- Haskell-Cafe - Haskell.org

Reaching Max Bolingbroke
by Roman Cheplyaka 19 Nov '12

19 Nov '12

For the last two months I've been trying to reach Max Bolingbroke via his hotmail address, github and linkedin, but did not succeed. Does anyone know if he's well? If someone could help by telling him that I'd like to get in touch, I'd appreciate that. Roman

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Re: [Haskell-cafe] List all multiply/add combinations
by Rune Harder Bak 19 Nov '12

19 Nov '12

On Sun, Nov 18, 2012 at 2:04 PM, Stefan Klinger <all-lista(a)stefan-klinger.de> wrote: > Sounds like you would want to enumerate all possible *abstract* syntax > trees, these implicitly have exactly the necessary parentheses. I'd do > this recursively, splitting the sequence of numbers in two at all > possible places, and then combine the corresponding results with all > possible operators. That was my second idea, but just doing it naively resulted in many equivalent calculations, so I thought there might be a better way to view the problem. But as Artyom showed On Sat, Nov 17, 2012 at 11:37 PM, Artyom Kazak <artyom.kazak(a)gmail.com> wrote: > Indentation messed up… I have pasted the code here: http://hpaste.org/77864 enumerating the abstract syntax tree is actually (or at least could be) the way to go! Thanks a lot! What I need is a little bit different, but now I feel I'm on the right track! This is my first question to the café and it makes me a lot more certain using Haskell for production, when you can get this kind of quick and thorough help when in doubt. Great community! -Rune

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Re: [Haskell-cafe] Automation of external library installation in haskell package.
by Maksymilian Owsianny 18 Nov '12

18 Nov '12

Hi Johan, Originally, that was my intention, to have the user install the library himself, and I have specified the extra-libraries field in the .cabal file. However, the problem with hackage not being able to build the package, and therefore generate documentation, remains. So conversely my question would be if there is any way to get around this? Best regards, Max. On Sun, Nov 18, 2012 at 8:21 PM, Johan Tibell <johan.tibell(a)gmail.com>wrote: > Hi Maksymilian, > > Generally I don't recommend that you bundle C libraries with your > Cabal package. On Linux users should get C libraries through their > distribution's package manager. The people who package Cabal packages > for Debian (as Debian packages) can then have the Debian package they > create depend on the C library's package, making sure it gets > installed automatically. > > You can list the library name in an extra-libraries field in your > .cabal file to communicate to users that they need this library > installed. There's a little bit of documentation on the > extra-libraries field in the Cabal User Guide > (http://www.haskell.org/cabal/users-guide/developing-packages.html#library > ). > > Cheers, > Johan >

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Automation of external library installation in haskell package.
by Maksymilian Owsianny 18 Nov '12

18 Nov '12

Hello Haskell-Cafe, I have created a package that requires an external library to be present on the machine. Because of that, obviously, hackage fails to build it, and subsequently fails to generate documentation. So my question is, how should I go about configuring Cabal/Setup.hs to automatize the process to allow hackage to build it properly and also simplify the package installation for the user. The packages that I'm talking about are bindings to Awesomium<http://awesomium.com/> : awesomium-raw <http://hackage.haskell.org/package/awesomium-raw> and subsequently, the higher level ones that depend on it awesomium <http://hackage.haskell.org/package/awesomium> awesomium-glut <http://hackage.haskell.org/package/awesomium-glut> Specifically, the installation process for linux is described as follows: To install Awesomium, you'll need to add the shared library to your > system's library search path. On Ubuntu, you can use the following > commands: > > cd awesomium_v1.6.5_sdk_linux32 > sudo mkdir /usr/lib/awesomium-1.6.5 > sudo cp -r build/bin/release/* /usr/lib/awesomium-1.6.5 > sudo ldconfig /usr/lib/awesomium-1.6.5 > > So if anyone could give me some hints, or point me to package that have a similar, external-library-copying-and-registering installation process i would be grateful.

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Parsing different types, same typeclass
by José Lopes 18 Nov '12

18 Nov '12

Hello everyone, I was wondering if you could help me! So, I have a typeclass "Action" which defines method "run": class Action a where run :: a -> Int and two data types that are instances of this typeclass: data A = A Int deriving (Read, Show) instance Action A where run (A n) = n data B = B Int deriving (Read, Show) instance Action B where run (B n) = n Now, I want to parse either "A" or "B" from a String. I was thinking about something like this... parseAction :: (Action a, Read a) => String -> a parseAction str | "(A " `isPrefixOf` str = (read :: String -> A) str | "(B " `isPrefixOf` str = (read :: String -> B) str The problem is that when calling "parseAction" I get "ambiguous type constraints". How to implement a parse function for two distinct types that share the same typeclass "Action". Because after calling "parseAction" I don't whether "A" or "B" was returned: I only care that they are "Action" instances so I can call "run". Best regards, José

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A small step towards solving cabal hell.
by timothyhobbs＠seznam.cz 18 Nov '12

18 Nov '12

I've read a lot of negative regarding the problems with cabal and hackage. I've written quite a few of them myself. I want to propose a simple change in philosophy of packages. Haskell has inherited a philosophy from the imperative world, that there are two types of packages: Libraries and applications. Libraries are big collections of modules. Applications are big collections of modules. There is a difference from the perspective of the build system. Applications are big collections of modules that belong together and mutually rely upon each other for the application to work. Such that if one module is missing, the application cannot build and thus cannot do useful tasks. If an application doesn't build, then the maintainer has to go and fix that problem. This isn't perfect, I do not solve this problem in this email. Libraries don't always have this property. For example, XMonad.Layout.Column (http://hackage.haskell.org/packages/archive/xmonad-contrib/0.10/doc/html/XM…) has no mutual dependency on XMonad.Layout.Circle (http://hackage.haskell.org/packages/archive/xmonad-contrib/0.10/doc/html/XM…) . This is a feature of libraries that can and should be taken advantage of wherever possible :) We can and should, break up libraries into "books". I hold this opinion, because libraries are hard to maintain. If xmonad-contrib refuses to build, I open up it's source code, and see some hundred modules. I cannot, as a non-xmonad developer, imagine myself fixing such a large library. But if it was just one small module from that library I wanted, and it refused to build, and I opened it up, and there were just 3 files there. I wouldn't feel so overwhelmed. I would fix the problem myself. What I'm trying to say, is that "books" (small packages containing 3 modules or less) are so easy to maintain, that they really need no maintainer at all. Any idiot can fix one. But libraries, with their hundreds of modules, and seemingly endless dependencies, Can be much harder to maintain, and require a knowledgeable maintainer. So lets stop publishing libraries, and maintaining libraries, and listening to people crying when libraries don't build, and lets start writing books, and publishing really small packages, that are so simple anyone can fix them when they break even when they have no in depth knowledge of the package. Lets make a new guideline for "libraries" that if it is possible to split a library into two or more, non-mutually dependent parts, than we *should* split them such. Timothy PS, I noticed that yesod is already a collection of "books", but it appears to me, that these packages are STILL mutually dependent :(

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I killed performance of my code with Eval and Strategies
by Janek S. 18 Nov '12

18 Nov '12

Dear Haskellers, I am reading Simon Marlow's tutorial on parallelism and I have problems with correctly using Eval monad and Strategies. I *thought* I understand them but after writing some code it turns out that obviously I don't because parallelized code is about 20 times slower. Here's a short example (code + criterion benchmarks): {-# LANGUAGE BangPatterns #-} module Main where import Control.Parallel.Strategies import Criterion.Main main :: IO () main = defaultMain [ bench "Seq" $ nf calculateSeq xs , bench "Par" $ nf calculatePar xs ] where xs = [1..16384] calculateSeq :: [Double] -> [Double] calculateSeq [] = [] calculateSeq (x:xs) = (sin . sqrt $ x) : xs calculatePar :: [Double] -> [Double] calculatePar xss = runEval $ go xss where go :: Strategy [Double] go [] = return [] go xs = do lsh <- (rpar `dot` rdeepseq) $ calculateSeq as lst <- go bs return (lsh ++ lst) where !(as, bs) = splitAt 8192 xs Compiling and running with: ghc -O2 -Wall -threaded -rtsopts -fforce-recomp -eventlog evalleak.hs ./evalleak -oreport.html -g +RTS -N2 -ls -s I get: benchmarking Seq mean: 100.5990 us, lb 100.1937 us, ub 101.1521 us, ci 0.950 std dev: 2.395003 us, lb 1.860923 us, ub 3.169562 us, ci 0.950 benchmarking Par mean: 2.233127 ms, lb 2.169669 ms, ub 2.296155 ms, ci 0.950 std dev: 323.5201 us, lb 310.2844 us, ub 344.8252 us, ci 0.950 That's a hopeless result. Looking at the spark allocation everything looks fine: SPARKS: 202 (202 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled) But analyzing eventlog with ThreadScope I see that parallel function spends most of the time doing garbage collection, which suggests that I have a memory leak somewhere. I suspected that problem might be caused by appending two lists together in the parallel implementation, but replacing this with difference lists doesn't help. Changing granularity (e.g. splitAt 512) also brings no improvement. Can anyone point me to what am I doing wrong? Janek PS. This is of course not a real world code - I know that I'd be better of using unboxed data structures for doing computations on Doubles.

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How to determine correct dependency versions for a library?
by Janek S. 18 Nov '12

18 Nov '12

Recently I started developing a Haskell library and I have a question about package dependencies. Right now when I need my project to depend on some other package I only specify the package name in cabal file and don't bother with providing the package version. This works because I am the only user of my library but I am aware that if the library were to be released on Hackage I would have to supply version numbers in the dependencies. The question is how to determine proper version numbers? I can be conservative and assume that version of libraries in my system are the minimum required ones. This is of course not a good solution, because my library might work with earlier versions but I don't know a way to check that. What is the best way to determine a minimal version of a package required by my library? I also don't see any sane way of determining maximum allowed versions for the dependencies, but looking at other packages I see that this is mostly ignored and package maintainers only supply lower versions. Is this correct approach? Janek

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List all multiply/add combinations
by Rune Harder Bak 17 Nov '12

17 Nov '12

Given a list of numbers of fixed length I need to list all possible values (and the associated computation) you get by inserting +,-,*,/ between the numbers, and also set parentheses where you please. It shouldn't list computations with unnecessary parentheses. Example list of length 3 [a,b,c] and only with + and *: a*b+c, a*(b+c),a*b*c,a+b*c,(a+b)*c,a+b+c What would be a good way to do this, and a good representation in Haskell? Best, Rune

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Call for Participation: Programming Languages Mentoring Workshop - a POPL workshop.
by Gareth Smith 17 Nov '12

17 Nov '12

Apologies for any duplicates: CALL FOR PARTICIPATION SIGPLAN Programming Languages Mentoring Workshop, Rome Tuesday January 22, 2013 Co-located with POPL 2013 PLMW web page: http://www.doc.ic.ac.uk/~gds/PLMW/index.html After the resounding success of the first Programming Languages Mentoring Workshop at POPL 2012, we proudly announce the 2nd SIGPLAN Programming Languages Mentoring Workshop (PLMW), co-located with POPL 2013 and organised by Nate Foster, Philippa Gardner, Alan Schmitt, Gareth Smith, Peter Thieman and Tobias Wrigstad. The purpose of this mentoring workshop is to encourage graduate students and senior undergraduate students to pursue careers in programming language research. This workshop will provide technical sessions on cutting-edge research in programming languages, and mentoring sessions on how to prepare for a research career. We will bring together leaders in programming language research from academia and industry to give talks on their research areas. The workshop will engage students in a process of imagining how they might contribute to our research community. We especially encourage women and underrepresented minority students to attend PLMW. Since PLMW will be in Rome this year, we particularly look forward to seeing Eastern European students at the workshop. This workshop is part of the activities surrounding POPL, the Symposium on Principles of Programming Languages, and takes place the day before the main conference. One goal of the workshop is to make the POPL conference more accessible to newcomers. We hope that participants will stay through the entire conference, and will also attend the POPL tutorials on Monday 21st January which are free to PLMW registered attendees. Through the generous donation of our sponsors, we are able to provide scholarships to fund student participation. These scholarships will cover reasonable expenses (airfare, hotel and registration fees) for attendance at both the workshop and the POPL conference. Students attending this year will get one year free student membership of SIGPLAN The workshop registration is open to all. Students with alternative sources of funding are welcome. APPLICATION for PLMW scholarship: The scholarship application can be accessed from the workshop web site (http://www.doc.ic.ac.uk/~gds/PLMW/index.html) The deadline for full consideration of funding is 9th December, 2012. Selected participants will be notified from Friday 14th December, and will need to register for the workshop by December 24th. SPONSORS: Imperial College London Jane Street Monoidics NSF Resource Reasoning SIGPLAN vmware

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Custom Enum instance with non-consecutive indices
by Nicolas Trangez 17 Nov '12

17 Nov '12

All, I've been working on a server implementation of an existing networking protocol. The protocol uses "magic constants" in some places (e.g. to tag message types), as well as bitfields, or a combination of both packed in a single value. I created data types for both the identifiers as well as the bitfield masks, e.g. > import Data.Bits > data MessageType = Message1 > | Message2 > | Message5 > data MessageFlag = Flag1 > | Flag2 Since I need to be able to get a numeric representation of them, I thought making a custom Enum instance would make sense: > instance Enum MessageType where > fromEnum a = case a of > Message1 -> 1 > Message2 -> 2 > Message5 -> 5 > toEnum n > | n == 1 = Message1 > | n == 2 = Message2 > | n == 5 = Message5 > instance Enum MessageFlag where > fromEnum a = case a of > Flag1 -> 1 `shiftL` 0 > Flag2 -> 1 `shiftL` 1 > toEnum n > | n == 1 `shiftL` 0 = Flag1 > | n == 1 `shiftL` 1 = Flag2 This is not a complete definition (not to mention non-exhaustive pattern matches), so I was wondering what the best practices are to extend this so these instances are 'correct'? I've been trying several approaches, but all of them failed on code like > getFlags :: Int -> [MessageFlag] > getFlags i = filter (\v -> (i .&. fromEnum v) /= 0) [Flag1 ..] Unless I hard-code all options in the last list, of course, but this is obviously not the intention. Any help or pointers (including "This is not how Enum is intended to be used") would be appreciated! Thanks, Nicolas

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Re: [Haskell-cafe] I killed performance of my code with Eval and Strategies
by Janek S. 17 Nov '12

17 Nov '12

> Do you really mean to calculate the 'sin . sqrt' of just the head of the list, or do you mean: > calculateSeq = map (sin . sqrt) ? Argh.. of course not! That's what you get when you code in the middle of a night. But in my code I will not be able to use map because elements will be processed in pairs, so let's say that my sequential function looks like this: calculateSeq :: [Double] -> [Double] calculateSeq [] = [] calculateSeq [x] = [sin . sqrt $ x] calculateSeq (x:y:xs) = (sin . sqrt $ x) : (cos . sqrt $ y) : calculateSeq xs > I don't think there's a memory leak. It looks more like you're just > allocating much more than is sane for such a simple function. > On a recent processor, sin . sqrt is two instructions. Meanwhile, you have > a list of (boxed?) integers being split up, then recombined. That's bound > to hurt the GC. I am not entirely convinced that my idea of using eval+strategies is bound to be slow, because there are functions like parListChunk that do exactly this: split the list into chunks, process them in parallel and then concatenate the result. Functions in Control.Parallel.Strategies were designed to deal with list so I assume it is possible to process lists in parallel without GC problems. However I do not see a way to apply these functions in my setting where elements of lists are processed in pairs, not one at a time (parList and parMap will not do). Also, working on a list of tuples will not do. > Also, you might want to configure criterion to GC between > runs. That might help. The -g flag passed to criterion executable does that. > What I'd suggest doing instead, is breaking the input into chucks of, say, > 1024, and representing it with a [Vector]. Then, run your sin.sqrt's on > each vector in parallel. Finally, use Data.Vector.concat to combine your > result. As stated in my post scriptum I am aware of that solution :) Here I'm trying to figure what am I doing wrong with Eval. Thanks! Janek > > Hope that helps, > - Clark > > On Wed, Nov 14, 2012 at 4:43 PM, Janek S. <fremenzone(a)poczta.onet.pl> wrote: > > Dear Haskellers, > > > > I am reading Simon Marlow's tutorial on parallelism and I have problems > > with correctly using Eval > > monad and Strategies. I *thought* I understand them but after writing > > some code it turns out that > > obviously I don't because parallelized code is about 20 times slower. > > Here's a short example > > (code + criterion benchmarks): > > > > {-# LANGUAGE BangPatterns #-} > > module Main where > > > > import Control.Parallel.Strategies > > import Criterion.Main > > > > main :: IO () > > main = defaultMain [ > > bench "Seq" $ nf calculateSeq xs > > , bench "Par" $ nf calculatePar xs ] > > where xs = [1..16384] > > > > calculateSeq :: [Double] -> [Double] > > calculateSeq [] = [] > > calculateSeq (x:xs) = (sin . sqrt $ x) : xs > > > > calculatePar :: [Double] -> [Double] > > calculatePar xss = runEval $ go xss > > where > > go :: Strategy [Double] > > go [] = return [] > > go xs = do > > lsh <- (rpar `dot` rdeepseq) $ calculateSeq as > > lst <- go bs > > return (lsh ++ lst) > > where > > !(as, bs) = splitAt 8192 xs > > > > Compiling and running with: > > > > ghc -O2 -Wall -threaded -rtsopts -fforce-recomp -eventlog evalleak.hs > > ./evalleak -oreport.html -g +RTS -N2 -ls -s > > > > I get: > > > > benchmarking Seq > > mean: 100.5990 us, lb 100.1937 us, ub 101.1521 us, ci 0.950 > > std dev: 2.395003 us, lb 1.860923 us, ub 3.169562 us, ci 0.950 > > > > benchmarking Par > > mean: 2.233127 ms, lb 2.169669 ms, ub 2.296155 ms, ci 0.950 > > std dev: 323.5201 us, lb 310.2844 us, ub 344.8252 us, ci 0.950 > > > > That's a hopeless result. Looking at the spark allocation everything > > looks fine: > > > > SPARKS: 202 (202 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled) > > > > But analyzing eventlog with ThreadScope I see that parallel function > > spends most of the time doing > > garbage collection, which suggests that I have a memory leak somewhere. I > > suspected that problem > > might be caused by appending two lists together in the parallel > > implementation, but replacing > > this with difference lists doesn't help. Changing granularity (e.g. > > splitAt 512) also brings no > > improvement. Can anyone point me to what am I doing wrong? > > > > Janek > > > > PS. This is of course not a real world code - I know that I'd be better > > of using unboxed data > > structures for doing computations on Doubles. > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe

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ANN: hledger-web 0.19
by Simon Michael 17 Nov '12

17 Nov '12

I'm pleased to announce hledger-web 0.19, compatible with the latest hledger and Yesod. Release notes: * web: builds with yesod 1.1.3 * web: obeys command-line query options at startup again * the autogenerated session file is now a dot file (.hledger-web_client_session.aes) hledger is a command-line tool and haskell library for tracking financial transactions, which are stored in a human-readable plain text format. In addition to reporting, it can also help you record new transactions, or convert CSV data from your bank. Add-on packages include hledger-web, a web interface. hledger is inspired by and compatible with John Wiegley's Ledger. For more, see http://hledger.org . Install it: cabal update; cabal install hledger-web For more installation help, see http://hledger.org/MANUAL.html#installing . Or, sponsor a ready-to-run binary for your platform: http://hledger.org/DOWNLOAD.html .

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Polymorphic Type Safe URL building -- I need advice on my project
by Kyle Hanson 17 Nov '12

17 Nov '12

Hello, So I started working on a project involving polymorphic type safe urls. I know there are other type-safe url's out there, but I want some advice before I start reorganizing it. It is located here: https://github.com/dracule/web-scatter One might ask why you would want polymorphic types in your URL. Current implementation of type-safe URLs use a single type and then have it pattern match on the constructors. However, It is a pretty common occurrence that you might want to use a type from other parts of code. Where this becomes prevalent is in IxSet and AcidState. For IxSet your Index keys all have to have separate types . So with this, you can convert your url into your index key's type and then just make your handler like this: myHandler :: BlogId -> SomeWebHandler I know my code probably has some terrible inefficiencies or something, so I am looking for some feedback on some jarring problems. Thank You

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Taking over ghc-core
by Shachaf Ben-Kiki 16 Nov '12

16 Nov '12

With Don Stewart's blessing (<https://twitter.com/donsbot/status/267060717843279872>), I'll be taking over maintainership of ghc-core, which hasn't been updated since 2010. I'll release a version with support for GHC 7.6 later today. Shachaf

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using createProcess and waitForProcess, but mpg123 plays for only 6 seconds
by rnons 15 Nov '12

15 Nov '12

Hi guys, I'm building an application that needs to wrap an external mp3 player, in this case mpg123. Here is my code: import System.IO import System.Process import Control.Concurrent mpgLoop = do let sh = "mpg123 -R" (Just hin, Just hout, _, hdl) <- createProcess (shell sh){ std_in = CreatePipe, std_out=CreatePipe } --hPutStrLn hin "SILENCE" hPutStrLn hin "LOAD /home/rnons/Music/test.mp3" hFlush hin waitForProcess hdl return () main = do forkIO mpgLoop -- threadDelay 20000000 getChar I expected with "waitForProcess", this program will be able to run till the song ends. However, mpg123 plays for only 6 seconds. Maybe I missed something? rnons

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local type denotation
by Serge D. Mechveliani 14 Nov '12

14 Nov '12

Please, how to correctly set an explicit type for a local value in the body of a polymorphic function? Example (tested under ghc-7.6.1): data D a = D1 a | D2 a (a -> a) f :: Eq a => D a -> a f (D1 x) = x f (D2 x g) = let -- y :: Eq a => a y = g x in if x == y then x else g y main = putStr $ shows (f (D2 (1 :: Int) succ)) "\n" This is compiled by ghc --make Main Now I need, for a certain reason, to explicitly set the type for y in `let', with the meaning: "this very `a' which is in the signature for f" (and I think that this type Haskell assignes to y in "y = g x"). I need to declare this type in a separate line: y :: <what ever it is>. Both `y :: a' and `y :: Eq a => a' are not compiled. Please, copy the answer to mechvel(a)botik.ru Thanks, ------ Sergei

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mtl-2.1 severly broken, cabal needs blacklisting
by Andreas Abel 14 Nov '12

14 Nov '12

After 2 days of shrinking 251 modules of source code to a few lines I realized that modify in MonadState causes <<loop>> in mtl-2.1. http://hackage.haskell.org/packages/archive/mtl/2.1/doc/html/src/Control-Mo… The bug has been fixed, apparently seven month ago. https://github.com/ekmett/mtl/pull/1 However, the "malicious" mtl-2.1 still lingers on: it is available from hackage and installed in many systems. This calls for a means of blacklisting broken or malicious packages. cabal update should also pull a blacklist of packages that will never be selected by cabal install (except maybe by explicit user safety overriding). I think such a mechanism is not only necessary for security purposes, but also to safe the valuable resources of our community. Cheers, Andreas -- Andreas Abel <>< Du bist der geliebte Mensch. Theoretical Computer Science, University of Munich Oettingenstr. 67, D-80538 Munich, GERMANY andreas.abel(a)ifi.lmu.de http://www2.tcs.ifi.lmu.de/~abel/

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Does anyone know where George Pollard is?
by Myles C. Maxfield 14 Nov '12

14 Nov '12

Hello, I sent a message to George Pollard (porges(a)porg.es) about his 'idna' package [1] a couple days ago, but he hasn't responded. I'd like to depend on his package for something that I'm working on, but his package fails to build (The email I sent him includes a patch that should fix up the build problems). The package hasn't been updated for 3 years and he hasn't listed a source control repository. Does anyone know where he is? If not, is there an accepted practice to resolve this situation? Should I upload my own 'idna2' package? Thanks, Myles [1] http://hackage.haskell.org/package/idna

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Re: [Haskell-cafe] Strange behavior with listArray
by oleg＠okmij.org 14 Nov '12

14 Nov '12

Alex Stangl wrote: > To make this concrete, here is the real solve function, which computes > a border array (Knuth-Morris-Pratt failure function) for a specified > string, before the broken memoization modification is made: > solve :: String -> String > solve w = let h = length w - 1 > wa = listArray (0, h) w > pI = 0 : solveR (tail w) 0 > solveR :: String -> Int -> [Int] > solveR [] _ = [] > solveR cl@(c:cs) k = if k > 0 && wa!k /= c > then solveR cl (pI!!(k-1)) > else let k' = if wa!k == c > then k + 1 > else k > in k' : solveR cs k' > in (intercalate " " . map show) pI > > Here solveR corresponds to f in the toy program and pI is the list > I would like to memoize since references to earlier elements could > get expensive for high values of k. Ok, let's apply a few program transformations. First we notice that the list pI must have the same length as the string w. Since we have already converted the string w to an array, wa, we could index into that array. We obtain the following version. > solve1 :: String -> String > solve1 w = (intercalate " " . map show) pI > where > h = length w - 1 > wa = listArray (0, h) w > pI = 0 : solveR 1 0 > solveR :: Int -> Int -> [Int] > solveR i k | i > h = [] > solveR i k = > let c = wa!i in > if k > 0 && wa!k /= c > then solveR i (pI!!(k-1)) > else let k' = if wa!k == c > then k + 1 > else k > in k' : solveR (i+1) k' > > t1s1 = solve1 "the rain in spain" > t1s2 = solve1 "aaaaaaaaaaaa" > t1s3 = solve1 "abbaabba" We don't need to invent an index: it is already in the problem. The unit tests confirm the semantics is preserved. The _general_ next step is to use the pair of indices (i,k) as the key to the two dimensional memo table. Luckily, our case is much less general. We do have a very nice dynamic programming problem. The key is the observation k' : solveR (i+1) k' After a new element, k', is produced, it is used as an argument to the solveR to produce the next element. This leads to a significant simplification: > solve2 :: String -> Array Int Int > solve2 w = pI > where > h = length w - 1 > wa = listArray (0, h) w > pI = listArray (0,h) $ 0 : [ solveR i (pI!(i-1)) | i <- [1..] ] > solveR :: Int -> Int -> Int > solveR i k = > let c = wa!i in > if k > 0 && wa!k /= c > then solveR i (pI!(k-1)) > else let k' = if wa!k == c > then k + 1 > else k > in k' > > t2s1 = solve2 "the rain in spain" > t2s2 = solve2 "aaaaaaaaaaaa" > t2s3 = solve2 "abbaabba" The unit tests pass.

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Re: [Haskell-cafe] Strange behavior with listArray
by oleg＠okmij.org 14 Nov '12

14 Nov '12

Alex Stangl posed a problem of trying to efficiently memoize a function without causing divergence: > solve = let a :: Array Int Int > a = listArray (0, 3) (0 : f 0) > f k = if k > 0 > then f (a!0) > else 0 : f 1 > in (intercalate " " . map show . elems) a Daniel Fischer explained the reason for divergence: > The problem, Alex, is that > > f k = if k > 0 > then f (a!0) > else 0 : f 1 > > is strict, it needs to know the value of (a!0) to decide which branch to take. > But the construction of the array a needs to know how long the list (0 : f 0) > is (well, if it's at least four elements long) before it can return the array. > So there the cat eats its own tail, f needs to know (a part of) a before it > can proceed, but a needs to know more of f to return than it does. But the problem can be fixed: after all, f k is a list of integers. A list is an indexable collection. Let us introduce the explicit index: > import Data.Array((!),Array,elems,listArray) > import Data.List(intercalate) > > solve = (intercalate " " . map show . elems) a > where > a :: Array Int Int > a = listArray (0, 3) (0 : [f 0 i | i <- [0..]]) > > f 0 0 = 0 > f 0 i = f 1 (i-1) > f k i = f (a!0) i This converges. Here is a bit related problem: http://okmij.org/ftp/Haskell/AlgorithmsH.html#controlled-memoization

2 1

Quickcheck
by graham＠fatlazycat.com 13 Nov '12

13 Nov '12

Hi, Trying to find some good docs on QuickCheck, if anyone has one ? Been scanning what I can find, but a question. What would be the best way to generate two different/distinct integers ? Use arbitrary ( if so do you have an example ) or a conditional on the property. Though I read the later can reduce passing test count. Thanks Graham

5 6

hackageDB haddock errors
by timothyhobbs＠seznam.cz 13 Nov '12

13 Nov '12

Hello, There are several packages without haddock documentation on hackage. The one that bugs me right now, is the latest version of gtk2hs: http://hackage. haskell.org/package/gtk I've been told that this may be due to hackage updating docs with an infrequent cron job, so that new packages don't get their docs for a long time. Whatever is causing it, the fact that hayoo http://holumbus.fh-wedel. de/hayoo/hayoo.html always uses the most frequent docs, means that I cannot currently search for anything gtk2hs related :( Can someone please look into prodding this cron job so it doesn't take days to run, or look into resolving any errors that might be causing this unfortunate behaviour? Thank you for your time, and happy hacking, Timothy

3 2

Wrong install-name path for shared/dynamic C-library on OS X
by Christiaan Baaij 13 Nov '12

13 Nov '12

Dear List, The context of my problem is the following: - ghc-HEAD uses shared-library _only_ on OS X - A cabalized package that builds a shared/dynamic C-library (GLFW-b) And my problem is: A dynamically linked Haskell library gets the wrong install-name for a C-library: otool -L ~/.cabal/lib/GLFW-b-0.1.0.5/ghc-7.7.20121111/libHSGLFW-b-0.1.0.5-ghc7.7.20121111.dylib: /Users/baaijcpr/.cabal/lib/GLFW-b-0.1.0.5/ghc-7.7.20121111/libHSGLFW-b-0.1.0.5-ghc7.7.20121111.dylib: .... build/libglfw.dylib (compatibility version 1.0.0, current version 1.0.0) .... All the other paths reported by 'otool -L' are absolute, while the C-library (libglfw.dylib) that was part of the Cabalized package is relative. The problem presents itself when I now want to use the GLFW-b package: Loading package GLFW-b-0.1.0.5 ... can't load .so/.DLL for: /Users/baaijcpr/.cabal/lib/GLFW-b-0.1.0.5/ghc-7.7.20121111/libHSGLFW-b-0.1.0.5-ghc7.7.20121111.dylib (dlopen(/Users/baaijcpr/.cabal/lib/GLFW-b-0.1.0.5/ghc-7.7.20121111/libHSGLFW-b-0.1.0.5-ghc7.7.20121111.dylib, 9): Library not loaded: build/libglfw.dylib Referenced from: /Users/baaijcpr/.cabal/lib/GLFW-b-0.1.0.5/ghc-7.7.20121111/libHSGLFW-b-0.1.0.5-ghc7.7.20121111.dylib Reason: image not found) I can update the install-name to the correct path using: install_name_tool -change build/libglfw.dylib @loader_path/libglfw.dylib libHSGLFW-b-0.1.0.5-ghc7.7.20121111.dylib So my question are: - Where does the original install_name path (build/libglfw.dyli) come from? - What should be specified in the .cabal and/or Setup.hs file so that the haskell library is installed using the _correct_ relative path? Cheers, Christiaan

1 0

JavaScript (SpiderMonkey, V8, etc) embedded in GHC?
by Bob Hutchison 13 Nov '12

13 Nov '12

Hi, I've looked around with no success… this surprises me actually. Has anyone embedded SpiderMonkey, V8, or any other relatively decent JavaScript interpreters in GHC (using the FFI)? I did find http://justinethier.github.com/husk-scheme/ which is a scheme R5RS implementation (I could make this work). There's also some work done embedding Lua. I also found a number of packages that compile javascript to Haskell, or the other way around, but I don't need that kind of thing. All I really need is to allow users to write some JavaScript that accepts a single JSON 'file/string' from my Haskell program and produces another JSON 'file/string' that my Haskell program will accept. Thanks, Bob

4 7

Re: [Haskell-cafe] curl package broken in Windows
by Kevin Cantu 13 Nov '12

13 Nov '12

Heh, I was trying to build this yesterday and it was making by eyes bleed. With the curl package on Hackage, I merely need an SSL enabled version of libcurl, and every Linux distro I've tried this on has several variations of such a package. (You have a choice of OpenSSL or GNUTLS, for example.) Apparently neither OpenSSL nor cURL wants to provide such a Windows binary package, though, so I attempted to build a version of libcurl with SSL support... Sigbjorn's instructions are definitely out of date. (I couldn't use his ZIP package, either.) Using MSYS, though, I was able to compile a version of libcurl with winssl, but then every variation of cabal install command wasn't cooperating. What should I be using instead of the following, neither of which seems to tell cabal where to find libcurl? ``` $ CPPFLAGS=-Ic:/code/curl_install/include cabal install curl --ld-options=-Lc:/code/curl_install/lib $ cabal install curl --extra-lib-dirs=c:/code/curl_install/lib --extra-include-dirs=c:/code/curl_install/include ``` Detail, showing some verbose error messages: https://gist.github.com/4056331 Kevin

5 8

ANN: fixed-vector
by Aleksey Khudyakov 13 Nov '12

13 Nov '12

Hello cafe! I want to announce library for the small vectors of fixed length fixed-vector[1]. Fixed means that vector's length is determined by its type. Generic API can work with both ATD-based vector like complex or Vec written below and array-based ones. > data Vec a = Vec a a a It's based on post by Roman Leschinsiy[2]. Library also provide array-based vectors with memory representation similar to onves from vector package. It's more compact though because there is no need to store lengh and and offset to first element. [1] http://hackage.haskell.org/package/fixed-vector [2] http://unlines.wordpress.com/2010/11/15/generics-for-small-fixed-size-vecto…

3 5

Reminder and Forward: Wednesday 14 November 2012 NYHUG Inaugural Meetup: Ozgun Ataman on Practical Data Processing and Gershom Bazerman on Putting Cloud Haskell to Work
by Jay Sulzberger 12 Nov '12

12 Nov '12

Below is a version of the announcement sent to the haskell-cafe and the haskell mailing lists by Gershom Bazerman on 9 October 2012. oo--JS. <blockquote what="official New York Haskell Users Group announcement" rsvp="Yes, see below." more="http://en.wikipedia.org/wiki/Haskell_%28programming_language%29 [page was last modified on 11 November 2012 at 00:56] http://www.haskell.org/haskellwiki/Haskell http://www.reddit.com/r/haskell http://www.aosabook.org/en/ghc.html http://en.wikipedia.org/wiki/Type_inference [page was last modified on 15 September 2012 at 15:37] http://en.wikipedia.org/wiki/Hindley%E2%80%93Milner [page was last modified on 9 October 2012 at 17:40] http://www.users.waitrose.com/~hindley/ http://www.cs.ru.nl/~henk/ http://www.usingz.com/ http://en.wikipedia.org/wiki/Curry-Howard [page was last modified on 14 September 2012 at 22:01] http://www.paultaylor.eu/stable/prot.pdf ftp://ftp.cs.cmu.edu/usr/rwh/www/home/courses/logic/www/handouts/curry-howa… http://www.students.science.uu.nl/~3448584/TypeSystems.pdf http://www.math.ias.edu/~vladimir/Site3/Univalent_Foundations_files/2011_Go… http://math.andrej.com/2012/11/08/how-to-implement-dependent-type-theory-i/ http://math.andrej.com/2012/11/11/how-to-implement-dependent-type-theory-ii/ http://math.andrej.com/2012/10/03/am-i-a-constructive-mathematician/" exclamation-of-old-lisper-upon-first-seeing-curry-howard-theorem: "But this is clearly wrong! Look I have a function string-length which has type String->Int, but 'String->Int' is not a theorem of any usual intuitionistic propositional logic." insight-of-newbie-coming-to-haskell-from-lisp: "Lisp's now buried background would require, if a student of Lisp were to ask for an outline of this background, a book of perhaps a hundred pages. Haskell, and more generally, all the New Crazy Types programming systems, would require three hundred pages." edits="some adventitious markup and some blank space removed"> Subject: Wednesday 14 November, NY Haskell Inaugural Meetup: Practical Data Processing _and_ Putting Cloud Haskell to Work Date: Fri, 12 Oct 2012 18:05:02 -0400 From: Gershom B <gershomb(a)gmail.com> To: lisp(a)lispnyc.org ====================================================== What: New York Haskell Users Group Meeting When: Wednesday, November 14, 2012 7 PM - 9 PM Where: Pivotal Labs, 841 Broadway, 8th floor (Just south of Union Square) http://maps.google.com/maps?q=841+Broadway,+New+York RSVP: http://www.meetup.com/NY-Haskell/ ====================================================== * 7 PM - Practical Data Processing With Haskell * 8 PM - Putting Cloud Haskell to Work for Distributed Computing -------------------------------------------------------------------------------------- 7 PM - Practical Data Processing With Haskell: Ozgun Ataman will give an introductory talk on Haskell, diving right in with how you can start using Haskell for practical data manipulation tasks today. You'll be introduced to a typical setup for Haskell development and given a demonstration of how a common data format (CSV, JSON, etc.) can be parsed, processed and finally output using Haskell. The talk will include a small actual Haskell program to be modified live and some commentary around using Haskell in practical applications. 8 PM - Putting Cloud Haskell to Work for Distributed Computing: Gershom Bazerman will give an overview of the new distributed-process library that implements Cloud Haskell, providing computation across heterogeneous nodes through a message passing interface. He will discuss what this new tool provides out of the box, and what you'll have to bring to the table. The talk will include some code samples using the new library, as well as some experience about what it's like to use Haskell for distributed computing in the real world. Speaker Biographies: Ozgun Ataman is the founder of Soostone, a management consulting and analytics company built on Haskell. He is the author of many open-source Haskell libraries, and is a contributor to the Snap web framework. Gershom Bazerman is a developer at S&P Capital IQ. He is most well known in the Haskell community as the author of the JMacro library for programmatic generation of JavaScript, and for one well-received April fools joke. -------------------------------------------------------------------------------------- We're kicking off the New York Haskell Users Group with two great talks, one for people just getting started and one for those with some experience already. Both should be accessible and enjoyable no matter how much (or little) you already know. Food and refreshments will be provided, courtesy of the generosity of Pivotal Labs, and after the talks, we're planning to keep the discussion going over food and drink at a nearby establishment. </blockquote> Distributed poC TINC: Jay Sulzberger <secretary(a)lxny.org> Corresponding Secretary LXNY LXNY is New York's Free Computing Organization. http://www.lxny.org

1 0

Hard drive thrashing with modern controllers
by timothyhobbs＠seznam.cz 12 Nov '12

12 Nov '12

Hello, I have a very general question that doesn't exactly relate to Haskell. Do I need to worry about hard drive thrashing with a modern controller? I understand, that such "thrashing" can significantly reduce the lifespan of modern SSD drives. For example, if I do: import Control.Monad foo = do forever $ writeFile "filename.foo" "Hello world!" will that destroy those sectors of my SSD after the rated 3000 write cycles? The reason I ask, is that I have a program here where I would like to save a set of files after every change/operation and I'm wondering if I should check if the files I'm saving have actually changed or if this will be handled by the OS... Thank you Timothy

3 5

Strange behavior with listArray
by Alex Stangl 12 Nov '12

12 Nov '12

I'm stymied trying to figure out why the program below blows up with <<<loop>>> when I use "f 0" for building the array, but if I substitute g or h in place of f, they work fine. Is this a bug or am I overlooking something? I am using GHC 7.4.2. Thanks, Alex P.S. Forgive the seemingly pointless program; I distilled this test from a longer actual program that was exhibiting this behavior. -------- import Data.Array((!),Array,elems,listArray) import Data.List(intercalate) solve = let a :: Array Int Int a = listArray (0, 3) (0 : f 0) f k = if k > 0 then f (a!0) else 0 : f 1 g k = k : a!(k+1) : a!(k+1) : a!(k+2) : a!(k+3) : [] h k = a!k : h (k+1) in (intercalate " " . map show . elems) a main = putStrLn solve

3 5

how to inject another source into conduit
by Alexander V Vershilov 12 Nov '12

12 Nov '12

Hello. I have problems with writing next code (using network-conduit) slightly simplified version: > app ad = appSource ad $$ sink > where > cMap = M.fromList [ ("upload", cmdUpload), ("download", cmdDownload) ] > sink = takeLine >>= \c -> case c of Just run -> run ; Nothing -> return () > cmdUpload = {- ... -} CB.sinkFile path > cmdDownload = do > {- code here -} > CB.sourceFile path $$ appSink ad -- this will not work because of > -- type error And I'm catching "cannot construct the infinite type". I've found an example in [1], but it's not exactly solves my problem, as all the logic is inside conduit, and it will break upload function. [1] http://www.yesodweb.com/blog/2012/06/conduit-0-5 -- Alexander Vershilov

2 1

GHC for mobile devices?
by Andrew Pennebaker 12 Nov '12

12 Nov '12

I'd love to use Haskell directly for making mobiles apps. How can we make this happen, porting GHC to Android, iOS, and Windows Phone? -- Cheers, Andrew Pennebaker www.yellosoft.us

4 3

Re: [Haskell-cafe] Motion to unify all the string data types
by John Lato 12 Nov '12

12 Nov '12

> > From: Francesco Mazzoli <f(a)mazzo.li> > > At Sat, 10 Nov 2012 15:16:30 +0100, > Alberto G. Corona wrote: > > There is a ListLike package, which does this nice abstraction. but I > don't > > know if it is ready for and/or enough complete for serious usage. I?m > > thinking into using it for the same reasons. > > > > Anyone has some experiences to share about it? > > I've used it in the past and it's solid, it's been around for a while and > the > original author knows his Haskell. > > Things I don't like: > > * The classes are huge: > < > http://hackage.haskell.org/packages/archive/ListLike/3.1.6/doc/html/Data-Li… > >. > I'd much rater prefer to have all those utilities functions outside the > type > class, for no particular reason other then the ugliness of the type > class. > Speaking as the ListLike maintainer, I'd like this too. But it's difficult to do so without sacrificing performance. In some cases, sacrificing *a lot* of performance. So they have to be class members. However, there's no reason ListLike has to remain a single monolithic class. I'd prefer an API that's split up into several classes, as was done in Edison. Then 'ListLike' itself would just be a type synonym, or possibly a small type class with the appropriate superclasses. However this seems like a lot of work for relatively little payoff, which makes it a low priority for me. * It defines its own wrappers for `ByteString': > < > http://hackage.haskell.org/packages/archive/ListLike/3.1.6/doc/html/Data-Li… > >. > The community's view on newtypes is funny. On the one hand, I see all the time the claim "Just use a newtype wrapper to write instances for ..." (e.g. the recent suggestion of 'instance Num a => Num (a,a)'. On the other, nobody actually seems to want to use these newtype wrappers. Maybe it clutters the code? I don't know. I couldn't think of a better way to implement this functionality, patches would be gratefully accepted. Anyway, you really shouldn't use these wrappers unless you're using a ByteString to represent ASCII text. Which you shouldn't be doing anyway. If you're using a ByteString to represent a sequence of bytes, you needn't ever encounter CharString. > * It doesn't have instances for `Text', you have to resort to the > `listlike-instances' package. > Given that text and vector are both in the Haskell Platform, I wouldn't object to these instances being rolled into the main ListLike package. Any comments on this? John L.

2 2

Motion to unify all the string data types
by Andrew Pennebaker 12 Nov '12

12 Nov '12

Frequently when I'm coding in Haskell, the crux of my problem is converting between all the stupid string formats. You've got String, ByteString, Lazy ByteString, Text, [Word], and on and on... I have to constantly lookup how to convert between them, and the overloaded strings GHC directive doesn't work, and sometimes ByteString.unpack doesn't work, because it expects [Word8], not [Char]. AAAAAAAAAAAAAAAAAAAH!!! Haskell is a wonderful playground for experimentation. I've started to notice that many Hackage libraries are simply instances of typeclasses designed a while ago, and their underlying implementations are free to play around with various optimizations... But they ideally all expose the same interface through typeclasses. Can we do the same with String? Can we pick a good compromise of lazy vs strict, flexible vs fast, and all use the same data structure? My vote is for type String = [Char], but I'm willing to switch to another data structure, just as long as it's consistently used. -- Cheers, Andrew Pennebaker www.yellosoft.us

7 7

Re: [Haskell-cafe] GHC for mobile devices?
by Casey Basichis 12 Nov '12

12 Nov '12

I've been looking for the last two days through every nook and cranny of google to find information about building for iOS. It seems to be the case that it was once possible to get ios running and now its a bit questionable. I've tried ghc-ios, ghc-iphone and a few others which are all somewhat out of date. If anyone has any experience on that platform I would greatly appreciate some guidance in getting it up and running. As for you notion of "hard truth," and "dumb apps acting as web front ends" its pretty blase to assume that anyone interested in this thread will share that perspective in terms of their own goals on these platforms. I compose professionally on my phone, its certainly not a toy for my purposes. I also have no interest whatsoever in getting Cocoa commands into Haskell. I just want a functional way of working with data. I would greatly prefer to go the Haskell route, but have been considering OCaml as well as they seem to have an active and enthused interest in iOS. I would love a bit of perspective on whether OCaml would be worth pursing in the long run for the short term benefit of having a more mature mobile implementation.' Thanks, Casey >Yes, I've seen some of the work done on this and I think he's also >looked into OCaml on Android. > >To be completely honest it's not really the language that's such the >barrier: the hard truth is that any Android app doing anything of >interest is necessarily going to be using the Android framework. >Android programming (at least, presubaly iOS too) is not a lot of >advanced construction, it's mostly dumb apps acting as web frontends, >I'd posit that this is probably why people have been mostly content >with the Android API for so long anyway. > >So the main barrier is being able to interface with the Java side of >things (obviously) in a way that doesn't completely kill perf either >(this sounds nontrivial...). Then you need to give a sane >reimplementation of the Android API within Haskell. > >kris -- Casey James Basichis Composer - Cartoon Network http://www.caseyjamesbasichis.com caseybasichis(a)gmail.com 310.387.7540

3 6

size of Haskell Platform
by Doug McIlroy 11 Nov '12

11 Nov '12

This note is an offshoot of "curl package broken in Windows", where this item appeared: > Did you know that Strawberry Perl includes a cygwin gcc? > ... > Maybe Haskell Platform could do the same. The suggestion brought to mind a true-life parable: the pump station at Tok. (Tok is the third corner--after Anchorage and Fairbanks--of Alaska's triangular core of long-distance highways.) When I visited Tok long ago, it was a village of several hundred souls, almost all of whom were employed by one government agency or another, principal among which were the highway department, the Alaska Communication Service and the pump station, which kept fuel flowing to Eielson Air Force Base. The mission of the station was to keep one pump running 24 hours a day. Most of the time, of course, the pump hummed along by itself. To assure that, there had to be a standby machine, an operator to watch over both, and a mechanic who could fix them if need be. For such a lonely job it was deemed well to have two operators. And there had to be two operators for each of several shifts. A little redundancy on the mechanical side seemed wise, too. The crew and their families, say nothing of the pumps themselves, needed to be housed, and the installation needed to be supplied with the necessities of life. (The nearest supermarket was in Fairbanks, 300 miles away.) These needs demanded a motor pool and property maintenance cadre, whose very presence reinforced the need. Thus the support team to keep one pump going ballooned to about 100 people--a chain reaction that barely avoided criticality. So it seems to be with Haskell Platform, which aims to include "all you need to get up and running"--"an extensive set of standard libraries and utilities with full documentation." I get the impression that the Platform is bedeviled by the same prospect of almost unfettered growth. [One ominous sign: the description of the Haskell Platform at lambda.haskell.org/platform/doc/current/start.html suggests that one must join some mysterious Cabal, whose nature is hidden by a link to nowhere, simply to get started.] What principles guide the selection of components for "all you need to get up and running"? Doug McIlroy

4 3

Not exporting a class
by Joachim Breitner 11 Nov '12

11 Nov '12

Hi, dear package authors. If you define a useful class, please think twice before you do not export the methods, otherwise something like this will happen: Am Samstag, den 10.11.2012, 21:39 -0800 schrieb David Fox: > It looks like there is an export copy of the whole prettyprint system > in template-haskell. I could switch over to that. Oh, and another > one in uulib. And also in ansi-wl-pprint. I think I will switch to > this last one. > > On Sat, Nov 10, 2012 at 6:17 PM, David Fox <dsf(a)seereason.com> wrote: > It may belong in Text.PrettyPrint. > > On Sat, Nov 10, 2012 at 6:16 PM, David Fox <dsf(a)seereason.com> > wrote: > About as small as a package can get! Yeah, the Pretty > class is in haskell-src, but they don't export the > pretty method. I will drop them a line, but I sort of > assumed they had a reason for doing it that way > > > On Thu, Nov 8, 2012 at 2:02 PM, Joachim Breitner > <mail(a)joachim-breitner.de> wrote: > Hi David, > > your pretty-class package is pretty small. In > the interest of preventing > package proliferation: Have you talked to the > maintainers of pretty or > haskell-src-exts whether they’d include the > class in their code? Thanks, Joachim -- Joachim "nomeata" Breitner Debian Developer nomeata(a)debian.org | ICQ# 74513189 | GPG-Keyid: 4743206C JID: nomeata(a)joachim-breitner.de | http://people.debian.org/~nomeata

2 1

Re: [Haskell-cafe] curl package broken in Windows
by Andrew Pennebaker 10 Nov '12

10 Nov '12

Yes, that's a very good point. Did you know that Strawberry Perl includes a cygwin gcc? http://strawberryperl.com Maybe Haskell Platform could do the same. On Nov 10, 2012 5:28 PM, "Stephen Tetley" <stephen.tetley(a)gmail.com> wrote: > The only short cut would be to have it in the Platform - otherwise it > is DIY I'm afraid. > > This is as much Microsoft's "fault" as Haskell's for not providing a C > compiler, build tools etc. in the baseline environment. >

1 0

Profiling a project with cabal and cabal-dev
by Mike Craig 10 Nov '12

10 Nov '12

Hi all, So I've been working on a project and I'd like to run it with profiling to diagnose the performance hotspots. It's a cabal project and I've been using cabal-dev for sandboxing. Normally I would just run `cabal-dev install` to get everything built and the executables "installed" in ./cabal-dev/bin. Since I want to compile with profiling, I tried this: $ cabal-dev install enable-executable-profiling The build fails with an error log that includes something like "Perhaps you haven't installed the profiling libraries for package `http-types-0.7.3.0.1'?" If I then try ... $ cabal-dev install -p --reinstall --force-reinstalls http-types-0.7.3.0.1 ... I find myself walking down the dependency tree. (http-types -> case-insensitive -> hashable -> etc.) Presumably this would eventually walk me out of the sandbox and I would be reinstalling the base libraries in my sandbox with profiling enabled. This doesn't seem right. (And if it is right, it doesn't seem like I should have to do it manually, package by package.) What am I doing wrong? Cheers, Mike

3 3

Re: [Haskell-cafe] Motion to unify all the string data types
by Tobias Brandt 10 Nov '12

10 Nov '12

On 10 November 2012 04:00, Johan Tibell <johan.tibell(a)gmail.com> wrote: > As for type classes, I don't think we use them enough. Perhaps because > Haskell wasn't developed as an engineering language, some good software > engineering principles (code against an interface, not a concrete > implementation) aren't used in out base libraries. One specific example is > the lack of a sequence abstraction/type class, that all the string, list, > and vector types could implement. Right now all these types try to > implement a compatible interface (i.e. the traditional list interface), > without a language mechanism to express that this is what they do. > Data.Collections<http://hackage.haskell.org/packages/archive/collections-api/1.0.0.0/doc/htm…> has (among others) a Sequence type class and provides instances for the base data types in a separate package. However, it appears that not many people are using it.

1 0

Computed promoted natural
by Arie Peterson 10 Nov '12

10 Nov '12

Hi, I'm trying to use data kinds, and in particular promoted naturals, to simplify an existing program. The background is as follows: I have a big computation, that uses a certain natural number 'd' throughout, which is computed from the input. Previously, this number was present as a field in many of my data types, for instance > data OldA = OldA Integer … . There would be many values of this type (and others) floating around, with all the same value of 'd'. I would like to move this parameter to the type level, like this: > data NewA (d :: Nat) = NewA … The advantage would be, that the compiler can verify that the same value of 'd' is used throughout the computation. Also, it would then be possible to make 'NewA' a full instance of 'Num', because 'fromInteger :: Integer -> NewA d' has a natural meaning (where the value of 'd' is provided by the type, i.e. the context in which the expression is used), while 'fromInteger :: Integer -> OldA' does not, because it is not possible to create the right value of 'd' out of thin air. Is this a sane idea? I seem to get stuck when trying to /use/ the computation, because it is not possible to create 'd :: Nat', at the type level, from the computed integer. Can one somehow instantiate the type variable 'd :: Nat' at an integer that is not statically known? Formulated this way, it sounds like this should not be possible, because all types are erased at compile time. However, it feels as though it might not be unreasonable in this situation, because the computation is polymorphic in the type 'd :: Nat'. I just want to substitute a specific value for 'd'. Maybe there is another way to approach this? Thanks for any advice, Arie

3 7

Re: [Haskell-cafe] GHC for mobile devices?
by Andrew Pennebaker 10 Nov '12

10 Nov '12

Awesome! Jeffrey Scofield has ported OCaml to iOS<http://psellos.com/ocaml/compile-to-iossim.html>, so there's also experience there. On Fri, Nov 9, 2012 at 11:46 PM, Kristopher Micinski <krismicinski(a)gmail.com > wrote: > If you have interest in doing this, I have quite a bit of experience > in Android hacking at the system level and above and would be glad to > talk about what might need to happen. (Though I don't know the GHC > internals / toolchain so well.) > > One potential choice is Scala, though from my limited experience > that's a very rough imitation of the uses for Haskell. (Though, > obviously it works mostly out of the box because of the JVM compiler > target..) > > http://www.haskell.org/haskellwiki/Android > > I've been writing up some thoughts on the Android activity lifecycle > already interpreted with respect to FP, apps are quite functional > already for a variety of reasons. > > kris > > On Fri, Nov 9, 2012 at 8:51 PM, Andrew Pennebaker > <andrew.pennebaker(a)gmail.com> wrote: > > I'd love to use Haskell directly for making mobiles apps. How can we make > > this happen, porting GHC to Android, iOS, and Windows Phone? > > > > -- > > Cheers, > > > > Andrew Pennebaker > > www.yellosoft.us > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > -- Cheers, Andrew Pennebaker www.yellosoft.us

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curl package broken in Windows
by Andrew Pennebaker 10 Nov '12

10 Nov '12

When I do "cabal install curl" in Windows, a missing cygwin dll message pops up, and packages that depend on curl, like shpider, fail to install. Specs: * cabal-install 0.14.0 * Cabal 1.14.0 * GHC 7.4.2 * Haskell Platform 2012 * Windows 7 Proffesional x64 -- Cheers, Andrew Pennebaker www.yellosoft.us

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haskell.org committee: Call for nominations
by Jason Dagit 09 Nov '12

09 Nov '12

Dear Haskellers, The second year of the haskell.org committee is drawing to a close and it is therefore time to seek replacements for those members whose term is expiring. This year one member has a term ending, Ganesh Sittampalam. Ganesh has been an excellent contributor and the rest of the committee would like to thank him for his excellent service. To nominate yourself, please send an e-mail to committee at haskell.org by 30 November 2012. As usual, retiring members are eligible to re-nominate themselves. Please feel free to include any information about yourself that you think will help us to make a decision. Being a member of the committee does not require a significant amount of time, but committee members should aim to be responsive during discussions when the committee is called upon to make a decision. More details about the committee's roles and responsibilities are on http://www.haskell.org/haskellwiki/Haskell.org_committee If you have any questions about the process, please feel free to e-mail us at committee at haskell.org or to contact one of us individually. Regards, Jason On behalf of the haskell.org committee

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Waiting for garbage collection can kill parallelism?
by Janek S. 09 Nov '12

09 Nov '12

Today I was reading "Parallel Performance Tuning for Haskell" by Jones, Marlow and Singh and wanted to replicate the results for their first case study. The code goes like this: module Main where import Control.Parallel main :: IO () main = print . parSumFibEuler 38 $ 5300 parSumFibEuler :: Int -> Int -> Int parSumFibEuler a b = f `par` (e `pseq` (e + f)) where f = fib a e = sumEuler b fib :: Int -> Int fib 0 = 0 fib 1 = 1 fib n = fib (n - 1) + fib (n - 2) mkList :: Int -> [Int] mkList n = [1..n-1] relprime :: Int -> Int -> Bool relprime x y = gcd x y == 1 euler :: Int -> Int euler n = length (filter (relprime n) (mkList n)) sumEuler :: Int -> Int sumEuler = sum . (map euler) . mkList sumFibEuler :: Int -> Int -> Int sumFibEuler a b = fib a + sumEuler b This is the version shown on page 3 of the paper, after adding the pseq combinator to enforce correct evaluation order. I compile and run it with: ghc -O2 -rtsopts -threaded -eventlog parallel.hs ./parallel +RTS -s -ls -N2 In the paper authors show that this version does in fact perform computation in parallel and that good speedup is achieved. However, when I run the code what happens is that HEC 1 blocks very quickly requesting GC. HEC 0 (if I am correct the one calculating sumEuler) does not interrupt but instead continues the computations until they are finished. Then the GC is performed and the HEC 1 resumes computation. In this way there is no parallelism, because first HEC 0 does all its computations and after first GC HEC 1 does its computation. My question is why this might be happening? I don't expect the results of the paper to be fully reproducible, because the paper is 3 years old and GHC has developed a lot since then. This however looks like a regression of some sort. I would appreciate if anyone could explain why this. Janek

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Why doesn't GHC optimize recursive functions by converting them into `let`?
by Petr P 09 Nov '12

09 Nov '12

Hi, recently I found out that some recursive functions can be greatly optimized just by rewriting them using `let` so that they're not recursive themselves any more (only the inner let is). To give an example: > fix :: (a -> a) -> a > fix f = f (fix f) isn't optimized, because it's a recursive function and so it isn't inlined or anything. However, defining it as > fix f = let x = f x in x makes it much more efficient, since `fix` is not recursive any more, so it gets inlined and then optimized for a given argument `f`. (See http://stackoverflow.com/questions/12999385/why-does-ghc-make-fix-so-confou… ) Another example is the well-known generic catamorphism function: > newtype Fix f = Fix { unfix :: f (Fix f) } > > catam :: (Functor f) => (f a -> a) -> (Fix f -> a) > catam f = f . fmap (catam f) . unfix is not optimized. When `catam` is used on a data structure such as [] or a tree, at each level `fmap` creates new constructors that are immediately consumed by `f`. But when written as > catam f = let u = f . fmap u . unfix in u this gets inlined and then optimized, and constructor creation/consumption is fused into very effective code. (See http://stackoverflow.com/q/13099203/1333025) As one comment asked, this seems like something GHC could do automatically. Would it be difficult? Is there a reason against it? I suppose in cases where it doesn't help, the additional `let` would get optimized away, so no harm would be done. And in cases like `fix` or `catam` the gain would be substantial. Best regards, Petr

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cabal-install prompt problem
by Magicloud Magiclouds 08 Nov '12

08 Nov '12

Hi, I am using git version of cabal-install (1.17.0). It was fine until today I got this: 920 $ cabal install yesod Resolving dependencies... cabal: dependencies conflict: ghc-7.6.1 requires an installed instance of directory ==1.2.0.1 however: There is nothing more after the ":". What is happening? -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Haskell Weekly News: Issue 250
by Daniel Santa Cruz 08 Nov '12

08 Nov '12

Welcome to issue 250 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of October 28 to November 3, 2012. Quotes of the Week * Cale: A list is a bit like a loop which hasn't happened yet. This operation is similar to "how do I make the 5th iteration of my loop do something different from the others?" * broombs: Why are there multiple packages for EitherT? broombs: edwardk: which is the better one? edwardk: broombs: either And today, we have our first quote submited via twitter! Thank you @gersei for sending this one in. If you'd like to submit quotes via twitter, add @HWNQuotes to your tweet. * @kmett:We now generate pretty core but have ugly code. Top Reddit Stories * Cartesian Closed Comic #15: Iteratees Domain: ro-che.info, Score: 75, Comments: 2 On Reddit: [1] http://goo.gl/9NpD2 Original: [2] http://goo.gl/SB7HZ * Are there any, or is there interest in starting, an online Haskell course like the Scala course on Coursera? Domain: class.coursera.org, Score: 69, Comments: 16 On Reddit: [3] http://goo.gl/UzBdF Original: [4] http://goo.gl/1Zzct * Why is this simple text processing program so much slower than in dynamic languages? Domain: honza.ca, Score: 65, Comments: 189 On Reddit: [5] http://goo.gl/jW5ow Original: [6] http://goo.gl/pPzIb * Fast Code Nation: The Bright Side of High-Level Languages Domain: bos.github.com, Score: 59, Comments: 11 On Reddit: [7] http://goo.gl/KmBRt Original: [8] http://goo.gl/jm9s7 * Fay slides Domain: fay-lang.org, Score: 56, Comments: 42 On Reddit: [9] http://goo.gl/0UM4T Original: [10] http://goo.gl/h8iZE * Data.Records: extensible records using the latest GHC features Domain: github.com, Score: 55, Comments: 44 On Reddit: [11] http://goo.gl/64wUj Original: [12] http://goo.gl/XVdB2 * Why Cabal Has Problems Domain: softwaresimply.blogspot.com, Score: 50, Comments: 41 On Reddit: [13] http://goo.gl/ZX1Ia Original: [14] http://goo.gl/TgnOG * The cabal/hackage situation, and what you can do about it Domain: alpmestan.com, Score: 49, Comments: 14 On Reddit: [15] http://goo.gl/jEGIj Original: [16] http://goo.gl/EK2DB * Brian O'Sullivan's favorite bug in GHC Domain: twitter.com, Score: 48, Comments: 12 On Reddit: [17] http://goo.gl/nMjeK Original: [18] http://goo.gl/odPH4 * Haskell for all: "Hello, core!" Domain: haskellforall.com, Score: 45, Comments: 6 On Reddit: [19] http://goo.gl/qXRd2 Original: [20] http://goo.gl/9cBeo * Michael Snoyman: Yesod, AngularJS and Fay Domain: yesodweb.com, Score: 43, Comments: 26 On Reddit: [21] http://goo.gl/8yfVd Original: [22] http://goo.gl/0xtPP * Cellular automata as comonads Domain: blog.sigfpe.com, Score: 41, Comments: 10 On Reddit: [23] http://goo.gl/73nr2 Original: [24] http://goo.gl/vH4TK * pipes-2.5: Faster and slimmer Domain: haskellforall.com, Score: 41, Comments: 56 On Reddit: [25] http://goo.gl/wtUZg Original: [26] http://goo.gl/rKR2D Top StackOverflow Questions * Writing cojoin or cobind for n-dimensional grid type votes: 41, answers: 3 Read on SO: [27] http://goo.gl/qbz7z * Why not be dependently typed? votes: 39, answers: 3 Read on SO: [28] http://goo.gl/1TSVh * Concrete example showing that monads are not closed under composition (with proof)? votes: 32, answers: 4 Read on SO: [29] http://goo.gl/sys7F * Monad Transformers vs Passing parameters to functions votes: 24, answers: 1 Read on SO: [30] http://goo.gl/UpU57 * Why is there no IO transformer in Haskell? votes: 16, answers: 2 Read on SO: [31] http://goo.gl/04Xc7 * Is it possible to make GHC optimize (deforest) generic functions such as catamorphisms? votes: 16, answers: 1 Read on SO: [32] http://goo.gl/f4ZlU * Is it possible to use irrefutable tilde (`~`) patterns in lambda expressions? votes: 14, answers: 1 Read on SO: [33] http://goo.gl/BmGY0 * Can Scala's Cake Pattern be implemented in Haskell? votes: 13, answers: 2 Read on SO: [34] http://goo.gl/QiMfg * Confused by the meaning of the 'Alternative' type class and its relationship to other type classes votes: 13, answers: 5 Read on SO: [35] http://goo.gl/RXNuV * How to add fields that only cache something to ADT? votes: 13, answers: 0 Read on SO: [36] http://goo.gl/FjLlX * Type algebra and Knuth's up arrow notation votes: 13, answers: 1 Read on SO: [37] http://goo.gl/kyYIn Until next time, Daniel Santa Cruz References 1. http://ro-che.info/ccc/15.html 2. http://www.reddit.com/r/haskell/comments/11t7ru/cartesian_closed_comic_15_i… 3. https://class.coursera.org/course/progfun 4. http://www.reddit.com/r/haskell/comments/12865b/are_there_any_or_is_there_i… 5. http://honza.ca/2012/10/haskell-strings 6. http://www.reddit.com/r/haskell/comments/120h6i/why_is_this_simple_text_pro… 7. http://bos.github.com/reaktor-dev-day-2012/reaktor-talk-slides.html 8. http://www.reddit.com/r/haskell/comments/11sl47/fast_code_nation_the_bright… 9. http://fay-lang.org/fay-talk/ 10. http://www.reddit.com/r/haskell/comments/11yrpi/fay_slides/ 11. https://github.com/jonsterling/Data.Records 12. http://www.reddit.com/r/haskell/comments/12835b/datarecords_extensible_reco… 13. http://softwaresimply.blogspot.com/2012/11/why-cabal-has-problems.html 14. http://www.reddit.com/r/haskell/comments/12hbzb/why_cabal_has_problems/ 15. http://alpmestan.com/posts/2012-11-02-cabal-hackage-what-you-can-do-about-i… 16. http://www.reddit.com/r/haskell/comments/12ijs7/the_cabalhackage_situation_… 17. https://twitter.com/bos31337/status/116372971509121025 18. http://www.reddit.com/r/haskell/comments/127emo/brian_osullivans_favorite_b… 19. http://www.haskellforall.com/2012/10/hello-core.html 20. http://www.reddit.com/r/haskell/comments/11vuzz/haskell_for_all_hello_core/ 21. http://www.yesodweb.com/blog/2012/10/yesod-fay-js 22. http://www.reddit.com/r/haskell/comments/12ct6p/michael_snoyman_yesod_angul… 23. http://blog.sigfpe.com/2006/12/evaluating-cellular-automata-is.html 24. http://www.reddit.com/r/haskell/comments/11o6nu/cellular_automata_as_comona… 25. http://www.haskellforall.com/2012/10/pipes-25-faster-and-slimmer.html 26. http://www.reddit.com/r/haskell/comments/12gikh/pipes25_faster_and_slimmer/ 27. http://stackoverflow.com/questions/12963733/writing-cojoin-or-cobind-for-n-… 28. http://stackoverflow.com/questions/12961651/why-not-be-dependently-typed 29. http://stackoverflow.com/questions/13034229/concrete-example-showing-that-m… 30. http://stackoverflow.com/questions/12968351/monad-transformers-vs-passing-p… 31. http://stackoverflow.com/questions/13056663/why-is-there-no-io-transformer-… 32. http://stackoverflow.com/questions/13099203/is-it-possible-to-make-ghc-opti… 33. http://stackoverflow.com/questions/13185632/is-it-possible-to-use-irrefutab… 34. http://stackoverflow.com/questions/12947176/can-scalas-cake-pattern-be-impl… 35. http://stackoverflow.com/questions/13080606/confused-by-the-meaning-of-the-… 36. http://stackoverflow.com/questions/13098448/how-to-add-fields-that-only-cac… 37. http://stackoverflow.com/questions/13170803/type-algebra-and-knuths-up-arro…

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Compiling with hsc2hs and -Wextra
by Niklas Hambüchen 08 Nov '12

08 Nov '12

Hi, when I compile my C binding using hsc2hs with the -Wextra enabled in my cabal file: cc-options: -std=c99 -Wall -Wextra -Werror This breaks the compilation started by hsc2hs: MyHsc.hsc: In function ‘main’: MyHsc.hsc:16:15: error: unused parameter ‘argc’ [-Werror=unused-parameter] MyHsc.hsc:16:27: error: unused parameter ‘argv’ [-Werror=unused-parameter] cc1: all warnings being treated as errors compiling dist/build/MyHsc_hsc_make.c failed (exit code 1) I would really like to compile my code with -Wextra -Werror though. Can't we just (void) argc; (void) argv; the arguments in the generated code, like described in [1]? Thanks Niklas [1] http://stackoverflow.com/questions/3599160/unused-parameter-warnings-in-c-c…

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Where is the documentation on exception types kept?
by timothyhobbs＠seznam.cz 07 Nov '12

07 Nov '12

Hello, I am trying to catch an "thread blocked on MVar indefinitely exception." Of course I can use ::SomeException as explained in http://hackage.haskell.org/ packages/archive/base/latest/doc/html/Control-Exception.html#g:3 but there is no explanation as to how to find the more case specific exceptions. http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control- Concurrent-MVar.html#t:MVar does not discuss exceptions at all! Indeed, I don't know of any haddock pages that include descriptions of exception types :( :O Thanks, Timothy

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