April 2009 - Haskell-Cafe

GHC 6.10.2 compatible problem.
by Andy Stewart 24 Apr '09

24 Apr '09

Hi, all! I saw GHC release new version now, and fix some bug. I want to know below pacakges whether works fine with GHC-6.10.2 before i upgrade it: cabal, gtk2hs, xmonad, yi, leksah Thanks! -- Andy

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Being impure within a 'pure' function
by Daniel K. 24 Apr '09

24 Apr '09

Hello, imagine the following situation: You want to implement e.g. Dijkstra's algorithm to find a shortest path between nodes u and v in a graph. This algorithm relies heavily on mutating arrays, so the type signature would look something like getDistance :: Graph -> Node -> Node -> IO Int Not mutating the underlying arrays would probably result in poor performance. BUT: For a constant graph, the distance between two nodes stays the same all the time, so in fact getDistance should be a pure function! So here is my question: Is there a way to write functions in Haskell that do some IO internally, but that are guaranteed to be side-effect free? Of course one would have to make sure that the array that is mutated inside getDistance must not be accessible from outside the function. Is that possible? If not, wouldn't that be desirable? If not, why not? Thanks Daniel

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ANNOUNCE: vacuum-cairo: a cairo frontend to vacuum for live Haskell data visualization
by Don Stewart 24 Apr '09

24 Apr '09

I am pleased to announce the release of vacuum-cairo, a Haskell library for interactive rendering and display of values on the GHC heap using Matt Morrow's vacuum library. This library takes vacuum's output, generates dot graph format from it, renders it to SVG with graphviz, and displays the resulting structure using the gtk2hs Cairo vector graphics bindings ... all at the GHCi command line. This tool is useful for examining Haskell data structures as they are represented directly in the heap. In particular, it makes sharing visible for the first time, as well as unboxed values. It should be useful for teaching Haskell, or for library authors debugging the design of their data structures. You can see pictures of the rendered display here: http://code.haskell.org/~dons/images/vacuum/intmap.png And youtube screencasts of vacuum-cairo in action: http://www.youtube.com/watch?v=oujaqo9GAmA Get it: cabal install vacuum-cairo And on Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/vacuum-cairo -- Don

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darcs hacking sprint 2 and hac5 report
by Eric Kow 24 Apr '09

24 Apr '09

Dear darcs users and Haskellers, Here's our report from the 2nd darcs hacking sprint, held as part of the Haskell Hackathon. http://blog.darcs.net/2009/04/darcs-hacking-sprint-2-report.html Thanks very much to the Hac5 team, to Hac5 sponsors, darcs donors, and to all participants at the Hackathon. We really enjoyed meeting everybody and hope to see you soon at the next event! -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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Confusing change causing space leak
by Denis Bueno 24 Apr '09

24 Apr '09

Hi all, I discussed this with a few people in #haskell-in-depth, and we thought I should send this to the list. The following paste shows the only changes required to remove a space leak: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=4185#a4185. Summary: if I reduce a tuple-from-IO to whnf in the "allocate" function passed to "bracket", no leak. For example, this also doesn't leak: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=4185#a4189 That paste page also contains the code for "externalSort". The "externalSort" function used in the example corresponds to the "sort" function provided by the Algorithms.ExternalSort module. Can anyone explain why the leak happens at all, or why it goes away? Denis

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Template Haskell + Profiling
by Krzysztof Skrzętnicki 24 Apr '09

24 Apr '09

Hi I'm trying to build some modules that use TH with profiling enabled. The normal build works: $ ghc -fforce-recomp --make -cpp -O -DXQUERY2_TEST Main [1 of 8] Compiling Text.XML.Expat.FormatCustom ( Text/XML/Expat/FormatCustom.hs, Text/XML/Expat/FormatCustom.o ) [2 of 8] Compiling DataStructures ( DataStructures.hs, DataStructures.o ) Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Loading package syb ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package packedstring-0.1.0.1 ... linking ... done. Loading package containers-0.2.0.1 ... linking ... done. Loading package pretty-1.0.1.0 ... linking ... done. Loading package template-haskell ... linking ... done. Loading package mtl-1.1.0.2 ... linking ... done. Loading package filepath-1.1.0.2 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.2 ... linking ... done. Loading package unix-2.3.2.0 ... linking ... done. Loading package directory-1.0.0.3 ... linking ... done. Loading package process-1.0.1.1 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. Loading package derive-0.1.4 ... linking ... done. [3 of 8] Compiling Utils ( Utils.hs, Utils.o ) [4 of 8] Compiling Semantics.Pure ( Semantics/Pure.hs, Semantics/Pure.o ) [5 of 8] Compiling Database ( Database.hs, Database.o ) [6 of 8] Compiling XPathDSL ( XPathDSL.hs, XPathDSL.o ) [7 of 8] Compiling QueryParse ( QueryParse.hs, QueryParse.o ) [8 of 8] Compiling Main ( Main.hs, Main.o ) Linking Main ... But profiling one fails: $ ghc -prof -auto -auto-all -fforce-recomp --make -cpp -O -DXQUERY2_TEST Main [1 of 8] Compiling Text.XML.Expat.FormatCustom ( Text/XML/Expat/FormatCustom.hs, Text/XML/Expat/FormatCustom.o ) [2 of 8] Compiling DataStructures ( DataStructures.hs, DataStructures.o ) Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. DataStructures.hs:1:0: Dynamic linking required, but this is a non-standard build (eg. prof). You need to build the program twice: once the normal way, and then in the desired way using -osuf to set the object file suffix. The error message is somewhat cryptic to me. Does anyone know how to perform profiling build when using TH? Regards Christopher Skrzętnicki

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(integer) linear programming
by Johannes Waldmann 24 Apr '09

24 Apr '09

Hi. I'm looking for linear programming solvers in Haskell (or a Haskell binding for glpk, or similar). - Thanks, J.W.

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ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
by atze＠cs.uu.nl 24 Apr '09

24 Apr '09

Utrecht Haskell Compiler -- first release, version 1.0.0 ======================================================== The UHC team is happy to announce the first public release of the Utrecht Haskell Compiler (UHC). UHC supports almost all Haskell98 features plus many experimental extensions. The compiler runs on MacOSX, Windows (cygwin), and various Unix flavors. Features: * Multiple backends, including a bytecode interpreter backend and a GRIN based, full program analysing backend, both via C. * Experimental language extensions, some of which have not been implemented before. * Implementation via attribute grammars and other high-level tools. * Ease of experimentation with language variants, thanks to an aspect-oriented internal organisation. Getting started & Download -------------------------- UHC is available for download as source distribution via the UHC home page: http://www.cs.uu.nl/wiki/UHC Here you will also find instructions to get started. Status of the implementation ---------------------------- Like any university project UHC is very much work in progress. We feel that it is mature and stable enough to offer to the public, but much work still needs to be done; hence we welcome contributions by others. UHC grew out of our Haskell compiler project (called Essential Haskell Compiler, or EHC) over the past 5 years. UHC internally is organised as a combination of separate aspects, which makes UHC very suitable to experiment with; it is relatively easy to build compilers for sublanguages, or to generate related tools such as documentation generators, all from the same code base. Extensions to the language can be described separately, and be switched on or of as need arises. Warning ------- Although we think that the compiler is stable enough to compile subtantial Haskell programs, we do not recommend yet to use it for any serious development work in Haskell. We ourselves use the GHC as a development platform! We think however that it provides a great platform for experimenting with language implementations, language extensions, etc. Mailing lists ------------- For UHC users and developers respectively: http://mail.cs.uu.nl/mailman/listinfo/uhc-users http://mail.cs.uu.nl/mailman/listinfo/uhc-developers Bug reporting ------------- Please report bugs at: http://code.google.com/p/uhc/issues/list The UHC Team -- Atze Dijkstra, Department of Information and Computing Sciences. /|\ Utrecht University, PO Box 80089, 3508 TB Utrecht, Netherlands. / | \ Tel.: +31-30-2534118/1454 | WWW : http://www.cs.uu.nl/~atze . /--| \ Fax : +31-30-2513971 .... | Email: atze(a)cs.uu.nl ............ / |___\

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"Search" vs "search" vs "search"
by haskell＠kudling.de 24 Apr '09

24 Apr '09

Hi, I have some usability issues with the Haskell homepage. What is the best place to discuss them? 1) "Search" vs "search" vs "search" The front page has 3 search boxes: * at the top with the buttons "Go" and "Search" * the third link on the left called "Search" * the next link below "Search haskell.org" What is the difference? Is it really necessary to have 3 search boxes? Most webpages have only one. 2) "Libraries" vs "Packages" The front pages says "Find a library" and links to Hackages which lists "packages". To reduce confusion, i think we should settle to the usage of only "packages". Opinions? 3) Haskell frontpage in general I also would like to discuss a general simplification. The Haskell wiki provides a lot of deep information but the front page is overwhelming. The Haskell front page offers more than 50 links = choices. Compare that with http://rubyonrails.org/ (my favorite) http://www.ruby-lang.org/ http://www.python.org/ (also a bit laden) Regarding the rule of 7 we should reduce the information we present on the front page. Who are our visitors and what do they want? * Beginners. People who have no clue what Haskell is want to find answers to the following questions ** What is Haskell? ** How does Haskell look and "feel"? ** Why should i care? ** Who else i using Haskell? ** How do i get started? * Intermediates. People who are getting an idea what Haskell is but want to find out more about it, want to find answers to the following questions ** Where can i read some tutorials? ** Where can i read the API documentation? ** What is going on in the Haskell comunity? * Experts. People who know a lot about Haskell, but come back here, because they want to find answers to the following questions ** What is going on in the Haskell comunity? ** Where can i read about Haskell's future? I'd propose something like the following grouping: * About Haskell ** What is Haskell? ** How does Haskell feels like? ** Haskell success stories ** Getting Started *** Installation guide *** First steps guides * Download * Documentation ** Tutorials ** How to install packages via Cabal/Hackages ** API documentation *** Static pages *** Hoogle ** Community *** Blogs *** Events *** Projects * News See also http://www.haskell.org/haskellwiki/Talk:Haskell bye, lenny

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Re: [Haskell-cafe] Overriding a Prelude function?
by michael rice 24 Apr '09

24 Apr '09

OK, I changed the operator from (>>) to (~>>). When I try to use it I get this: [michael@localhost ~]$ ghci rand GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( rand.hs, interpreted ) Ok, modules loaded: Main. *Main> rollDie ~>> (rollDie ~>> rollDie) <interactive>:1:0: No instance for (Show (Seed -> (Int, Seed))) arising from a use of `print' at <interactive>:1:0-32 Possible fix: add an instance declaration for (Show (Seed -> (Int, Seed))) In a stmt of a 'do' expression: print it *Main> Michael --- On Wed, 4/22/09, Luke Palmer <lrpalmer(a)gmail.com> wrote: From: Luke Palmer <lrpalmer(a)gmail.com> Subject: Re: [Haskell-cafe] Overriding a Prelude function? To: "michael rice" <nowgate(a)yahoo.com> Cc: "Ross Mellgren" <rmm-haskell(a)z.odi.ac>, "Dan Weston" <westondan(a)imageworks.com>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Wednesday, April 22, 2009, 5:02 PM On Wed, Apr 22, 2009 at 1:47 PM, michael rice <nowgate(a)yahoo.com> wrote: Here's what I get: [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> import Prelude hiding ((>>)) You know, to avoid this nonsense you could just name the operator something else, like >>~, or ~>>, or $@**!. Operators are just names. Luke <interactive>:1:0: parse error on input `import' Prelude> ===== I was passing seed0 to rollDie and getting back (r1,seed1) passing seed1 to rollDie and getting back (r2,seed2) passing seed2 to rollDie and getting back (r3,seed3) Just based on the problem text, I would guess that passing rollDie and seed0 to (>>) I would get back (r3,seed3), losing the intermediate random numbers r1 and r2 along the way, at least that's what I understood it to say. So, I know that next I'm probably going to have to do something to remedy that, but I haven't gotten to that next step yet. What is unsugar? Thanks in advance for your patience. Michael --- On Wed, 4/22/09, Dan Weston <westondan(a)imageworks.com> wrote: From: Dan Weston <westondan(a)imageworks.com> Subject: Re: [Haskell-cafe] Overriding a Prelude function? To: "Ross Mellgren" <rmm-haskell(a)z.odi.ac> Cc: "michael rice" <nowgate(a)yahoo.com>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Wednesday, April 22, 2009, 12:37 PM Be aware that the do unsugars to (Prelude.>>), not your (>>), even if you hide (Prelude.>>): import Prelude hiding ((>>)) m >> f = error "Call me!" main = putStrLn . show $ do [3,4] [5] The desugaring of the do { [3,4]; [5] } is (Prelude.>>) [3,4] [5] = [5,5], whereas you might have hoped for [3,4] >> [5] = error "Call me!" Dan Ross Mellgren wrote: > I think > > import Prelude hiding ((>>)) > > does that. > > -Ross > > On Apr 22, 2009, at 11:44 AM, michael rice wrote: > >> I've been working through this example from: http://en.wikibooks.org/wiki/Haskell/Understanding_monads >> >> I understand what they're doing all the way up to the definition of (>>), which duplicates Prelude function (>>). To continue following the example, I need to know how to override the Prelude (>>) with the (>>) definition in my file rand.hs. >> >> Michael >> >> ============== >> >> [michael@localhost ~]$ cat rand.hs >> import System.Random >> >> type Seed = Int >> >> randomNext :: Seed -> Seed >> randomNext rand = if newRand > 0 then newRand else newRand + 2147483647 >> where newRand = 16807 * lo - 2836 * hi >> (hi,lo) = rand `divMod` 127773 >> >> toDieRoll :: Seed -> Int >> toDieRoll seed = (seed `mod` 6) + 1 >> >> rollDie :: Seed -> (Int, Seed) >> rollDie seed = ((seed `mod` 6) + 1, randomNext seed) >> >> sumTwoDice :: Seed -> (Int, Seed) >> sumTwoDice seed0 = >> let (die1, seed1) = rollDie seed0 >> (die2, seed2) = rollDie seed1 >> in (die1 + die2, seed2) >> >> (>>) m n = \seed0 -> >> let (result1, seed1) = m seed0 >> (result2, seed2) = n seed1 >> in (result2, seed2) >> >> [michael@localhost ~]$ >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org <mailto:Haskell-Cafe@haskell.org> >> http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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