December 2005 - Haskell-Cafe

Killer pickler combinators (was Time leak)
by Joel Reymont 21 Dec '05

21 Dec '05

The original paper is at http://research.microsoft.com/ ~akenn/fun/ picklercombinators.pdf My adaptation is at http://wagerlabs.com/timeleak.tgz. This is a full repro case, data included. The issue is that with 1000 threads my serialization is taking a few seconds. Inserting a delay or passing in trace (see README and code) gets serialization time down to 0-1s, the way it should be. What gives? Try it for yourself and please let me know! Thanks, Joel -- http://wagerlabs.com/

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Re: problems with square roots...
by Scherrer, Chad 21 Dec '05

21 Dec '05

------------ From: Daniel Carrera <daniel.carrera(a)zmsl.com> Hey, The sqrt function is not doing what I want. This is what I want: round sqrt(2) --------------------------------------------------- Daniel, A lot of Haskell folks like to avoid parentheses as much as possible, and there's a really convenient way to do this. There is a Prelude function ($) f x = f x which is right-associative, so you can write round $ sqrt x == round (sqrt x) This becomes really convenient when multiple application is involved: print $ round $ sqrt x == print (round (sqrt x)) -Chad

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Fwd: Monads and pedagogy (Functions with side-effects?)
by Greg Woodhouse 21 Dec '05

21 Dec '05

Oops...meant to send this to the list, too. --- Greg Woodhouse <gregory.woodhouse(a)sbcglobal.net> wrote: > Date: Wed, 21 Dec 2005 09:42:44 -0800 (PST) > From: Greg Woodhouse <gregory.woodhouse(a)sbcglobal.net> > Subject: Monads and pedagogy (Functions with side-effects?) > To: David Barton <dlb(a)patriot.net> > > --- David Barton <dlb(a)patriot.net> wrote: > > > Wolfgang Jeltsch writes: > > ----- Original Message ----- > > > > > Am Mittwoch, 21. Dezember 2005 13:15 schrieb Creighton Hogg: > > >> [...] > > > > > >> Monads, I believe, can be just thought of as containers for > state. > > > > > > I would say that you are talking especially about the I/O monad > > here. A > > > monad > > > as such is a rather general concept like a group is in algebra. > > > > While this is correct, I'm afraid that for most of us it is a > > flavorless > > answer. I wish I had the mathematical mind that made the word > > "group" in > > this context instantly intuitively recognizable, but I don't. > > Groups are actually relatively intuitive structures, representing (no > pun intended) the symmetries of some type of object. So, in teaching > group theory it is easy to immediately provide a number of easy to > grasp examples: the symmetric group (any kind of rearrangement), the > cyclic group (cyclic permutation), dihedral group (geometric > symmetries > of a "square"), SL(n, R) ("area preserving" linear transformations), > SO(n, R) (transformation preserving angles), Isom(M) (all geomteric > motions of an object so that in the end it occupies the same "space" > without any stretching or other deformation), etc., etc. > > Rings are a little more tricky, but R[x] (polynomials over R) is the > universal example, with homomorphic images like Z[i] just being > "polynomials in i" where you "reduce" according to i^2 = -1, and so > on. > Matrices over a ring give you the prototypical example of a module. I > could go on, but the point is that in abstract algebra it is usually > easy to provide simple intuitive examples of the structures > illustrating most of the basic properties of the category. (Okay, > okay, > topoi and coherent sheaves may be an exception! But even then, > analytic > functions go a long way.) > > With monads, it is not hard to give a formal definition, just as is > the > case with groups, but much harder (for me, anyway) to find a simple > intuitive set of examples that capture the essence of monads in such > a > way that the definition is obvious. Think about it: If groups are > just > symmetries, then "doing nothing" is clearly a symmetry (1), "doing > nothing" before or after a symmetry doesn't change anything. > "Undoing" > a symmetry gives you an inverse. Associativity is less obvious, > unless > you think of groups realized as permutations of a set, so that the > group product just becomes function composition. > > > > === > Gregory Woodhouse <gregory.woodhouse(a)sbcglobal.net> > "Interaction is the mind-body problem of computing." > --Philip L. Wadler >

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Learning about haskell compilers
by Creighton Hogg 21 Dec '05

21 Dec '05

Hi guys, I was wondering where I should get started in learing about how to implement a haskell compiler? Are there papers, wiki entries, or other things people think would be helpful or should I just start looking at the source of one of the compilers?

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Re: [Haskell-cafe] Killer pickler combinators (was Time leak)
by Joel Reymont 21 Dec '05

21 Dec '05

On Dec 21, 2005, at 2:56 PM, Cale Gibbard wrote: > By the way, when I was doing threadDelays, I meant: > trace s = withMVar lock $ const $ threadDelay 20 > In case you didn't try that. I'm trying 5k threads and I still get delays at times. Using thread delay of 1, though. ./unstuff trace.dat unstuff: user error (ORANGE ALERT: 0s, 7s, SrvServerInfo, ix1: 6, size: 49722) unstuff: user error (ORANGE ALERT: 0s, 8s, SrvServerInfo, ix1: 6, size: 49722) unstuff: user error (ORANGE ALERT: 0s, 7s, SrvServerInfo, ix1: 6, size: 49722) unstuff: user error (ORANGE ALERT: 0s, 5s, SrvServerInfo, ix1: 6, size: 49722) unstuff: user error (ORANGE ALERT: 0s, 5s, SrvServerInfo, ix1: 6, size: 49722) unstuff: user error (ORANGE ALERT: 0s, 4s, SrvServerInfo, ix1: 6, size: 49722) unstuff: user error (ORANGE ALERT: 0s, 6s, SrvServerInfo, ix1: 6, size: 49722) -- http://wagerlabs.com/

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Re: [Haskell-cafe] Killer pickler combinators (was Time leak)
by Joel Reymont 21 Dec '05

21 Dec '05

Cale is a genius! This totally solves the timeouts. Why, though? The lock is only used at the end of the unpickling. Why does it help? On Dec 21, 2005, at 2:56 PM, Cale Gibbard wrote: > By the way, when I was doing threadDelays, I meant: > trace s = withMVar lock $ const $ threadDelay 20 > In case you didn't try that. > > - Cale > > On 21/12/05, Joel Reymont <joelr1(a)gmail.com> wrote: >> I'm not sure I buy this. Again, this helps: >> >> {-# NOINLINE lock #-} >> lock :: MVar () >> lock = unsafePerformIO $ newMVar () >> >> trace s = withMVar lock $ const $ putStrLn s >> >> and then in read_ >> >> cmd <- read h trace >> >> trace is called _after_ all the timings in read so it should not >> affect the timings. >> >> You could basically say that the lock is at the end of read, after >> all the unpickling has been done. The other interesting thing is that >> replacing trace with >> >> delay _ = threadDelay 1 >> >> does not solve the issue. -- http://wagerlabs.com/

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Re: [Haskell-cafe] Re: Tutorial uploaded
by John Hughes 21 Dec '05

21 Dec '05

> >---------------------------------------------------------------------- > >Message: 1 >Date: Wed, 21 Dec 2005 10:48:08 +0000 >From: Robin Green <greenrd(a)greenrd.org> >Subject: Re: [Haskell-cafe] Re: Tutorial uploaded > > > > > > >>Beginners should start with non-monadic functions in order to later avoid >>IO in their functions whereever possible. >> >> > >Whilst localising IO to a small part of the program is generally a good >idea, beginners should not be scared off by the thought that IO in >Haskell is so hard it has to be covered on page 94. This is not the >case. It should be introduced on page 1. > >If people want Haskell to be treated as a practical language, not just >something for doing academic teaching and research with, it should be >taught as a practical language - which means that things like IO and >space/time usage come to the forefront. > > > Couldn't agree more. I show my students IO in the first lecture of their first programming course in the first term of their first year. A few lectures later I discuss it in more detail, and tell to think of an IO a as instructions on a piece of paper to produce an a. My students have no difficulty understanding the difference between being given 500 crowns (a value), and being given my ATM card and code with instructions how to work an ATM! Haskell IO is mystifed far too often in my opinion--it needn't be hard at all. John

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YAHT Tutorial
by Christian Maeder 21 Dec '05

21 Dec '05

Folks, I'm surprised Hal's tutorial is so less known. I recommend (and send) it to everybody who asks me. I like that it starts early with some IO and mainly promotes ghc/ghci. Hugs is a good tool as well (and I sometimes use it, too), but ghc is definitively the flagship with better error reporting, active maintainance and for producing fast code (if that is important). It's good that this tutorial is freely available, but some people prefer to have a proper textbook. I just wonder what happened to "An Introduction to Computing (with Haskell). Manuel M. T. Chakravarty and Gabriele C. Keller. Pearson Education Australia, 2002." which is also available in german as "Einführung in die Programmierung mit Haskell" http://www.pearson-studium.de/main/main.asp?page=bookdetails&ProductID=81781 It's not listed under http://www.haskell.org/bookshelf/ nor http://www.haskell.org/learning.html. Maybe it is hard to recommend a single tutorial without spoiling selling chances of others. In our teaching courses "Simon Thompson: Haskell: The Craft of Functional Programming," is mostly used. Others find "Paul Hudak: The Haskell School of Expression" the best. I just see that the recent tutorial version contains Parsec (good) and FiniteMap (deprecated) examples. I would appreciate the chapter "Advanced Techniques" being filled (but know how much time that would cost). Thanks for your tutorial so far, Hal! Cheers Christian

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Re: [Haskell] A simple server (or how to do io).
by Pupeno 21 Dec '05

21 Dec '05

On Tuesday 20 December 2005 23:43, Einar Karttunen wrote: > Your problem is that the ghci prompt *is* blocking everything. ghci is > builded non-threaded. Ghci uses a blocking call to read the lines > which means that all other computations are stopped. Ok, then pressing enter each time I want one of my servers to reply. I can live with that. > > I compile it and run it. It returns immediately and the server of course, > > is not running. > > It spawns the server threads and returns immediately. Indeed. > > I think what I am missing is that server, after running runStreamServer > > should block and be able to reply to all the queries (in the final > > version I'll also have runDgramServer as well) and be able to receive a > > signal, upon which it should close the socket and finish. How can do that > > ? Any help will be appreciated. > > Catch the signal and kill the listening threads. If you > are using the new 0.3.1 release then you can use the > killServer call. Oh, new release, I am downloading it right now! So, I install a signal handler with installHandler... and then ? how do I prevent the program for quiting ? am I missing some kind of event loop here ? > ps. I think it may be best to continue on haskell-cafe@ rather > than the main list. I am posting to there as well (to continue there). -- Pupeno <pupeno(a)pupeno.com> (http://pupeno.com)

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Proposal for a first tutorial.
by Daniel Carrera 20 Dec '05

20 Dec '05

Hello, I've almost finished my proposal for a very first Haskell tutorial. I notice that the Wiki is restricted. How can I upload it to show it to you? Can I send it to the list as an attachment? (it's only 4k). Cheers, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/ I am not over-weight, I am under-tall. /

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