December 2010 - Haskell-Cafe

Re: [Haskell-cafe] Offer to mirror Hackage
by Brandon Moore 14 Dec '10

14 Dec '10

On Dec 13, 2010, at 6:15 PM, wren ng thornton <wren(a)freegeek.org> wrote: On 12/13/10 8:25 AM, Paul Sargent wrote: How about, as a cheep and cheerful method to get up running. If the premise is that the original server is trustworthy and the mirrors aren't, then: 1) Hash all packages on the original server. 2) Hash goes into a side car file (e.g.<packagename>.sha) that lives next to the package If hashed are added to the package information "cabal update" downloads, installing packages from mirrors will continue to work during a central outage. I still contend that we shouldn't have to trust the central server either. The hash can be created alongside the sdist on the maintainer's computer, and then both are uploaded to central. Thus, the maintainer can verify that the hash on central matches their own, which ensures that: For now, it's enough to find a simple scheme where adding untrusted mirrors is no worse than the current situation. Hashes seem to work for that: 1. cabal update always reads from the central server (if uploads are impossible when the central server is down, the package lit won't even get stale) 2. The package descriptions are extended with hashes 3. Cabal may download packages from mirrors, but checks the hash. Your proposal doesn't narrow trust to the maintainers (which is currently open to the public anyway), because an adversary as described could return the correct hash and package for the maintainer, and the corrupted version to others. Brandon

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[ANNOUNCE] haskell-mpi-1.0.0
by Dmitry Astapov 14 Dec '10

14 Dec '10

Dear Haskellers, We are pleased to announce the release of haskell-mpi-1.0.0, a suite of Haskell bindings to the C MPI library and convenience APIs on top of it. About MPI --------- MPI, the Message Passing Interface, is a popular communications protocol for distributed parallel computing (http://www.mpi-forum.org/) MPI applications consist of independent computing processes which share information by message passing. It supports both point-to-point and collective communication operators, and manages much of the mundane aspects of message delivery. There are several high-quality implementations of MPI available, all of which conform to the standard API specification (the latest version of which is 2.2). The MPI specification defines interfaces for C, C++ and Fortran, and bindings are available for many other programming languages. About Haskell-MPI ----------------- As the name suggests, Haskell-MPI provides a Haskell interface to MPI, and thus facilitates distributed parallel programming in Haskell. It is implemented on top of the C API via Haskell's foreign function interface. Haskell-MPI provides three different ways to access MPI's functionality: * A direct binding to the C interface (see Control.Parallel.MPI.Internal). * A convenient interface for sending arbitrary serializable Haskell data values as messages (see Control.Parallel.MPI.Simple). * A high-performance interface for working with (possibly mutable) arrays of storable Haskell data types (see Control.Parallel.MPI.Fast). We do not currently provide exhaustive coverage of all the functions and types defined by MPI 2.2, although we do provide bindings to the most commonly used parts. In future we plan to extend coverage based on the needs of projects which use the library. The package is available from http://hackage.haskell.org/package/haskell-mpi. Examples and comprehensive testsuite are included in the source distribution. Code was tested on 32- and 64-bit platforms, with MPICH2 and OpenMPI. The Fast API shows performance comparable to C, and the Simple API is generally 2-7 time slower due to (de)serialization overhead and necessity to issue additional MPI requests behind the curtains in some cases. Bernie Pope started this project as a rewrite of hMPI which was written by Michael Weber and Hal Daume III. He was later joined by Dmitry Astapov, working on the library as part of Well-Typed LLP's Parallel Haskell Project. Development is happening on GitHub, in git://github.com/bjpop/haskell-mpi. Please join in! Sincerely yours, Dmitry Astapov, Bernie Pope -- Dmitry Astapov Well-Typed LLP, http://www.well-typed.com/

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Re: [Haskell-cafe] admission to graduate program in CS
by Johann Bach 13 Dec '10

13 Dec '10

Can I get some recommendations for CS textbooks to study for the CS GRE? Note that I am already fluent in C++ and Python (and getting better at Haskell), so I don't need hand-holding in learning the very basics... however, I do need to review the standard theory. For example, if the GRE asks: "What are the fundamental elements of OO?" I'm actually not sure what the answer is. (There's a difference between using an OO language, and talking about the theory behind it. In garden-variety programming we never talk about the theory.) I assume that any textbook is going to include the basics---I can skim through those parts. But I would be curious if there is a textbook that quickly gets into the deep stuff (and is particularly relevant to the GRE). Thanks, J

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Conditional compilation for different versions of GHC?
by Jinjing Wang 13 Dec '10

13 Dec '10

Dear list, >From ghc 7.0.1 release notes: > The Language.Haskell.TH.Quote.QuasiQuoter type has two new fields: quoteType and quoteDec. Some of my code needs to be conditionally compiled to support both version 6 and 7, what is the recommended way to do it? ref: * http://new-www.haskell.org/ghc/docs/7.0.1/html/users_guide/release-7-0-1.ht… -- jinjing

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Monad Syntax
by caseyh＠istar.ca 13 Dec '10

13 Dec '10

From: A Neighborhood of Infinity Monday, August 07, 2006 You Could Have Invented Monads! (And Maybe You Already Have.) http://blog.sigfpe.com/2006/08/you-could-have-invented-monads-and.html The following part: do let x = 7 y <- Writer (x+1,"inc\n") z <- Writer (2*y,"double\n") Writer (z-1,"dec\n") The notation is very suggestive. When we write y <- ... it's as if we can pretend that the expression on the right hand side is just x+1 and that the side-effect just looks after itself. Why pretend? Couldn't the syntax be: do let x = 7 y <- x+1, Writer ("inc\n") z <- 2*y, Writer ("double\n") z-1, Writer ("dec\n") Or some other delimiter than the comma? I'm thinking this syntax might be easier to understand. :)

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Weird error during derive-2.3.0.2 build
by Alex 13 Dec '10

13 Dec '10

Hi all. I am trying to install the "yi" editor using cabal-install, which in turns installs the package 'derive-2.3.0.2'. I get the following error during derive's compilation: [53 of 58] Compiling Data.Derive.Internal.Traversal ( Data/Derive/Internal/Traversal.hs, dist/build/Data/Derive/Internal/Traversal.o ) Data/Derive/Internal/Traversal.hs:34:0: Illegal instance declaration for `Applicative (Writer w)' (All instance types must be of the form (T t1 ... tn) where T is not a synonym. Use -XTypeSynonymInstances if you want to disable this.) In the instance declaration for `Applicative (Writer w)' cabal: Error: some packages failed to install: derive-2.3.0.2 failed during the building phase. The exception was: ExitFailure 1 Of course, I tried to build it with --ghc-option=-XTypeSynonymInstances, which in turn gives another compilation error. I am using ubuntu maverick 32-bit, with ghc-6.12.1 and cabal-install 0.8.2. The ghc was installed from the package repository. Cabal was installed using: cabal-install cabal Alex.

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Connecting to a running Haskell program using GHCI
by aditya siram 13 Dec '10

13 Dec '10

Hi all, I'd like to load a Haskell program into GHCi A and then connect to GHCi A from GHCI B. Is there any way to do this? This would be handy (handier than telnet) for demoing and debugging server applications that use shared resources. -deech

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Re: [Haskell-cafe] [Haskell] reference to DoCon
by Henning Thielemann 13 Dec '10

13 Dec '10

On Mon, 13 Dec 2010, Serge D. Mechveliani wrote: > Dear administration of www.haskell.org, > > (I am sorry for not finding a more appropriate list for this letter) > > In old days, my program system DoCon > > (computer algebra written in Haskell) had its copy on > www.haskell.org/docon/ > -- if I remember correct. It certainly disappeared on the server migration 2010-11-30. That's not nice. A solution will certainly be to move www.haskell.org/docon/ to community.haskell.org/docon/ and set a redirection from www.haskell.org/docon/.

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Re: [Haskell-cafe] A home-brew iteration-alike library: some extension quiestions
by John Lato 13 Dec '10

13 Dec '10

From: oleg(a)okmij.org > > Just for the record: the library IterateeM.hs, uses NO extensions to > Haskell98, let alone Haskell2010. The library as written requires > LowLevelIO.hs, which uses FFI (which has been Haskell98 addendum and > is in proper Haskell2010). The sample code, Wc.hs, for example, is > Haskell98. So, the iteratee can be written with no extensions > whatsoever. > Related to this, IterateeM.hs and LowLevelIO.hs don't use runtime exceptions either, which I consider a particularly nice feature. > > In particular, IterateeM does not use any monad transformer library > (although it could have). I found that the trouble of writing a state > monad for a particular state is negligible compared to the pain of > choosing a particular monad transformer library, and especially the > pain inflicted on the users who have to deal with many a conflicts of > monad transformer libraries. > > > > The problem was that I wished Zippee. It means that external enumerator > > must be "suspended" at some points so Zippee can process elements from > > both left and right streams in desired order. It makes any other > > approach I considered impossible to use. > > The file IterateeN.hs demonstrates zipping two streams together (in > lock-step and not in-lockstep). It turns out, the existing Iteratee > interface and type suffices. This is described in more detail in: > > Parallel composition of iteratees: one source to several sinks > http://okmij.org/ftp/Streams.html#1enum2iter > > Parallel composition of streams: several sources to one sink > http://okmij.org/ftp/Streams.html#2enum1iter > It seems I was mistaken in my last reply. That's what I get for answering before I read through all my weekend email. John

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admission to graduate program in CS
by Johann Bach 13 Dec '10

13 Dec '10

Hello list, I am a programmer who just got laid off. I have been studying Haskell and learning a lot about CS, *as a hobby*. I have a B.S. from Caltech in engineering (didn't take many CS classes). Now that my options are open, I'd like to think about graduate school in CS which is probably the academic subject I love most in the world. But I didn't take many CS classes in college, and my job was garden-variety programming, mostly to someone else's design and process rules. My real CS experience comes from my hobbies such as learning Haskell. But that leaves me without a way to prove my ability, at least not by showing recent schoolwork or having any recommendations from CS professors. I hardly know anything about how graduate schools evaluate candidates. So what I am wondering is, could I use a demonstration of my self-study to demonstrate ability? I'm not sure what form that would take, but I'm just trying to look at every angle right now. Thanks, Johann

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