- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] [Haskell] select(2) or poll(2)-like function?
by Don Stewart 18 Apr '11

18 Apr '11

Redirecting to haskell-cafe@, where this kind of long discussion belongs. On Mon, Apr 18, 2011 at 9:07 AM, Colin Adams <colinpauladams(a)googlemail.com> wrote: > > > On 18 April 2011 16:54, Ertugrul Soeylemez <es(a)ertes.de> wrote: >> >> > >> > Well, *someone* has to worry about robustness and scalability. Users >> > notice when their two minute system builds start taking four minutes >> > (and will be at my door wanting me to fix it) because something didn't >> > scale fast enough, or have to be run more than once because a failing >> > component build wasn't restarted properly. I'm willing to believe that >> > haskell lets you write more scalable code than C, but C's tools for >> > handling concurrency suck, so that should be true in any language >> > where someone actually thought about dealing with concurrency beyond >> > locks and protected methods. The problem is, the only language I've >> > found where that's true that *also* has reasonable tools to deal with >> > scaling beyond a single system is Eiffel (which apparently abstracts >> > things even further than haskell - details like how concurrency is >> > achieved or how many concurrent operations you can have are configured >> > when you start an application, *not* when writing it). Unfortunately, >> > Eiffel has other problems that make it undesirable. >> >> I can't make a comparison, because I don't know Eiffel. > > I do, and I don't recognize what the OP is referring to - I suspect he meant > Erlang. > > -- > Colin Adams > Preston, Lancashire, ENGLAND > () ascii ribbon campaign - against html e-mail > /\ www.asciiribbon.org - against proprietary attachments > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > >

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Adjoint Folds and Unfolds Or: Scything through the Thicket of Morphisms
by KC 18 Apr '11

18 Apr '11

Just tripped over this: Adjoint Folds and Unfolds Or: Scything through the Thicket of Morphisms Folds and unfolds are at the heart of the algebra of programming. They allow the cognoscenti to derive and manipulate programs rigorously... Ralf Hinze Lecture Notes in Computer Science, 2010, Volume 6120, Mathematics of Program Construction, Pages 195-228 -- -- Regards, KC

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Adjoint Folds and Unfolds Or: Scything through the Thicket of Morphisms
by KC 18 Apr '11

18 Apr '11

Just tripped over this: Adjoint Folds and Unfolds Or: Scything through the Thicket of Morphisms Folds and unfolds are at the heart of the algebra of programming. They allow the cognoscenti to derive and manipulate programs rigorously... Ralf Hinze Lecture Notes in Computer Science, 2010, Volume 6120, Mathematics of Program Construction, Pages 195-228 -- -- Regards, KC

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Summer school on Applied Functional Programming at Utrecht University; deadline for registration May 15
by S. Doaitse Swierstra 18 Apr '11

18 Apr '11

Again we will teach an "Applied Functional Programming Summer in Haskell" school this year at Utrecht University. In the previous two occasions students were all very happy with the school and we plan to repeat this success this year. The intended audience are prospective master students who have been in contact with Functional Programming, e.g. by taking a general course on programming languages, and want to learn more about Haskell and its typical programming patterns. In the previous two years we have taught an introductory part (advanced bachelor level), an advanced part (beginning master level) and a shared part for both groups. Topics covered are, besides some examples of domain specific languages, also monads, monad transformers, arrows, parser combinators and self-analysing programs, underlying principles, type inferencing, etc. Half of the course time is spent on a larger programming exercise; you can also come with a problem of your own if you want, and get help from the Utrecht University Software Technology group in finding the proper Haskell idioms, tools and libraries, for solving it. Important links: -- our own page where we supply information based on questions asked http://www.cs.uu.nl/wiki/bin/view/USCS2011/WebHome -- the poster you can print and hang somewhere (why not your office door): http://www.cs.uu.nl/wiki/pub/USCS2011/WebHome/USCSpos11.pdf -- the official summerschool site where you can register: http://www.utrechtsummerschool.nl/index.php?type=courses&code=H9 Furthermore we ask for your cooperation to bring this announcement under the attention of potential participants. Best, Doaitse Swierstra PS: apologies if you get this mail more than once

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Re: [Haskell-cafe] How to contact OpenGL package maintainer (where is Sven?)
by Joachim Breitner 18 Apr '11

18 Apr '11

[Moving to haskell-cafe, not really library specific any more] Hi, Am Sonntag, den 17.04.2011, 12:39 -0700 schrieb Jason Dagit: > In this specific case, I'll do what I can to clean things up but your > request makes me pause and think that the debian packaging for cabal > packages is not automated enough. As haskell developers it seems a > little odd to me that we need to consider the cost of creating new > packages for the sake of debian. I like debian, so please don't take > that the wrong way :) the observation is correct, but unfortunately hard to change – Debian is large, slow moving, and unlikely to change policies for a corner-case such as Haskell libraries. Also, the policies that make life a bit more harder for us are also the policies that make Debian a good distribution: E.g. manually creating a copyright file describing who all has copyright on the files, and what license the files are under – .cabal has fields for that, but unfortunately, they very often do not reflect the reality of the .hs files. Or the other one, requiring that an upload of a source package is accompanied by a manually built and signed binary package ensures that the maintainer actually checked that the package builds and hopefully has tested it. If you think more about it, the lack of some of the policies on Hackage, e.g. nobody checking that licenses and copyright are correctly specified and actually compatible with dependencies, make Hackage a dangerous source for serious users. I am not saying that this should change, but the cheapness of Hackage uploads has its downsides. Greetings, Joachim -- Joachim Breitner e-Mail: mail(a)joachim-breitner.de Homepage: http://www.joachim-breitner.de ICQ#: 74513189 Jabber-ID: nomeata(a)joachim-breitner.de

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Killing threads in foreign calls.
by Jason Dusek 18 Apr '11

18 Apr '11

I am building an application that uses Postgres for storage. If a query runs too long, I would like to kill the querying thread, releasing its lock on the connection; if the connection is a in a bad state -- for example, busy -- I would like to clean up the connection. Unfortunately, killing calls in to libpq seems not to work. I have put a minimal example on hpaste: http://hpaste.org/45774/minimal_pg_contention_example If you install libpq with Cabal, you can run it. In the example, the main thread spawns a worker thread that queries Postgres, running "SELECT pg_sleep(10);"; the main thread waits half a second and then tries to kill the worker. Unfortunately, the worker always manages to get as far as printing "complete". In the code, I call `Database.PQ.exec': http://hackage.haskell.org/packages/archive/libpq/0.4.1/doc/html/src/Databa… This in turn calls a `safe' binding, `c_PQexec', to `PQexec' in the C library: http://hackage.haskell.org/packages/archive/libpq/0.4.1/doc/html/src/Databa… There are async interfaces, too; they do not seem to be any more killable then the sync ones. Maybe the problem is that you can't kill a thread while it's in a foreign call? I do not see any documentation to this effect; but I may have missed it. -- Jason Dusek () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments

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Template Haskell question
by Alexey Karakulov 18 Apr '11

18 Apr '11

I'm interested if it's possible to use functions from some module without explicitly importing it. In ghci it's done on the fly, like this: Prelude> Data.Map.empty Loading package array-0.3.0.2 ... linking ... done. Loading package containers-0.4.0.0 ... linking ... done. fromList [] But without gchi it seems impossible. I have the file Test.hs: > {-# LANGUAGE TemplateHaskell #-} > module Test where > import Language.Haskell.TH > > x :: ExpQ > x = global $ mkName "Data.Map.empty" When I load it in ghci, all works: $ ghci -XTemplateHaskell Test.hs *Test> $x Loading package pretty-1.0.1.2 ... linking ... done. Loading package array-0.3.0.2 ... linking ... done. Loading package containers-0.4.0.0 ... linking ... done. Loading package template-haskell ... linking ... done. fromList [] But when I try to use it from other module, it fails. File Main.hs: > {-# LANGUAGE TemplateHaskell #-} > module Main where > import Test > > main = do > print $x $ runhaskell Main.hs Main.hs:5:9: Not in scope: `Data.Map.empty' In the result of the splice: $x To see what the splice expanded to, use -ddump-splices In the first argument of `print', namely `$x' In the expression: print ($x) -- All the best, Alexey

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"import" functionality in DSLs
by Nikhil A. Patil 18 Apr '11

18 Apr '11

Hi, I am planning a simple monadic DSL (frankly, calling it a DSL is a bit of a stretch; it's just a somewhat glorified state monad), and I wish to implement some kind of "import" functionality for it. The DSL code looks something like this: > doit :: DSL Term > doit = do (+) <- define_function "+" 2 > n0 <- define_constant "0" > k <- define_constant "k" > -- begin beautiful DSL code > let x = k + n0 > return $ x + x The code above adds identifiers "+", "0", "k" to the DSL monad and conveniently exposes haskell identifiers (+), n0, k for the user to use in code that follows. (Note that these define_* functions make state updates in the underlying state monad.) Needless to say, most functions like doit have very similar define_* calls in the beginning. Thus, I want to implement some kind of import functionality. Ideally, the code would look like this: > module DSLPrelude where > > prelude :: DSL () > prelude = do (+) <- define_function "+" 2 > n0 <- define_constant "0" > k <- define_constant "k" > return () > module Main where > import DSLPrelude > > doit :: DSL Term > doit = do prelude > -- begin beautiful DSL code > let x = k + n0 > return $ x + x ...but of course that won't work since (+), n0, k are not in scope. I can think of two solutions, both of which I dislike: Solution 1: > module DSLPrelude where > > prelude :: DSL (Term -> Term -> Term, Term, Term) > prelude = do (+) <- define_function "+" 2 > n0 <- define_constant "0" > k <- define_constant "k" > return ((+), n0, k) > module Main where > import DSLPrelude > > doit :: DSL Term > doit = do ((+), k, n0) <- prelude > -- begin beautiful DSL code > let x = k + n0 > return $ x + x This is quite unsafe: I have mistakenly swapped k and n0 in doit, without failing typecheck. Solution 2: > module DSLPrelude where > > (+) :: DSL (Term -> Term -> Term) > n0 :: DSL Term > k :: DSL Term > (+) = define_function "+" 2 > n0 = define_constant "0" > k = define_constant "k" > module Main where > import DSLPrelude > > doit :: DSL Term > doit = do (+) <- (+) > n0 <- n0 > k <- k > -- begin beautiful DSL code > let x = k + n0 > return $ x + x ...which works, but still has quite a bit of boilerplate crap. I feel this would be a common problem with a lot of DSLs, so I am curious to know how others solve it. Any pointers and suggestions are most welcome and greatly appreciated. Thanks! nikhil

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Oddness with binary-0.5.0.2 user install
by Antoine Latter 18 Apr '11

18 Apr '11

Hello folks, The docs for binary-0.5.0.2 on Hackage claim that their is a Binary instance for lazy ByteStrings, but there does not seem to be such an instance in my local install of binary-0.5.0.2. In addition, the docs on Hackage claim that there is an Applicative instance for the 'Get' type, which my install is also lacking. I've verified this in ghci and by looking at my own locally generated haddocks. The module structure for the 'binary' package is pretty straightforward - so I don't think I'm missing any imports in ghci. Has anyone seen this before? Antoine

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Haskell Platform 2011.2.0.1 now available
by Don Stewart 17 Apr '11

17 Apr '11

We're pleased to announce the 2011.2.0.1 release of the Haskell Platform: a single, standard Haskell distribution for everyone. Download the Haskell Platform 2011.2.0.1: http://haskell.org/platform/ This release adds support for GHC 7.0.3, and significant improvements for Mac OS X users. The specification, along with installers (including Windows, Mac and Unix installers for a full Haskell environment) are available. The Haskell Platform is a single, standard Haskell distribution for every system, in the form of a blessed library and tool suite for Haskell distilled from the thousands of libraries on Hackage, along with installers for a wide variety of systems. It saves developers work picking and choosing the best Haskell libraries and tools to use for a task. When you install the Haskell Platform, you get the latest stable compiler, an expanded set of core libraries, additional development tools, and cabal-install – so you can download anything else you need from Hackage. This release ships with GHC 7.0.3. What you get is specified here: http://hackage.haskell.org/platform/contents.html Thanks! -- The Platform Infrastructure Team P.S. a big thanks to Mark Lentczner and Mikhail Glushenkov who built the Mac and Windows installers!

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