevin

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ANNOUNCE: highlighting-kate 0.2.7
by John MacFarlane 16 Jul '10

16 Jul '10

I'm pleased to announce a new release of highlighting-kate [1], a syntax highlighting library based on syntax definitions from the kate editor. !! Important note !! The new release uses new, two-letter class names in its HTML output. If you use highlighting-kate, you will need to update your css files. Three sample css files can be found in the css directory of the source. This release also incorporates improvements to the kate syntax definition for Haskell by Nicolas Wu. There are two minor API changes: 1. A new function, languagesByFilename, has been added. This will give better results than languagesByExtension for things like "Makefile" or "CMake.txt". 2. A new formatting option, OptDetailed, has been added. Use this only if you need access to detailed, language-specific lexical information, and not generic styles. [1]: http://hackage.haskell.org/package/highlighting-kate

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point-free ADT pattern matching ?
by Alexey Karakulov 16 Jul '10

16 Jul '10

I wonder if pattern matching could be less verbose. Maybe this sounds weird, but here is example of what I mean: > type A = (Int, String) > > f :: String -> A -> A > f s (i,s') = (i, s ++ s') > > data B = B Int String deriving Show > >g :: String -> B -> B >g s (B i s') = B i $ s ++ s' Types A/B and functions f/g are quite similar: (x :: A) or (x :: B) means that x contains some integer and string values, and f/g functions take some string and prepend it to the string part of x. The code for f and g has the same level of verbosity, but -- ta-dah! -- we can use arrows and define f in a highly laconic manner: > import Control.Arrow > f' :: String -> A -> A > f' = second . (++) So my queastion is how I could define (g' :: String -> B -> B) in the same way.

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Haddock anchors
by Mark Lentczner 16 Jul '10

16 Jul '10

I've been re-working on the Haddock HTML back end to get it to generate "semantic markup" and legal XHTML. One of the problems is that the anchors that Haddock currently generate aren't always legal in HTML, XHTML, or XML. I'd like to fix the anchor generation so that they are. If I do, then links between old and new generated Haddock pages will land on the right page, but won't always get to the right spot on the page. Will this be a problem for anyone? On one's own machine, I imagine we can come up with a simple script that will just rebuild all the Haddock docs and that will take care of it. What about on Hackage? Eventually, as packages update, the issue will diminish. Can we just bulk re-build the haddock for the latest rev of all packages? - Mark Mark Lentczner http://www.ozonehouse.com/mark/ IRC: mtnviewmark

3 2

cryptohash and an incremental API
by Thomas M. DuBuisson 16 Jul '10

16 Jul '10

Vincent, Due to spam-like comments on -cafe I hadn't been subscribed for a while and missed your cryptohash discussion! Particularly: > The main reason for this library is the lack of incremental api exposed by > current digest libraries, and filling the void about some missing digest > algorithms; Also the speed comes as a nice bonus. I've been working on a new crypto library specifically to provide a unified API for packages implementing cryptographic algorithms. You can see the discussions on libraries(a)haskell.org [1] [2]. Please feel free to take a look, comment, contribute, and hopefully move to the interface. I should be finishing up BlockCipher modes and adding hash tests soon. Cheers, Thomas [1] http://www.haskell.org/pipermail/libraries/2010-May/013688.html [2] http://www.haskell.org/pipermail/libraries/2010-June/013782.html

3 3

Expression dye
by Andrew Coppin 15 Jul '10

15 Jul '10

I'm trying to write a function that builds a series of results in a very complicated way. Eventually I ended up writing things like > newtype Dye = Dye String deriving (Eq, Show) > > instance Num Dye where > (Dye x) + (Dye y) = Dye (x ++ " + " ++ y) > (Dye x) - (Dye y) = Dye (x ++ " - " ++ y) > (Dye x) * (Dye y) = Dye (x ++ " * " ++ y) > abs (Dye x) = Dye ("abs " ++ x) and so on. In this way, you can do something like > sum [Dye "x", Dye "y", Dye"z"] and get "0 + x + y + z" as the result. (In reality you probably want to keep track of bracketing and so forth.) In this way, you can take functions that accept any Num instance and feed the "dye" through them to see what they're actually computing on a given run. Has anybody ever put anything like this on Hackage? I'd prefer to not invent this stuff if somebody has already done it... (The small problem with the approach above, of course, is that as soon as the function wants to do comparisons or take flow control decisions, you've got trouble. It's not impossible to solve, but it *is* a lot of work...)

4 8

IFL 2010: 3rd CFP and 1st Call for Participation
by Don Stewart 14 Jul '10

14 Jul '10

3RD CALL FOR PAPERS and 1ST CALL FOR PARTICIPATION 22nd Symposium on Implementation and Application of Functional Languages (IFL 2010) September 1-3, 2010 Utrecht University Alphen aan den Rijn, The Netherlands http://www.cs.uu.nl/ifl2010 ******************************************************************************************* ** Submission and Registration are now both OPEN ** ** If you intend to participate in IFL 2010, help us by registering as soon as possible. ** ** ** ** Submission closes July 25, Registration closes August 1 ** ******************************************************************************************* After a first successful visit to the USA, the Symposium on Implementation and Application of Functional Languages returns to Europe for its 22nd edition. The hosting institution is Utrecht University in the Netherlands, although the conference itself will take place in the ornithological theme park Avifauna in Alphen aan den Rijn, situated conveniently close to Schiphol (Amsterdam Airport). The symposium dates are September 1-3, 2010. The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2010 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Following the IFL tradition, IFL 2010 will use a post-symposium review process to produce formal proceedings which will be published by Springer Verlag in the Lecture Notes in Computer Science series. All participants in IFL 2010 are invited to submit either a draft paper or an extended abstract describing work to be presented at the symposium. At no time may work submitted to IFL be simultaneously submitted to other venues. Here we follow the ACM Sigplan republication policy as defined on http://www.sigplan.org/republicationpolicy.htm. The submissions will be screened by the program committee chair to make sure they are within the scope of IFL, and will appear in the draft proceedings distributed at the symposium. Submissions appearing in the draft proceedings are not peer-reviewed publications. After the symposium, authors will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full article for the formal review process. These revised submissions will be reviewed by the program committee using prevailing academic standards to select the best articles, which will appear in the formal proceedings. INVITED SPEAKER Johan Nordlander of Lulea University, the designer and developer of the Timber language, is the invited speaker at IFL 2010. Timber is a functional programming language that draws some of its concepts from object-oriented programming, and has built-in facilities for concurrent execution. The language is specifically targeted at implementing real-time embedded systems. TOPICS IFL welcomes submissions describing practical and theoretical work as well as submissions describing applications and tools. If you are not sure that your work is appropriate for IFL 2010, please contact the PC chair at jur(a)cs.uu.nl. Topics of interest include, but are not limited to: language concepts type checking contracts compilation techniques staged compilation runtime function specialization runtime code generation partial evaluation (abstract) interpretation generic programming techniques automatic program generation array processing concurrent/parallel programming concurrent/parallel program execution functional programming and embedded systems functional programming and web applications functional programming and security novel memory management techniques runtime profiling and performance measurements debugging and tracing virtual/abstract machine architectures validation and verification of functional programs tools and programming techniques industrial applications of functional programming PAPER SUBMISSIONS Prospective authors are encouraged to submit papers or extended abstracts to be published in the draft proceedings and to present them at the symposium. All contributions must be written in English, conform to the Springer-Verlag LNCS series format and not exceed 16 pages. The draft proceedings will appear as a technical report of the Department of Computer Science of Utrecht University. SPONSORS IFL 2010 is sponsored by Microsoft Research. As a result we can offer decreased participation fees for Master students and PhD students who plan to attend or present at IFL 2010. PETER LANDIN PRIZE The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honored article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 Euros. IMPORTANT DATES Draft proceedings submission deadline July 25, 2010 Registration deadline August 1, 2010 IFL 2010 Symposium September 1-3, 2010 Submission for review process deadline October 25, 2010 Notification Accept/Reject December 22, 2010 Camera ready version February 17, 2011 PROGRAM COMMITTEE Jost Berthold University of Copenhagen (DIKU), Denmark Olaf Chitil University of Kent, UK John Clements California Polytechnic State University, USA Matthew Fluet Rochester Institute of Technology, USA Andy Gill Kansas University, USA Jurriaan Hage (Chair) University of Utrecht, Netherlands Bastiaan Heeren Open University, Netherlands Ralf Hinze University of Oxford, UK John Hughes Chalmers University of Technology, Sweden Yukiyoshi Kameyama University of Tsukuba, Japan Gabriele Keller University of New South Wales, Australia Pieter Koopman Radboud University Nijmegen, Netherlands Luc Maranget INRIA, France Simon Marlow Microsoft Research, UK Marco T. Morazán Seton Hall University, USA Rex Page University of Oklahoma, USA Ricardo Peña Universidad Complutense de Madrid, Spain Sven-Bodo Scholz University of Hertfordshire, UK Tom Schrijvers Catholic University of Leuven, Belgium Don Stewart Galois, USA Wouter Swierstra Vector Fabrics, Netherlands Don Syme Microsoft, UK Peter Thiemann University of Freiburg, Germany Phil Trinder Heriott-Watt University, Scotland Janis Voigtländer University of Bonn, Germany Viktória Zsók Eötvös Loránd University, Hungary

1 0

Call for Copy: Monad.Reader Issue 17
by Brent Yorgey 14 Jul '10

14 Jul '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 will be: **Friday, October 1, 2010** 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.

1 0

Comments on Haskell 2010 Report
by Alex Stangl 14 Jul '10

14 Jul '10

based upon final version PDF created 7/6/2010 11:44:27 1. I.E. and e.g. should be followed by commas -- unless UK usage differs from US standards. (Page 3 and elsewhere, although FFI chapter seems to have the commas.) Also, inconsistent mix of British and American usage throughout (-ize and -ise, flavour, behaviour, behavior, analyse, color, colour, finaliser, finalizer, etc.) 2. Inconsistent spelling (lowercase/uppercase in section 1.4; lower-case/upper-case in section 2.4) 3. In section 3.11, boolean misspelled boolen. 4. LaTeX macros involving @ showing up in text. See sects 3.12, 6.4.2, 11.1, and in Chapter 9. 5. In section 3.15.2, reference to third bullet seems like it should be fourth bullet. 6. In section 3.17.2, the example that is supposed to return [undefined,undefined,undefined] seems like it really ought to return undefined, although I can see in the description above for ~apat matching where the other interpretation would hold. I actually tried this in GHC 10.4.2, binding the result to a variable and then applying the function length to it, and it comes back with undefined, whereas performing "length [undefined,undefined,undefined]" returns 3. So it appears that in this case, at least GHC 10.4.2 is returning undefined rather than [undefined,undefined,undefined]. 7. In section 6.3.4, "[LT..]" needs whitespace in the middle to parse correctly. 8. I'm curious about last part of section 6.4.3 that says 0**y is undefined. It seems to work reasonably under GHC, returning 0 in every case except 0**0, which returns 1 (the ^0 == 1 rule taking precedence, apparently.) Saying 0**0 is undefined seems reasonable, but why 0**y? 9. In section 6.4.6, statement about b^(d-1) <= m < b^d doesn't seem to hold when m < 0. Should it be |m| in the equation, rather than m? 10. Section 7.1 uses "function" in places where it ought to use "action". It seems more correct to describe print as returning an action that outputs a value. Most of the "Input Functions" (e.g., getChar, getLine, getContents, readLn) should be described as actions, not functions. It switches to using the term "operation", which seems better, but then reverts back to "function". 11. In section 8.4, variable misspelled varibale. 12. In section 8.5.1, in "we require that chname ends on .h", "on" should be "in". 13. Also in section 8.5.1, dependent misspelled "dependant". 14. Last paragraph of section 8.5,1 has extra "a" -- "... defined to a accept a ...". Following sentence has a semicolon where it should have a comma. 15. In section 10.3, "(i.e. the programmer..." has no closing paren. 16. In section 10.4, "... other lines are comment", "comments" sounds better. This occurs twice. 17. Also in section 10.6, anything misspelled "anyything". 18. In Chapter 15, it would be more clear to describe "bit i" as producing a value with the ith bit set, and all other bits clear. 19. In section 20.3.1, caveats about finiteness of lists similar to ones given for "and" and "or" could also apply to "any" and "all". Ditto for elem, notElem, etc. Maybe a single paragraph could summarize this short-circuiting behavior for all of them. 20. In heading for 20.9.2, quotes around "Set" are not balanced. Both are closing quotes. Ditto for 20.10.1, 20.10.2. 21. In section 29.1.1, "finaliser" and "finalizer" used in same paragraph. 22. In section 38.2, first occurrence of 'dual' has mismatched quotes. 23. In section 41.1, quotes around "perform" are mismatched. Word "function" is mildly misused again here. 24. In section 41.3.1, it would be nice to document what happens if act terminates abnormally, and then a secondary exception occurs during the closing of the handle. Oftentimes systems lose the primary exception and propagate out the secondary exception, whereas in reality we may be more interested in the original primary exception. 25. In section 41.4.4, bullet before isPermissionError isn't rendered correctly. Alex

15 20

Applying a value to a function generically
by Arnaud Bailly 14 Jul '10

14 Jul '10

Hello, I would like to construct a collection of function-like objects on which I could apply some value, in a typesafe and clean way. Let's say I have something like this: > data Fun = forall a b . F (a -> b) > type Callables = Map String Fun I would like to be able to write: > invoke :: Callables -> a -> String -> b > invoke m d k = case lookup k m of > Just (F f) -> f d > Nothing -> error $ "unable to find function for key " ++ k which of course does not compile nor even make sense. I suspect I need to have more information in Fun to be able to apply functions correctly, but don't know where to look at. I read recently an article about implicit configurations that used some type wizardry to ensure correct and typesafe use of external data and sounded pretty cool, (http://www.cs.rutgers.edu/~ccshan/prepose/prepose.pdf) but I suspect this is 1) too complex for me an 2) overkill for my problem. Could someone point me in the right direction ? Thanks in advance Arnaud

3 2

Collecting MonadError errors generically
by oleg＠okmij.org 14 Jul '10

14 Jul '10

Leon Grynszpan wrote: > What I want, instead, is to run a whole bunch of > computations that may throw errors. If there are any errors, I want to > collect them all into one big master error. If not, I want a list of > results. Here's an example of usage: > > couldThrowError :: (Error e, MonadError e m) => t1 -> m t2 > getParams :: (Error e, MonadError e m) => [t1] --> m [t2] > getParams = groupErrors . map couldThrowError > > I found it pretty easy to implement groupErrors for Either String: > ... > The problem, though, is that running this function now causes type > inference to provide "Either String" as my MonadError implementation... > It would be nice if I could stay generic. You might find the following two functions useful: > reify :: (Error e, MonadError e m) => m a -> m (Either e a) > reify m = (m >>= return . Right) `catchError` (return . Left) > > -- Not needed here, but very nice to have anyway > reflect :: (Error e, MonadError e m) => m (Either e a) -> m a > reflect m = m >>= either throwError return Then groupErrors can be written almost the same way you wrote for the Either String monad -- but now generically > groupErrors :: (Monoid e, Error e, MonadError e m) => [m a] -> m [a] > groupErrors lst = mapM reify lst >>= \lst -> > case partitionEithers lst of > ([],xs) -> return xs > (errs,_) -> throwError (mconcat errs) If you don't like the Monoid constraint, this is fine. But you have to provide then some other way to group errors. For example, you could use the new extensible exceptions. Here is the rest of the code: > couldThrowError :: (MonadError String m) => String -> m Int > couldThrowError s = case reads s of > [(n,"")] -> return n > _ -> throwError $ "parse error: " ++ s ++ "\n" > > getParams :: (Monoid e, Error e, MonadError e m) => > (t1 -> m t2) -> [t1] -> m [t2] > getParams f = groupErrors . map f > > test :: Either String [Int] > test = getParams couldThrowError ["1","2","a","b"] If you wish to know more about reify and reflect, a good paper to start is the following. Section 1.2 talks directly about exception monads. @InProceedings{ filinski-representing, author = "Andrzej Filinski", title = "Representing Monads", pages = "446--457", crossref = "popl1994", url = "http://www.diku.dk/~andrzej/papers/RM-abstract.html http://www.diku.dk/~andrzej/papers/RM.dvi http://www.diku.dk/~andrzej/papers/RM.ps.gz", abstract = "We show that any monad whose unit and extension operations are expressible as purely functional terms can be embedded in a call-by-value language with ``composable continuations''. As part of the development, we extend Meyer and Wand's characterization of the relationship between continuation-passing and direct style to one for continuation-passing vs.~general ``monadic'' style. We further show that the composable-continuations construct can itself be represented using ordinary, non-composable first-class continuations and a single piece of state. Thus, in the presence of two specific computational effects---storage and escapes---\emph{any} expressible monadic structure (e.g., nondeterminism as represented by the list monad) can be added as a purely definitional extension, without requiring a reinterpretation of the whole language. The paper includes an implementation of the construction (in Standard ML with some New Jersey extensions) and several examples." } There is a follow-up: @InProceedings{ filinski-layered, author = "Andrzej Filinski", title = "Representing Layered Monads", pages = "175--188", crossref = "popl1999", url = "http://www.diku.dk/~andrzej/papers/RLM-abstract.html http://www.diku.dk/~andrzej/papers/RLM.dvi http://www.diku.dk/~andrzej/papers/RLM.ps.gz", } The term reification has been introduced 26 years ago: @InProceedings{ friedman-reification, author = {Daniel P. Friedman and Mitchell Wand}, title = {Reification: Reflection without Metaphysics}, pages = {348--355}, crossref = "lfp1984", abstract = {We consider how the data structures of an interpreter may be made available to the program it is running, and how the program may alter its interpreter's structures. We refer to these processes as reification and reflection. We show how these processes may be considered as an extension of the fexpr concept in which not only the form and the environment, but also the continuation, are made available to the body of the procedure. We show how such a construct can be used to effectively add an unlimited variety of ``special forms'' to a very small base language. We consider some trade-offs in how interpreter objects are reified. Our version of this construct is similar to one in 3-Lisp [Smith 82, 84], but is independent of the rest of 3-Lisp. In particular, it does not rely on the notion of a tower of interpreters.} }

1 0

Collecting MonadError errors generically
by Leon Grynszpan 14 Jul '10

14 Jul '10

Hello, This is my first post to the Haskell Café, and I'm hoping the issue I'm tackling here isn't one that's been thoroughly explored elsewhere. If that's the case, I'll apologize in advance for my insufficient google chops. Otherwise, here goes... I do a lot of work within the error monad. Having read Eric Kidd's blog on error reporting in Haskell (http://www.randomhacks.net/articles/2007/03/10/haskell-8-ways-to-report-err…) I'm always a bit paranoid about dealing with errors in the most generic, flexible way possible. As a result, a lot of the functions I use that could result in errors have signatures along these lines: couldThrowError :: (Error e, MonadError e m) => t1 -> t2 -> m t3 The way I figure it, this means that I can use these functions within any particular MonadError, which keeps me happy if I decide to change implementations at any particular point during development. Usually, I use these couldThrowError functions in a standard do block. Something along the lines of: couldThrowError :: (Error e, MonadError e m) => t1 -> t2 -> m t3 couldThrowError = do x <- processWithPossibleError t1 y <- processWithPossibleError t2 ... return (z :: t3) Things stay generic at this level. I can still choose any MonadError implementation I want, and if any of the computations in the do block throws an Error, the whole function short-circuits. Often enough, however, I don't want this kind of short-circuiting behavior. What I want, instead, is to run a whole bunch of computations that may throw errors. If there are any errors, I want to collect them all into one big master error. If not, I want a list of results. Here's an example of usage: couldThrowError :: (Error e, MonadError e m) => t1 -> m t2 getParams :: (Error e, MonadError e m) => [t1] --> m [t2] getParams = groupErrors . map couldThrowError I found it pretty easy to implement groupErrors for Either String: groupErrors :: (Error e, MonadError e m) => [Either String a] -> m [a] groupErrors eithers = case partitionEithers eithers of ([], xs) -> return xs (es, _) -> strError $ unlines es The problem, though, is that running this function now causes type inference to provide "Either String" as my MonadError implementation. If any of the potential error sources are dealing with a different concrete implementation, I'm stuck! It would be nice if I could stay generic. I made an effort at this that you can see below, but it strikes me as very awkward. collectErrors :: (Monoid e, Error e, MonadError e m) => m a -> m [a] -> m [a] collectErrors m1 m2 = do m1r <- (m1 `catchError` (\e1 -> (m2 `catchError` (\e2 -> throwError $ mappend e1 e2)) >> throwError e1)) m2r <- m2 return $ m1r : m2r groupErrors' :: (Monoid e, Error e, MonadError e m) => [m a] -> m [a] groupErrors' = foldr collectErrors (return []) As you can see, I now require an error type that implements monoid. An example I've tested is: newtype ErrorString = ES String deriving (Read, Show, Eq, Ord, Error) instance Monoid ErrorString where mempty = ES "" mappend (ES s1) (ES s2) = ES $ s1 `mappend` "\n" `mappend` s2 This seems to work, but something strikes me as being very hackish here. Maybe it's just that collectErrors looks very ugly, and in Haskell my intuition tells me that what looks ugly on the surface is often ugly down below. I suspect that I might really not be approaching this problem the right way, but after a lot of time spent wrestling with it, I'm eager to read your suggestions. Is there a better way to do groupErrors, or should I be looking at an entirely different philosophy? Thanks, Grynszpan

2 1

library for objective-j?
by Günther Schmidt 14 Jul '10

14 Jul '10

Hi all, is there a haskell library for generating Objective-J? Günther

2 3

AngloHaskell 2010
by Derek Wright 13 Jul '10

13 Jul '10

Are you interested in AngloHaskell 2010? AngloHaskell is a gathering of all people Haskell-related from beginners to seasoned hackers. Previously it has been held in Cambridge (three times) and London (once). Last year I volunteered to organise it but I want know when you can attend. Traditionally it is the first Friday and Saturday in August, which this year is the 6th and 7th August. But we may only have enough time to organise one day so the question for you is: Q) Which day is better for you? Friday, Saturday or either? I am looking for venues in London but if you can suggest or offer a venue then let me know. As well as attendees we are looking for talks on all and any Haskell related subjects. There is a section on the wiki page http://www.haskell.org/haskellwiki/AngloHaskell/2010 for talks. Please go ahead and add your talk to it. Let me know by email, the wiki or the #anglohaskell IRC channel. I planning to have the venue confirmed this week and then a full announcement will be made. Thanks, Derek.

1 0

The site has been exploited (again)
by Christopher Done 13 Jul '10

13 Jul '10

http://haskell.org/ It says "TO BUY Cilamox ONLINE", etc. Whoever has power please fix this and upgrade the bloody wiki. This is ridiculous. Point the domain at tryhaskell.org or something. I'll put a holder page up. Anything. Cheers

6 10

Marshalling
by Phyx 13 Jul '10

13 Jul '10

Hello I'm wondering if anyone ever benchmarked marshalling in Haskell/GHC. No matter how much I optimize my Haskell code my program still seems to run slow, which leads me to beleive that Marshalling is painfully slow. Does anyone know a way I can test this and fix it? Regards, Phyx

3 4

ByteString, zipWith', and rewrite rules
by Thomas M. DuBuisson 13 Jul '10

13 Jul '10

Comments on the zipWith' function inside of Data.ByteString say: -- Rewrite rules -- are used to automatically covert zipWith into zipWith' when a pack is -- performed on the result of zipWith. This is only true internally to Data.ByteString because the zipWith' function could be inlined away by GHC once that module is compiled, right? If I want "pack (zipWith xor bs1 bs2)" to be efficient then I'll have to either get zipWith' exported or write my own lower level zipWith'? If I'm wrong here then the comment needs to be moved to somewhere that will get haddockized, perhaps with zipWith. Cheers, Thomas

2 1

Functional dependencies and Peano numbers
by Oscar Finnsson 13 Jul '10

13 Jul '10

Hi, there is a lot of buzz around functional dependencies on the mailing list and Planet Haskell. I've read some of the tutorials and I think I understand how they work but I still haven't figured out where they can be useful. * Can someone give me a real world (preferably hackagedb) example where functional dependencies are used? * Can someone give me a real world (preferably hackagedb) example where Peano numbers a la "data Zero; data Succ n" are used? Finally... * All the examples involving functional dependencies are on the form > a b c | a b -> c but can they also be on a form similar to > a b c d e f g h| b c -> d e f | b d g -> h (i.e. d,e,f are decided by the b,c-combination while h is decided by the b,d,g-combination)? -- Oscar

6 5

64-bit #const with hsc2hs?
by Dean Herington 13 Jul '10

13 Jul '10

Does anyone know if I can bring a 64-bit (long long or unsigned long long) value from C-land into Haskell via hsc2hs? #const on such a value seems to provide only the low 32 bits, at least in my environment (Haskell Platform 2009.2.0.2 on Windows). I've tried to create a 64-bit variant of #const without success. Dean

1 0

PNG sample on haskellwiki
by C K Kashyap 13 Jul '10

13 Jul '10

Hi, I tried out the code on this page http://haskell.org/haskellwiki/Library/PNGbut the png file that'e emitted does not seem to open properly with image viewing tools. Has anyone tried it out? I added this bit for supplying the data for image creation - count=100 row = take count (cycle [True,False]) rows = take 100 (repeat row) image = png rows main=writeFile "hello.png" image -- Regards, Kashyap

2 2

ghc-6.12 can't works with Cabal-1.8.0.4?
by Andy Stewart 13 Jul '10

13 Jul '10

Hi all, I upload package http://hackage.haskell.org/package/poppler-0.11.1 I can compile it on my box, but it build failure at hackage with below error: ------------------------------> error start <------------------------------ *** setup configure Reading installed packages... /usr/local/bin/ghc-pkg dump --global /usr/local/bin/ghc-pkg dump --user /usr/local/bin/ghc --print-libdir Creating setup (and its parents) /usr/local/bin/ghc -package Cabal-1.8.0.4 --make Setup.hs -o setup/setup -odir setup -hidir setup <command line>: cannot satisfy -package Cabal-1.8.0.4 (use -v for more information) *** Trying again with preferred-versions *** setup configure Reading installed packages... /usr/local/bin/ghc-pkg dump --global /usr/local/bin/ghc-pkg dump --user /usr/local/bin/ghc --print-libdir Creating setup (and its parents) /usr/local/bin/ghc -package Cabal-1.8.0.4 --make Setup.hs -o setup/setup -odir setup -hidir setup <command line>: cannot satisfy -package Cabal-1.8.0.4 (use -v for more information) ------------------------------> error end <------------------------------ My poppler.cabal is ------------------------------> poppler.cabal start <------------------------------ Name: poppler Version: 0.11.1 License: GPL-2 License-file: COPYING Copyright: (c) 2001-2010 The Gtk2Hs Team Author: Andy Stewart Maintainer: gtk2hs-users(a)sourceforge.net Build-Type: Custom Cabal-Version: >= 1.6 Stability: stable homepage: http://www.haskell.org/gtk2hs/ bug-reports: http://hackage.haskell.org/trac/gtk2hs/ Synopsis: Binding to the Poppler. Description: Poppler is a fork of the xpdf PDF viewer, to provide PDF rendering functionality as a shared library, to centralize the maintenance effort. And move to forward in a number of areas that don't fit within the goals of xpdf. Category: Graphics Tested-With: GHC == 6.12.3 Extra-Source-Files: Gtk2HsSetup.hs hierarchy.list template-hsc-gtk2hs.h x-Types-File: Graphics/UI/Gtk/Poppler/Types.chs x-Types-ModName: Graphics.UI.Gtk.Poppler.Types x-Types-Import: System.Glib.GObject x-Types-Lib: poppler x-Types-Prefix: poppler x-Types-Tag: poppler x-Types-Hierarchy: hierarchy.list Data-Dir: demo Data-Files: PdfViewer.hs Makefile Source-Repository head type: darcs location: http://patch-tag.com/r/AndyStewart/poppler Library build-depends: base >= 4 && < 5, array, containers, haskell98, mtl, bytestring, glib >= 0.11 && < 0.12, cairo >= 0.11 && < 0.12, gtk >= 0.11 && < 0.12 build-tools: gtk2hsC2hs, gtk2hsHookGenerator, gtk2hsTypeGen exposed-modules: Graphics.UI.Gtk.Poppler.Action Graphics.UI.Gtk.Poppler.Attachment Graphics.UI.Gtk.Poppler.Document Graphics.UI.Gtk.Poppler.FormField Graphics.UI.Gtk.Poppler.Layer Graphics.UI.Gtk.Poppler.Page Graphics.UI.Gtk.Poppler.Poppler other-modules: Graphics.UI.Gtk.Poppler.Enums Graphics.UI.Gtk.Poppler.Structs Graphics.UI.Gtk.Poppler.Types extensions: ForeignFunctionInterface include-dirs: . x-c2hs-Header: glib/poppler.h pkgconfig-depends: poppler-glib >= 0.12.4, gobject-2.0, glib-2.0, cairo >= 1.2.0, gdk-2.0, gdk-pixbuf-2.0, pango ------------------------------> poppler.cabal end <------------------------------ Something is wrong? Thanks, -- Andy

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haskell platform questions
by Warren Harris 12 Jul '10

12 Jul '10

I installed haskell platform some time ago, and now I'm wondering what version I have. How do I find out? Also, when the new one comes along on the 21st, is there a way to upgrade? Or if I must first uninstall the one I have now, how do I uninstall it? Is it recommended to periodically upgrade packages that came with the platform (cabal upgrade), or is it recommended that they be left alone to avoid dependency incompatibilities. Similar question for ghc itself -- can/should it be upgraded in the context of haskell platform? (I was hoping to try Leksah, but it dies without ghc 6.12.1. I seem to have 6.10.4.) Apologies in advance if this is all documented somewhere, but I couldn't find it on the haskell platform site/trac. BTW, I'm on Mac/ Leopard -- love the fact that it didn't take hours to build everything! Warren

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darcs 2.5 beta 1 release
by Reinier Lamers 12 Jul '10

12 Jul '10

Hi all, The darcs team would like to announce the immediate availability of darcs 2.5 beta 1. 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 * obliterate has a -o flag to save obliterated patch * amend-record now supports --ask-deps * apply now supports --match * 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 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-2.4.98.1.tar.gz and build it by hand as explained in the README file. Kind Regards, the darcs release manager, Reinier Lamers

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Float instance of Data.Bits
by Sam Martin 12 Jul '10

12 Jul '10

Hi, Is there a particular reason Float, Double, etc do not have instances of Data.Bits in the standard libraries? I note the Haskell 2010 report doesn't include them either. In fact, I'm not actually sure how you'd implement the instance for floating point types without having some kind of compiler-specific extension, or a c-binding. Could anyone fill in my missing knowledge here? Cheers, Sam

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ANNOUNCE: fgl-5.4.2.3
by Ivan Lazar Miljenovic 12 Jul '10

12 Jul '10

I'm pleased to present the first new release of fgl [1] since Thomas Bereknyei took over maintaining it from Martin Erwig. [1] http://hackage.haskell.org/package/fgl Before people start panicking, rioting, etc., please check the version number: this is just a bug-fix release, and not the complete re-write version which we've been talking about (since we got a little sidetracked, etc.). As such, the API hasn't changed, and this should fit right in to packages already using fgl (sorry to all those people who followed my advice and put "fgl == 5.4.2.2" in the build-depends fields of their packages' .cabal files, but I didn't expect to make another 5.4.y release). The exact change that has been made is to fix a bug pointed out to me by Tristan Allwood, in that Data.Graph.Inductive.PatriciaTree didn't support multiple edges (and furthermore this wasn't specified in the documentation). This has now been rectified. As an indication of what these changes mean, see this sample call graph produced by my SourceGraph program; when using PatriciaTree from fgl-5.4.2.2 the lines were all the same thickness; now there is among other things a loop of width 32 on getExp and a line of width 7 from getExp to maybeEnt. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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