- Haskell-Cafe - Haskell.org

GSoC project idea: haskell-type-exts
by Niklas Broberg 28 Mar '12

28 Mar '12

See ticket here: http://hackage.haskell.org/trac/summer-of-code/ticket/1620 == What? == The haskell-src-exts library (HSE) is very popular for manipulating Haskell source code in a wide range of scenarios, such as (syntactic) preprocessors (e.g. hsx), syntax highlighting, refactoring and code inspection (e.g. hlint), etc. A commonly sought feature, that would enable a multitude of new and improved scenarios, is the ability to type-check the code, and annotate the AST with type information. Type-driven refactoring, type-aware code inspection, type-based preprocessing, ... Lots of pure awesomeness lies in that direction. Sadly, HSE has no type-checking abilities. For those who seek such, we are forced to point in the direction of the dark cave that is the GHC API. While GHC is an utterly awesome compiler, there are several reasons why this is not an ideal situation. First, GHC was designed and built for optimization, which means that even the early stages of the compilation pipeline contain features that cater to later optimization, but that are (at best) unnecessary for a front-end tool. Second, the API into GHC internals is defined as an afterthought. Some good work has been done to make it somewhat palatable, but it remains a fact that GHC's primary purpose is not to be a library for integration in tools, and this shows. Overall, depending on GHC when all you want is front-end capabilities like parsing and typechecking, is really like using the proverbial sledge hammer. Also, at the meta level, GHC deserves "competition". Right now GHC is de facto the only implementation of Haskell, and as such it more or less defines the progress of Haskell. Too often this leads to overly pragmatic choices, which lead to inconsistencies and headaches for tool programmers. Having a separate, stand-alone front end library would help set higher standards. So - what we need is a type checker that works on the HSE AST: haskell-type-exts! == How? == I now hear you think: "This is too much, an enormous project, surely it can't be done as a GSoC project?" Sure it can! By walking in the footsteps of giants, and following GHC's approach. This can be achieved by closely observing the documentation provided by [1] and [2], with a bit of [3] thrown into the mix. I already have a basic library that provides most of the core framework necessary to cover extensions, but at this time it's mostly just scaffolding. It needs a lot of love, to provide cover for all the various available extensions, but I'm confident it can be done satisfactorily as a GSoC project. Still, it would be reasonable to modularize the task. Certain features are more crucial than others to support, simply because they have a more profound impact on the overall type-checking. Higher-rank types, GADTs and type families (and fundeps) are likely the most important ones to get right. There are also some design issues to get right (e.g. using pure substitution vs efficient mutable variables). I (Niklas Broberg) am more than willing to mentor this project. == Further reading: == [1] S. Peyton Jones, D. Vytiniotis, S. Weirich, and M. Shields. Practical type inference for arbitrary-rank types. [2] D. Vytiniotis, S. Peyton Jones, T. Schrijvers, M. Sulzmann. OutsideIn(X) – Modular type inference with local assumptions [3] M. P. Jones. Typing Haskell in Haskell

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Regarding Haskell FFI
by rajendra prasad 28 Mar '12

28 Mar '12

Hi, I am trying to load the DLL(Wrapper.dll) in my code(Main.hs). When I am placing the dll in local directory, I am able to load it through following command: ghci Main.hs -L. -lWrapper But, I am not able to load it if I am putting it in some other directory(../../bin). I used the following command: ghci Main.hs -L../../bin/ -lWrapper I doubt I am not using the correct way to specify the path of dll in the command. Please correct me if I am wrong. Thanks in advance. Regards, Rajendra

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Happstack routing + boomerang + non-ascii urls
by Semen Trygubenko 28 Mar '12

28 Mar '12

Dear Haskell-Cafe, Last weekend I was test driving Web.Routes.Boomerang. A great package … Is there any way I could make it work with urls containing non-ASCII characters? Happstack.Server routing using "path" handles non-ASCII, as in, for instance, dir "work" $ path $ workHandler , where workHandler gets a correctly decoded path segment passed in as a String. The following, however, sitemap :: Router Sitemap sitemap = ( rWork . anyString ) passes to Work constructor an incorrectly decoded String. All the plumbing re embedding boomerang route into a normal Happstack one, etc. is done as per example from crash course here: http://happstack.com/docs/crashcourse/WebRoutes.html#web-routes-boomerang Many thanks, Sem -- Семен Тригубенко

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Arguments against an hypothetical Data.ByteString.Generic?
by Yves Parès 27 Mar '12

27 Mar '12

Hello, As vector provides a class generalizing all flavours (Data.Vector.Generic.Vector), it occurs to me that the same could be done for ByteString. Then, packages based on it would have the choice between hardcoded and generic, they wouldn't have to duplicate a module to handle both strict and lazy versions, as (with the exception of functions for communication with C code) they already provide the same API. I would be willing to make it, it's a concern I've had in mind for a long time, but as I'm pretty sure the idea isn't new, I would very much like to know if and what arguments (related to performance maybe ? I don't know...) were raised against that.

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ANNOUNCE: haskell-src-exts-1.12.0
by Niklas Broberg 27 Mar '12

27 Mar '12

Fellow Haskelleers, I'm pleased to announce the release of haskell-src-exts-1.12.0! * On hackage: http://hackage.haskell.org/package/haskell-src-exts * Via cabal: cabal install haskell-src-exts * Darcs repo: http://code.haskell.org/haskell-src-exts It's been a long while since the last update before this, but it is my definite hope that I will once again be able to update regularly. This major release primarily enables the (relatively) new quasi-quoter syntax. The "major" version bump is questionable, but better safe than sorry. The only backwards incompatible change is that pretty- and exact-printing now use the new quasi-quoter syntax, which makes the round-tripping 'exactPrint . parse' cycle fail for code using the old syntax. There are more changes in the pipeline, some larger and some smaller. Stay tuned! Changelog: 1.11.1 --> 1.12.0 =============== * Move from old [$...| quasi-quote syntax to the new [...| one. The old syntax is still recognized while parsing. * Allow symbols as variables when TypeOperators is enabled. * Rename ExplicitForall in ExplicitForAll, to be consistent with GHC and the Haskell' process. Cheers, /Niklas

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Reminder Munich Haskell Meeting
by Heinrich Hördegen 27 Mar '12

27 Mar '12

Dear all, just to remind you: Tomorrow, 28th of March, our Haskell get-together will take place in Munich. If you plan to join, go to http://www.haskell-munich.de/dates and click the button. Have a nice day, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

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Regarding Haskell FFI for C/C++ (Passing of String)
by rajendra prasad 27 Mar '12

27 Mar '12

Hi, I have just started learning Haskell FFI. I am trying to send a string from hastell to a C function. For this, I am required to convert the haskell string to byte string. I have two methods to achieve this task. Both are listed below: 1) import Foreign.C.String let arg1 = map castCharToCChar "Hello" :: [CChar] 2) import qualified Data.ByteString.Char8 as B f = B.pack "Hello" I just wanted to know the optimal way to achieve this task. Please suggest the optimal way of doing this. If there is any other way, please share it. Also, please suggest me any good tutorial to start with Haskell FFI for C/C++. Thank you very much. Regards, Rajendra

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good lightweight web-framework like sinatra?
by serialhex 27 Mar '12

27 Mar '12

i'm looking for something lightweight, that dosnt need it's own server, can easily run on cgi on an apache with minimal work, and dosn't have many dependancies. i was looking at yesod, but it is bigger than i need for my site (at this point) and would take too much work to get running on my webhost. though i am looking forward to learning it and using it in the future, i just need something that will play nicely with apache & cgi... justin p.s. if anyone is interested to know i'm using nearlyfreespeach.net as my host... haskell is one of the many languages they support via cgi, but, at the moment, it is kind of difficult to get yesod or rails or the like to work on it... :-/ -- * The wise man said: "Never argue with an idiot. They bring you down to their level and beat you with experience." * As a programmer, it is your job to put yourself out of business. What you do today can be automated tomorrow. ~Doug McIlroy No snowflake in an avalanche ever feels responsible. --- CFO: “What happens if we train people and they leave?” CTO: “What if we don’t and they stay?”

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Is there a generic way to detect "mzero"?
by Ting Lei 27 Mar '12

27 Mar '12

Hi, I was writing a code trying to use MonadPlus to detect some error cases (representing missing values etc. in pure code). With the Maybe monad, I can do this: can0 :: (a -> Maybe b) -> a -> Bool can0 f x = case f x of Nothing -> False Just x -> True And I got the expected result: *Main> can0 (\x -> Just x) 1 True But, when I try to generalize this using MonadPlus, as follows: can :: (MonadPlus m) => (a -> m b) -> a -> Bool can f x = case f x of mzero -> False _ -> True I got a warning: __testError.hs:31:11: Warning: Pattern match(es) are overlapped In a case alternative: _ -> ... Ok, modules loaded: Main. And the result is also not as intended (see also can0): *Main> can (\x -> Just x) 1 False Can anyone help to explain why this wouldn't work or if there is a workaround to use Monadplus and mzero (or Monad and fail) to achieve this? Thanks in advance for your help Ting

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Need a backup GSOC to apply for? Last minute new proposal -- memory reuse analysis for GHC
by Ryan Newton 27 Mar '12

27 Mar '12

Hi potential GSOC'ers, If there are multiple students interested in the project you're applying to it's a good idea to put in more than one application. This is a project proposal that would focus on performance analysis -- in particular in reusing some of the tools for analyzing memory access patterns in the context of Haskell: http://hackage.haskell.org/trac/summer-of-code/ticket/1619 I think this project would yield pretty good bang for the buck because almost all the pieces are already there. They just need to be combined, polished, documented, and evangelized. Cheers, -Ryan

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ANN: Latest package versions on Hackage as JSON/JSONP
by Simon Hengel 26 Mar '12

26 Mar '12

Hi, I provide a relation between /package name/ and /latest version/ on Hackage as both JSON and JSONP. http://www.typeful.net/~tbot/hackage/ This are static files, and they are regenerated whenever new packages are uploaded (with a delay of about one minute). Two simple usage examples are given in the README[1]. Feel free to access this information for whatever purpose! Cheers, Simon [1] https://github.com/sol/hackage-jsonp#readme

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difficulty building (some) TH packages with ghc-7.4.1
by John Lato 26 Mar '12

26 Mar '12

Hello, I've run into an odd problem when building certain packages that use Template Haskell with GHC-7.4.1. For example, RepLib: $ cabal --version cabal-install version 0.13.3 using version 1.14.0 of the Cabal library $ cabal install -O2 -w ~/.ghc-7.4.1/bin/ghc RepLib --reinstall Resolving dependencies... Configuring RepLib-0.5.2... Building RepLib-0.5.2... Preprocessing library RepLib-0.5.2... [ 1 of 12] Compiling Generics.RepLib.R ( Generics/RepLib/R.hs, dist/build/Generics/RepLib/R.o ) [ 2 of 12] Compiling Generics.RepLib.R1 ( Generics/RepLib/R1.hs, dist/build/Generics/RepLib/R1.o ) [ 3 of 12] Compiling Generics.RepLib.Derive ( Generics/RepLib/Derive.hs, dist/build/Generics/RepLib/Derive.o ) [ 4 of 12] Compiling Generics.RepLib.PreludeReps ( Generics/RepLib/PreludeReps.hs, dist/build/Generics/RepLib/PreludeReps.o ) Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package type-equality-0.1.0.2 ... linking ... done. Loading package array-0.4.0.0 ... linking ... done. Loading package deepseq-1.3.0.0 ... linking ... done. Loading package containers-0.4.2.1 ... linking ... done. Loading package pretty-1.1.1.0 ... linking ... done. Loading package template-haskell ... linking ... done. Loading package transformers-0.2.2.0 ... linking ... done. Loading package mtl-2.0.1.0 ... linking ... done. cabal: Error: some packages failed to install: RepLib-0.5.2 failed during the building phase. The exception was: ExitFailure 11 This is without library profiling enabled. If I enable library profiling, the unprofiled build is fine but the profiled build exits with the same error, at the same location. If I pass the "-v" argument to cabal-install, the package builds fine with profiling disabled. The "-v" flag makes no difference with profiling enabled, although some packages (but not RepLib) work with "-v2" I haven't seen this with ghc-7.2.*, but I'm guessing this is some sort of cabal-install bug. Has anyone else witnessed anything like this? Thanks, John L.

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Using HaXml
by Yves Parès 26 Mar '12

26 Mar '12

Hello café, Provided what I read, HaXml seems to be the recommended library for parsing XML files (I'm trying to browse and alter spreadsheets (ODS) and possibly release a package when I'm done), is there somewhere tutorials on how to use it? I used 'xml' package by the past, but HaXml is more substantial. Thanks.

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Execute only one step in Hood-Melville Real time queue.
by Xinyu LIU 26 Mar '12

26 Mar '12

Hi, I read the Hood-Melville real time queue realization in [1]. There are 2 lists maintained in queue, front and rear. When the queue gets unbalanced due to push/pop, it amortized the f ++ reverse r incrementally to build the new front list based on the below mechanism reverse r = reverse' r [] where reverse' [] acc = acc reverse' (x:xs) = reverse' xs (x:acc) and f ++ reverse r == reverse (reverse f) ++ reverse r == reverse' (reverse f) [] ++ reverse r == reverse' (reverse' f []) (reverse' r []) And the re-balance happens once | f | + 1 = | r |. Let's denote m = |f|. incremental f ++ reverse r takes total 2*m + 2 steps. In the the realization mentioned in [1], it execute 2 steps every push and pop to make sure the incremental computation finish before next queue re-balancing. However, I found it's possible to execute only one step per push/pop. because: 1. Next re-balance happens at earliest | f' | + 1 = |r|+|f|+1+1 = 2*m + 2 times by continuously push operation; 2. If we keep a copy of f, and a counter of how many elements left in f, which need to be incrementally 'appended', by continuously m times popping operation. we can finish the f ++ reverse r (actually, only reverse r is needed, as all elements in f are popped) Based on this fact, I rewrite the program as the following: data State a = Empty | Reverse Int [a] [a] [a] [a] -- n, f', acc_f' r, acc_r | Append Int [a] [a] -- n, rev_f', acc | Done [a] -- result: f ++ reverse r deriving (Show, Eq) -- front, length of front, on-goint reverse state, rear, length of reverse data RealtimeQueue a = RTQ [a] Int (State a) [a] Int deriving (Show, Eq) -- we skip the empty error for pop and front empty = RTQ [] 0 Empty [] 0 isEmpty (RTQ _ lenf _ _ _) = lenf == 0 -- O(1) time push push (RTQ f lenf s r lenr) x = balance f lenf s (x:r) (lenr + 1) -- O(1) time pop pop (RTQ (_:f) lenf s r lenr) = balance f (lenf - 1) (abort s) r lenr front (RTQ (x:_) _ _ _ _) = x balance f lenf s r lenr | lenr <= lenf = step f lenf s r lenr | otherwise = step f (lenf + lenr) (Reverse 0 f [] r []) [] 0 -- execute f ++ reverse r step by step step f lenf s r lenr = case s' of Done f' -> RTQ f' lenf Empty r lenr s' -> RTQ f lenf s' r lenr where s' = if null f then next $ next s else next s next (Reverse n (x:f) f' (y:r) r') = Reverse (n+1) f (x:f') r (y:r') next (Reverse n [] f' [y] r') = Append n f' (y:r') next (Append 0 _ acc) = Done acc next (Append n (x:f') acc) = Append (n-1) f' (x:acc) next s = s -- Abort unnecessary appending as the element is popped abort (Append 0 _ (_:acc)) = Done acc -- Note! we rollback 1 elem abort (Append n f' acc) = Append (n-1) f' acc abort (Reverse n f f' r r') = Reverse (n-1) f f' r r' abort s = s Note the 'where' clause in step function. This is because we need an extra step to change the state from (Append 0 _ xs) to (Done xs). I tested this program with invariant testing with QuickCheck. The behavior is correct as other queue implementation. The program can be found here: https://github.com/liuxinyu95/AlgoXY/blob/algoxy/datastruct/elementary/queu… Reference: [1]. Chris Okasaki. ``Purely functional data structures''. P102. Section 8.2.1. Cambridge University Press. ISBN 0521663504 -- Larry, LIU Xinyu https://sites.google.com/site/algoxy/ https://github.com/liuxinyu95/AlgoXY

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What happened with goa package repo?
by Dmitry Malikov 26 Mar '12

26 Mar '12

Cloning into bare repository '/usr/portage/distfiles/egit-src/goa.git'... fatal: The remote end hung up unexpectedly * ERROR: dev-haskell/goa-9999 failed (unpack phase): * git-2_initial_clone: can't fetch from git://github.com/chrisdone/goa.git https://github.com/chrisdone/goa returns 404. So it seems like Chris dropped that repo. Did someone have clone of it? It will be nice to put it back on github.

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Google Summer of Code: a NUMA wishlist!
by Sajith T S 26 Mar '12

26 Mar '12

Dear Cafe, It's last minute-ish to bring this up (in my part of the world it's still March 25), but graduate students are famously a busy and lazy lot. :) I study at Indiana University Bloomington, and I wish to propose^W rush in this proposal and solicit feedback, mentors, etc while I can. Since student application deadline is April 6, I figure we can beat this into a real proposal's shape by then. This probably also falls on the naive and ambitious side of things, and I might not even know what I'm talking about, but let's see! That's the idea of proposal, yes? Broadly, idea is to improve support for NUMA systems. Specifically: -- Real physical processor affinity with forkOn [1]. Can we fire all CPUs if we want to? (Currently, the number passed to forkOn is interpreted as number modulo the value returned by getNumCapabilities [2]). -- Also kind of associated with the above: when launching processes, we might want to specify a list of CPUs rather than the number of CPUs. Say, a -N [0,1,3] flag rather than -N 3 flag. This shall enable us to gawk at real pretty htop [3] output. -- From a very recent discussion on parallel-haskell [4], we learn that RTS' NUMA support could be improved. The hypothesis is that allocating nurseries per Capability might be a better plan than using global pool. We might borrow/steal ideas from hwloc [5] for this. -- Finally, a logging/monitoring infrastructure to verify assumptions and determine if/how local work stays. (I would like to acknowledge my fellow conspirators and leave them unnamed, lest they shall be embarrassed by my... naivete.) Thanks, Sajith. [1] http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurre… [2] http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurre… [3] http://htop.sourceforge.net/ [4] http://groups.google.com/group/parallel-haskell/browse_thread/thread/7ec1eb… [5] http://www.open-mpi.org/projects/hwloc/ -- "the lyf so short, the craft so long to lerne." -- Chaucer.

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How to handle C library dependencies on Hackage?
by . 25 Mar '12

25 Mar '12

Hello Cafe, I recently did my first upload to Hackage: <http://hackage.haskell.org/package/jalla>. The library does not build (and therefore unfortunately the documentation is not built either), because the C library that it needs to wrap is usually not installed with normal Linux distributions. It's LAPACKE (note the E at the end). I /could/ upload the source code of that library alongside my Haskell library and build it using Setup.hs (this works on my computer). However, LAPACKE is large compated to my own library (about 16MB sources) and I don't want to flood Hackage with it if it's not necessary. Can anyone here give me some advice on how to solve this on the system that builds the hackage packages? Thanks very much, Christian

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Re: [Haskell-cafe] Open-source projects for beginning Haskell students?
by John Lato 25 Mar '12

25 Mar '12

> From: Heinrich Apfelmus <apfelmus(a)quantentunnel.de> > > Tom Murphy wrote: >> If you want to do Haskell audio synthesis, you could also use >> hsc3 (good start here: http://slavepianos.org/rd/ut/hsc3-texts/) With >> hsc3 you can start on serious audio synthesis with only a few lines of >> Haskell. In my opinion it could use a much larger community. > > While Rohan's bindings to SuperCollider are great, I have found that > SuperCollider itself is quite difficult to understand for a new user. > (My tomata-rubato project aims to be much easier to learn.) > > Also, as far as I am aware, you can't do low-level audio programming in > SuperCollider, i.e. play a list of samples that you've calculated > yourself. That's cool if you're only interested in sound design, but bad > for learning how audio programming works. I think this charge is a bit unfair. If you really want to do low-level stuff, it's possible within SC. You just have to work in SuperCollider, not Haskell (AFAIK). However, it is possible to transfer audio data between Haskell and Csound, in several ways. The hCsound package comes with some examples of transferring the audio input and output streams between csound and haskell. Named channels provide for even more complicated routing if you like. John L.

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GSoC project ideas for web technology
by Jeremy Shaw 25 Mar '12

25 Mar '12

Hello, If you are looking for some ideas for a GSoC project, I have written down some web technology related projects I would like to see. So far I mostly have ideas for improvements to HSX (a templating solution) and acid-state (a pure, haskell persistent datastore). Both these technologies can be used with any Haskell web framework (though they are embraced most fully by Happstack). http://code.google.com/p/happstack/wiki/GoogleSummerOfCode I am an officially registered Haskell GSoC mentor. I am happy to help expand any of these ideas into a genuine GSoC proposal and to act as a mentor on the project. If you have other ideas you wish to pitch, I am happy to hear those as well. - jeremy

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sinkfork using conduit 3.0
by grant 25 Mar '12

25 Mar '12

I am trying to update sinkfork to use conduits 3.0. The difficulty I am having is trying to figure out how to push a value into a SinkM constructor. Any ideas? Here is the old! conduits code 2.0: https://gist.github.com/1975383 Thanks in advance. Grant

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Alex character sets
by Omari Norman 24 Mar '12

24 Mar '12

I'm wrinting (or at least, trying to write) an Alex lexer. Say I want to group character ranges together and specify that I want a character that is *not* in any of those ranges. Example --- { module Main where } %wrapper "posn-bytestring" $lowAscii = \x00-\x1f -- unprintable stuff $lowSymbol = \x21-\x2f -- exclamation point to solidus $space = \x20 $special = [^$lowAscii $lowSymbol] lex :- $special { Word } { data Token = Word AlexPosn ByteString.ByteString } When I run this through Alex, I get "Prelude.Enum.Char.pred: bad argument". What am I doing wrong? How would I accomplish this sort of thing? Thanks. --Omari

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ANNOUNCE: options, an easy-to-use command-line option parser
by John Millikin 24 Mar '12

24 Mar '12

Hackage: http://hackage.haskell.org/package/options Home page: https://john-millikin.com/software/haskell-options/ API reference: https://john-millikin.com/software/haskell-options/reference/haskell-option… The options package lets library and application developers easily work with command-line options. The following example is a full program that can accept two options, --message and --quiet: ---------------------------------------------------------- {-# LANGUAGE TemplateHaskell #-} import Options defineOptions "MainOptions" $ do stringOption "optMessage" "message" "Hello world!" "A message to show the user." boolOption "optQuiet" "quiet" False "Whether to be quiet." main :: IO () main = runCommand $ \opts args -> do if optQuiet opts then return () else putStrLn (optMessage opts) ---------------------------------------------------------- ---------------------------------------------------------- $ ./hello Hello world! $ ./hello --message='ciao mondo' ciao mondo $ ./hello --quiet $ ---------------------------------------------------------- In addition, this library will automatically create documentation options such as --help and --help-all: ---------------------------------------------------------- $ ./hello --help Help Options: -h, --help Show option summary. --help-all Show all help options. Application Options: --message A message to show the user. --quiet Whether to be quiet. ----------------------------------------------------------

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Un-memoization
by Victor Miller 23 Mar '12

23 Mar '12

I was writing a Haskell program which builds a large labeled binary tree and then does some processing of it, which is fold-like. In the actual application that I have in mind the tree will be *huge*. If the whole tree is kept in memory it would probably take up 100's of gigabytes. Because of the pattern of processing the tree, it occurred to me that it might be better (cause much less paging) if some large subtrees could be replaced by thunks which can either recalculate the subtree as needed, or write out the subtree, get rid of the references to it (so it can be garbage collected) and then read back in (perhaps in pieces) as needed. This could be fairly cleanly expressed monadically. So does anyone know if someone has created something like this? Victor

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Open-source projects for beginning Haskell students?
by Brent Yorgey 23 Mar '12

23 Mar '12

Hi everyone, I am currently teaching a half-credit introductory Haskell class for undergraduates. This is the second time I've taught it. The last time, for their final project I gave them the option of contributing to an open-source project; a couple groups took me up on it and I think it ended up being a modest success. So I'd like to do it again this time around, and am looking for particular projects I can suggest to them. Do you have an open-source project with a few well-specified tasks that a relative beginner (see below) could reasonably make a contribution towards in the space of about four weeks? I'm aware that most tasks don't fit that profile, but even complex projects usually have a few "simple-ish" tasks that haven't yet been done just because "no one has gotten around to it yet". If you have any such projects, I'd love to hear about it! Here are a few more details: * The students will be working on the projects from approximately the end of this month through the end of April. * By "relative beginner" I mean someone familiar with the material listed here: http://www.cis.upenn.edu/~cis194/lectures.html and just trying to come to terms with Applicative and Monad. They definitely do not know much if anything about optimization/profiling, GADTs, the mtl, or Haskell-programming-in-the-large. (Although part of the point of the project is to teach them a bit about programming-in-the-(medium/large)). * What I would hope from you is a willingness to exchange email and/or chat with the student(s) over the course of the project, to give them a bit of guidance/mentoring. I am certainly willing to help on that front, but of course I probably don't know much about your particular project. thanks! -Brent

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Parallelism causes space leaks
by Yavuz Yetim 23 Mar '12

23 Mar '12

Hi, For the code below, where it says "HERE" in comments, if I remove the part after `using` the code works fine. However, with this version it causes a Stack space overflow (if allowed uses GBs of memory). You just need to input a file with around 1M lines each having something like "Int Value: 3 @x". What is wrong with adding the parList to this code? (Same thing happens for parMap, and parListChunk, etc) Yavuz import System.IO import System.Environment import System.IO.Error import Control.Parallel import Control.Parallel.Strategies import Control.Monad import Data.Binary as DB import Data.Binary.Put import Data.Word import Data.Maybe import qualified Data.ByteString.Lazy as B import Text.Regex.TDFA import Text.Regex.Base.Context main = do { args <- getArgs; x <- getLines (head args); mapM_ writeMaybeIntBinary ((map perLineOperator x) `using` parList rdeepseq); -- HERE return (); } writeMaybeIntBinary :: Maybe Word32 -> IO () writeMaybeIntBinary Nothing = return (); writeMaybeIntBinary (Just intB) = do { B.hPut stdout (runPut (putWord32host intB));}; getLines :: FilePath -> IO [String] getLines = liftM lines . readFile perLineOperator :: String -> Maybe Word32 perLineOperator line = let { getIntStr :: String -> String; getIntStr "" = ""; getIntStr line = let { matches = (line =~ "Int Value: (-?[0-9]*) .*" :: [[String]]); } in if matches == [] then "" else (last (head matches)); } in let { intStr = (getIntStr line); } in if intStr == "" then Nothing else Just (read intStr :: Word32)

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[ANNOUNCE] vector-conduit
by Jared Hance 23 Mar '12

23 Mar '12

I've released a small (one-module) library [1] inspired by functions in Data.Conduit.List like sourceList, consumeList, and take. The most recent (by a few days) depends on conduit-0.3.*, but vector-conduit-0.2.1.0 depends on conduit-0.2.*. Notably, however, the two have different APIs (0.3 has an API more consistent with other vector operations). Use of any versions below 0.2.1 is discouraged as consumeVector is notably less efficient since in those versions it relies on a dlist. This should be useful for replacing consumeList with consumeVector, since consumeVector is much more efficient for reading in a large amount of values with, at this point, very little code overhead for using a Vector rather than a list. The library uses Data.Vector.Generic so unboxed, boxed, immutable, and mutable vectors are all supported, along with conduits for converting between mutable and immutable. [1] http://hackage.haskell.org/package/vector-conduit-0.3.0.0

2 4

haskell-platform vs macports
by Warren Harris 23 Mar '12

23 Mar '12

I assume that many haskell users out there on macs who are also users of macports, and I bet they've hit this same issue that I've hit numerous times. The problem is that there are 2 incompatible versions of libiconv -- one that ships with the mac, and one that's built with macports and that many macports-compiled libraries depend on. Work-arounds have been documented in numerous places (notably here: http://blog.omega-prime.co.uk/?p=96) but if you are trying to link with a macports-compiled libraries that depends on /opt/local/lib/libiconv.2.dylib, your only alternative seems to be building ghc from source in order to avoid the incompatibility. I hit this problem while trying to build a foreign-function interface to libarchive. So I built ghc from scratch -- in fact I built the whole haskell-platform. This was relatively painless, and fixed the libiconv problem. However, it brings me to the real point of my message, which is that the version of haskell-platform available via macports is way out of date (2009.2.0.2 http://www.macports.org/ports.php?by=name&substr=haskell-platform) I'm wondering whether the haskell-platform team has considered maintaining a macports version of the haskell-platform for us mac users in addition to the binary install that's available now. This would avoid the incompatibilities such as this nagging one with libiconv. Perhaps it's just a matter of maintaining template versions of the port files? Warren Undefined symbols for architecture x86_64: "_locale_charset", referenced from: _localeEncoding in libHSbase-4.3.1.0.a(PrelIOUtils.o) "_iconv_close", referenced from: _hs_iconv_close in libHSbase-4.3.1.0.a(iconv.o) (maybe you meant: _hs_iconv_close) "_iconv", referenced from: _hs_iconv in libHSbase-4.3.1.0.a(iconv.o) (maybe you meant: _hs_iconv, _hs_iconv_open , _hs_iconv_close ) "_iconv_open", referenced from: _hs_iconv_open in libHSbase-4.3.1.0.a(iconv.o) (maybe you meant: _hs_iconv_open) ld: symbol(s) not found for architecture x86_64 collect2: ld returned 1 exit status cabal: Error: some packages failed to install: Libarchive-0.1 failed during the building phase. The exception was: ExitFailure 1

2 2

Puzzled about inlining and specialise inline under ghc -O2
by Rafal Kolanski 22 Mar '12

22 Mar '12

Dear Haskell-Cafe, I'm computing a histogram of a bunch of symbols with up to 8 bits of information each, stored in a unboxed vector of Word8. The histogram is represented as an unboxed vector of Int with size 2^bits. I compute the histogram by folding an increment function. The problem: depending on what types and what annotations I give to the increment and histogram function (see below), the GC gets through a different amount of memory. I'm using GHC 7.0.3 and -O2. I'd like to better understand how and why the optimisation does/doesn't kick in. Here are the functions with the most generic types I can think of: import qualified Data.Vector.Unboxed as UV import qualified Data.Vector.Unboxed.Mutable as UMV import Control.Monad.Primitive (PrimMonad, PrimState) increment :: (PrimMonad m, UMV.Unbox a, Num a, Integral b) => UMV.MVector (PrimState m) a -> b -> m (UMV.MVector (PrimState m) a) increment v x = do n <- UMV.read v (fromIntegral x) UMV.write v (fromIntegral x) (n+1) return v histogram :: (Integral a, UMV.Unbox a) => Int -> UV.Vector a -> UV.Vector Int histogram bitsPerSym v = runST $ do a <- UMV.replicate (2^bitsPerSym) (0::Int) a' <- UV.foldM' increment a v UV.unsafeFreeze a' Running my test load, I get: total alloc = 33,206,568 bytes Looking at the core, ghc is not specialising the functions, even if I tell it to inline them. So let's brutally change the types to be as specific as I need for my application: increment :: UMV.MVector s Int -> Word8 -> ST s (UMV.MVector s Int) histogram :: Int -> UV.Vector Word8 -> UV.Vector Int result: total alloc = 19,581,152 bytes and if I put INLINE pragmas for both functions: 16,952,512 bytes I should be able to achieve the same effect with SPECIALISE INLINE pragmas, right? Let's try that: {-# SPECIALISE INLINE increment :: UMV.MVector s Int -> Word8 -> ST s (UMV.MVector s Int) #-} {-# SPECIALISE INLINE histogram :: Int -> UV.Vector Word8 -> UV.Vector Int #-} result: 33,139,856 bytes (GHC can't figure out application of the first rule, giving: Warning: RULE left-hand side too complicated to desugar) So unfortunately my most generic form won't work here, I need to specialise increment to be in ST (which sucks, because I want it to work for both IO and ST): increment :: (UMV.Unbox a, Num a, Integral b) => UMV.MVector s a -> b -> ST s (UMV.MVector s a) {-# SPECIALISE INLINE increment :: UMV.MVector s Int -> Word8 -> ST s (UMV.MVector s Int) #-} result: 17,016,192 bytes This is very close to the most specific function instantiations and INLINE, but: - I've lost being generic between ST and IO - it's still a little bigger than the specific instances + INLINE So my questions are: what is going on? Can I have genericity between ST and IO while keeping the low GC usage? How come SPECIALISE INLINE does not give the same result as specific instances + INLINE? Obviously, for this example, I don't really *need* increment to work inside IO, since I'm using runST... but I want to understand what is going on. Profiling Haskell performance and memory usage has always been difficult for me. Much thanks in advance, Rafal Kolanski.

1 0

Munich Haskell Meeting
by Heinrich Hördegen 22 Mar '12

22 Mar '12

Dear all, once again, it's time for our monthly Haskell get-together in Munich. On Wed, 28 Feb 2012, at 19h30, we will meet at Cafe Puck (near the universities). Everybody is invited. If you plan to join, please go to: http://www.haskell-munich.de/dates and add yourself. This will help to reserve tables. Have a nice time until Wednesday, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

1 0

Re: [Haskell-cafe] Are there arithmetic composition of functions?
by oleg＠okmij.org 22 Mar '12

22 Mar '12

> At present we can easily express different flavors of conjunction, but > expressing disjunction is hard. Disjunction is not particularly difficult. See, for example, http://okmij.org/ftp/Haskell/TTypeable/TTypeable.hs and search for ORELSE. The code demonstrates higher-order type-level programming (for example, higher-order function Member with the equality predicate as the argument). The file implements closed-world negation for type predicates. See http://okmij.org/ftp/Haskell/typeEQ.html for explanations. Incidentally, one application of that machinery is precisely your original problem. The code http://okmij.org/ftp/Haskell/TTypeable/NotAFunctionT.hs implements vector spaces as function spaces, so you can use the same operation <+> :: Vspace v => v -> v -> v to add arguments of the type (Num n =>n), Num n => a -> n, Num n => a -> b -> n, etc. (there is also a scalar-vector multiplication).

1 0

GSoC and Data Parallel Haskell
by Dmitry Dzhus 22 Mar '12

22 Mar '12

How are you gentlemen. I'm interested about what additions to DPH would really benefit the community. The only ticket on DPH in GSoC Trac seems a bit stale: http://hackage.haskell.org/trac/summer-of-code/ticket/1537. Any new tips? My field is currently molecular simulation. (I'm doing a Direct Simulation Monte-Carlo implementation for rarefied gas dynamics with application to space satellites&stuffs aerodynamics in Haskell for my Master's thesis, I believe using DPH as well would be my final goal as molecular simulations vectorize nicely. Simulation domain is partitioned into cells and stochastic collision sampling (as in DSMC) may be performed in every cell in parallel.)

3 4

Haskell Weekly News: Issue 219
by Daniel Santa Cruz 22 Mar '12

22 Mar '12

Welcome to issue 218 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of March 11 to 17, 2012. Quotes of the Week * cmccann: * cmccann needs to hurry up and get his stream processing library finished before all the possible synonyms for "thing that other things flow through" are taken * shergill: silver bullets don't respect eta equivalency. it needs to be made of silver, and not simply *act* like one :P Top Reddit Stories * Shelly: Write your shell scripts in Haskell Domain: yesodweb.com, Score: 65, Comments: 34 On Reddit: [1] http://goo.gl/lZdyV Original: [2] http://goo.gl/HHIU0 * The Glasgow Haskell Compiler (draft chapter for "The Architecture of Open Source Applications, Volume 2") Domain: community.haskell.org, Score: 53, Comments: 28 On Reddit: [3] http://goo.gl/qVoFj Original: [4] http://goo.gl/Rc42A * Announcing the Yesod Platform Domain: yesodweb.com, Score: 52, Comments: 13 On Reddit: [5] http://goo.gl/TzW0w Original: [6] http://goo.gl/z3fwC * The Cabal of my dreams Domain: blog.johantibell.com, Score: 42, Comments: 19 On Reddit: [7] http://goo.gl/VTpJu Original: [8] http://goo.gl/wrrNP * Announcing: Snap Framework v0.8 Domain: snapframework.com, Score: 39, Comments: 6 On Reddit: [9] http://goo.gl/19EyG Original: [10] http://goo.gl/USfny * Leksah 0.12 Domain: haskell.org, Score: 32, Comments: 9 On Reddit: [11] http://goo.gl/5Ft8w Original: [12] http://goo.gl/K6OEx * uncurryWith and concatenative programming Domain: self.haskell, Score: 30, Comments: 4 On Reddit: [13] http://goo.gl/dzzXH Original: [14] http://goo.gl/dzzXH * New course on parallel functional programming at Chalmers Domain: cse.chalmers.se, Score: 29, Comments: 2 On Reddit: [15] http://goo.gl/vwH6F Original: [16] http://goo.gl/zKQjh * Announcing Haskell project support on travis-ci.org Domain: about.travis-ci.org, Score: 28, Comments: 8 On Reddit: [17] http://goo.gl/uH7g3 Original: [18] http://goo.gl/KbE7I * Anyone in the Toronto area looking for a job? Domain: self.haskell, Score: 28, Comments: 22 On Reddit: [19] http://goo.gl/NHdwz Original: [20] http://goo.gl/NHdwz * Building a Concurrent Web Scraper With Haskell Domain: adit.io, Score: 26, Comments: 1 On Reddit: [21] http://goo.gl/G7QXM Original: [22] http://goo.gl/FV2LO * Using Text.Parsec.Indent to parse an indentation-sensitive language with Haskell’s Parsec library. Domain: spin.atomicobject.com, Score: 26, Comments: 7 On Reddit: [23] http://goo.gl/dXkzO Original: [24] http://goo.gl/9BFa3 * Coapplicative Functor? Domain: self.haskell, Score: 21, Comments: 37 On Reddit: [25] http://goo.gl/4eluT Original: [26] http://goo.gl/4eluT * HalVM: Easy install on Fedora 16 + 64bit support Domain: community.galois.com, Score: 21, Comments: 2 On Reddit: [27] http://goo.gl/goN4p Original: [28] http://goo.gl/6EQAu Top StackOverflow Questions * How do you represent a graph in Haskell? votes: 18, answers: 4 Read on SO: [29] http://goo.gl/t5vij * Impredicative types vs. plain old subtyping votes: 16, answers: 0 Read on SO: [30] http://goo.gl/3cmCj * Can GHC really never inline map, scanl, foldr, etc.? votes: 14, answers: 3 Read on SO: [31] http://goo.gl/se91V * Redefine IO to simplify debugging? votes: 11, answers: 1 Read on SO: [32] http://goo.gl/s62s9 * Quadratic programming in Haskell votes: 11, answers: 1 Read on SO: [33] http://goo.gl/jYWEP * How can I test a higher-order function using QuickCheck? votes: 11, answers: 2 Read on SO: [34] http://goo.gl/1YHql * Haskell: how to map a tuple? votes: 10, answers: 7 Read on SO: [35] http://goo.gl/fk7LE * Repacking monads — any generic way? votes: 9, answers: 4 Read on SO: [36] http://goo.gl/QklWI * Is there any reason not to use the INLINABLE pragma for a function? votes: 9, answers: 1 Read on SO: [37] http://goo.gl/t75CV * Repa 3 performance and correct usage of 'now' votes: 9, answers: 1 Read on SO: [38] http://goo.gl/PbyzK Until next time, Daniel Santa Cruz References 1. http://www.yesodweb.com/blog/2012/03/shelly-for-shell-scripts 2. http://www.reddit.com/r/haskell/comments/qt17p/shelly_write_your_shell_scri… 3. http://community.haskell.org/~simonmar/papers/aos.pdf 4. http://www.reddit.com/r/haskell/comments/qvyup/the_glasgow_haskell_compiler… 5. http://www.yesodweb.com/blog/2012/03/announcing-yesod-platform 6. http://www.reddit.com/r/haskell/comments/qxopq/announcing_the_yesod_platfor… 7. http://blog.johantibell.com/2012/03/cabal-of-my-dreams.html 8. http://www.reddit.com/r/haskell/comments/qwj5j/the_cabal_of_my_dreams/ 9. http://snapframework.com/blog/2012/03/16/snap-0.8-released 10. http://www.reddit.com/r/haskell/comments/qzzfe/announcing_snap_framework_v0… 11. http://www.haskell.org/pipermail/haskell-cafe/2012-March/100023.html 12. http://www.reddit.com/r/haskell/comments/qro91/leksah_012/ 13. http://www.reddit.com/r/haskell/comments/qy8h2/uncurrywith_and_concatenativ… 14. http://www.reddit.com/r/haskell/comments/qy8h2/uncurrywith_and_concatenativ… 15. http://www.cse.chalmers.se/edu/course/pfp/ 16. http://www.reddit.com/r/haskell/comments/quc64/new_course_on_parallel_funct… 17. http://about.travis-ci.org/blog/announcing_support_for_haskell_on_travis_ci/ 18. http://www.reddit.com/r/haskell/comments/qsyr5/announcing_haskell_project_s… 19. http://www.reddit.com/r/haskell/comments/qw99e/anyone_in_the_toronto_area_l… 20. http://www.reddit.com/r/haskell/comments/qw99e/anyone_in_the_toronto_area_l… 21. http://adit.io/posts/2012-03-10-building_a_concurrent_web_scraper_with_hask… 22. http://www.reddit.com/r/haskell/comments/qvp1u/building_a_concurrent_web_sc… 23. http://spin.atomicobject.com/2012/03/16/using-text-parsec-indent-to-parse-a… 24. http://www.reddit.com/r/haskell/comments/qzeyk/using_textparsecindent_to_pa… 25. http://www.reddit.com/r/haskell/comments/qsrmq/coapplicative_functor/ 26. http://www.reddit.com/r/haskell/comments/qsrmq/coapplicative_functor/ 27. http://community.galois.com/pipermail/halvm-devel/2012-March/000022.html 28. http://www.reddit.com/r/haskell/comments/qynuq/halvm_easy_install_on_fedora… 29. http://stackoverflow.com/questions/9732084/how-do-you-represent-a-graph-in-… 30. http://stackoverflow.com/questions/9732766/impredicative-types-vs-plain-old… 31. http://stackoverflow.com/questions/9658438/can-ghc-really-never-inline-map-… 32. http://stackoverflow.com/questions/9656734/redefine-io-to-simplify-debugging 33. http://stackoverflow.com/questions/9672686/quadratic-programming-in-haskell 34. http://stackoverflow.com/questions/9686665/how-can-i-test-a-higher-order-fu… 35. http://stackoverflow.com/questions/9722689/haskell-how-to-map-a-tuple 36. http://stackoverflow.com/questions/9684252/repacking-monads-any-generic-way 37. http://stackoverflow.com/questions/9709823/is-there-any-reason-not-to-use-t… 38. http://stackoverflow.com/questions/9713378/repa-3-performance-and-correct-u…

1 0

Rewrite rules not firing for rules matching multiple instance methods
by Acshi Haggenmiller 22 Mar '12

22 Mar '12

Hi, I have some rewrite rules set up and am finding that in the case where my rule pattern matches to instance methods and there are more than one of them, my rules do not fire. If they are simply taken out from being instance methods, they match just fine. I have posted more details and a code example to http://stackoverflow.com/questions/9811294/rewrite-rules-not-firing-for-rul… I appreciate the help, Acshi

1 0

MSIL manipulation libraries
by ARJANEN Loïc Jean David 21 Mar '12

21 Mar '12

Hello, Someone knows of a good MSIL library, to manipulate and output MSIL ? Regards, ARJANEN Loïc

2 1

Quickest way to pass Text to C code
by Yves Parès 21 Mar '12

21 Mar '12

Hello, I have to interact with a C++ library that accepts as string types (putting c++ strings aside) pointers of wchar_t (CWString in Haskell) or unsigned 32-bit int (Ptr Word32 for UTF-32 codepoints). I have read what text, bytestring and base provide, but Text can only be directly converted to (Ptr Word16), and if I use encodeUTF32 to get a ByteString, then I only get useAsCString, no direct conversion to CWString or Ptr WordXX is possible. Not to mention the extra memory allocations due to intermediate conversions. base provides Foreign.C.String.useAsCWString, but it requires that either I use simple Strings at the first place or (same thing than before) I convert from Text to String before passing to C. Is there something I'm missing or isn't this kind of conversion that easy?

3 4

ANN: Haskell Platform Versions Comparison Chart
by Simon Hengel 21 Mar '12

21 Mar '12

Hi, I compiled a chart that gives a side-by-side comparison of package versions in various Haskell Platform releases. http://sol.github.com/haskell-platform-versions-comparison-chart/ This includes both, packages that come with ghc and platform packages. Source is on GitHub[1]. Cheers, Simon [1] https://github.com/sol/haskell-platform-versions-comparison-chart

6 11

Are there arithmetic composition of functions?
by sdiyazg＠sjtu.edu.cn 21 Mar '12

21 Mar '12

By arithmetic I mean the everyday arithmetic operations used in engineering. In signal processing for example, we write a lot of expressions like f(t)=g(t)+h(t)+g'(t) or f(t)=g(t)*h(t). I feel it would be very natural to have in haskell something like g::Float->Float --define g here h::Float->Float --define h here f::Float->Float f = g+h --instead of f t = g t+h t --Of course, f = g+h is defined as f t = g t+h t I guess as long as all operands have the same number of arrows in there types then they should have the potential to be composed like this. Or g::Float->Float->Float --define g here h::Float->Float->Float --define h here f::Float->Float->Float f = g+h --means f x y = g x y + h x y -- f = g+h is defined as f x = g x+h x which in turn is defined as f x y = g x y+h x y This should be easy to implement, with TH perhaps. And I thought there would be a library (not in the language itself, of course) for this, but I haven't find one. Can someone tell me whether there is some implementation of such composition? If there isn't then I may build one.

15 25

good lightweight web-framework like sinatra?
by Greg Weber 21 Mar '12

21 Mar '12

WAI has CGI handlers, so you should be able to deploy a WAI-based framework like Yesod to CGI. If you want a simple framework, you can just use WAI directly. Scotty is a WAI framework that mimics Sinatra. Greg Weber

2 1

ANN: doctest-0.6.0 (support for ghc-7.4, please test)
by Simon Hengel 21 Mar '12

21 Mar '12

Hi, I just uploaded a new version of Doctest that adds support for ghc-7.4. http://hackage.haskell.org/package/doctest Doctest now comes with it's own parser and does not depend on Haddock anymore. This is a quite involved change, and it may still have issues. So pleases test, and do not be shy in reporting any bugs (either as a reply to this post or on [1]). Cheers, Simon [1] https://github.com/sol/doctest-haskell/issues

1 0

scoutess & Google Summer of Code project ideas for prospective students
by Alp Mestanogullari 21 Mar '12

21 Mar '12

Hi, I have written a blog post about potential GSoC projects that revolve around the scoutess project [1] and would, I believe, be useful to the Haskell community: http://alpmestan.wordpress.com/2012/03/21/scoutess-continuous-integration-c… Feel free to give your opinion and ask questions, in particular if you're a student interested in working on this. There also is a ticket on the Haskell GSoC trac [2]. [1] http://patch-tag.com/r/alpmestan/scoutess/wiki/ [2] http://hackage.haskell.org/trac/summer-of-code/ticket/1612 -- Alp Mestanogullari

1 0

struggling with blocking shell cmd when using System.Process
by Rouan van Dalen 20 Mar '12

20 Mar '12

Hi Everyone, I am strugling to get the following code to work correctly and I'm hoping someone will be able to shed some light as to why the code is behaving as it does: {-| runs the supplied command and args as a SHELL command -} runShell :: String -> IO (String, ExitCode) runShell shellCmd = do (_, Just hOut, Just hErr, hProcess) <- createProcess (shell shellCmd) { std_out = CreatePipe, std_err = CreatePipe } exitCode <- waitForProcess hProcess outTxt <- hGetContents hOut errTxt <- hGetContents hErr return $ (outTxt ++ errTxt, exitCode) If I try to su the runShell function with the following command, it simply blocks forever: git --no-pager blame <your git file here> but if I do: git status The runShell function works as expected. Can someone shed some light on why the 'git blame' command blocks? Interrestingly enough, if I remove the waitForProcess action, the 'git blame' command starts working as expected, but I no longer have my blocking behaviour, waiting for the command to finish and return the ExitCode. Regards Rouan

2 2

HughesPJ vs. Wadler-Leijen
by Evan Laforge 20 Mar '12

20 Mar '12

I've been trying to get my head around the Wadler-Leijen pretty printing combinators for a while now and having some trouble. Specifically, I have trouble getting them to pick optimal line breaks. The existing combinators like 'sep' (and everything built from it) merge all elements with <$> and then 'group' the whole thing, with the result that they either all go on one line, or get one line each. This is quite ugly for large lists of small elements. The other alternative is 'fillSep', which does a separate 'group' on each element. Unfortunately, it then tends to make very bad line wrapping decisions, e.g. you get: Rec { hi = "there" } Rec { hi = "there", hi = "there" , hi = "there" } Rec { lab = "short", label = [ 0, 1, 2 , 3, 4, 5 , 6, 7, 8 , 9, 10 , 11, 12 ] } No matter how much fancy 'group's and 'nest's and whatnot I threw in, it was always a choice between forcing wrapping on every element and looking ugly for many small elements, or trying to fit more into one line and having it wrap in the wrong place and wind up scrunched up on the right margin. Here's my latest attempt: list = commas PP.lbracket PP.rbracket . map format commas :: Doc -> Doc -> [Doc] -> Doc commas left right xs = PP.group $ left <+> punctuate (\x -> PP.group (x <$$> PP.comma <> PP.space)) xs <$> right punctuate :: (Doc -> Doc) -> [Doc] -> Doc punctuate f [] = mempty punctuate f [x] = x punctuate f (x:xs) = f x <> punctuate f xs record :: Doc -> [(String, Doc)] -> Doc record title fields = PP.group $ PP.hang 2 $ title <$> (commas PP.lbrace PP.rbrace (map f fields)) where f (label, field) = PP.hang 2 $ PP.group $ PP.text label <+> PP.equals <$> field But the thing is, the HughesPJ-using Language.Haskell.Pretty in haskell-src gets the line wrapping just right. So I investigated how it works, and it's very simple, here's the reduced version: class Pretty a where format :: a -> Doc list :: (Pretty a) => [a] -> Doc list = bracket_list . PP.punctuate PP.comma . map format fsep' :: [Doc] -> Doc fsep' [] = PP.empty fsep' (d:ds) = PP.nest 2 (PP.fsep (PP.nest (-2) d:ds)) bracket_list :: [Doc] -> Doc bracket_list = PP.brackets . PP.fsep brace_list :: [Doc] -> Doc brace_list = PP.braces . PP.fsep record :: Doc -> [(String, Doc)] -> Doc record title fields = title <> (brace_list (map field fields)) where field (name, val) = fsep' [PP.text name, PP.equals, val] ---- This formats records like so: Rec{hi = "there"} Rec{hi = "there" hi = "there" hi = "there"} Rec{label = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30] label = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]} Rec{lab = "short" label = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]} Much better, even if it's not my preferred style! Of course WL doesn't have fsep or the negative nest craziness (I don't even know what it's doing there), but it has the more general 'group'. However, no matter how complicated I got with WL it just never came out right. In contrast, a very simple HughesPJ implementation gets it right. The thing is, when trying to figure out which pretty print library to use, the consensus is that WL is just all around better, even though HughesPJ is somewhat standard (but there are 6 WL variants on hackage, and no (?) HughesPJ ones). So am I just using it wrong? If I translate the HughesPJ one over directly into LW, here's what I get: Rec{hi = "there"} Rec{hi = "there", hi = "there", hi = "there"} Rec{label = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30], label = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]} Rec{lab = "short", label = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]} So is WL really all it's cracked up to be? Am I using it wrong? I was going to suggest some consolidation in the pretty printing library packages, but now I'm not even sure which style should "win"...

3 6

hasktags - etags backend, any reason to not remove function arguments from tag files?
by Marc Weber 20 Mar '12

20 Mar '12

Hi list, hasktags -e generates lines containing the function arguments. For Example: f0 foo\x7f5,6 The "foo" is the function arg, not the keyword to be tagged. Emacs still finds f0, but it does not show it in tab completion using m-x find-tag .. I don't use Emacs myself. That's why I want to ask whether this is intention or a bug. The fix is trivial: use "name" of FoundThing instead of "concat (take (token + 1) $ spacedwords fullline)" Is there any reason to keep current behaviour? Marc Weber

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Re: [Haskell-cafe] Are there arithmetic composition of functions?
by Denis Moskvin 20 Mar '12

20 Mar '12

> > > From: Felipe Almeida Lessa <felipe.lessa(a)gmail.com> > To: sdiyazg(a)sjtu.edu.cn > Cc: haskell-cafe(a)haskell.org > Date: Mon, 19 Mar 2012 14:24:13 -0300 > Subject: Re: [Haskell-cafe] Are there arithmetic composition of functions? > import Control.Applicative > > f, g :: Float -> Float > f x = x + 1 > g x = 2 * x > > h = (+) <$> f <*> g > > > Cheers, =) > > -- > Felipe. > > Monadic version: import Control.Monad import Control.Monad.Instances (+.) :: Num a => (a -> a) -> (a -> a) -> a -> a (+.) = liftM2 (+) (+..) :: Num a => (a -> a -> a) -> (a -> a -> a) -> a -> a -> a (+..) = liftM2 $ liftM2 (+) infixl 6 +., +.. > (sin +. cos) (pi/4) 1.414213562373095 > ((*) +.. (/)) 4 2 10.0 Deniok

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Google Summer of Code idea of project & application
by Damien Desfontaines 20 Mar '12

20 Mar '12

Dear members of Haskell Cafe, My name is Damien Desfontaines, and I'm currently following a Theoretical Computer Science Major at the École Normale Supérieure in Paris, which is one of the most selective universities in France. To complete my curriculum, I am to find a three-month internship related to something I have learnt, including lambda-calculus, compilers, graph theory, languages theory, and complexity & calculability theories. I wish to make it useful for the open-source community, for example by working for the Haskell group through the Google Summer of Code Program. The project I suggest is mainly inspired by Ticket #1555 [1] : I think that would be a great idea to make it possible to call some Haskell code into OCamL. In particular, this would contribute to the spreading of Haskell in countries where OCamL is proeminent, mainly France and Italy. The idea would be the following : building a translator which would turn Haskell code into (purely functional) OCamL code, in order to enable the use of Haskell functions and libraries within OCamL programs, in a "human-readable" way (the OCamL source code generated would ideally be understandable enough to be manually modified). I am well aware that one of the main issues lies in dealing with the conversion between the two types systems, especially because of Haskell's lazy evaluation and typeclasses. However, such a challenge really motivates me to spend hours looking for the most efficient and usable solution, I would really enjoy spending three months on such problems. However, I sincerely think I have the skills needed to go through such a project. Indeed, I have a long experience with OCamL. I used this language to automatically write the undecidable Gödel proposition in Peano arithmetics, using Tarski's original proof [2]. I have written a mini-Java compiler with it [3], a MISP microprocessor simulator along with three friends [4], and many other things for studies and personal entertainment. My experience of Haskell is shorter, I learnt it with two books, "Learn You a Haskell for Great Good !" and "Real World Haskell" (you are the co-author of this last book, aren't you ? Well, congratulations, it's really well-written and precise !). I use it mainly for mathematical-oriented works, and for coding competitions such as Google Jam or Prologin. I also have a little experience with open-source software : I have been using Debian Linux for 4 years, so I know the basics of UNIX administration. I know how to write a bug report, a man-page, a README, etc. I spend a lot of time helping others on french IRC channels, Linux- or programmation-oriented. I contribute to the Weboob project [5] by writing documentation and new modules. This, in my opinion, makes me quite autonomous, and able to solve issues by myself, by reading documentation, manuals or language specifications. Apart from that, I have never been engaged in development of "large" projects such as Haskell - but I am ready and excited to start ! I am confident I can interact efficiently with the Haskell community: if you would like me to create a blog syndicated to Planet Haskell, in which I would explain my work every one or two weeks, I can do that. I know how to read a man-page, a language specification or a system documentation, so I consider myself as quite autonomous to answer questions I could ask myself during a project. Furthermore, please let me know if you think my skills could be useful to another Haskell project. I look forward to the opportunity to discuss this idea with one or several developers, Damien Desfontaines [1] http://hackage.haskell.org/trac/summer-of-code/ticket/1555 [2] http://www.eleves.ens.fr/home/desfonta/godel/godel.ml [3] http://www.eleves.ens.fr/home/desfonta/ProjetCompil/ [4] https://github.com/kyoDralliam/CircuitSimulator [5] http://weboob.org/

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Google Summer of Code 2012
by Edward Kmett 19 Mar '12

19 Mar '12

It is that time of year again; the Google Summer of Code is upon us! If you are a student and want to sign up to make $5,000 for hacking on the code you love over the summer or are willing to help out as a mentor, now is the time to act. Please sign up by adding your name to the list at: http://hackage.haskell.org/trac/summer-of-code/wiki/People2012 Our main wiki entry for the Summer of Code 2012 is now online: http://hackage.haskell.org/trac/summer-of-code/wiki/Soc2012 If you are interested in applying as a student, here are some guidelines: http://hackage.haskell.org/trac/summer-of-code/wiki/StudApply2012 Student signups with Google start next week, but it is a good idea to start discussing things with potential mentors as soon as possible. The official * deadline* for student applications with Google is April 6th, but if you are interested please add your name to the People2012 page above, as soon as possible! *Interested in helping out, but not sure how?* There are a number of ideas that we've been collecting for projects available through the trac http://hackage.haskell.org/trac/summer-of-code/report/1 and which have been collecting over time in a reddit we used to use in years past: http://www.reddit.com/r/haskell_proposals/top/ While that reddit and the larger http://reddit.com/r/haskell reddit is a great place to discuss proposals, we had to choose one mechanism to supply Google with our ideas officially, so please submit proposals to the trac if you can think of something you like to work on, or which would be an appropriately beneficial project for a summer worth of work. *Helping Out Darcs* Haskell.org is planning to continue to act as an umbrella organization covering darcs for the Summer of Code once more. If you are interested in helping them out, they also have a page full of project ideas as well: http://wiki.darcs.net/GoogleSummerOfCode Again, please submit proposals as tickets to the trac if you can think of something you'd like to work on or have a neat idea for something someone else might want to work on (especially if you'd be willing to mentor it!) Of course, you can always feel free to email me or inquire on irc.freenode.net on #haskell or #haskell-soc or with the darcs folks over on #darcs for more information. If you don't have an IRC client, you can get there from: http://webchat.freenode.net Thank you all very much and we look forward to another successful Summer of Code! -Edward Kmett

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ANN: unification-fd 0.7.0
by wren ng thornton 19 Mar '12

19 Mar '12

-------------------------------------------- -- unification-fd 0.7.0 -------------------------------------------- The unification-fd package offers generic functions for single-sorted first-order structural unification (think of programming in Prolog, or of the metavariables in type inference)[1][2]. The library *is* sufficient for implementing higher-rank type systems a la [Peyton Jones, Vytiniotis, Weirich, Shields], but bear in mind that unification variables are the metavariables of type inference--- not the type-variables. An effort has been made to make the package as portable as possible. However, because it uses the ST monad and the mtl-2 package it can't be H98 nor H2010. However, it only uses the following common extensions which should be well supported[3]: Rank2Types MultiParamTypeClasses FunctionalDependencies -- Alas, necessary for type inference FlexibleContexts -- Necessary for practical use of MPTCs FlexibleInstances -- Necessary for practical use of MPTCs UndecidableInstances -- For Show instances due to two-level types -------------------------------------------- -- Changes (since 0.6.0) -------------------------------------------- This release is another major API breaking release. Apologies, but things are a lot cleaner now and hopefully the API won't break again for a while. The biggest change is that the definition of terms has changed from the previous: data MutTerm v t = MutVar !v | MutTerm !(t (MutTerm v t)) To the much nicer: data UTerm t v = UVar !v | UTerm !(t (UTerm t v)) The old mnemonic of "mutable terms" was inherited from the code's previous life implementing a logic programming language; but when I was playing around with implementing a type checker I realized that the names don't really make sense outside of that original context. So the new mnemonic is "unification terms". In addition to being a bit shorter, it should help clarify the separation of concerns (e.g., between unification variables vs lambda-term variables, type variables, etc.). The swapping of the type parameters is so that UTerm can have instances for Functor, Monad, etc. This change should've been made along with the re-kinding of variable types back in version 0.6.0, since the UTerm type is the free monad generated by t. I've provided all the category theoretic instances I could imagine some plausible reason for wanting. Since it's free, there are a bunch more I haven't implemented since they don't really make sense for structural terms (e.g., MonadTrans, MonadWriter, MonadReader, MonadState, MonadError, MonadCont). If you can come up with some compelling reason to want those instances, I can add them in the future. Since the order of type parameters to BindingMonad, UnificationFailure, Rank, and RankedBindingMonad was based on analogy to the order for terms, I've also swapped the order in all of them for consistency. I've removed the eqVar method of the Variable class, and instead added an Eq superclass constraint. Again, this should've happened with the re-kinding of variables back in version 0.6.0. A major benefit of this change is that now you can use all those library functions which require Eq (e.g., many of the set-theoretic operations on lists, like (\\) and elem). I've added new functions: getFreeVarsAll, applyBindingsAll, freshenAll; which are like the versions without "All", except they're lifted to operate over Foldable/Traversable collections of terms. This is crucial for freshenAll because it allows you to retain sharing of variables among the collection of terms. Whereas it's merely an optimization for the others (saves time for getFreeVarsAll, saves space for applyBindingsAll). The type of the seenAs function has also changed, to ensure that variables can only be seen as structure rather than as any UTerm. Thanks to Roman Cheplyaka for suggesting many of these changes. -------------------------------------------- -- Description -------------------------------------------- The unification API is generic in the type of the structures being unified and in the implementation of unification variables, following the two-level types pearl of Sheard (2001). This style mixes well with Swierstra (2008), though an implementation of the latter is not included in this package. That is, all you have to do is define the functor whose fixed-point is the recursive type you're interested in: -- The non-recursive structure of terms data S a = ... -- The recursive term type type PureTerm = Fix S And then provide an instance for Unifiable, where zipMatch performs one level of equality testing for terms and returns the one-level spine filled with pairs of subterms to be recursively checked (or Nothing if this level doesn't match). class (Traversable t) => Unifiable t where zipMatch :: t a -> t b -> Maybe (t (a,b)) The choice of which variable implementation to use is defined by similarly simple classes Variable and BindingMonad. We store the variable bindings in a monad, for obvious reasons. In case it's not obvious, see Dijkstra et al. (2008) for benchmarks demonstrating the cost of naively applying bindings eagerly. There are currently two implementations of variables provided: one based on STRefs, and another based on a state monad carrying an IntMap. The former has the benefit of O(1) access time, but the latter is plenty fast and has the benefit of supporting backtracking. Backtracking itself is provided by the logict package and is described in Kiselyov et al. (2005). In addition to this modularity, unification-fd implements a number of optimizations over the algorithm presented in Sheard (2001)--- which is also the algorithm presented in Cardelli (1987). * Their implementation uses path compression, which we retain. Though we modify the compression algorithm in order to make sharing observable. * In addition, we perform aggressive opportunistic observable sharing, a potentially novel method of introducing even more sharing than is provided by the monadic bindings. Basically, we make it so that we can use the observable sharing provided by the modified path compression as much as possible (without introducing any new variables). * And we remove the notoriously expensive occurs-check, replacing it with visited-sets (which detect cyclic terms more lazily and without the asymptotic overhead of the occurs-check). A variant of unification which retains the occurs-check is also provided, in case you really need to fail fast. * Finally, a highly experimental branch of the API performs *weighted* path compression, which is asymptotically optimal. Unfortunately, the current implementation is quite a bit uglier than the unweighted version, and I haven't had a chance to perform benchmarks to see how the constant factors compare. Hence moving it to an experimental branch. These optimizations pass a test suite for detecting obvious errors. If you find any bugs, do be sure to let me know. Also, if you happen to have a test suite or benchmark suite for unification on hand, I'd love to get a copy. -------------------------------------------- -- Notes and limitations -------------------------------------------- [1] At present the library does not appear amenable for implementing higher-rank unification itself; i.e., for higher-ranked metavariables, or higher-ranked logic programming. To be fully general we'd have to abstract over which structural positions are co/contravariant, whether the unification variables should be predicative or impredicative, as well as the isomorphisms of moving quantifiers around. It's on my todo list, but it's certainly non-trivial. If you have any suggestions, feel free to contact me. [2] At present it is only suitable for single-sorted (aka untyped) unification, a la Prolog. In the future I aim to support multi-sorted (aka typed) unification, however doing so is complicated by the fact that it can lead to the loss of MGUs; so it will likely be offered as an alternative to the single-sorted variant, similar to how the weighted path-compression is currently offered as an alternative. [3] With the exception of fundeps which are notoriously difficult to implement. However, they are supported by Hugs and GHC 6.6, so I don't feel bad about requiring them. Once the API stabilizes a bit more I plan to release a unification-tf package which uses type families instead, for those who feel type families are easier to implement or use. There have been a couple requests for unification-tf, so I've bumped it up on my todo list. -------------------------------------------- -- References -------------------------------------------- Luca Cardelli (1987) /Basic polymorphic typechecking/. Science of Computer Programming, 8(2):147--172. Atze Dijkstra, Arie Middelkoop, S. Doaitse Swierstra (2008) /Efficient Functional Unification and Substitution/, Technical Report UU-CS-2008-027, Utrecht University. <http://www.cs.uu.nl/research/techreps/repo/CS-2008/2008-027.pdf> Simon Peyton Jones, Dimitrios Vytiniotis, Stephanie Weirich, Mark Shields /Practical type inference for arbitrary-rank types/, to appear in the Journal of Functional Programming. (Draft of 31 July 2007.) Oleg Kiselyov, Chung-chieh Shan, Daniel P. Friedman, and Amr Sabry (2005) /Backtracking, Interleaving, and/ /Terminating Monad Transformers/, ICFP. <http://www.cs.rutgers.edu/~ccshan/logicprog/LogicT-icfp2005.pdf> Tim Sheard (2001) /Generic Unification via Two-Level Types/ /and Paramterized Modules/, Functional Pearl, ICFP. <http://web.cecs.pdx.edu/~sheard/papers/generic.ps> Tim Sheard & Emir Pasalic (2004) /Two-Level Types and/ /Parameterized Modules/. JFP 14(5): 547--587. This is an expanded version of Sheard (2001) with new examples. <http://web.cecs.pdx.edu/~sheard/papers/JfpPearl.ps> Wouter Swierstra (2008) /Data types a la carte/, Functional Pearl. JFP 18: 423--436. <http://www.cs.ru.nl/~wouters/Publications/DataTypesALaCarte.pdf> -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/unification-fd Darcs: http://community.haskell.org/~wren/unification-fd Haddock (Darcs version): http://community.haskell.org/~wren/unification-fd/dist/doc/html/unification… -- Live well, ~wren

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Re: [Haskell-cafe] Best way to use the google visualization (javascript) API rather than static image charts (hs-gchart)?
by Ryan Newton 19 Mar '12

19 Mar '12

Thanks David, As an update, HJScript is a bit of a nightmare to figure out (missing documentation, examples). And the model of JS values: http://hackage.haskell.org/packages/archive/language-javascript/0.4.10/doc/… Seems less human comprehensible than, for example, WebBits': http://hackage.haskell.org/packages/archive/WebBits/2.0/doc/html/BrownPLT-J… However, jmacro is a breeze for spitting out JS code. The little script below will make a simple line plot with google charts. I'll give flot a try latter. Zooming / panning sounds nice. Cheers, -Ryan Aforementioned script: ------------------------------------------------------------------------------- {-# LANGUAGE QuasiQuotes #-} import Language.Javascript.JMacro hdr :: String hdr = "<html> <head> <script type=\"text/javascript\" src=\" https://www.google.com/jsapi\"></script> <script type=\"text/javascript\">" ftr :: String ftr = " </script> </head> <body> <div id=\"chart_div\" style=\"width: 900px; height: 500px;\"></div> </body> </html>" testdata :: [(String, Int, Int)] testdata = [ ("2004", 100, 400), ("2005", 1170, 460), ("2006", 860, 580), ("2007", 1030, 540) ] -- | This provides a set of basic functional programming primitives, a few utility functions -- and, more importantly, a decent sample of idiomatic jmacro code. View the source for details. -- body :: JStat body :: (ToJExpr a3, ToJExpr a2, ToJExpr a1, ToJExpr a) => (a, a1, a2) -> a3 -> JStat body (title,line1,line2) testdata = [$jmacro| google.load("visualization", "1", {packages:["corechart"]}); fun drawChart { var dat = new google.visualization.DataTable(); dat.addColumn('string', `(title)` ); dat.addColumn('number', `(line1)` ); dat.addColumn('number', `(line2)` ); // -- Here's our data... this can get BIG: dat.addRows( `(testdata)` ); var options = { title: `(title)` }; var chart = new google.visualization.LineChart(document.getElementById('chart_div')); chart.draw(dat, options); } google.setOnLoadCallback(drawChart); |] main = do putStrLn hdr print$ renderJs$ body ("blah","line1","line2") testdata putStrLn ftr

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Stack overflow while programming imperatively
by Clark Gaebel 18 Mar '12

18 Mar '12

Hey list. I was recently fixing a space leak by dropping down to imperative programming in a section of my code, when it started developing space leaks of its own. I found the problem though - it was my "for loop": http://hpaste.org/65514 Can anyone provide suggestions on why that stack overflows? It seems ridiculously tail recursive. I tried to do it more haskell-like with http://hpaste.org/65517, but it was still spending 75% of its time in GC. Is there any way to write such a loop with ~100% productivity? I don't think there should be _any_ garbage generated, as all values may be stack allocated. Is this a performance regression in GHC? Thanks, - clark

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Summer of Code idea: Haskell Web Toolkit
by Alejandro Serrano Mena 18 Mar '12

18 Mar '12

Hi, I'm really looking forward to helping in the Summer of Code, if Haskell goes into it this year (something I take for granted :). I would like to propose an idea for a project, and I'm looking for suggestions about whether it's good, should be improved or it's just unfeasible. My idea is to make a client-side Haskell Web Toolkit, in the spirit of Google Web Toolkit, which would allow to program in Haskell the client part of a web application, and would complement the web frameworks already existing for Haskell (such as Yesod and Snap). The point is coming about with a Haskell-ish way to program applications, to reuse all the existing knowledge for our beloved language. I've added more details in a pre-proposal in Google Docs, available in https://docs.google.com/document/d/1FnTNO9uTobDHRTDXWurKns7vGTjeauw0nRhbtt6… Tell me if you prefer to see it in other format, but I didn't want to generate a bigger e-mail. Thanks in advance.

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