- Haskell-Cafe - Haskell.org

Hackage download statistis
by Maurício CA 12 Feb '10

12 Feb '10

Hi, all, Some time ago a download statistic of hackage was made available, and analysed in a few ways. Googling for it still find that at a Galois web page. I though that, since the tools to get that were there, this would be output once in a while, but it seems it hasn't been done since then. Does anyone know if there are plans about that? Thanks, Maurício

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Seen on reddit: or, foldl and foldr considered slightly harmful
by Johann Höchtl 12 Feb '10

12 Feb '10

In a presentation of Guy Steele for ICFP 2009 in Edinburgh: http://www.vimeo.com/6624203 he "considers foldl and foldr" harmful as they hinder parallelism because of "Process first element, then the rest" Instead he proposes a divide and merge aproach, especially in the light of going parallel. The slides at http://docs.google.com/viewer?url=http%3A%2F%2Fresearch.sun.com%2Fprojects%… [Bware: Google docs] are somewhat geared towards Fortress, but I wonder what Haskellers have to say about his position. Greetings, Johann

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ANN: Happstack 0.4.1
by Jeremy Shaw 11 Feb '10

11 Feb '10

Happstack 0.4.1 STABLE is now available. We recommend that all users of Happstack update to 0.4.1 immediately. Migration from Happstack 0.3 should be trivial. There was one race condition in happstack-state which could cause a state event to fail under heavy congestion. Happstack is the Haskell Application Server Stack. It is a scalable framework for developing web applications. It is similar in purpose to Django, RoR, Turbogears, Grails, Seaside, etc. Using less buzzwords, it is a collection of libraries which can be used to develop a self-contained server application which does not require SQL or Apache (though it can integrate with both). It includes: * an HTTP server * a rich DSL for deconstructing HTTP Requests and generating Responses * HTML/XML/etc templating via HStringTemplate, HSP, or Text.XHtml * a persistent data store based on user defined Haskell data types with ACID properties and state migration * and more!! Happstack's modular nature means it is easy to integrate unrelated 3rd party modules such as Pandoc, or to add support for an alternative templating system. Additionally, it does not exclude you from using SQL, or supporting other protocols besides HTTP. HOMEPAGE: http://www.happstack.com/ SUPPORTED PLATFORMS: Happstack supports GHC 6.8, 6.10, and 6.12, under GNU/Linux, FreeBSD, OS X, and Windows. GHC 6.12 currently requires a minor patch to HJScript. Details here: http://groups.google.com/group/haskell-server-pages/browse_thread/thread/56… INSTALL: 1. ensure that ~/.cabal/bin is in your $PATH 2. cabal update 3. cabal install happstack * see SUPPORTED PLATFORMS for information about GHC 6.12 NOTABLE CHANGES: For a complete list of changes see: http://www.patch-tag.com/r/mae/happstack/snapshots/all/history Removed: * happstack-contrib has been completely removed. If you were using something in it, please complain on the mailing list. * removed Happstack.Server.S3 -- newer, better maintained S3 libraries are on hackage now * removed unsupported spread-based multimaster support fileServe: * now with sendfile() support! On supported platforms sendfile allows files to be served from the disk to the network in a zero-copy fashion. This means faster transfers, less memory and CPU usage, and no file handle leaks. (hopefully). sendfile uses native sendfile() support on Linux, Windows, OS X, and FreeBSD. Other platforms will use a sendfile emulation mode. We recommend that you use sendFile instead of sendFileStrict or sendFileLazy, unless you have specific needs and know some reason why you must use sendFileStrict or sendFileLazy. * FileServe internals majorly refactored. FileServe now provides a modular way of building your own fileServe-like functions. Also includes functions for serving a single file instead of the contents of a directory. * added more complete list of mime-types * automatically redirect from foo to foo/ if foo is a directory * filter . and .. from request path, but allow filenames and directories which start with . happstack-server: * ServerMonad instance for ErrorT * Added ServerMonad, FilterMonad, and WebMonad for XMLGenT * Added secure flag to cookies. Defaults to False. * fix supportsIPv6 to work with newer versions of network library * simpleHTTPWithSocket + bindPort for H.S.SimpleHTTP (you can use this to start the server as root, bind to the socket, and then drop privileges.) * Fixed Accept-Encoding parser to accepts encodings with a - in them. Added additional tests to acceptEncodingParserTest * fix so basicAuth sets correct response code and headers. * SimpleHTTP: added trailingSlash. Guard which checks that Request URI ends with a / * decode body of PUT requests not just POST * added ifModifiedSince guard * added host and withHost guards to SimpleHTTP (similar to dir and path, but for the requested hostname) other: * happstack-state: Correct an embarrassing race condition bug. * happstack-data: Exported getSafeGet and getSafePut * happstack: add state transition code to guestbook (add email to form, handler, and state), and tar of example _local state before transition experimental: * happstack-state: now has experimental replication support using Amazon Web Services. This support is disabled by default. This feature is highly experimental, may not work, and may even be completely removed in future versions (though, hopefully not). THANKS: Alex Jacobson David Himmelstrup / Lemmih Matthew Elder / mae Antoine Latter Erik Hesselink Pasqualino 'Titto' Assini Nils / McManiaC Thomas Hartman Creighton Hogg / wchogg John MacFarlane Jochem Berndsen / koeien MightByte And everyone who filed bug reports, tested RC1 and RC2, and anyone else I missed.

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Examples/docs for simulated annealing bindings
by Dougal Stanton 11 Feb '10

11 Feb '10

I found the HsASA library [1] on Hackage, but there's no documentation and it's not particularly intuitive. I can't see any obvious way of choosing initial config or generating new configurations. Google reveals no one using it. Does anyone have ideas? [1]: http://hackage.haskell.org/package/HsASA-0.1 Cheers, D -- Dougal Stanton dougal(a)dougalstanton.net // http://www.dougalstanton.net

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Streamed creation of STArray from list?
by Jason Dagit 11 Feb '10

11 Feb '10

Hello, I was wondering if there is a trick for generating a new STArray from a list in such a way that you do not have to hold both the list and array in memory? http://www.haskell.org/ghc/docs/latest/html/libraries/array-0.3.0.0/Data-Ar… As far as I can tell, that is the interface for creating ST Arrays. For example, newListArray looks almost perfect: *newListArray* :: (MArray<http://www.haskell.org/ghc/docs/latest/html/libraries/array-0.3.0.0/Data-Ar…>a e m, Ix<http://www.haskell.org/ghc/docs/latest/html/libraries/base-4.2.0.0/Data-Ix.…>i) => (i, i) -> [e] -> m (a i e) If I know the length of the list, I might expect newListArray to have the memory behavior I want. In my case, the code calls (length xs) to calculate the length of the list. As I understand it, that will force the spine of the list into memory. Can this be avoided? The only trick that comes into mind is the one used by the C++ vector class where you dynamically resize the array and copy the values to the new array. That's potentially a lot of copying, right? Is some other array better suited for this, such as vector or uvector? Thanks, Jason

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Haskell and the Job Market, e.g. with Google
by Hans van Thiel 11 Feb '10

11 Feb '10

Hello, Somewhat in response to the original post about Haskell engineers I, II and III. This confirms the remark that Haskell experience is now being appreciated, though not (yet) used (very much). Steven Grant, recruiter from Google, asked me to bring to his attention anyone who might be suitable, so that's what I'm doing. <start quote> We are currently aggressively recruiting for a large number of engineers in EMEA. I spotted your extensive open source experience and was particularily interested to see you have worked with Haskell. I am not looking for a Haskell developer but more interested in people that have worked in exotic languages such as Haskell or Erlang or Scheme. The roles we have are heavily open sourced based and will be mainly working with Python, C, Linux, shell etc and are based in Dublin, London or Zurich. If you have any interest in discussing these further, drop me an email to stevengrant(a)google.com and we can discuss. <end quote> >From a second email: <start quote> The job specs are below. http://www.google.ie/support/jobs/bin/answer.py?answer=34884 http://www.google.ie/support/jobs/bin/answer.py?answer=34883 The roles are within a very specialist team within Google. They are a hybrid type role and are responsible for making our products reliable scalable and more efficient. <end quote> Get in touch with Steven: Steven Grant European IT Staffing Phone: +353 1 543 5083 Google Ireland Ltd., Barrow Street, Dublin 4, Ireland Registered in Dublin, Ireland Registration Number: 368047 I think this is interesting even to those who are not looking for a job right now, since it shows the current mind-set regarding Haskell, at a major and leading IT company. Best Regards, Hans van Thiel

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If monads are single/linearly threaded, doesn't that reduce parallelism?
by caseyh＠istar.ca 11 Feb '10

11 Feb '10

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HDBC convert [SqlValue] without muchos boilerplate
by Iain Barnett 11 Feb '10

11 Feb '10

Hi, I'm trying to get to grips with HDBC and have the following problem. When I run a query that returns a result set, each row comes back as a [SqlValue]. Naively, I thought the following function would convert a [SqlValue] into a string, but instead I get the error below. convrow2 :: [SqlValue] -> String convrow2 (x:xs) = foldl (\i j -> i ++ " | " ++ show j ) (show (fromSql x)) xs Prelude> :l TasksSimple.lhs [1 of 1] Compiling Main ( TasksSimple.lhs, interpreted ) TasksSimple.lhs:126:65: No instance for (convertible-1.0.5:Data.Convertible.Base.Convertible SqlValue a) arising from a use of `fromSql' at TasksSimple.lhs:126:65-73 Possible fix: add an instance declaration for (convertible-1.0.5:Data.Convertible.Base.Convertible SqlValue a) In the first argument of `show', namely `(fromSql x)' In the second argument of `foldl', namely `(show (fromSql x))' In the expression: foldl (\ i j -> i ++ " | " ++ show j) (show (fromSql x)) xs Failed, modules loaded: none. I tried looking at how to add an instance declaration for convertible, but was stumped. This code, however, works in GHCi. Would anyone know how to convert from [SqlValue] in a straightforward way without having to specify every field "by hand" ? I don't fancy doing this for each sql statement I need to run. convrow1 :: [SqlValue] -> String convrow1 [tasksid,title,added] = show ( (fromSql tasksid)::Integer ) ++ " | " ++ fromSql title ++ " | " ++ show ((fromSql added)::LocalTime) Any help is much appreciated, especially as I haven't looked at any Haskell in a while and wasn't any good with it before! Regards, Iain This is my set up: GHC is 6.10.4 HDBC is HDBC-2.1.1, HDBC-2.2.2, HDBC-postgresql-2.1.0.0,HDBC-postgresql-2.2.0.0 Convertible is convertible-1.0.5, convertible-1.0.8 OSX 10.6

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lazy'foldl
by Sebastian Fischer 10 Feb '10

10 Feb '10

Hello, I have implemented the following function: lazy'foldl :: (a -> b -> Maybe a) -> Maybe a -> [b] -> Maybe a lazy'foldl _ Nothing _ = Nothing lazy'foldl _ m [] = m lazy'foldl f (Just y) (x:xs) = lazy'foldl f (f y x) xs After hoogling its type, I found that Control.Monad.foldM :: (a -> b -> Maybe a) -> a -> [b] -> Maybe a seems like a perfect replacement because lazy'foldl f (Just x) xs == foldM f x xs holds for all finite lists xs. Here is an inductive proof: lazy'foldl f (Just x) [] == Just x == foldM f x [] lazy'foldl f (Just x) (y:ys) == lazy'foldl f (f x y) ys (if f x y == Nothing) == lazy'foldl f Nothing ys == Nothing == Nothing >>= \z -> foldM f z ys == f x y >>= \z -> foldM f z ys == foldM f x (y:ys) lazy'foldl f (Just x) (y:ys) == lazy'foldl f (f x y) ys (if f x y == Just z) == lazy'foldl f (Just z) ys (induction) == foldM f z ys == Just z >>= \z -> foldM f z ys == f x y >>= \z -> foldM f z ys == foldM f x (y:ys) I think the above equation holds even for infinite lists xs. Both functions terminate on infinite lists, if the accumulator is eventually Nothing. Do you see any differences in terms of strictness, i.e., a counter example to the above equation that involves bottom? I don't. Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)

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Generate random UArray in constant memory space.
by Vasyl Pasternak 10 Feb '10

10 Feb '10

Hello Cafe, I tried to generate memory-efficient list of random numbers, so I've used uvector library for this task. But it doesn't work, it still allocates more than 6Gb of memory for the random list of 10 million elements. Here is the code: > import Text.Printf > import System.Random > import Control.Applicative > import System.Environment > import Data.Array.Vector > randomListU :: (Int, Int) -> StdGen -> Int -> (UArr Int) > randomListU b g size = unfoldU size gen g > where gen g = let (x, g') = randomR b g > in JustS (x :*: g') > main = do > [size] <- map read <$> getArgs > let ints = randomListU (-10, 10) (mkStdGen 1) size > printf "%d\n" (sumU ints) Could someone give a hint, how to implement this function in constant memory space? Thank you in advance. Best regards, Vasyl Pasternak

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Two GET HTTP requests
by Chris Eidhof 10 Feb '10

10 Feb '10

Hi everyone, I'm trying to do a number of successive HTTP requests in one program. Here's what I tried: Approach 1: I used the 'download' package, which failed to install on OS X. It fails with "error: libio.h: No such file or directory". Approach 2: I installed the 'download-curl' package, and tried again. This seems to fail on the following example: > import Network.Curl.Download > > main = do x <- openURI "http://haskell.org" > y <- openURI "http://haskell.org/hoogle" > return () If I put a print statement around the second line of the do-statement it looks like openURI never returns. Approach 3: I used the simpleHTTP function from the HTTP package. This crashed, after I dug a little deeper into the code, it threw an error on calling the parseURI function (openFile: no such file exists). I installed the latest network package and upgraded my HTTP package, and the parseURI error went away. I felt like I was almost there, and tried the following: > simpleHTTP (getRequest "http://haskell.org") This failed with just the text "Bus error". I searched the HTTPBis git repository, but couldn't find the text "Bus error". I don't have a clue of how to fix this. I'm a bit stuck here, I would love to help fix the errors, but don't know what would be the best place to begin. If anyone can point me in the right direction, I will try to patch at least one of these packages. Thanks, -chris

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Lazy language on JVM/CLR
by Tony Morris 10 Feb '10

10 Feb '10

I have hypothesised a pure, lazy language on the JVM and perhaps the .NET CLR with FFI to .NET/Java libraries. I foresee various problems but none that are catastrophic; just often requiring a compromises, sometimes very unattractive compromises. I have authored several libraries in the same vain as pure, lazy programming to run on the JVM in Java and Scala programming languages. I expect others have forethought and perhaps even experimented with such a language. Are there any dangers to be wary of that undo the entire endeavour? Thanks for any insights. -- Tony Morris http://tmorris.net/

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Jobs
by Roderick Ford 10 Feb '10

10 Feb '10

We need two developers at the company I work for; that is, I am NOT a recruiter. My boss is asking me to search for developers for me to work with ... like minded, haskell loving, driven, c/c++ (c# if we have to) for an INITIALLY c# full-time salaried position in Arizona (or FL or UT). He is open to starting new projects using Haskell, where applicable. So, he is involving me in the interview process, and I've been less than pleased with candidate selection so far. I told him we need to find people that see the beauty of mathematics and Haskell. If you have a resume you want me to forward on, reply to me directly and I will forward on through the review process. Here is a clip from posting 1 (don't take too literally): Software Engineer with strong 'Design, Programming, Debugging' experience Required Experience 3+yrs MS C# *AND* C++ MS SQL Server MS ASP.NET MS SQL Applicants must have at least 3 years practical work experience designing and building commercial software products that have shipped on the Windows OS platform. Requires proven experience building Web GUI on Windows platform, familiarity with distributed programming and networking. Must also be able to write and debug SQL Stored Procedures. Developer will be required to take ownership of the assigned modules through design, code & unit test. Past team lead experience would be valuable. Familiarity with gcc and Linux / Unix programming would be a plus. 4 year degree in a Computer Science related field with at least 5-7 years of experience in a software development environment or similar combination of education/work experience. This position is available in the following offices: Scottsdale, Arizona Provo, Utah Melbourne, Florida Here is a clip from posting 2: Job Description As a Senior C# - Asp.net Software Engineer in this position you will operate within a group of engineers, engaged in product design and implementation of enterprise applications. This role has a very high level of technical and product knowledge spanning multiple development technologies and target platforms. The Senior Software Engineer operates unsupervised and is a self-starter. Essential Duties and Responsibilities: Performs OOD, implementation and maintenance for products composed with a variety of technologies, mostly for Windows platforms but also Linux and other platforms Provides expertise for multiple software layers and levels of abstraction: UI, business layer, database, and distributed operations. Builds a tactical consensus with the development team whilst demonstrating strong technical leadership skills Experience working in a scrum agile environment a plus Performs highly complex product design activities which may require extensive research and analysis Performs complex bug verification, release testing and beta support for assigned products Researches problems discovered by QA or product support and identifies solutions to the problems Conducts unit testing and integration testing for functionality and performance limits In absence of product architects, may serve as a product architect Researches and understands non-functional requirements beyond the marketing requirements for a product Researches new technology or development tools to remain informed of current technology Required Skills Advanced technical knowledge in software development methodologies, design and implementation in C#. Expert analytical and design skills at multi-product/multi-environment level. Understands cross-platform and/or internationalisation development. In-depth experience in research and analysis, project planning and implementation. Excellent understanding of entire development process, including specification, documentation and quality assurance. Aware of business issues as they impact overall project plans. Recognised reference and central point for technical issues across groups and products. Strong communications and interpersonal skills. Advanced PC skills. Required Experience A university degree in computer science or software engineering, preferably a masters or higher, is desireable. Requires 10 to 15 years of experience in product design Good knowledge of .Net, and ASP.NET, for C# developers. Minimum 5 Years Experience of C# language and Asp.net

1 0

Navigating Graphs
by Matthias Görgens 09 Feb '10

09 Feb '10

Using something like zippers it is easy to navigate an acyclic graph in O(1) operations per arc you follow. Inspired by Chris Okasaki's work on queues I wondered if there is a similar approach to navigating cyclic graphs. If the graph you are navigating is static (i.e. does not have to support addition or removal of vertices in any reasonable amount of time), I guess you can get away with "Tying the know" (http://www.haskell.org/haskellwiki/Tying_the_Knot) But is there a technique that allows navigation, insertion and removal at focus in (at least amortised) O(1) operations each? As a generalisation being able to have multiple points of focus (in an acyclic graph for a start) would also be interesting. Any thoughts?

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Re: [Haskell-cafe] Generate random UArray in constant memory space.
by Daniel Fischer 09 Feb '10

09 Feb '10

Am Tuesday 09 February 2010 19:19:18 schrieben Sie: > Daniel, > > I've just run venum2 program locally and here is my results: > > $ ./venum2 10000000 +RTS -s > ./venum2 10000000 +RTS -s > 50000005000000 > 22,736 bytes allocated in the heap > 688 bytes copied during GC > 17,184 bytes maximum residency (1 sample(s)) > 19,680 bytes maximum slop > 1 MB total memory in use (0 MB lost due to fragmentation) > > $ ./venum2 1000000000 +RTS -s > ./venum2 1000000000 +RTS -s > 500000000500000000 > 24,152 bytes allocated in the heap > 688 bytes copied during GC > 17,184 bytes maximum residency (1 sample(s)) > 19,680 bytes maximum slop > 1 MB total memory in use (0 MB lost due to fragmentation) > > So my PC shows that there is constant memory allocation. Maybe I'm > doing something wrong ? Unlikely. However, I notice that you seem to have a 64-bit system. I don't know the details, but usually those have more registers than a 32- bit system, so you can get more complicated loops to run completely in the registers. I think that's what happens here, enumFromToU' is just complicated enough to not run in the registers on my 32-bit system, but still runs in the registers on your 64-bit system. The PRNG code is too complicated (too many temporary variables) to run in the registers on either system (BTW, have you tried it with the specialised PRNG code in the source file? I'm not sure whether that might be just small enough to run in the registers on a 64-bit system.). > > 2010/2/9 Daniel Fischer <daniel.is.fischer(a)web.de>: > > Am Tuesday 09 February 2010 15:43:13 schrieben Sie: > >> Update: > >> > >> I've implemented `enumFromToU` through `unfoldU`: > >> > enumFromToU' from to = unfoldU (to - from) f from > >> > where f i = let i' = i + 1 in JustS (i' :*: i') > >> > >> This code behaves similarly to `enumFromToU` (i.e. constantly uses > >> ~25 kb of memory, runs in the same time as above). > > > > Wait, > > > > $ cat venum2.hs > > module Main (main) where > > > > import Text.Printf > > import Control.Applicative > > import System.Environment > > import Data.Array.Vector > > > > main = do > > [size] <- map read <$> getArgs > > let ints = enumFromToU' 0 size :: UArr Int > > printf "%d\n" (sumU ints) > > > > enumFromToU' from to = unfoldU (to - from) f from > > where f i = let i' = i + 1 in JustS (i' :*: i') > > > > $ ghc -O2 --make venum2 > > [1 of 1] Compiling Main ( venum2.hs, venum2.o ) > > Linking venum2 ... > > $ ./venum2 +RTS -sstderr -RTS 1000000 > > ./venum2 1000000 +RTS -sstderr > > 1784293664 > > 48,256,384 bytes allocated in the heap > > 6,256 bytes copied during GC > > 26,804 bytes maximum residency (1 sample(s)) > > 25,524 bytes maximum slop > > 1 MB total memory in use (0 MB lost due to > > fragmentation) > > > > $ ./venum2 +RTS -sstderr -RTS 10000000 > > ./venum2 10000000 +RTS -sstderr > > -2004260032 > > 481,937,552 bytes allocated in the heap > > 19,516 bytes copied during GC > > 26,804 bytes maximum residency (1 sample(s)) > > 25,512 bytes maximum slop > > 1 MB total memory in use (0 MB lost due to > > fragmentation) > > > > So we have constant memory, but linear allocation, just like with the > > random numbers. > > > > With enumFromToU, also the allocation is constant, so unfoldU > > allocates, enumFromToU not. > > > >> So the question is why random number list generator eats O(n) memory > >> ? > > > > It doesn't, not here, at least. Using System.Random, the allocation > > figures are about ten times as high, but the residency is about the > > same. Putting the PRNG code in the same file and tweaking things a bit > > (eliminating all intermediate Integers, e.g.), I get > > > > $ cat ran2Vec.hs > > {-# LANGUAGE BangPatterns #-} > > module Main (main) where > > > > import Text.Printf > > import Control.Applicative > > import System.Environment > > import Data.Array.Vector > > > > randomListU :: (Int, Int) -> StdGen -> Int -> (UArr Int) > > randomListU b@(l,h) g size = unfoldU size gen g > > where > > !k = h-l+1 > > !b' = 2147483561 `rem` k > > gen !g = let (!x, !g') = stdNext g > > in JustS ((l+ (x+b') `rem` k) :*: g') > > > > main = do > > [size] <- map read <$> getArgs > > let ints = randomListU (-10, 10) (mkStdGen 1) size > > printf "%d\n" (sumU ints) > > > > data StdGen = StdGen {-# UNPACK #-} !Int {-# UNPACK #-} !Int > > > > mkStdGen :: Int -> StdGen > > mkStdGen s > > | s < 0 = mkStdGen (-s) > > | otherwise = StdGen (s1+1) (s2+1) > > where > > (q, s1) = s `divMod` 2147483562 > > s2 = q `mod` 2147483398 > > > > {-# INLINE stdNext #-} > > stdNext :: StdGen -> (Int, StdGen) > > -- Returns values in the range stdRange > > stdNext (StdGen s1 s2) = z' `seq` g' `seq` (z', g') > > where > > !g' = StdGen s1'' s2'' > > !z' = if z < 1 then z + 2147483562 else z > > !z = s1'' - s2'' > > > > !k = s1 `quot` 53668 > > !s1' = 40014 * (s1 - k * 53668) - k * 12211 > > !s1'' = if s1' < 0 then s1' + 2147483563 else s1' > > > > !k' = s2 `quot` 52774 > > !s2' = 40692 * (s2 - k' * 52774) - k' * 3791 > > !s2'' = if s2' < 0 then s2' + 2147483399 else s2' > > $ ghc -O2 -funbox-strict-fields -funfolding-use-threshold=16 --make > > ran2Vec -o ran2AVec5 > > Linking ran2AVec5 ... > > $ ./ran2AVec5 +RTS -sstderr -RTS 10000000 > > ./ran2AVec5 10000000 +RTS -sstderr > > 5130 > > 521,828,888 bytes allocated in the heap > > 8,664 bytes copied during GC > > 26,788 bytes maximum residency (1 sample(s)) > > 25,636 bytes maximum slop > > 1 MB total memory in use (0 MB lost due to > > fragmentation) > > > > nearly the same allocation figures as for the *much* simpler > > enumFromToU', and it's about ten times as fast as System.Random. > >

2 1

Strange Haddock output
by Sean Leather 09 Feb '10

09 Feb '10

I was browsing Hackage pointlessly and came across what appears to be strange Haddock formatting here: http://hackage.haskell.org/packages/archive/pointless-haskell/0.0.3/doc/htm… The following source -- | Polytypic 'Prelude.Functor' class for functor representations class Functor (f :: * -> *) where fmap :: Fix f -- ^ For desambiguation purposes, the type of the functor must be passed as an explicit paramaeter to 'fmap' -> (x -> y) -> Rep f x -> Rep f y -- ^ The mapping over representations produces the following view from the browser class Functor f where Polytypic Prelude.Functor class for functor representations Methods fmap :: => Fix f -> x -> y -> Rep f x The mapping over representations -> Rep f y with the Haddock 2.6.0 on Hackage. There seems to be an extraneous => in the signature of fmap, and the comment on Fix f does not show up. Sean

2 2

automatically exclude unused modules at compile time
by Hong Yang 09 Feb '10

09 Feb '10

When I turned on "-Wall" knob at compile time, I got a message: Warning: Module `System.Cmd' is imported, but nothing from it is used, except perhaps instances visible in `System.Cmd' To suppress this warning, use: import System.Cmd() Then I did four experiments as follows at compile time: 1) import System.Environment 2) import System.Environment() 3) import System.Environment(only the functions I used) 4) -- import System.Environment It turned out that case 1-3) have the size after compilation, and that case 4) has smaller size than the others. Can we make GHC a little bit more intelligent to automatically exclude unused modules at compile time as in case 4)? (Am I just too lazy to add several dashes?) Thanks, Hong

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Re: [Haskell-cafe] Generate random UArray in constant memory space.
by Daniel Fischer 09 Feb '10

09 Feb '10

Am Tuesday 09 February 2010 15:43:13 schrieben Sie: > Update: > > I've implemented `enumFromToU` through `unfoldU`: > > enumFromToU' from to = unfoldU (to - from) f from > > where f i = let i' = i + 1 in JustS (i' :*: i') > > This code behaves similarly to `enumFromToU` (i.e. constantly uses ~25 > kb of memory, runs in the same time as above). Wait, $ cat venum2.hs module Main (main) where import Text.Printf import Control.Applicative import System.Environment import Data.Array.Vector main = do [size] <- map read <$> getArgs let ints = enumFromToU' 0 size :: UArr Int printf "%d\n" (sumU ints) enumFromToU' from to = unfoldU (to - from) f from where f i = let i' = i + 1 in JustS (i' :*: i') $ ghc -O2 --make venum2 [1 of 1] Compiling Main ( venum2.hs, venum2.o ) Linking venum2 ... $ ./venum2 +RTS -sstderr -RTS 1000000 ./venum2 1000000 +RTS -sstderr 1784293664 48,256,384 bytes allocated in the heap 6,256 bytes copied during GC 26,804 bytes maximum residency (1 sample(s)) 25,524 bytes maximum slop 1 MB total memory in use (0 MB lost due to fragmentation) $ ./venum2 +RTS -sstderr -RTS 10000000 ./venum2 10000000 +RTS -sstderr -2004260032 481,937,552 bytes allocated in the heap 19,516 bytes copied during GC 26,804 bytes maximum residency (1 sample(s)) 25,512 bytes maximum slop 1 MB total memory in use (0 MB lost due to fragmentation) So we have constant memory, but linear allocation, just like with the random numbers. With enumFromToU, also the allocation is constant, so unfoldU allocates, enumFromToU not. > > So the question is why random number list generator eats O(n) memory ? It doesn't, not here, at least. Using System.Random, the allocation figures are about ten times as high, but the residency is about the same. Putting the PRNG code in the same file and tweaking things a bit (eliminating all intermediate Integers, e.g.), I get $ cat ran2Vec.hs {-# LANGUAGE BangPatterns #-} module Main (main) where import Text.Printf import Control.Applicative import System.Environment import Data.Array.Vector randomListU :: (Int, Int) -> StdGen -> Int -> (UArr Int) randomListU b@(l,h) g size = unfoldU size gen g where !k = h-l+1 !b' = 2147483561 `rem` k gen !g = let (!x, !g') = stdNext g in JustS ((l+ (x+b') `rem` k) :*: g') main = do [size] <- map read <$> getArgs let ints = randomListU (-10, 10) (mkStdGen 1) size printf "%d\n" (sumU ints) data StdGen = StdGen {-# UNPACK #-} !Int {-# UNPACK #-} !Int mkStdGen :: Int -> StdGen mkStdGen s | s < 0 = mkStdGen (-s) | otherwise = StdGen (s1+1) (s2+1) where (q, s1) = s `divMod` 2147483562 s2 = q `mod` 2147483398 {-# INLINE stdNext #-} stdNext :: StdGen -> (Int, StdGen) -- Returns values in the range stdRange stdNext (StdGen s1 s2) = z' `seq` g' `seq` (z', g') where !g' = StdGen s1'' s2'' !z' = if z < 1 then z + 2147483562 else z !z = s1'' - s2'' !k = s1 `quot` 53668 !s1' = 40014 * (s1 - k * 53668) - k * 12211 !s1'' = if s1' < 0 then s1' + 2147483563 else s1' !k' = s2 `quot` 52774 !s2' = 40692 * (s2 - k' * 52774) - k' * 3791 !s2'' = if s2' < 0 then s2' + 2147483399 else s2' $ ghc -O2 -funbox-strict-fields -funfolding-use-threshold=16 --make ran2Vec -o ran2AVec5 Linking ran2AVec5 ... $ ./ran2AVec5 +RTS -sstderr -RTS 10000000 ./ran2AVec5 10000000 +RTS -sstderr 5130 521,828,888 bytes allocated in the heap 8,664 bytes copied during GC 26,788 bytes maximum residency (1 sample(s)) 25,636 bytes maximum slop 1 MB total memory in use (0 MB lost due to fragmentation) nearly the same allocation figures as for the *much* simpler enumFromToU', and it's about ten times as fast as System.Random. > > Regards, > Vasyl Pasternak > > 2010/2/9 Vasyl Pasternak <vasyl.pasternak(a)gmail.com>: > > Sorry, maybe I should ask more clearer. > > > > I've looked at dons article "Haskell as fast as C"[1], and tried to > > implement similar algorithm but for list of random numbers. > > > > Please look at code: > >> import Text.Printf > >> import Control.Applicative > >> import System.Environment > >> import Data.Array.Vector > >> > >> main = do > >> [size] <- map read <$> getArgs > >> let ints = enumFromToU 0 size :: UArr Int > >> printf "%d\n" (sumU ints) > > > > This code runs in constant space (on my pc ~25kb allocates on the > > heap) regardless of array size. So I tried to achieve similar with > > random list, just to replace `enumFromToU` with my own list generator. > > > > So the question - is it possible to implement random list similary to > > enumFromToU? > > > > > > [1]http://donsbot.wordpress.com/2008/06/04/haskell-as-fast-as-c-workin > >g-at-a-high-altitude-for-low-level-performance/ > > > > Thank you, > > Vasyl Pasternak > > > > 2010/2/9 Daniel Fischer <daniel.is.fischer(a)web.de>: > >> Am Dienstag 09 Februar 2010 13:18:23 schrieb Vasyl Pasternak: > >>> Hello Cafe, > >>> > >>> I tried to generate memory-efficient list of random numbers, so I've > >>> used uvector library for this task. But it doesn't work, > >>> it still allocates more than 6Gb of memory for the random list of 10 > >>> > >>> million elements. Here is the code: > >> > >> Hmm, > >> > >> $ ghc -O2 --make ranVec > >> [1 of 1] Compiling Main ( ranVec.hs, ranVec.o ) > >> Linking ranVec ... > >> $ ./ranVec 10000000 +RTS -sstderr > >> 5130 > >> 4,919,912,080 bytes allocated in the heap > >> 883,256 bytes copied during GC > >> 26,896 bytes maximum residency (1 sample(s)) > >> 25,620 bytes maximum slop > >> 1 MB total memory in use (0 MB lost due to > >> fragmentation) > >> > >> maximum residency is just eight bytes more than for 100,000 or > >> 1,000,000 numbers. I think that is constant space. > >> > >> The ~5 GB total allocation is sequential, ten million new StdGens are > >> produced and allocated, then immediately garbage collected. I see no > >> problem (except that StdGen is slow, e.g. the Mersenne twister is > >> much faster [and allocates less, but still linear in size]). > >> > >>> > import Text.Printf > >>> > import System.Random > >>> > import Control.Applicative > >>> > import System.Environment > >>> > import Data.Array.Vector > >>> > > >>> > randomListU :: (Int, Int) -> StdGen -> Int -> (UArr Int) > >>> > randomListU b g size = unfoldU size gen g > >>> > where gen g = let (x, g') = randomR b g > >>> > in JustS (x :*: g') > >>> > > >>> > main = do > >>> > [size] <- map read <$> getArgs > >>> > let ints = randomListU (-10, 10) (mkStdGen 1) size > >>> > printf "%d\n" (sumU ints) > >>> > >>> Could someone give a hint, how to implement this function in > >>> constant memory space? > >>> > >>> Thank you in advance. > >>> > >>> Best regards, > >>> Vasyl Pasternak

1 0

Re: [Haskell-cafe] Iteratee, parsec & co.
by John Lato 09 Feb '10

09 Feb '10

> From: Edward Kmett <ekmett(a)gmail.com> > > 3. Why Seek FileOffset is error message? >> > > I'm not quite sure what you're asking here. One of the infelicities of iteratee-0.3 is that it defines a data type: > data ErrMsg = Err String > | Seek FileOffset > deriving (Show, Eq) I believe this is what the OP is referring to. Oleg originally implemented seek using IORefs; I wanted to try this and didn't know if it would work so I used a minimal implementation. I've decided this is the correct approach, but the details need to be tweaked a bit. Cheers, John

1 0

various dependencies breakages
by Thomas Girod 09 Feb '10

09 Feb '10

Hi there. I'm trying to reinstall gitit after updating to ghc 6.12 and it's driving me crazy. I hope you will be able to help me troubleshoot this mess ... I'm starting from scratch again, using cabal. I will post more problems as they appears. $ cabal install gitit Resolving dependencies... cabal: cannot configure datetime-0.2. It requires QuickCheck ==2.* For the dependency on QuickCheck ==2.* there are these packages: QuickCheck-2.1, QuickCheck-2.1.0.1, QuickCheck-2.1.0.2 and QuickCheck-2.1.0.3. However none of them are available. QuickCheck-2.1 was excluded because happstack-util-0.4.1 requires QuickCheck <2 QuickCheck-2.1.0.1 was excluded because happstack-util-0.4.1 requires QuickCheck <2 QuickCheck-2.1.0.2 was excluded because happstack-util-0.4.1 requires QuickCheck <2 QuickCheck-2.1.0.3 was excluded because happstack-util-0.4.1 requires QuickCheck <2 Can't cabal handle several versions of the same library in parallel ? If not, how can I solve this problem ? cheers tom

3 3

Iteratee, parsec & co.
by Maciej Piechotka 09 Feb '10

09 Feb '10

I read a lot about iteratee IO and it seemed very interesting (Unfortunately it lacks tutorial). Especially features like 'no input yet' in network programming (with lazy IO + parsec I run into problems as it tried to evaluate the first response character before sending output). I decided first write a simple program and then attempt write a Stream implementation for parsec. > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE MultiParamTypeClasses #-} > import Data.Iteratee > import Text.Parse > > data Buffer a = Buffer > instance Monad m => Stream (Buffer a) (IterateeG [] a m) a where > uncons Buffer = IterateeG loop > where loop (Chunk []) = > return $! Cont (IterateeG loop) Nothing > loop (Chunk (x:xs)) = > return $! Done (Just (x, Buffer)) > (Chunk xs) > loop (EOF Nothing) = > return $! Done Nothing (EOF Nothing) > loop (EOF (Just e)) = > return $! throwErr e 1. I'm not quite sure what Cursor was suppose to do from A Parsing Trifecta presentation. 2. Is there any package which contains this stream definition? 3. Why Seek FileOffset is error message? Regards PS. I guess iteratee does qualify as cafe but if beginner would be more appropriate group then I'm sorry - I'll remember next time.

4 3

haskelldb in-memory driver?
by Johannes Waldmann 09 Feb '10

09 Feb '10

Is there a haskelldb-driver that works with a datasource that is in memory, as a Haskell object? (like LINQ can work on C# collections) - J.

4 4

Compiling on Windows - using a particular Manifest file
by Günther Schmidt 09 Feb '10

09 Feb '10

Hi, how can I make ghc use a particular manifest file for embedding? (on Windows) Günther

2 1

cannot post to haskellmode-emacs@projects.haskell.org
by Jose A. Ortega Ruiz 09 Feb '10

09 Feb '10

Hi, all my posts to the haskellmode-emacs list (using the address in the subject) are being rejected with the error: 451 451 Temporary local problem - please try later (state 18) (after several retries by the Google smtp server, which i use as a smarthost). Any idea of what could be causing this problem? Thanks! jao -- There are two ways to write error-free programs; only the third one works. - Alan Perlis, Epigrams in Programing

4 3

Using ShowS or Difference Lists
by Mark Spezzano 08 Feb '10

08 Feb '10

Hi, Just wondering whether I can use ShowS or tupling or Difference Lists to speed up the following code? It's basic text processing. It takes in a list of Lines where each Line is a list of Words and intersperses " " between them then concatenates them into a longer String. Note that there is a recursive call and the ++ operator. Thanks Mark -- Function: joinLines -- Joins the Words within Lines together with whitespace and newline characters -- Argument: Lines to pad with whitespace and newlines -- Evaluate: The processed and concatenated String joinLines :: [Line] -> String joinLines (l:[]) = concat (intersperse " " l) joinLines (l:ls) = (concat (intersperse " " l)) ++ ('\n':joinLines ls)

6 6

Links to darcs.haskel.org in the haskellwiki
by Henk-Jan van Tuyl 08 Feb '10

08 Feb '10

L.S., There are a lot of links in the haskellwiki that point to projects at darcs.haskel.org; I hope that anyone who moves a project, looks the links up and updates them. An example of a page with several obsolete links is http://www.haskell.org/haskellwiki/Applications_and_libraries/Web_programmi… Validity of links can automatically be checked with the linkchecker: http://validator.w3.org/checklink I have done quite a lot of updating of links in the past, I hope more people are willing to check and update pages that haven't been touched for a while. Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

4 3

Spelling checker exercise
by Matthew Phillips 08 Feb '10

08 Feb '10

Hello all, sorry to bring up an old chestnut, but I’m trying to improve my Haskell-fu by writing a small program, and chose Peter Norvig’s spelling checker as my exercise material (http://norvig.com/spell-correct.html) While I’ve gotten it working, and found it quite illuminating, I also found it to to be very slow. And then I discovered that, of course, others have been here well before me ([1] and [2]). Those discussions were very interesting, but the code they refer to is mostly not available, so the most I’ve gotten out of it so far is that: (a) I should be using strict left folds and strict Map insertions for the word frequency map (this shaved off about a second: ~5s -> ~4s for a single word on my 2.4GHz MacBook Pro, GHC 6.10.4) (b) I should probably be using ByteString’s (c) Using Set’s for the edit permutations probably isn’t worth it (although I found using plain lists made it about a second slower) (b) is difficult because I’ve used matching patterns plus list comprehensions to generate the potential edits, and I really like how elegantly it pans out that way. Because ByteString’s are not lists, I can’t see a way to keep the current structure and use them. The code is at [3] (link to version at time of post). Profiling [4] shows: $ ./spelling becuase +RTS -p becuase -> because $ cat spelling.prof total time = 4.02 secs (201 ticks @ 20 ms) total alloc = 1,544,257,792 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc train Main 52.7 19.7 readFile Main 28.9 8.6 wordsBy Main 10.9 49.5 toLower Main 7.0 21.8 ... So it appears that “train" (building the freq map) and “readFile” in “nwords" are the places to hone. I will look at using Bloom Filters or Trie’s instead of Data.Map, but I wonder if readFile should be taking nearly %30 of the run time, even for a 6MB file? Sorry to dump such a long post on the list — I’ll understand if no one can be bothered rehashing this. But, in summary I’d like to know: (a) how could I use ByteString’s for this to speed up I/O and reduce memory usage without losing the nice readability? (b) should readFile be so slow? (c) any other tips Possibly all my questions could be answered if someone has the code from the old posts. Cheers, Matthew. [1]: http://haskell.markmail.org/search/?q=norvig%20spelling#query:norvig%20spel… [2]: http://thread.gmane.org/gmane.comp.lang.haskell.cafe/21780 [3]: http://github.com/scramjet/spelling/blob/44fd336ef4f62d49c7087dbc1ffb9c009b… [4]: http://github.com/scramjet/spelling/blob/f5146b24b77443c22975ff69fb91aa922c…

3 11

Interpreting profiling results
by Gregory Collins 08 Feb '10

08 Feb '10

Hello all, A friend and I are trying to benchmark some network server code which runs fine on my OSX laptop (~7500 req/s) but which is >10x slower on a much faster 8-core Xeon Linux box. The profiling run on Linux looks like this: individual inherited COST CENTRE MODULE no. entries %time %alloc %time %alloc MAIN MAIN 1 0 95.7 0.2 100.0 100.0 ... How should we interpret this result? MAIN doesn't seem to correspond to any user code so we're wondering where the time is going. G. -- Gregory Collins <greg(a)gregorycollins.net>

2 4

cabal install time
by Ozgur Akgun 08 Feb '10

08 Feb '10

I hate sending such messages to cafe, but... When I try to install "time" I get the following error: > sudo cabal install time ld: duplicate symbol ___hscore_d_name in /Users/username/.cabal/lib/unix-2.4.0.0/ghc-6.10.4/libHSunix-2.4.0.0.a(dirUtils.o) and /Library/Frameworks/GHC.framework/Versions/610/usr/lib/ghc-6.10.4/base-4.1.0.0/libHSbase-4.1.0.0.a(PrelIOUtils.o) UsingSnow Leopard with the following versions of ghc and cabal > ghc --version The Glorious Glasgow Haskell Compilation System, version 6.10.4 > cabal --version cabal-install version 0.6.4 using version 1.6.0.3 of the Cabal library Thanks for any help. Best, -- Ozgur Akgun

1 0

Haskell Weekly News: Issue 149 - February 08, 2010
by jfredett＠gmail.com 08 Feb '10

08 Feb '10

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20100208 Issue 149 - February 08, 2010 --------------------------------------------------------------------------- Welcome to issue 149 of HWN, a newsletter covering developments in the [1]Haskell community. Hello Haskellers, this week's HWN was delayed a bit in the hopes of making it a bit more substantial. I hate putting up thin HWNs, but of course this must occasionally happen. We have several new CFPs for workshops this week, a new benchmarking package, and some fun quotes. Till next week, Haskellers, your Haskell Weekly News! Announcements Call for Papers: Haskell Symposium 2010. Jeremy.Gibbons [2]announced a call for papers for the 2010 ACM SIGPLAN Haskell Symposium 2010 in Baltimore, Maryland; on 30th September. 2nd CfP: LOPSTR 2010. Temur Kutsia [3]announced a second call for papers for the 20th International Symposium on Logic-Based Program Synthesis and Transformation, being held in Hagenberg, Austria, July 23-25, 2010 (co-located with PPDP 2010). PAR 2010: First CFP. Koen Claessen [4]announced a first call for papers for PAR'10, the Workshop on Partiality and Recursion in Interactive Theorem Provers. data-ordlist-0.2. Leon Smith [5]announced a new release of ordlist, including a change to the module name and bug fixes. progression-0.1. Neil Brown [6]announced Progression, a metalibrary which consolidates various existing tools for Haskell optimization (notably Criterion). HList darcs repo. Oleg He who inhabits all types, but is not _|_, has [7]announced a new darcs repo for HList (and OOHaskell) on community.haskell.org. Discussion a beginner question: decorate-op-undecorate. Aran Donohue [8]asked about how to write a function which can 'inspect' inside a datatype. Translation of Haskell type classes. Enrique Martin [9]talked about some experiments he's done with type classes, and asked some questions regarding optimization related to them. Category Theory woes. Mark Spezzano [10]asked about resources for learning about category theory. Blog noise [11]Haskell news from the [12]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Dan Piponi (sigfpe): [13]The Categorification of the Naturals. * Roman Cheplyaka: [14]hledger. * Luke Palmer: [15]Associative Alpha Blending. * Neil Brown: [16]Progression: Supporting Optimisation in Haskell. * Holumbus: [17]Hayoo! Webservice API. * apfelmus: [18]The Operational Monad Tutorial. * Dan Piponi (sigfpe): [19]Tagging Monad Transformer Layers. * Darcs: [20]darcs weekly news #53. * JP Moresmau: [21]EclipseFP 1.110 released, with debugging support. Quotes of the Week * lispy|web: This curses binding appears to be terminally broken * lispy: I did, 'cabal install mage' and it complains about curses * lament: Just use fix to find the least funny joke * copumpkin: A monad is just a lax functor from a terminal bicategory, duh. fuck that monoid in category of endofunctors shit About the Haskell Weekly News New editions are posted to [22]the Haskell mailing list as well as to [23]the Haskell Sequence and [24]Planet Haskell. [25]RSS is also available, and headlines appear on [26]haskell.org. To help create new editions of this newsletter, please see the information on [27]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [28]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.general/17778 3. http://article.gmane.org/gmane.comp.lang.haskell.general/17770 4. http://article.gmane.org/gmane.comp.lang.haskell.general/17769 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70035 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/69929 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/69898 8. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/69984 9. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/69914 10. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/69803 11. http://planet.haskell.org/ 12. http://haskell.org/haskellwiki/Blog_articles 13. http://blog.sigfpe.com/2010/02/decategorification-of-naturals.html 14. http://ro-che.blogspot.com/2010/02/hledger.html 15. http://lukepalmer.wordpress.com/2010/02/05/associative-alpha-blending/ 16. http://chplib.wordpress.com/2010/02/04/progression-supporting-optimisation-… 17. http://holumbus.fh-wedel.de/blog/?p=33 18. http://apfelmus.nfshost.com/articles/operational-monad.html 19. http://blog.sigfpe.com/2010/02/tagging-monad-transformer-layers.html 20. http://blog.darcs.net/2010/02/darcs-weekly-news-53.html 21. http://jpmoresmau.blogspot.com/2010/02/eclipsefp-1110-released-with-debuggi… 22. http://www.haskell.org/mailman/listinfo/haskell 23. http://sequence.complete.org/ 24. http://planet.haskell.org/ 25. http://sequence.complete.org/node/feed 26. http://haskell.org/ 27. http://haskell.org/haskellwiki/HWN 28. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

2 1

Re: [Haskell-cafe] a beginner question: decorate-op-undecorate
by John Lato 08 Feb '10

08 Feb '10

Hi Stephen, > From: Stephen Tetley <stephen.tetley(a)gmail.com> > > Hi John > > I'm not sure about making Binding polymorphic to get Functor, > Traversable, Foldable... > > While I think you're correct that partitionEithers might not be a > useful example to draw from in this case, I'd assume that Binding > would be part of a larger syntax-tree, thus there might not be a > appropriate single leaf to make the tree polymorphic on. Felipe > Lessa's point - to use Uniplate or one of the Generics packages - > might be a better candidate for implementing traversals. I did not consider that, and hadn't seen any other replies when I first replied. If true, I would agree that one of the Generics packages would be the best option. Regardless, I think this is a valid approach. By thinking of data in terms of Functor, Traversable, etc., one can recognize these patterns. Even if Binding is part of a larger syntax tree and can't reasonably be made polymorphic, creating an fmap-like function to do what the OP requested is IMHO simple and idiomatic Haskell. Using Generics then simplifies the implementation. Cheers, John

2 1

Number formatting
by Günther Schmidt 08 Feb '10

08 Feb '10

Hello, is there a function which will format a number with separators, something that will turn 14509.8674 into 14,509.86? Günther

3 4

GUI programming
by Victor Nazarov 08 Feb '10

08 Feb '10

Hello, I've been writing some GUI application with Gtk2hs. It's an interpreter for lambda-calculus and combinatory logic, it's GPL and if you interested I can share it with cafe. The problem is that the GUI code has become very ugly and I'm tempted to rewrite it totally. I've been looking forward to the FRP stuff, but I've never seen a single definition of the term. Conal Eliot's "denotational programming" is too general to be definition. I want to try Grapefruit, but I got totally lost when I see arrow notation. I consider more lightweight and more imperative approach, something closer to CSP (Communicating Secuential Processes) then FRP. I've just crafted some sample program to illustrate my idea. The behaviour is a monad and it's IO monad so you can do any IO (Gtk2hs) programming you wish. The differences is that you don't attach static event handlers and tries to determine what to do dependent on application state. You attach and detach handlers as much as possible. Behaviour looks like a process that can stop execution and wait for some GUI event. When event arrived it continues execution. Do you see this approach viable. There are steel some details to emerge: * How to wait for several events * How to handle IO exceptions Here is the code: {-# LANGUAGE ExistentialQuantification #-} module Main where import Data.IORef import System.Glib import Graphics.UI.Gtk import Control.Monad.Trans type Event obj = IO () -> IO (ConnectId obj) data Behaviour a = forall b. BBind (Behaviour b) (b -> Behaviour a) | BIO (IO a) | forall obj. GObjectClass obj => BWaitEvent (Event obj) (Behaviour a) instance Monad Behaviour where action >>= generator = BBind action generator return a = BIO (return a) instance MonadIO Behaviour where liftIO action = BIO action runBehaviour :: Behaviour a -> IO a runBehaviour (BBind (BWaitEvent event after) f) = runBehaviour (BWaitEvent event (after >>= f)) runBehaviour (BBind (BIO a) f) = a >>= \x -> runBehaviour (f x) runBehaviour (BBind (BBind a f) g) = runBehaviour (a >>= (\x -> f x >>= g)) runBehaviour (BIO a) = a runBehaviour (BWaitEvent event after) = do sigIdRef <- newIORef (error "You can't access sigIdRef before signal is connected") sigId <- event $ do sigId <- readIORef sigIdRef signalDisconnect sigId runBehaviour after return () writeIORef sigIdRef sigId return (error "You can't expect result from behaviour") waitEvent :: GObjectClass obj => Event obj -> Behaviour () waitEvent event = BWaitEvent event (return ()) main :: IO () main = do initGUI window <- windowNew onDestroy window mainQuit set window [windowTitle := "Hello World"] button <- buttonNew let buttonB label = do liftIO $ set button [buttonLabel := label] waitEvent (onClicked button) buttonB (label ++ "*") runBehaviour (buttonB "*") set window [containerChild := button] widgetShowAll window mainGUI -- Victor Nazarov

7 9

cabal fun (not)
by Johannes Waldmann 08 Feb '10

08 Feb '10

while trying to live with different versions of packages, I found cabal-install's behaviour strange because: you can "cabal install" a package without any complaint but the next "ghc-pkg list" is all red because of conflicts. and it's too late then - there's no way to get back. even if you re-install some package, it seems it is getting a different hashcode, so it's not recognized. if you were doing "cabal install --global" for some ghc package (e.g. old-locale), then ghc seems completely hosed and you've got to re-install the compiler. well that's not a big deal by itself but afterwards you have to re-install all packages ... so please please please have "cabal install" fail with some error message if (that is, before) the install would break anything. - J. $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.1 $ cabal --version cabal-install version 0.8.0 using version 1.8.0.2 of the Cabal library

4 5

Darcs fundraising 2010 - send us to ZuriHac! ($792 more by 15 Feb)
by Eric Kow 08 Feb '10

08 Feb '10

Hi everybody, Only one more week of fundraising to go! We've raised $1208 so far, which gives us another $792 to go in our final push. Can we do it? I sure hope so. Even if we do not, we'll still have raised more than we did last year, so thanks to all! If you're still thinking of helping out, now is the perfect time :-) Otherwise, stay tuned at http://darcs.net/donations.html for our progress. Eric PS. If you send me an email telling me how much you donated, I can update the donations site immediately so that folks can watch our progress bar obsessively :-) Donating by Google Checkout ---------------------------------------------------------------------- Donating via Google Checkout puts more of your donation to work, since Google charges no Checkout fees to 501(c)(3) organizations. Please visit http://darcs.net/donations.html for more details. Donating by check (USA) ---------------------------------------------------------------------- The next best option is to donate by check if you have a US bank account. Checks should be made payable to "Software Freedom Conservancy, Inc.", with "Directed donation: Darcs" in the memo. They can be mailed to Software Freedom Conservancy 1995 BROADWAY FL 17 NEW YORK NY 10023-5882 USA Donating by Paypal ---------------------------------------------------------------------- Please visit http://darcs.net/donations.html for more details. -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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ANN: data-ordlist-0.2
by Leon Smith 07 Feb '10

07 Feb '10

I have released data-ordlist 0.2, with a number of changes: 1. The module name is now Data.List.Ordered, instead of Data.OrdList 2. Three bugfixes: (ack!) insertSet and insertBag assumed reverse-ordered lists, nub failed to remove duplicates. Thanks to Topi Karvonen for reporting the first problem. 3. One semantic change: old_nubBy f == new_nubBy (not . f). The new version is better keeping with the spirit of the rest of the library, and makes the old nub bug much more obvious. Now nubBy is the greedy algorithm that returns a sublist such that for all binary predicates: isSortedBy pred (nubBy pred xs) == True 4. Improved documentation, I hope! Please consider taking a look and letting me know what you think. http://hackage.haskell.org/package/data-ordlist Cheers, Leon

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Re: [Haskell-cafe] a beginner question: decorate-op-undecorate
by John Lato 06 Feb '10

06 Feb '10

> From: Aran Donohue <aran.donohue(a)gmail.com> > > Hi Haskell-Cafe, > > Consider a data type such as > > data Binding = Binding Var (Either Value [Value]) > > representing a variable bound either to a fixed value or that has a list of > possible values. > > I'd like to perform an operation on say, the fixed-value members of a list > of bindings. Data.Either has "partitionEithers"---I'd essentially like to > use partitionEithers, but in a way that it "peeks" into the value field of > the binding. For the sake of argument, let's say I can't or can't modify > Binding to move the Either to the outside. I think that partitionEithers is leading you down the wrong trail. If what you want to do is modify some values inside the binding, I would start with this: > mapVal :: (Either Value [Value] -> Either Value [Value]) -> Binding -> Binding > mapVal f (Binding v e) = Binding v (f e) > > mapLeft :: (a -> b) -> Either a c -> Either b c > mapLeft f = either (Left . f) Right > > -- mapRight is just fmap, but for symmetry > mapRight :: (b -> c) -> Either a b -> Either a c > mapRight = fmap > > modifyFixed :: (Value -> Value) -> Binding -> Binding > modifyFixed f b = mapVal (mapLeft f) b > > modifyList :: ([Value] -> [Value]) -> Binding -> Binding > modifyList f b = mapVal (mapRight f) b > > -- note that modifyFixed and modifyList have very nice point-free representations > -- modifyFixed = mapVal . mapLeft > -- modifyList = mapVal . mapRight Now to apply this to a list: > modifyFixedBindings :: (Value -> Value) -> [Binding] -> [Binding] > modifyFixedBindings f binds = map (modifyFixed f) binds > -- or point-free > modifyFixedBindings' = map . modifyFixed In my opinion, this would be more idiomatic if Binding were polymorphic: > data Binding' k v = Binding' k v > > instance Functor (Binding' k) where > fmap f (Binding' k v) = Binding' k (f v) > > type Binding2 = Binding' Var (Either Value [Value]) now mapVal is just fmap, and these functions are: > modifyFixed2 :: (Val -> Val) -> [Binding2] -> [Binding2] > modifyFixed2 = fmap . fmap . mapLeft > > modifyList2 :: ([Val] -> [Val]) -> [Binding2] -> [Binding2] > modifyList2 = fmap . fmap . mapRight I've typed out all the steps for clarity, but to be honest, I wouldn't bother with the Fixed and List variants, unless you're going to use them frequently. I would do just: > mapVals :: (Either Value [Value] -> Either Value [Value]) -> [Binding] -> [Binding] > mapVals f = map (\(Binding var val) -> Binding var (f val)) and leave it at that, using "mapVals" with the "either" function when necessary. I would consider making Binding polymorphic, though, so you can write the Functor instance. You may also want to look at Data.Traversable. Cheers, John

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safe-lazy-io on GHC 6.12?
by Tom Tobin 06 Feb '10

06 Feb '10

After convincing myself the hard way that you can't be lazy across strict monadic results (by writing myself a "foldrM" -- yeah, I'm still a beginner), I noticed the recent discussion of safe-lazy-io vs. iteratee with interest. The safe-lazy-io package seems much easier to understand than iteratee, but it doesn't compile on GHC 6.12 (and I haven't had any luck in figuring out how to update it myself). The Hackage build log shows the same build result I'm getting [1]; is there any chance of it getting updated to work on 6.12? [1] http://hackage.haskell.org/packages/archive/safe-lazy-io/0.1/logs/failure/g…

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Re: [Haskell-cafe] safe lazy IO or Iteratee?
by John Lato 06 Feb '10

06 Feb '10

> Subject: Re: [Haskell-cafe] safe lazy IO or Iteratee? > > Downside: iteratees are very hard to understand. I wrote a > decently-sized article about them trying to figure out how to make > them useful, and some comments in one of Oleg's implementations > suggest that the "iteratee" package is subtly wrong. Oleg has written > at least three versions (non-monadic, monadic, monadic CPS) and I've > no idea why or whether their differences are important. Even dons says > he didn't understand them until after writing his own iteratee-based > IO layer. More significant than, and orthogonal to, the differences between non-monadic and monadic are the two primary implementations Oleg has written. They are[1]: Design 1: newtype Iteratee el m a = Iteratee{runIter:: Stream el -> m (IterV el m a)} data IterV el m a = IE_done a (Stream el) | IE_cont (Iteratee el m a) (Maybe ErrMsg) Design 2: newtype Iteratee el m a = Iteratee{runIter:: m (IterV el m a)} data IterV el m a = IE_done a (Stream el) | IE_cont (Stream el -> Iteratee el m a) (Maybe ErrMsg With the first design, it's impossible to get the state of an iteratee without feeding it a chunk. There are other consequences too. The second design seems to require some specialized combinators, that is (>>==) and ($$), which are not required for the first version. Neither situation is ideal. The CPS version appears to remedy both flaws, but at the expense of introducing CPS at a low level (this can be hidden from the end user in many cases). I already think of iteratees as holding continuations, so to me the so-called "CPS version" is to me a double CPS. Both designs appear to offer similar performance in aggregate, although there are differences for particular functions. I haven't yet had a chance to test the performance of the CPS variant, although Oleg has indicated he expects it will be higher. The monadic/non-monadic issue is related. Non-monadic iteratees are iteratees that can't perform monadic effects when they're running (although they can still be fed from a monadic enumerator). Essentially it's the difference between "fold" and "foldM". They are simpler and more efficient because of this, but also much less powerful. Any iteratee design can support both non-monadic and monadic, but *I* don't want to support both. At least, I don't want to have double modules for everything for nearly identical functions, and polymorphic code that can handle non-monadic and monadic iteratees is non-trivial[2]. Much of my recent work has been in the consequences of these various design considerations for the next version of the iteratee library. Currently undecided, although I'm leaning towards CPS. It seems to solve a lot of problems, and the implementation details are generally cleaner too. Cheers, John [1] Both taken from http://okmij.org/ftp/Haskell/Iteratee/IterateeM.hs. Design 1 is commented out on that page. [2] At least for me. Maybe others can provide a better solution.

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Simulation of interconnect network
by Roger King 06 Feb '10

06 Feb '10

I am building a simulator for an interconnect network for a multiprocessor computer. I would like to develop it in Haskell as an opportunity to learn Haskell. The network will have a number of routers with input ports and output ports and crossbars between them. I would like to simulate the protocol. This would be an event driven simulator. It would be at a high level, leaving out many details. I would like it to be fast and be able to run it on several processors. I would like to know if you have any advice. Has anyone done this before? Are there any discrete event simulators written in Haskell? R

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a beginner question: decorate-op-undecorate
by Aran Donohue 06 Feb '10

06 Feb '10

Hi Haskell-Cafe, Consider a data type such as data Binding = Binding Var (Either Value [Value]) representing a variable bound either to a fixed value or that has a list of possible values. I'd like to perform an operation on say, the fixed-value members of a list of bindings. Data.Either has "partitionEithers"---I'd essentially like to use partitionEithers, but in a way that it "peeks" into the value field of the binding. For the sake of argument, let's say I can't or can't modify Binding to move the Either to the outside. What would be an idiomatic Haskell way to accomplish this? Currently I've got "liftedPartitionEithers :: [a] -> (a -> Either b c) -> ([a], [a])" which is my own version of partitionEithers that calls a selector first. Another option would be to map each Binding to a new datatype that has the Either on the outside, use partitionEithers, and map back. Thanks, Aran

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Re: [Haskell-cafe] Re: programmatic DB interface?
by Magnus Therning 06 Feb '10

06 Feb '10

On Fri, Feb 5, 2010 at 10:38, Johannes Waldmann <waldmann(a)imn.htwk-leipzig.de> wrote: > >> As I posted earlier, using 'T' to separate date and time is correct >> ISO8601 according to wikipedia[1]. > > So you're saying "2009-12-20 16:55:57.297" > (returned by the psql server) is wrong, > and it should instead send "2009-12-20T16:55:57.297Z" ? Well, it isn't necessarily *wrong* ;-) "2009-12-20 16:55:57.297" is made up of two pieces, one for date and one for time. "2009-12-20T16:55:57.297Z" is made up of one piece, which specifies both date and time. This is why I believe that the function ISO8601DateFormat is correct when it provides a format for a combined date and time. > What would be the query to obtain this format? > > I don't see this mentioned anyplace on > http://www.postgresql.org/docs/8.3/interactive/functions-datetime.html Neither do I, I don't find anything relating to ISO8601 dates (they seem more concerned with ISO DOW and year). > Perhaps we take this back to haskell-cafe (or libraries?) as it might be > of general interest. Sure, why not. It was most likely a mistake on my side to drop haskell-cafe in the first place. /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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Trapping getChar before echo
by Mark Spezzano 05 Feb '10

05 Feb '10

Hi, Is there any way of trapping keystrokes in Haskell, modifying them, and then echoing? Basically I want to give the user a prompt like: > and then have whatever they type appear in UPPERCASE regardless of whether caps lock was on or not. By default Haskell seems to echo characters in whatever case they were typed. I want to sneak in a toUpper in before the Chars get echoed. Thanks Mark Spezzano

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Generating repeatable arbitrary values with QuickCheck 2
by Sean Leather 05 Feb '10

05 Feb '10

I would like to generate an arbitrary (large) value to benchmark the performance of constructing that value with isomorphic types. It seems like QuickCheck might be useful in this regards. Has anyone done something similar? In versions 1.*, there was a generate function: generate :: Int -> StdGen -> Gen a -> a > generate n rnd (Gen m) = m size rnd' > where (size, rnd') = randomR (0, n) rnd > That seems to have disappeared in versions 2.*, and I didn't find a clear replacement. I came up with using the destructor for Gen: unGen :: Gen a -> StdGen -> Int -> a > The function generate seems to have a little something extra, though I'm not sure if it's necessary. Is this true, or should I write an equivalent generate function? As an aside, it would be nice to have a generate function in the library, even if it is only a wrapper for unGen. In the end, I would write something like the following: unGen arbitrary (mkStdGen 11) 5 :: [Int] > This produces, for example, [5,1,-2,-4,2]. I also want to generate the same value for a type isomorphic to [Int]. unGen arbitrary (mkStdGen 11) 5 :: List Int > Unfortunately, this produces Cons 4 (Cons 3 (Cons (-2) (Cons 0 (Cons (-1) Nil)))): same length but different values. The Arbitrary instances are the same. I had similar results with generate from QC 1. Any suggestions on how to do this? With another library perhaps? Thanks, Sean

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online Real World Haskell - tool used?
by Sophie 05 Feb '10

05 Feb '10

Not a Haskell question, pardon me. I believe the online Real World Haskell book with commenting system was generated with an open-source tool (written by one of the authors?). Any idea where I might find this? [I think I had gotten the link long ago, lost it]. I am also curious what authoring tools were used to write the book. Thanks! Sophie

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could we get a Data instance for Data.Text.Text?
by Jeremy Shaw 05 Feb '10

05 Feb '10

Hello, Would it be possible to get a Data instance for Data.Text.Text? This would allow us to create a Serialize instance of Text for use with happstack -- which would be extremely useful. We (at seereason) are currently using this patch: http://src.seereason.com/haskell-text-debian/debian/patches/add_Data_instan… which basically adds: +textType = mkStringType "Data.Text" + +instance Data Text where + toConstr x = mkStringConstr textType (unpack x) + gunfold _k z c = case constrRep c of + (CharConstr x) -> z (pack [x]) + _ -> error "gunfold for Data.Text" + dataTypeOf _ = textType + This particular implementation avoids exposing the internals of the Data.Text type by casting it to a String in toConstr and gunfold. That is similar to how Data is implemented for some numeric types. However, the space usage of casting in Float to a Double is far less than casting a Text to a String, so maybe that is not a good idea? Alternatively, Data.ByteString just does 'deriving Data'. However, bytestring also exports Data.ByteString.Internal, wheres Data.Text.Internal is not exported. Any thoughts? I would like to get this handled upstream so that all happstack users can benefit from it. - jeremy

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Stack ADT?
by michael rice 05 Feb '10

05 Feb '10

Can't find a Stack datatype on Hoogle? Where should I look? Michael

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programmatic DB interface?
by Johannes Waldmann 05 Feb '10

05 Feb '10

Hello. What is currently the recommended higher level data base interface for Haskell? I want to construct queries in a programmatic/algebraic way, and I want typed queries and results. - I guess I want LINQ. The examples in http://book.realworldhaskell.org/read/using-databases.html seem to construct queries as strings. That's horrible! Looks exactly like PHP! Well, no, but it's a lower abstraction level than that I want to work at. I'm sure I'm just missing some package/framework here. Would takusen help? Where's its home page? this does not work: http://darcs.haskell.org/takusen/ Best, Johannes.

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multMM :: Matrix -> Matrix -> Matrix --multiplies two matrices question (Homework)
by 조광래 05 Feb '10

05 Feb '10

hi i was trying to solve it but All i got is type Matrix=[[Double]] multMM :: Matrix -> Matrix -> Matrix --multiplies two matrices multMM m t = [[sum (zipWith (*) (head m)(a)) ] ]where a = [head a | a<- t] Main> multMM [[2,1,-6],[1,-3,2]] [[1,0,-3],[0,4,20],[-2,1,1]] [[14.0]] from this i could get only the first multiplication 14 that is by 2*1 + 1*0 + -6*-2 = 14 how can i make this go throw all rows and colums to get multiple of matrixs? Thank you~

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Dutch HUG: meeting Monday 8th in Utrecht
by Tom Lokhorst 05 Feb '10

05 Feb '10

I'd like to invite functional programmers in The Netherlands to the Dutch Haskell User Group [1, 2] meeting next Monday at 19:00. Two talks are scheduled: - Sebastiaan Visser and Tom Lokhorst will give a talk entitled "AwesomePrelude, Liberating Haskell from data types!" - Martijn van Steenbergen will talk about serializing monadic computations. In particular, he will describe a monadic EDSL to describe compass-and-straightedge constructions and how to serialize and deserialize such values. The meeting will be in the Booth Hall [3] of the Utrecht University Library. There's free car parking [4] at the library, and it's easily accessible by bike and bus [5]. After the talks we will move to The Basket [6] for drinks. See you next Monday! - Tom Lokhorst [1]: http://www.haskell.org/haskellwiki/Dutch_HUG [2]: http://groups.google.com/group/dutch-hug [3]: http://www.uu.nl/EN/library/contact/university_library/zaalverhuur/Pages/de… [4]: http://www.uu.nl/EN/library/contact/university_library/Parkeren/Pages/defau… [5]: http://www.uu.nl/EN/library/contact/university_library/plattegrondenrou/Pag… [6]: http://utrecht.thebasket.nl/

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