- Haskell-Cafe - Haskell.org

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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threadWaitRead and threadWaitWrite on multiple fds
by Mathijs Kwik 13 Dec '10

13 Dec '10

Hi all, I read the paper about the new ghc7 event handling IO manager goodies. This is all very exciting stuff. I didn't know GHC's RTS had these smart async-IO facilities. The paper pointed me at threadWaitRead/threadWaitWrite. While very nice the way they are, I would also like to be able to wait on more than 1 fd until 1 of them becomes available. Would it be possible to create this functionality myself? Or do I need to request it and wait for a new GHC? It looks like the functionality I need is in System/Event/Thread. I think I can manage to write my own version of those functions which then loop through a list of fds, but the original file imports System/Event/Manager (registerFd, unregisterFd_) which I can't do because it's a hidden module. Is there a way around this? Thanks, Mathijs

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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: Permjacov Evgeniy <permeakra(a)gmail.com> > > Hi. I Wrote a simple iteration library. It was not intensively tested, > so it MAY contatin bugs, but it is very unlikely. The library is > currently on github: https://github.com/permeakra/iteration > > I'm not ready to upload it to hackage, as some testing and extension is > really needed. However, I'd like to know about possible flaws. > > Current goal is addition of byte-stream (de)compression and IO functions > extenstion. After this package will be cabalized and uploaded to > hackage. So, while design is not frozen yet, I'm interested in criticism > -)/ > First, I haven't examined your code in sufficient depth to fully understand it, so I may be off the mark with this comment. However, I'm not convinced it's possible to safely implement zipping in iteration-style IO without another technique (e.g. monadic regions), by which I mean I suspect any code which exposes enough control to pause an enumeration (or alternatively allows a user-supplied termination check to the enumerator) will also not provide guaranteed finalization/garbage collection of the underlying resource (e.g. handle). I also require zipping of streams, and in fact it is possible with iteratee. I haven't included the code in the library because I was formerly not convinced of its safety. I do think it's safe now, but only by use of a monadic region. John

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Re: [Haskell-cafe] A home-brew iteration-alike library: some extension quiestions
by oleg＠okmij.org 13 Dec '10

13 Dec '10

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. 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

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