March 2008 - Haskell-Cafe

linker question
by Galchin Vasili 13 Mar '08

13 Mar '08

Hello, I have made modifications against an existing Haskell library and installed in my --prefix=$HOME. How do I specify to link against this test .a and not the "global" archive? Regards, vasili

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Java interfacing and ghc-6.8?
by Marc Weber 12 Mar '08

12 Mar '08

Hi, I've read on haskell org about gcjni Haskel/Java VM bridge Lambada (< ghc 6.6.1) Do you know wether any of them can be compiled with ghc-6.8 ? If not does it need much effort to patch them? Sincerly Marc Weber

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Constructing Data.Map from ByteString
by Dave Tapley 12 Mar '08

12 Mar '08

Hi all, I've been plugging away at this all day and some discussion in #haskell has been fruitless. Perhaps you have the inspiration to see what's happening! Concerning this minimal example: http://hpaste.org/6268 It works as required, loading K/V pairs into a Data.Map, the concern is the amount of memory used. Pausing (using a getLine) after the readFile one can see (through 'ps v') that the whole file 'f' is loaded in to memory. However once the Map is created the memory footprint swells to around five times the size. Surely this can't be just overhead from Map? My reasoning was that by using ByteString and getting the whole file into memory a small and linear increase would be seen for Map overhead.. I have tried using both Data.ByteString.Char8 and Data.ByteString.Lazy.Char8 with negligible difference. For a hoot I tried it with String and yes, it's ridiculous :) Cheers, Dave

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Reminder: 4th Haskell Hackathon, April 11-13 in Gothenburg
by Bjorn Bringert 12 Mar '08

12 Mar '08

------------------------------------------------------------------------ Hac4 4th Haskell Hackathon April 11-13, 2008 Gothenburg, Sweden http://haskell.org/haskellwiki/Hac4 Sponsored by Credit Suisse and Galois. ------------------------------------------------------------------------ This is a reminder to register for the 4th Haskell Hackathon. The event will be held over 3 days, April 11-13, at Chalmers University of Technology in Gothenburg, Sweden. == NEWS == * Lunches and at least one dinner for the Hackathon attendees will be provided through our generous sponsors Credit Suisse and Galois. * If you plan to start a new project on community.haskell.org, please request an account and project in advance, so as to save valuable Hackathon time. See http://community.haskell.org/admin/ for more details. == What it is == The plan is to hack on Haskell infrastructure, tools, libraries and compilers. To attend please register, and get ready to hack those lambdas! Code to hack on: * Hackage * Cabal * Porting foreign libraries * Bug squashing * You decide! Before you attend, do start thinking and familiarizing yourself with 1 or 2 projects you wish to work on, to ensure no wasted effort during the Hackathon. A list of possible projects is available on the website. == Registration == We ask that you register your interest. Follow the instructions on the registration page: http://haskell.org/haskellwiki/Hac4/Register Once you've registered, do add your info to the attendees self-organizing page, http://haskell.org/haskellwiki/Hac4/Attendees if you are looking to share costs, or meet up prior to the hackathon, with other attendees. N.B. if you already expressed interest via the wiki, do confirm by registering `officially' anyway. == Important dates == Hackathon: April 11-13, 2008 == Organizers == Björn Bringert (local) Thomas Schilling (local) Lennart Kolmodin (local) Krasimir Angelov (local) Jean-Philippe Bernardy (local) Ian Lynagh Duncan Coutts

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Re: [Haskell-cafe] simple Haskell question - List comprehensions
by Philip Müller 12 Mar '08

12 Mar '08

Dan Licata schrieb: > Does that help? > Yeah, it did - Thanks! - Philip

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Exception handling when using STUArray
by oleg＠okmij.org 11 Mar '08

11 Mar '08

Sterling Clover wrote: > there's no standard way that I know of besides "inspection" to > determine if code might throw an exception, and this is particularly > the case with the dreaded lazy IO of prelude functions. The following old message showed even two ways of doing exactly that -- in Haskell, with no external tools. Exceptions in types and exception-free programming http://www.haskell.org/pipermail/haskell/2004-June/014271.html The third way described in the message -- ensuring that exceptions will never occur -- was further developed as lightweight static capabilities. > And as per a recent discussion on libraries, it's still a royal pain > to catch IO exceptions in monads with an IO base. That is again very easy. Takusen uses a CaughtMonadIO -- which includes a wide range of monads: IO itself and various transformations of it. I am extensively using a variant written by Jesse Tov based on the Takusen's idea. His code defines two classes: EMonad and EMonadIO -- which enclose most of the interesting monads. The latter is the subclass of the former and also allows arbitrary IO (via liftIO). In either case, I use gthrow, gbracket, gcatch, ghandle, gfinally, etc. -- without even thinking which Monad I'm in and how error handling is actually implemented (via ErrorT or via IO exceptions). It works universally for most of monads of interest.

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Reflective capabilities of Haskell
by Martin Hofmann 11 Mar '08

11 Mar '08

I am trying to port a programme written in Maude, which is a reflective language based on rewriting logic ( http://maude.cs.uiuc.edu/ ), to Haskell. I thought using Template Haskell might be a good idea, but I got stuck and now I am wondering if this is really possible in Haskell. Let me give an example of a Maude module defining the function last over a list of Peano numbers. fmod LAST is sorts Peano PeanoList . op 0 : -> Peano [ctor] . op s : Peano -> Peano [ctor] . op [] : -> PeanoList [ctor] . op cons : Peano PeanoList -> PeanoList [ctor] . op last : PeanoList -> Peano . vars X Y : Peano . var Xs : PeanoList . eq last(cons(X,[])) = X . eq last(cons(X,cons(Y,Xs))) = last(cons(Y,Xs)) . endfm So, last(cons(s(0),cons(s(s(0)),cons(s(s(s(0))),[])))) would reduce to s(s(s(0))). The cool thing about Maude is, that terms, modules, ... can be lifted to the meta level. For example: upModule('LAST, false) would yield

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Instance selection based on a class constraint [was: Issues(Bugs?) with GHC Type Families]
by oleg＠okmij.org 11 Mar '08

11 Mar '08

> Manuel M T Chakravarty: > Hugo Pacheco: > > I would simply like the compiler not to use that instance if the > > equality constraint does not hold, like some another instance > > dependency constraint, but I assume that is not possible. > > This is independent of type families. The selection of type class > instances does *never* dependent on the instance context, only on the > instance head. I know that this is sometimes annoying, but type > checking would be a a lot more complicated/expensive without that rule. What Hugo Pacheco wants *is* possible, in GHC as it is (and even as it was back in 2004). Although the selection of type class instances indeed depends only on the instance head, on the surface, the dependence on context is quite easy to accomplish. Here's is the outline of the idea. Suppose we wish to select one of the two instances, depending on whether the constraint C x hold or not: instance C x => CMain x instance CMain x We encode this as follows: class CMain x instance (CPred x flag, CMain' flag x) => CMain x The main class has only one instance, and there is no longer any overlapping. The main selection is done in an auxiliary class class CMain' flag x instance CMain' HTrue x -- will be selected when CPred holds instance CMain' HFalse x -- otherwise Note there are no overlapping instances required. The trick is to re-write constraint C x into the form CPred x flag (where flag functionally depends on x). Whereas C x succeeds, CPred x flag should succeed and unify flag with HTrue. Should C x fail, CPred should still succeed and unify flag with HFalse. TypeEq is such a predicate in the case of type equality. CPred is easy to implement with type-classes, and probably even easier with type families. The HList and OOHaskell papers use this technique to great extent. The following message describes that even backtracking is possible: http://okmij.org/ftp/Haskell/poly2.txt

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Where does ~> come from?
by Steve Lihn 11 Mar '08

11 Mar '08

While I am reading TypeCompose module, I am having trouble finding where ~> comes from? (And its son ~~> and grandson ~~~>, etc.) This is my immediate question. Help is appreciated. A bigger (higher-order?) issue I encountered and I think other people could also have is to look up all these special "symbols" during the learning curve. As things are getting higher and higher order in Haskell, more inventive symbols are appearing. It becomes harder to remember the exact definitions are. The difficulties are: 1) I used to see a page on haskell.org that lists many symbols, but I could not find that page any more. 2) You would hope there is a quick way to search those symbols. But most search engines do not treate symbols friendly, often just ignore them. I typed ~> in Hoogle, it also returned nothing. 3) If the module defining the symbol is not in standard library, it is not possible to look up the symbol in the core library index. The only way to find the symbol seems to go to the source code, check every module being imported and if I am lucky I will find it in the same package. If I am not lucky, I will have to search that module on the Internet, and hunt it down recursively. I wonder if anybody has the same problem. If enough crowd have the same problem, maybe there could be a better way to handle this kind of documentation/learning issue! Thanks, Steve

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automagically?
by Galchin Vasili 10 Mar '08

10 Mar '08

Hello, Say i am building package X that depends on A, H, D. How can I automagically check to A, H, D need to "darcs get" down and built/install before X? If this feature is not available it should be, i.e. an integration of darcs and cabal. Regards, Vasya

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