- Haskell-Cafe - Haskell.org

Existential type question
by Michael Snoyman 22 Nov '09

22 Nov '09

Hi all, I've come across some code I just can't figure out how to write appropriately. Below is a silly example that demonstrates what I'm trying to do. I don't really have the appropriate vocabulary to describe the issue, so I'll let the code speak for itself. In particular, I'm trying to understand what the correct type signatures for unwrapMyData and bin should be. Thanks, Michael --- {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FlexibleContexts #-} class Monad m => MonadFoo x m where foo :: x -> m a data MyData a = forall i. Integral i => MyLeft i | MyRight a instance Monad MyData where return = MyRight (MyLeft i) >>= _ = MyLeft i (MyRight x) >>= f = f x instance Integral i => MonadFoo i MyData where foo = MyLeft bar :: MonadFoo Int m => Int -> m String bar 0 = return "zero" bar i = foo i baz :: String -> MyData String baz "zero" = MyRight "Zero" baz _ = MyLeft (-1 :: Integer) --This works: unwrapMyData (MyLeft i) = foo (fromIntegral i :: Integer) unwrapMyData (MyLeft i) = foo i -- This is what I'd like to work unwrapMyData (MyRight a) = return a bin i = do a <- bar i b <- unwrapMyData $ baz a return $ b ++ "!!!" instance Show a => MonadFoo a IO where foo = fail . show -- I know, it's horrible... main = do res <- bin 0 putStrLn res

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What's wrong with code.haskell.org ?
by Andy Stewart 22 Nov '09

22 Nov '09

Hi all, I can't use `darcs get --partial http://code.haskell.org/gtk2hs/` to get source code, and can't push patch. What's wrong with code.haskell.org? Rest for weekend? :) -- Andy

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haskell code from hi
by Ozgur Akgun 22 Nov '09

22 Nov '09

Is there possibly a way of getting source code, given a *.hi file? Yes you're right I deleted all my *.hs files, while trying to remove *.hi ones!! Desperately, -- Ozgur Akgun

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ANN: GPipe-1.1.0 with greatly improved performance
by Tobias Bexelius 22 Nov '09

22 Nov '09

Hi, GPipe 1.1.0 is now available on Hackage. This update includes the following: * BIG performance increase: The previous shader cache keys grew exponential in size in relation to the GPipe program. This is now fixed with a completely new shader generator so the cache key sizes are linear relative to the program. In addition, the cache now uses patricia trees instead of an ordinary map, which will be a big performance win in programs with many shaders. In the simple example on http://www.haskell.org/haskellwiki/GPipe I got an performance increase of factor 2.7(!), and expect it to be even larger for more complex programs. * Other minor fixes: Fixed so min, max, abs and sign no longer works on Vertex Int and Fragment Int types, since its not supported in GLSL 1.20. Reexport of Vec-Boolean instances so you dont need to import that package on its own any more. Added IfB instances for the associated type Color. Cheers Tobias _________________________________________________________________ Nya Windows 7 gör allt lite enklare. Hitta en dator som passar dig! http://windows.microsoft.com/shop

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How to understand such a newtype ?
by zaxis 22 Nov '09

22 Nov '09

newtype X a = X (ReaderT XConf (StateT XState IO) a) #ifndef __HADDOCK__ deriving (Functor, Monad, MonadIO, MonadState XState, MonadReader XConf, Typeable) #endif In `X (ReaderT XConf (StateT XState IO) a)`, X is a type constructor, how to understand `(ReaderT XConf (StateT XState IO) a)` ? And why use `#ifndef __HADDOCK__` ? Sincerely! ----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/How-to-understand-such-a-newtype---tp26462332p2646233… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Haskell as an alternative to Java
by Philippos Apolinarius 21 Nov '09

21 Nov '09

I wonder whether the Haskell community tryed to reproduce the study Lisp as an alternative to Java, by Ron Garret / Erann Gat. However, since that study was reproduced in almost every other language (with a fair amount of cheating going on, I believe :-), I am sure that there is a Haskell version (without cheating, I am sure :-). Here is a paper about the original study: http://www.flownet.com/gat/papers/lisp-java.pdf Here is Norvig's solution to the original problem: http://www.norvig.com/java-lisp.html Could someone send me a link to a Haskell solution, with comments? I have seen a solution in Clean, but since it was a homework in the college where I study, I am quite sure that my fellow students have just translated Norvig's solution to Clean. Therefore, it would be great if one could send me a link to a Clean solution too. __________________________________________________________________ Be smarter than spam. See how smart SpamGuard is at giving junk email the boot with the All-new Yahoo! Mail. Click on Options in Mail and switch to New Mail today or register for free at http://mail.yahoo.ca

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Fwd: Error in building buster 1.51
by Лев Никитин 21 Nov '09

21 Nov '09

Hello. I try to install buster 1.51. It has been uploaded on hackage. During building there is an error. Same error can been seen on hackage (look at build failre: http://hackage.haskell.org/packages/archive/buster/2.51/logs/failure/ghc-6.… ). How to fix it? Thanks

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Fwd: [BostonHaskell] Next meeting: November 24th at MIT (32-G882)
by Ravi Nanavati 20 Nov '09

20 Nov '09

---------- Forwarded message ---------- From: Ravi Nanavati <ravi_n(a)alum.mit.edu> Date: Fri, Nov 20, 2009 at 4:00 PM Subject: Next meeting: November 24th at MIT (32-G882) To: bostonhaskell(a)googlegroups.com I'm pleased to announce the November meeting of the Boston Area Haskell Users' Group. Based on the available days remaining in November, the meeting has been scheduled for Tuesday, November 24th from 6:30pm to 8:30pm. Like recent meetings, it will be held in the MIT CSAIL Reading Room (32-G882, i.e. a room on the 8th floor of the Gates Tower of the MIT's Stata Center at 32 Vassar St in Cambridge, MA). Based on the response to my previous message, I'm developing a QuickCheck-focused joint programming exercise. If anyone wants to help with that, I'd appreciate it (since I'm trying to pull this together at the last minute). My plan is to introduce QuickCheck and start with a couple of warm-up exercises before our traditional break. After the break I'd like to use QuickCheck to find a subtle bug or two in some interesting, nontrivial (but not too large or complicated) Haskell program. I've found some introductory material to use as a starting point, but I don't have the right sort of Haskell program handy. If anyone has an interesting Haskell program they can share that is a good fit for this exercise, please let me know (the sooner the better). Since the meeting is going to be focused on a programming exercise, I don't think we'll have room for any larger presentations (at least based on our experience at the September meeting), but I'm there will be room for lightning talks or other advertisements if anyone has an interest in doing that. The November attendance poll is at: http://doodle.com/hyv5a76ib6untwtp As always, responding to this poll will help with two things: 1. Getting an idea of what fraction of the Boston-area Haskell community can and can't attend this meeting (to help with future scheduling). 2. Giving me an estimated count of attendees, since I have talked my wife into baking some more goodies and bringing drinks again. Helping with BostonHaskell (besides volunteering to speak at future meetings): 1. Sponsorship of refreshments is still being eagerly solicited (especially as, for personal reasons, the Cupcake Fund is running low). 2. Please let me know if you're willing to arrive early (6pm) to stand by the door and let people into the 8th floor. I'm guessing 1 or 2 volunteers is enough here. 3. If someone is willing to edit the HaskellWiki page, post to reddit or otherwise publicize the meeting please just go ahead and do that (especially as I'm sending this out a bit later than intended). If you have any questions about the meeting please send them to them BostonHaskell mailing list: bostonhaskell(a)googlegroups.com or contact me directly. I look forward to seeing many Boston-area Haskellers at the November meeting! Thank you, - Ravi Nanavati

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Applicative Functor or Idiom?
by David Sankel 20 Nov '09

20 Nov '09

After reading several recent papers I came to the understanding that there isn't consensus on the name of Applicative Functors. Several prefer to call them idioms: "'Idiom' was the name McBride originally chose, but he and Paterson now favour the less evocative term `applicative functor'. We have a slight preference for the former, not least because it lends itself nicely to adjectival uses, as in `idiomatic traversal'"[1] I also noticed use of the term Idiom in [2], [3], and [4]. I'm writing a set of classes that includes AF's and I'm trying to decide whether to call the class Idiom. Anyone have more information on this question? David [1] Gibbons, J. and Oliveira, B. c. 2009. The essence of the iterator pattern. J. Funct. Program. 19, 3-4 (Jul. 2009), 377-402. DOI= http://dx.doi.org/10.1017/S0956796809007291 [2] RALF HINZE (2009). The Bird Tree. Journal of Functional Programming, 19 , pp 491-508 doi:10.1017/S0956796809990116 [3] S. Lindley, P. Wadler, and J. Yallop. Idioms are oblivious, arrows are meticulous, monads are promiscuous. In Proc. of MSFP, 2008. [4] The Arrow Calculus, Sam Lindley, Philip Wadler, and Jeremy Yallop. Tech report, 2008. -- David Sankel Sankel Software www.sankelsoftware.com

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ANNOUNCE: gnome-keyring 0.1 (bindings to libgnome-keyring)
by John Millikin 20 Nov '09

20 Nov '09

The GNOME Keyring is a service for securely storing per-user secret information, such as passwords and encryption keys, on the GNOME desktop. This library is a binding to the libgnome-keyring C library. The API is still a bit too slave-ish to the original for my taste, some modules will be changing/merging in the next version. The innards are an absolute horror -- don't look too close if you're one of those people who faint at rampant unsafe casting. That said, it works surprisingly well for a first-release FFI binding -- mostly due to the excellent design of libgnome-keyring and c2hs. So if any Linux/BSD/OpenSolaris/etc users have needed secure password storage in Haskell, please try it out and let me know if you encounter any problems. Download: http://hackage.haskell.org/package/gnome-keyring/ API docs (copy of original library docs): https://dl.dropbox.com/u/1947532/gnome-keyring_0.1/index.html

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Iteratee question
by Valery V. Vorotyntsev 20 Nov '09

20 Nov '09

I am writing a binary data parser and use `iteratee' package. The following pattern appears quite often in my code: > results <- map someConversion `liftM` replicateM nbytes Iter.head The meaning is: take `nbytes' from stream, apply `someConversion' to every byte and return the list of `results'. But there's more than one way to do it[1]: > i1, i2, i3 :: Monad m => Int -> IterateeG [] Word8 m [String] > i1 n = map conv `liftM` replicateM n Iter.head > i2 n = map conv `liftM` joinI (Iter.take n stream2list) > i3 n = joinI $ Iter.take n $ joinI $ mapStream conv stream2list [1] http://en.wikipedia.org/wiki/There's_more_than_one_way_to_do_it If you try the program[2] I've hpasted, you will see that these three iteratees have equal results. [2] http://hpaste.org/fastcgi/hpaste.fcgi/view?id=12464#a12464 Of those i1, i2, i3 which one is "better" and why? Or is there another - preferable - way of applying iteratees to this task? My naïve guess is that i1 will have worse performance with big n's. It looks like `i1' is reading bytes one by one, while `i2' takes whole chunks of data... I'm not sure though. I would appreciate it a lot if you improve my understanding. -- vvv

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ANN: bindings-SDL 1.0.2, the domain specific language for FFI description
by Maurício CA 20 Nov '09

20 Nov '09

Hi, This package is to be used with hsc2hs. (hsc2hs is called automatically if you use Cabal.) It's a self-contained set of macros used to create a Haskell wrap for a C interface. They follow the idea that it's better to have a C-like wrap code first and then write Haskell-like code on top of that than going directly from C to Haskell style. Hackage link: http://hackage.haskell.org/package/bindings-DSL Wiki documentation: http://bitbucket.org/mauricio/bindings-dsl The fancy 'domain specific language' comes from 'binding-SDL' macros hability to describe a C interface. Instead of writing Haskell or preprocessor code, you can use 'bindings-DSL' macros to describe the interface you want to wrap without knowledge of how that description translates to preprocessor or Haskell code. Suppose, for instance, you have the following C struct: struct example_struct{ int n; struct example_struct *p; } It will be described like this: #starttype struct example_struct #field n , CInt #field p , Ptr <example_struct> #stoptype and this gives you a Haskell data type (automatically named C'example_struct) with the same fields and Storable instantiation. Here is a bigger example, with structs, macros, functions and global variables. Suppose you want to wrap this: http://git.savannah.gnu.org/cgit/gsl.git/tree/multimin/gsl_multimin.h Then this is how your 'bindings-DSL' code will look like: http://bitbucket.org/mauricio/bindings-gsl/src/tip/src/Bindings/Gsl/Multidi… That code translates to these Haskell declarations: http://hackage.haskell.org/packages/archive/bindings-gsl/0.1.1.6/doc/html/B… 'bindings-DSL' came from previous 'bindings-common' package. It started as an effort to have many Haskell interfaces to good libraries available on Hackage, and came to be a language to support writing such interfaces easily and reliably. Hope it's usefull to you. Best, Maurício

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ANNOUNCE: Haskell Communities and Activities Report (17th ed., November 2009)
by Janis Voigtländer 20 Nov '09

20 Nov '09

On behalf of the many, many contributors, I am pleased to announce that the Haskell Communities and Activities Report (17th edition, November 2009) http://www.haskell.org/communities/ is now available from the Haskell Communities home page in PDF and HTML formats. Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline ;-) to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ---------------------------------------- End of April 2010: target deadline for contributions to the May 2010 edition of the HC&A Report ---------------------------------------- Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in October (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org> -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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Fwd: [Haskell-cafe] ANNOUNCE: feldspar-language
by Tom Hawkins 20 Nov '09

20 Nov '09

---------- Forwarded message ---------- From: Tom Hawkins <tomahawkins(a)gmail.com> Date: Thu, Nov 19, 2009 at 9:31 PM Subject: Re: [Haskell-cafe] ANNOUNCE: feldspar-language To: Emil Axelsson <emax(a)chalmers.se> On Thu, Nov 19, 2009 at 5:58 PM, Emil Axelsson <emax(a)chalmers.se> wrote: > Hi Tom, thanks for the code! > > Is my understanding correct that `filter` in the C code performs evaluation > of one cycle, and someone else will take care of calling it repeatedly on > the input stream? This is correct. In our application there is no buffered steam. Instead this code is processing data immediately after it is sampled by the ADCs; in our case once every 1ms. > > Is there a way to avoid rolling out the summation loops fully? Yes, but only by creating a custom primitive (see 'action'). But then you're writing in C, not Atom.

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community.haskell.org is down
by Neil Mitchell 20 Nov '09

20 Nov '09

Hi, community.haskell.org isn't responding, I get connection failures. Thanks, Neil

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prologue junk on ppc (debian)
by Brian Denheyer 19 Nov '09

19 Nov '09

Got the following: Building uvector-0.1.0.5... [ 1 of 13] Compiling Data.Array.Vector.Prim.Text ( Data/Array/Vector/Prim/Text.hs, dist/build/Data/Array/Vector/Prim/Text.o ) Prologue junk?: .type sD7_entry, @function sD7_entry: # 25 "/tmp/ghc14266_0/ghc14266_0.hc" 1 That seems bad... The Glorious Glasgow Haskell Compilation System, version 6.10.4 This is the deb ghc6 package for unstable. Cabal was installed using ghc 6.8.x. I tried re-installing a couple of other cabal packages, including hps and bytestring, and they went through without a hitch. Brian

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Literate refactoring?
by Jacques Carette 19 Nov '09

19 Nov '09

I'd like to ask the cafe's advice on how to 'do this right'. The situation is that we've got 2 literate Haskell programs which we would like to refactor. In fact, this refactoring will ultimately unify these 2 separate programs into a single framework. Every refactoring step is well-defined and induces a discrete modification. These programs also come with tests of their own, which we would like to refactor along with the code. In the end, we would like to obtain: 1. For each discrete step, full working programs, with tests and literate comments 2. A technical report which describes all the steps from #1 above, as a literate Haskell document. For the TechReport, we don't need to be able to extract working code from it, but the code which is in it should be chunks of the working programs, not 'independent'. Especially as some of the 'discrete steps' will result in dead ends, which we want to document, but not otherwise pursue. In other words, the set of "discrete programs" do not form a total linear order, but is more like a linear order with a few short branches hanging off of it. We were thinking that using some combination of scripts, 'patches', features from subversion and literate Haskell documents should allow us to do this. In an ideal world, it would be nice if what we stored was only (generalized) patches, and we could generate #1 and #2 above from that. [These patches would be generalized because they would contain descriptions of what the patch is about, as well as actual patches; though perhaps that description is really just a patch to the TechReport, we're not sure]. We were able to envision a number of different partial solutions, but somehow none of them 'came together' well enough to feel satisfactory. Instead of biasing the cafe's thinking by laying those pieces out, I figured I would lay out the high-level requirements, and let creative solutions come in. Jacques

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(possibly) a list comprehensions question
by Ozgur Akgun 19 Nov '09

19 Nov '09

Hi Cafe! I am struggling with an interesting problem while defining a function. It looks quite easy to me, but I couldn't manage to have a proper implementation yet. To illustrate what I'm trying to achive, I'll introduce special cases of the desired function, and hopefully build towards a solution step by step. -- simplest case, taking 2 lists as parameters and returning a list of list containing every possible pair (but represented as lists) allPossibilities2 :: [a] -> [a] -> [[a]] allPossibilities2 listX listY = [ [x,y] | x <- listX, y <- listY] -- sample output -- allPossibilities2 [1,2,3] [7,8,9] -- [[1,7],[1,8],[1,9],[2,7],[2,8],[2,9],[3,7],[3,8],[3,9]] -- simplest case with 3 parameters instead of 2 allPossibilities3 :: [a] -> [a] -> [a] -> [[a]] allPossibilities3 listX listY listZ = [ [x,y,z] | x <- listX, y <- listY, z <- listZ] -- allPossibilities3 [1,2] [3,4,5] [6,7] -- [[1,3,6],[1,3,7],[1,4,6],[1,4,7],[1,5,6],[1,5,7],[2,3,6],[2,3,7],[2,4,6],[2,4,7],[2,5,6],[2,5,7]] These are easy and work just fine. All I want to do is to generalize this function receiving n lists as parameters and doing the simple action described above. Since I cannot pass variable number of parameters to a function, I'll use list of lists from now on. Following are the implementations of the same functions with different types (instead of two lists, a list of lists assumed to caontain those 2 elements) allPossibilities2' :: [[a]] -> [[a]] allPossibilities2' list = [ [x,y] | x <- list !! 0, y <- list !! 1] -- allPossibilities2' [[1,2,3],[7,8,9]] -- [[1,7],[1,8],[1,9],[2,7],[2,8],[2,9],[3,7],[3,8],[3,9]] allPossibilities3' :: [[a]] -> [[a]] allPossibilities3' list = [ [x,y,z] | x <- list !! 0, y <- list !! 1, z <- list !! 2] -- allPossibilities3' [[1,2],[3,4,5],[6,7]] -- [[1,3,6],[1,3,7],[1,4,6],[1,4,7],[1,5,6],[1,5,7],[2,3,6],[2,3,7],[2,4,6],[2,4,7],[2,5,6],[2,5,7]] This is ugly! Anyway, just forget the fact that these funstions do not do a check on the length of the input list for a moment. My question is, how can I generalize this function to accept a list of lists of arbitrary length, and produce the required result. I hope I managed to make my point clear enough. Waiting for suggestions. Regards, -- Ozgur Akgun

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How to show record syntax?
by Sean Leather 19 Nov '09

19 Nov '09

A while back, I was working on the Show function for EMGM [1] and verifying its output vs. that of GHC's when I discovered (I thought) a minor difference between what was necessary and what GHC did for record syntax. Labeled construction and update, according to the Report [2], have higher precedence than function application. This means that I can do the following > module Main where > data A = A {x :: Int} deriving Show > main = print $ Just A {x = 5} without putting parentheses around A {x = 5}. If I run this in GHCi, *Main> main Just (A {x = 5}) I see that the derived show in GHC adds parentheses around the record value. This led to an issue report [3] which developed a few responses. Some don't like the grammar as defined, because, at a glance, it appears too much like function application. Others noted that Hugs does not add the parentheses and wanted to see consistency between compilers. In the spirit of the GHC Bug Sweep [4] and finding resolution, I put the question to the wider community. How should we show record syntax? Should we try to ensure consistency between the compilers? Should we try to print as few parentheses as possible? Should we write a Haskell' proposal to "fix" the grammar and make Just A {x=5} illegal? What do you think? [1] http://hackage.haskell.org/packages/archive/emgm/0.3.1/doc/html/Generics-EM… [2] http://www.haskell.org/onlinereport/exps.html#sect3 [3] http://hackage.haskell.org/trac/ghc/ticket/2530 [4] http://hackage.haskell.org/trac/ghc/wiki/BugSweep Regards, Sean

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Re: [Haskell-cafe] If you haven't bought any of Knuth's fascicles yet, this is definitely the one to get. Bitwise Tricks & Techniques
by Doug McIlroy 19 Nov '09

19 Nov '09

One of Knuth's sources is the well-named book "Hacker's Delight" by Henry S. Warren; see hackersdelight.org

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Typefuck: Brainfuck in the type system
by Johnny Morrice 19 Nov '09

19 Nov '09

Greetings list, I was um well, drinking beer and thought it would be amusing to write a brainfuck interpreter which runs within the GHC type checker so I did, using type families. I haven't decided whether or not to put it on hackage (it is rather silly after all) but I have a link to a cabalized package and instructions on how to work it in an entry on my blog, here: http://killersmurf.blogspot.com/2009/11/typefuck.html Enjoy the ridiculousness. Johnny

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Cabal + gfortran?
by Gregory Crosswhite 19 Nov '09

19 Nov '09

Hi, I want to set up a build that compiles fortran sources in addition to the Haskell, and which maybe also eventually checks for the existence of a library or two (specifically, LAPACK and BLAS). Does anyone here have a suggestion for where I should be looking to figure out how to do this, such as a package which uses such a setup that would provide an example? Thanks, Greg

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pthread_kill missing?
by Gabor Greif 19 Nov '09

19 Nov '09

Hi all, I'd like to send a signal from the main thread to a forkOS-ed thread in GHC. The former should use raiseSignal and the second should sit in awaitSignal. I figured that the posix functionality in the unix-2.3 library does not cover this case. I would have expected that blocking/sending/receiving signals would utilize the pthread_* routines. This does not seem to be the case. Any idea when these will be available? Thanks in advance, cheers, Gabor PS: I guess some of you will say, "use condition variables". But that won't answer my question :-)

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faster compiling for ghc
by Evan Laforge 19 Nov '09

19 Nov '09

Recently the "go" language was announced at golang.org. There's not a lot in there to make a haskeller envious, except one real big one: compilation speed. The go compiler is wonderfully speedy. Anyone have any tips for speeding up ghc? Not using -O2 helps a lot. I spend a lot of time linking (ghc api drags in huge amounts of ghc code) and I'm hoping the new dynamic linking stuff will speed that up. I suppose it should be possible to run the whole thing under the bytecode compiler, and this works fine for rerunning tests, I can just stay in ghci, make changes, :r and rerun, but it runs into trouble as soon as code wants to link in foreign C. I also recently discovered -fobject-code, which indeed starts compiling right away, cutting out the ghc startup overhead. However, it doesn't appear to help with the final link, so I wind up having to reinvoke ghc anyway. According to Rob Pike, the main reason for 6g's speed is that in a dependency tree where A depends on B depends on C, the interface for B will pull up all the info needed from C. So compiling A only needs to look at B. Would it help ghc at all if it did the same with hi files? I've heard that ghc does more cross module inlining than your typical imperative language, but with optimization off maybe we can ignore all that? I've seen various bits of noise about supporting parallel builds with --make and it seems to involve making the whole compiler re-entrant which is non-trivial. Would it be simpler to parallelize pure portions of the compilation, say with parallel strategies? Or just start one ghc per core and have a locking scheme so they don't step on each others files?

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Re: [Haskell-cafe] Some help needed to start Haskell with Yi
by Robert Greayer 19 Nov '09

19 Nov '09

On Wed, Nov 18, 2009 at 8:15 PM, Daniel Fischer <daniel.is.fischer(a)web.de>wrote: > >> Am Donnerstag 19 November 2009 01:07:37 schrieb Henning Thielemann: >> > Kapil Hari Paranjape schrieb: >> > > Hello, >> > > >> > > On Sat, 14 Nov 2009, Jaco van Iterson wrote: >> > >> Only installation with 'cabal install yi' in a Cygwin shell under MS >> > >> Windows XP ended in: >> > >> Yi\Prelude.hs:182:9: >> > >> Duplicate instance declarations: >> > >> instance Category Accessor.T -- Defined at >> Yi\Prelude.hs:182:9-38 >> > >> instance Category Accessor.T >> > >> -- Defined in data-accessor-0.2.1:Data.Accessor.Private >> > >> cabal.exe: Error: some packages failed to install: >> > >> yi-0.6.1 failed during the building phase. The exception was: >> > >> exit: ExitFailure 1 >> > >> >> > >> Seems easy to fix but I can't even find where on my drive I can find >> the >> > >> source code. >> > >> >> > >> Where is the source? >> > >> > Seems to be that the author defined an orphan instance - something one >> > should never do! >> >> So what do you do if you need an instance ClassX TypeY but the author of >> the package that >> defines TypeY hasn't provided one? >> >> You can define an orphan instance or duplicate packageY but with the >> instance. Both are >> bad. Providing an orphan instance until packageY has one seems the lesser >> evil to me. >> Are there any good options? > > Whether it qualifies as 'good' or not, I'm not sure, but I think the standard recommendation is to newtype-wrap the type you want to make an orphan instance of, and make the instance on that (new) type. >

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ANNOUNCE: parallel-2.0.0.0
by Simon Marlow 19 Nov '09

19 Nov '09

I've just uploaded parallel-2.0.0.0 to Hackage. If you're using Strategies at all, I'd advise updating to this version of the parallel package. It's not completely API compatible, but if you're just using the supplied Strategies such as parList, the changes should be relatively minor. The Haddock docs include a full description of the changes, reproduced below. Haddock docs are also here, until Hackage catches up: http://www.haskell.org/~simonmar/parallel/index.html The Strategy-using programs I've tried so far go faster. Enjoy! Cheers, Simon --------------------------------------------------------------------- Version 1.x The original Strategies design is described in <http://www.macs.hw.ac.uk/~dsg/gph/papers/html/Strategies/strategies.html> and the code was written by Phil Trinder, Hans-Wolfgang Loidl, Kevin Hammond et al. Version 2.x Later, during work on the shared-memory implementation of parallelism in GHC, we discovered that the original formulation of Strategies had some problems, in particular it lead to space leaks and difficulties expressing speculative parallelism. Details are in the paper \"Runtime Support for Multicore Haskell\" <http://www.haskell.org/~simonmar/papers/multicore-ghc.pdf>. This module has been rewritten in version 2. The main change is to the 'Strategy a' type synonym, which was previously @a -> Done@ and is now @a -> a@. This change helps to fix the space leak described in \"Runtime Support for Multicore Haskell\". The problem is that the runtime will currently retain the memory referenced by all sparks, until they are evaluated. Hence, we must arrange to evaluate all the sparks eventually, just in case they aren't evaluated in parallel, so that they don't cause a space leak. This is why we must return a \"new\" value after applying a 'Strategy', so that the application can evaluate each spark created by the 'Strategy'. The simple rule is this: you /must/ use the result of applying a 'Strategy' if the strategy creates parallel sparks, and you should probably discard the the original value. If you don't do this, currently it may result in a space leak. In the future (GHC 6.14), it will probably result in lost parallelism instead, as we plan to change GHC so that unreferenced sparks are discarded rather than retained (we can't make this change until most code is switched over to this new version of Strategies, because code using the old verison of Strategies would be broken by the change in policy). The other changes in version 2.x are: * Strategies can now be defined using a convenient Applicative type Eval. e.g. parList s = unEval $ traverse (Par . s) * 'parList' has been generalised to 'parTraverse', which works on any 'Traversable' type. * 'parList' and 'parBuffer' have versions specialised to 'rwhnf', and there are transformation rules that automatically translate e.g. @parList rwnhf@ into a call to the optimised version. * 'NFData' is deprecated; please use the @DeepSeq@ class in the @deepseq@ package instead. Note that since the 'Strategy' type changed, 'rnf' is no longer a 'Strategy': use 'rdeepseq' instead.

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If you haven't bought any of Knuth's fascicles yet, this is definitely the one to get. Bitwise Tricks & Techniques
by Casey Hawthorne 18 Nov '09

18 Nov '09

If you haven't bought any of Knuth's fascicles yet, this is definitely the one to get. "The Art of Computer Programming: Volume 4 Bitwise Tricks & Techniques Binary Decision Diagrams" Fascicle 1 Donald E. Knuth 2009 Describes basic "broadword" operations and an important class of data structures that can make computer programs run dozens - even thousands - of times faster. I started a Haskell program using bitwise operations and then discarded them thinking them to low level. I think one of the dangers of Haskell, is that I get carried away with medium and high level abstractions and think "everything" MUST be done that way. :) -- Regards, Casey

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Re: Re[Haskell-cafe] cursive to foldr
by Ezra Lalonde 18 Nov '09

18 Nov '09

Using the same basic structure you did, and foldr, I think below is the simplest method: ==================== import Data.Maybe searchList :: (a -> Bool) -> [a] -> Maybe [a] searchList p xs = foldr (\x acc -> if p x then Just (x: fromMaybe [] acc) else acc) Nothing xs ==================== ghci> searchList (=='o') "A quick brown fox" Just "oo" ghci> searchList (==' ') "A quick brown fox" Just " " ghci> searchList (=='z') "A quick brown fox" Nothing >From maybe gets rid of the Maybe, so that our recursive call works: ghci> fromMaybe [] (Just [1..3]) [1,2,3] That's why we got the error below when we tried without fromMaybe; on subsequent applications of foldr, the type would have to change. ==================== <interactive>:1:51: Couldn't match expected type `[a]' against inferred type `Maybe [a]' In the expression: if p x then Just (x : acc) else acc In the first argument of `foldr', namely `(\ x acc -> if p x then Just (x : acc) else acc)' In the expression: foldr (\ x acc -> if p x then Just (x : acc) else acc) Nothing xs ==================== I have a feeling that using fromMaybe is not the best way, but it gets the job done for now. On that note; if somebody with some more experience would chime in, that'd be awesome. ;) Ezra dima.neg wrote: > > How can I do this using foldr? > > searchList p [] = Nothing > searchList p (x:xs) > | p x = Just (x:filter p xs) > | otherwise = searchList p xs > > > I try this: > searchList p xs = foldr (\x acc -> if p x then Just (x:acc) else acc) > Nothing xs > but don't work. > > Thanks > -- View this message in context: http://old.nabble.com/Recursive-to-foldr-tp26368900p26399795.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Call for Participation - PEPM'10 (co-located with POPL'10)
by Janis Voigtländer 18 Nov '09

18 Nov '09

=============================================================== CALL FOR PARTICIPATION ACM SIGPLAN 2010 Workshop on Partial Evaluation and Program Manipulation (PEPM'10) Madrid, January 18-19, 2010 (Affiliated with POPL'10) http://www.program-transformation.org/PEPM10 =============================================================== Abstracts of all papers and presentations are available from the above web site. INVITED TALKS: * Lennart Augustsson (Standard Chartered Bank, UK) Title: O, Partial Evaluator, Where Art Thou? * Jeremy Siek (University of Colorado at Boulder, USA) Title: General Purpose Languages Should be Metalanguages. SPECIAL FEATURE: * Andy Gill, Garrin Kimmell and Kevin Matlage. Capturing Functions and Catching Satellites. CONTRIBUTED TALKS: * Nabil el Boustani and Jurriaan Hage. Corrective Hints for Type Incorrect Generic Java Programs. * Johannes Rudolph and Peter Thiemann. Mnemonics: Type-safe Bytecode Generation at Run Time. * Elvira Albert, Miguel Gomez-Zamalloa and German Puebla. PET: A Partial Evaluation-based Test Case Generation Tool for Java Bytecode. * Martin Hofmann. Igor2 - an Analytical Inductive Functional Programming System. * José Pedro Magalhães, Stefan Holdermans, Johan Jeuring and Andres Löh. Optimizing Generics Is Easy! * Michele Baggi, María Alpuente, Demis Ballis and Moreno Falaschi. A Fold/Unfold Transformation Framework for Rewrite Theories extended to CCT. * Hugh Anderson and Siau-Cheng KHOO. Regular Approximation and Bounded Domains for Size-Change Termination. * Évelyne Contejean, Pierre Courtieu, Julien Forest, Andrei Paskevich, Olivier Pons and Xavier Urbain. A3PAT, an Approach for Certified Automated Termination Proofs. * Fritz Henglein. Optimizing Relational Algebra Operations Using Generic Equivalence Discriminators and Lazy Products. * Adrian Riesco and Juan Rodriguez-Hortala. Programming with Singular and Plural Non-deterministic Functions. * Martin Hofmann and Emanuel Kitzelmann. I/O Guided Detection of List Catamorphisms. * Andrew Moss and Dan Page. Bridging the Gap Between Symbolic and Efficient AES Implementations. * Christopher Brown and Simon Thompson. Clone Detection and Elimination for Haskell. * Stefan Holdermans and Jurriaan Hage. Making Stricterness More Relevant. * Arun Lakhotia, Davidson Boccardo, Anshuman Singh and Aleardo Manacero Júnior. Context-Sensitive Analysis of Obfuscated x86 Executables. * Xin Li and Mizuhito Ogawa. Conditional Weighted Pushdown Systems and Applications. * Ivan Lazar Miljenovic. The SourceGraph Program. * Florian Haftmann. From Higher-Order Logic to Haskell: There and Back Again. IMPORTANT DATES: * Early registration deadline: December 22, 2009 * Hotel registration deadline: December 28, 2009

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ANN: LambdaCube engine and Bullet physics binding
by Csaba Hruska 18 Nov '09

18 Nov '09

Hi everyone, I'm pleased to announce the first pre-beta-candidate-release of the LambdaCube 3D engine [1] and the bindings for the Bullet physics library [2]. The packages are just a cabal-install away. You should be able to install lambdacube-engine and lambdacube-examples right away. In order to get the physics engine running, please check the instructions on the wiki. After compiling bullet, you can try the lambdacube-bullet package as well, which shows how to combine the two. It can't be stressed enough that this project is in a very early stage, and everything is bound to change and improve a lot. I just made this publicly available in order to attract contributions, so if you have anything to share, be it bugfixes, code improvements or more and prettier examples, feel free to join in! Cheers, Csaba [1] http://haskell.org/haskellwiki/LambdaCubeEngine [2] http://haskell.org/haskellwiki/Bullet

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GHCi's :t misbehaving
by Philippa Cowderoy 18 Nov '09

18 Nov '09

I have some mildly complicated parsing code, that uses parsec to return a computation (in a state monad) that handles operator precedence - so I can handle scoped precedence/fixities, much like in Haskell. I just spent a while bolting on some new features. More time than I'd like, I'd left it alone for a while and it took a bit of time getting my head around it again enough to be okay doing folds on parse results and the like. Enough so that I've been leaning on the type checker to tell me when I've messed up! So, I have something that loads into ghci okay now. I go to check the type of one of the functions using :t and get this error: <interactive>:1:0: Ambiguous type variable `m' in the constraint: `Monad m' arising from a use of `constructor' at <interactive>:1:0-10 Probable fix: add a type signature that fixes these type variable(s) Now I'm not about the supply the type signature, that's what I asked it for! And it ought to typecheck okay, given that the code loaded in the first place. I'm about to turn in for the night, but I'm wondering what's going on here. Anyone? -- flippa(a)flippac.org

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Announcing the GHC Bug Sweep
by Simon Marlow 18 Nov '09

18 Nov '09

Help us weed the GHC ticket database, and get a warm fuzzy feeling from contributing to Haskell core technology! There are currently ~750 tickets against GHC. Many of them have not been looked at in months or years. Often when I go through old tickets I find easy targets: bugs that have already been fixed, duplicates, bugs that are not reproducible and the submitter has gone away. So the idea we have is this: do an incremental sweep of the whole database, starting from the oldest tickets. Check each one, and try to make some progress on it. If we get enough momentum going we can make sure every ticket gets looked at every few months at the least. This is a game for the whole family! We don't care how much progress you make on each ticket, just as long as someone has taken a look and moved the ticket forward in some way. For example, you might check for duplicates, update the metadata, ask for more information from the submitter, try to reproduce the bug against the latest version of GHC. To claim a ticket all you have to do is remove it from the list on the wiki. Full instructions are here http://hackage.haskell.org/trac/ghc/wiki/BugSweep including a list of suggestions for ways to make progress on a ticket. Cheers! Simon & the GHC team

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ICFP '10: Second call for workshop proposals
by Wouter Swierstra 18 Nov '09

18 Nov '09

CALL FOR WORKSHOP AND CO-LOCATED EVENT PROPOSALS ICFP 2010 15th ACM SIGPLAN International Conference on Functional Programming September 27 - 29, 2010 Baltimore, Maryland http://www.icfpconference.org/icfp2010 The 15th ACM SIGPLAN International Conference on Functional Programming will be held in Baltimore, Maryland on September 27-29, 2010. ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. Proposals are invited for workshops (and other co-located events, such as tutorials) to be affiliated with ICFP 2010 and sponsored by SIGPLAN. These events should be more informal and focused than ICFP itself, include sessions that enable interaction among the attendees, and be fairly low-cost. The preference is for one-day events, but other schedules can also be considered. ---------------------------------------------------------------------- Submission details Deadline for submission: November 20, 2009 Notification of acceptance: December 18, 2009 Prospective organizers of workshops or other co-located events are invited to submit a completed workshop proposal form in plain text format to the ICFP 2010 workshop co-chairs (Derek Dreyer and Chris Stone), via email to icfp10-workshops at mpi-sws.org by November 20, 2009. (For proposals of co-located events other than workshops, please fill in the workshop proposal form and just leave blank any sections that do not apply.) Please note that this is a firm deadline. Organizers will be notified if their event proposal is accepted by December 18, 2009, and if successful, depending on the event, they will be asked to produce a final report after the event has taken place that is suitable for publication in SIGPLAN Notices. The proposal form is available at: http://www.icfpconference.org/icfp2010/icfp10-workshops-form.txt Further information about SIGPLAN sponsorship is available at: http://acm.org/sigplan/sigplan_workshop_proposal.htm ---------------------------------------------------------------------- Selection committee The proposals will be evaluated by a committee comprising the following members of the ICFP 2010 organizing committee, together with the members of the SIGPLAN executive committee. Workshop Co-Chair: Derek Dreyer (MPI-SWS) Workshop Co-Chair: Chris Stone (Harvey Mudd College) General Chair: Paul Hudak (Yale University) Program Chair: Stephanie Weirich (University of Pennsylvania) ---------------------------------------------------------------------- Further information Any queries should be addressed to the workshop co-chairs (Derek Dreyer and Chris Stone), via email to icfp10-workshops at mpi-sws.org.

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hipmunkplayground on windows
by Anatoly Yakovenko 18 Nov '09

18 Nov '09

Anyone else seeing a bunch of linker errors when trying to install HipmunkPlayground? this is what i see: C:\Program Files\Haskell\OpenGL-2.2.3.0\ghc-6.10.4/libHSOpenGL-2.2.3.0.a(RenderMode.o):fake:(.text+0x5be): more undefined references to `glRenderMode' follow C:\Program Files\Haskell\OpenGL-2.2.3.0\ghc-6.10.4/libHSOpenGL-2.2.3.0.a(Environments.o):fake:(.text+0x1c25): undefined reference to `glGetTexEnvfv' C:\Program Files\Haskell\OpenGL-2.2.3.0\ghc-6.10.4/libHSOpenGL-2.2.3.0.a(Environments.o):fake:(.text+0x5779): undefined reference to `glTexEnvi' C:\Program Files\Haskell\OpenGL-2.2.3.0\ghc-6.10.4/libHSOpenGL-2.2.3.0.a(Environments.o):fake:(.text+0x58f5): undefined reference to `glGetTexEnviv' C:\Program Files\Haskell\OpenGL-2.2.3.0\ghc-6.10.4/libHSOpenGL-2.2.3.0.a(Environments.o):fake:(.text+0x5aba): undefined reference to `glTexEnvf' C:\Program Files\Haskell\OpenGL-2.2.3.0\ghc-6.10.4/libHSOpenGL-2.2.3.0.a(Environments.o):fake:(.text+0x5c7d): undefined reference to `glGetTexEnvfv' C:\Program Files\Haskell\OpenGL-2.2.3.0\ghc-6.10.4/libHSOpenGL-2.2.3.0.a(Environments.o):fake:(.text+0x6d51): undefined reference to `glTexEnvfv' C:\Program Files\Haskell\OpenGL-2.2.3.0\ghc-6.10.4/libHSOpenGL-2.2.3.0.a(Queries.o):fake:(.text+0x339): undefined reference to `glGetTexLevelParameteriv' collect2: ld returned 1 exit status cabal.exe: Error: some packages failed to install: HipmunkPlayground-5.0.0 failed during the building phase. The exception was: exit: ExitFailure 1

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omitting params in function bindings
by Doug McIlroy 18 Nov '09

18 Nov '09

Is there a deep reason (beyond saving a sentence or two in the language definition) for requiring all patterns in a function binding to have the same explicit arity? For example, in dropWhile0 :: Num a => [a] -> [a] dropWhile0 (0:xs) = dropWhile0 xs dropWhile0 xs = xs why shouldn't the last line be replaceable by dropWhile0 = id

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Re: [Haskell-cafe] Re: ANNOUNCE: deepseq-1.0.0.0
by Luke Palmer 18 Nov '09

18 Nov '09

On Tue, Nov 17, 2009 at 8:48 PM, Dean Herington <heringtonlacey(a)mindspring.com> wrote: > The documentation claim that "The default implementation of 'deepseq' is > simply 'seq'" is not exactly right, as `deepseq` and `seq` have different > signatures. Which raises the more interesting question: Why did you choose > a different signature? And, would a version of `seq` with the same > signature as `deepseq` be useful? The situation is analogous to, say, "null" having this signature: null :: [a] -> Bool Instead of this one: null :: [a] -> b -> b -> b Or the recent famous debate about returning Maybe a vs. Monad m => m a for failure. If we have seq' :: a -> (), then we have seq = m . seq' where m () = id And of course we can go the other way too. So it is a question of taste. deepseq is simpler by at least two standards: it is not polymorphic and it has only one argument. There are exactly two values of both () and forall b. b -> b, but that fact is more obvious of the former (IMO). I think it is the right choice. Luke

1 0

help with musical data structures
by Michael Mossey 18 Nov '09

18 Nov '09

I'm pretty new to Haskell so I don't know what kind of data structure I should use for the following problem. Some kind of arrays, I guess. One data item, called OrientedPCSet ("oriented pitch class set," a musical term) will represent a set whose members are from the range of integers 0 to 11. This could probably be represented efficiently as some kind of bit field for fast comparison. Another item, PitchMatrix, will be a 2-d matrix of midi pitch numbers. This matrix will be constructed via a backtracking algortithm with an evaluation function at each step. It will probably be constructed by adding one number at a time, starting at the top of a column and working down, then moving to the next column. This matrix should probably be implemented as an array of some sort for fast lookup of the item row x, column y. It doesn't require update/modification to be as fast as lookup, and it won't get very large, so some sort of immutable array may work. Thanks, Mike

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Simple hash table creation
by michael rice 17 Nov '09

17 Nov '09

I'm trying to create a hash table. Yeah, I know, don't use hash tables, but I need to create something I'm familiar with, not something I've never worked with before. What's wrong with this code? Michael ==================== import Prelude hiding (lookup) import Data.HashTable data MyHashTable = HashTable String Int dummy:: String -> Int dummy s = 7 ht = MyHashTable.new (==) dummy ==================== [michael@localhost ~]$ ghci hash1 GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( hash1.hs, interpreted ) hash1.hs:9:5: Not in scope: `MyHashTable.new' Failed, modules loaded: none. Prelude>

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ghci + user prelude
by Sean McLaughlin 17 Nov '09

17 Nov '09

Hello, If there's a file called Prelude.hs in a directory, and ghci is started from that directory, ghci dies. -- Prelude.hs module A.Prelude where $ ghci GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. <command line>: module `Prelude' is not loaded Is there a way around this? I often like to have a modified Prelude file in a subdirectory of my project, and this behavior keeps me from being able to start ghci there. Thanks! Sean

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Little errors in number calculations
by Abby Henríquez Tejera 17 Nov '09

17 Nov '09

Hi. I've seen that in GHC sometimes there are little errors in some basic number calculations: *Prelude> 123.35503 * 10.0 1233.5502999999999 *Prelude> properFraction 123.35503 (123,0.3550299999999993) whereas in Hugs no such errors seem to occur (that I have found, at least): *Hugs> 123.35503 * 10.0 1233.5503 (but:) *Hugs> properFraction 123.35503 (123,0.355029999999999) I understand that error may (and will) happen in floating point, but it surprises me that they do so easily, and, above all, the difference between GHC and Hugs. Does someone know why does this difference occur? (Thanks in advance, by the way :) ).

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FW: [Haskell-cafe] Status of TypeDirectedNameResolution proposal?
by Simon Peyton-Jones 17 Nov '09

17 Nov '09

[Resending, this time with link; sorry] | What's the status of the TDNR proposal [1]? Personally I think it is a | very good idea and I'd like to see it in Haskell'/GHC rather sooner | than later. Working around the limitations of the current record | system is one of my biggest pain points in Haskell and TDNR would be a | major improvement. Thus I wonder if someone is actively working on | this proposal? It's stalled. As far as I know, there's been very little discussion about it. It's not a trivial thing to implement, and it treads on delicate territory (how "." is treated). So I'd need to be convinced there was a strong constituency who really wanted it before adding it. I've added an informal straw poll to the bottom of [1] to allow you to express an opinion. Also I'm not very happy with the "stacking operations" part, and I'd like a better idea. Simon [1] http://hackage.haskell.org/trac/haskell-prime/wiki/TypeDirectedNameResoluti…

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Re: [Haskell-cafe] ANNOUNCE: control-monad-failure and safe-failure
by Kalman Noel 17 Nov '09

17 Nov '09

Michael Snoyman schrieb: > control-monad-failure provides a basic notion of failure which does not > commit to any concrete representation. > It is just a version of the MonadError class without the annoying bits. > >> class MonadFailure e m where failure :: e -> m a Why is it called "MonadFailure" (specifically, what's the "Monad" bit doing there)?

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Howto start a bigger project
by Günther Schmidt 17 Nov '09

17 Nov '09

Hi all, I'm stuck with a problem where I need serious help from other haskellers, in particular those that participate here on this list. It's a rather big project and I will need to set it up in an organized way, something with a blog, web page or other means. I tried to solve it by myself while asking the occasional question here but that turned out to be ineefective. The problem as such is certainly of interest for just about any programmer who is using Haskell for real world programming too. In short, to get started I'd appreciate some tips how to set this up. Günther

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ANN: wcwidth-0.0.1
by Jason Dusek 17 Nov '09

17 Nov '09

A small package with bindings to a function in <wchar.h> that assigns a column width to Unicode characters. http://hackage.haskell.org/package/wcwidth-0.0.1 This is the function that tells your terminal to make Chinese characters two columns wide while making the letter 'a' but one column wide. In addition, a little utility is provided that constructs a table of widths by character and a listing of character ranges with the same width. On different systems, `wcwidth` may assign different widths to the same character for some obscure characters. -- Jason Dusek

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How much time until dependencies are rebuilt in hackage?
by Maurício CA 17 Nov '09

17 Nov '09

Suppose package B depends on A. If a new version of A is uploaded to hackage, how much later package B will be rebuilt (either to show a problem with the new version or to solve a problem with a previous version of A)? Thanks, Maurício

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C headers in cabal dependencies
by Maurício CA 17 Nov '09

17 Nov '09

Hi, I've been using 'install-includes' in a package. I sometimes make small changes to those include files, and I've seen that cabal doesn't consider then dependencies, i.e., doesn't rebuild .hsc files depending on then. I'm not sure if this is an error, as parsing hsc2hs input may not be cabal task. Anyway, I would like to suggest that files in install-includes be considered dependencies and that files under 'includes' be also included when processing '.hsc' files. Thanks. Best, Maurício

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creating documentation links failed with cabal install --haddock-options=--hyperlink-source. is this a bug?
by Thomas Hartman 17 Nov '09

17 Nov '09

cabal haddock -–hyperlink-source installs documentation with links to source code, which also be on by defualt. cabal install –haddock-options=–hyperlink source does not install hyperlinked source. cabal install –haddock-options=-–hyperlink sourceblehblehbleh doesn’t throw an error, which suggests to me that the first command might also be passing the flags with an error. Whatever the case, seems to me the examples section in cabal --help should really include examples using --with-PROG and --PROG-options. I can't find a good one online. fwiw, cabal install --haddock-option=hyperlink-source DID generate an error in the output: target `hyperlink-source' is not a module name or a source file also, blogged about this here: http://blog.patch-tag.com/2009/10/23/cabal-install-a-package-with-documenta…

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DSL in Haskell
by CK Kashyap 16 Nov '09

16 Nov '09

Hi All, I was reading a Ruby book and in that it was mentioned that its capability to dynamically query and modify classes makes it suitable for implementing DSL's ... I am referring to Ruby's reflection and methods like "method_missing" here. It can allow things like not having to define constants for all possible unicode code points etc...For example, first use of U0123 could bring such a constant definition into existence etc I see multiple search hits when I look for Haskell and DSL - can someone please point me to a good primer or explain to me how equivalent of above mentioned features in Ruby can be done in Haskell ... or the Haskell alternative for it. Regards, Kashyap

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ANNOUNCE: attempt
by Michael Snoyman 16 Nov '09

16 Nov '09

We'd like to announce the second release (version 0.0.1) of the attempt package, for handling of failures. This release has been made to work with the new control-monad-failure package[1] in an effort to standardize failure handling. Notable changes to this release: * The MonadAttempt class was fully redundant with MonadFailure, and has thus been removed. * WrapFailure class has been moved to control-monad-failure (with minor modifications). * Data.Attempt.Helper (safe versions of some functions) has been moved to safe-failure[2]. * Full support for the monadloc package[3] to get monadic stack traces. As usual, this release is available on hackage[4]. API stability cannot be guaranteed with this release as well, but we're getting much closer to hitting the mark. All the best, Michael Snoyman, Nicolas Pouillard 1. http://hackage.haskell.org/package/control-monad-failure 2. http://hackage.haskell.org/package/safe-failure 3. http://hackage.haskell.org/package/monadloc 4. http://hackage.haskell.org/package/attempt

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ANNOUNCE: control-monad-failure and safe-failure
by Michael Snoyman 16 Nov '09

16 Nov '09

Folks, We are extremely happy to announce the control-monad-failure and safe-failure packages for error handling. control-monad-failure provides a basic notion of failure which does not commit to any concrete representation. It is just a version of the MonadError class without the annoying bits. > class MonadFailure e m where failure :: e -> m a Instances are provided for several concrete representations of failure handling: Maybe, Either, ErrorT, Control.Exception.Throw, and []. safe-failure is a fork of Neil Mitchell's Safe package providing MonadFailure versions of several partial functions in the Prelude, which instead of failing with a runtime error fail with Failure. > head :: MonadFailure HeadFailure m => [a] -> m a This is a joint release from the authors of the attempt and the control-monad-exception packages, and accompanying releases of those packages have been made making them adopt the MonadFailure interface. We have also created a wiki page explaining our reasons for following this path in: http://www.haskellwiki.org/Failure Thanks, Jose Iborra, Nicolas Pouillard and Michael Snoyman

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