July 2012 - Haskell-Cafe

Cabal error installing Yesod and nehe-tuts on Haskell Platform 2012.2.0.0
by Eric Macaulay 03 Jul '12

03 Jul '12

Hello all! I trust tried to install yesod using "cabal install yesod-platform" but the installation aborted with the following error: cabal: Error: some packages failed to install: authenticate-1.2.1.1 depends on zlib-conduit-0.4.0.2 which failed to install. http-conduit-1.4.1.10 depends on zlib-conduit-0.4.0.2 which failed to install. wai-extra-1.2.0.5 depends on zlib-conduit-0.4.0.2 which failed to install. xss-sanitize-0.3.2 failed while unpacking the package. The exception was: user error (truncated tar archive) yesod-1.0.1.6 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-auth-1.0.2.1 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-core-1.0.1.2 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-default-1.0.1.1 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-form-1.0.0.4 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-json-1.0.0.1 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-persistent-1.0.0.1 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-platform-1.0.5 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-routes-1.0.1.2 failed while unpacking the package. The exception was: user error (truncated tar archive) yesod-static-1.0.0.3 depends on zlib-conduit-0.4.0.2 which failed to install. zlib-conduit-0.4.0.2 failed while unpacking the package. The exception was: user error (truncated tar archive) Any idea what's wrong?

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how to check thunk
by Kazu Yamamoto 03 Jul '12

03 Jul '12

Hello, Are there any ways to see if a value is a thunk or memorized? I would like to have a function like: isThunk :: a -> IO Bool I'm implementing some data structures described in "Purely Functional Data Structures" and want to check my implementation is correct. E.g. https://gist.github.com/2987425 Thanks. --Kazu

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Re: [Haskell-cafe] Data.Bitmap, readBitmap exception
by Balazs Komuves 03 Jul '12

03 Jul '12

Hello, Author of Data.Bitmap here. The issue is caused by the fact that Data.Bitmap does *not* support bmp files at all. This should be clear from the documentation. The functions you tried to used use their own file format, which a really very simple format; they are only added for simple experimentation. (again, this is explicit in the docs). The same documentation points to the stb-image [1] package, which supports loading of some common formats (png, jpg, bmp). In general, I suggest that with package-specific question, unless it is a widely used package, mail the maintainer directly; you have better chance to get an answer that way. Regards, Balazs [1] http://hackage.haskell.org/package/stb-image Hello, > > I am using bitmap-0.2 [1] to load a Bitmap file [2]. However, the > following sample GHCi session fails for me. > > > Prelude Data.Bitmap Data.Bitmap.Pure.File> readBitmap "b.bmp" :: IO > (Bitmap Word8) > > *** Exception: Data/Bitmap/Internal.hs:(63,17)-(67,15): Non-exhaustive > patterns in case > > I downloaded the sources of bitmap-0.2 from Hackage and modified the > relevant function by appending the last line. > > > decodeCType :: CInt -> PixelComponentType > > decodeCType k = case k of > > 1 -> PctWord8 > > 2 -> PctWord16 > > 3 -> PctWord32 > > 4 -> PctFloat > > _ -> error $ "decodeCType: unexpected integer (" ++ show k ++ ")" > > With this modification the same GHCi session given above results in > the following error. > > > Prelude Data.Bitmap Data.Bitmap.Pure.File> readBitmap "b.bmp" :: IO > (Bitmap Word8) > > *** Exception: decodeCType: unexpected integer (7077888) > > I don't have any idea where 7077888 comes from. Am I doing something > wrong? Is my bitmap corrupted in some way? The number 7077888 has no > apparent significance except that it is a perfect cube [3]. So I guess > this is not a problem with endianness. > > In fact as far as I can tell `decodeCType` is only called on > `(PixelComponent t => c_type t)` and `c_type t` only yields integer > values in the (inclusive) range of 1 to 4. That is if I am not missing > some orphaned instance. > > Could anyone please enlighten me what is happening and if I am doing > something wrong? > > In case this is relevant: I am using bitmap-0.2 from Hackage with GHC > 7.4.1 on a 3.2.20 kernel i686 architecture Linux. > > Regards, > Alexander Foremny > > [1] http://hackage.haskell.org/package/bitmap > [2] https://www.dropbox.com/s/pyutvni9vx6f6mo/b.bmp > [3] http://www.wolframalpha.com/input/?i=7077888 >

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ICFP 2012: Call for participation
by Wouter Swierstra 02 Jul '12

02 Jul '12

===================================================================== Call for Participation The 17th ACM SIGPLAN International Conference on Functional Programming (ICFP 2012) and affiliated events http://www.icfpconference.org/icfp2012/ Copenhagen, Denmark, Sep 9-15, 2012 ===================================================================== ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. The conference covers the entire spectrum of work, from practice to theory, including its peripheries. A full week dedicated to functional programming: 1 conference, 1 symposium, 9 workshops, 8 tutorials, programming contest results, student research contest * Accepted Papers: http://www.icfpconference.org/icfp2012/accepted.html * Local arrangements (including travel and accommodation): http://icfpconference.org/icfp2012/local.html Conference hotel reservation cutoff: July 9, 2012 * Registration is available via: http://icfpconference.org/icfp2012/ Electronic registration will open shortly. * Follow @icfp_conference on twitter for the latest news: http://twitter.com/#!/icfp_conference There are several events affiliated with ICFP: September 9 Workshop on Cross-paradigm Language Design and Implementation Workshop on Generic Programming Workshop on Higher-Order Programming with Effects Workshop on Logical Frameworks and Meta-languages: Theory and Practice September 10-12 ICFP - main conference September 13 Commercial Users of Functional Programming – Day 1 (CUFP Tutorials) Haskell Symposium Workshop on ML September 14 Commercial Users of Functional Programming – Day 2 (CUFP Tutorials) Erlang Workshop Haskell Implementors' Workshop OCaml Users and Developers Workshop September 15 Commercial Users of Functional Programming – Day 3 (CUFP Talks) Workshop on Functional High-Performance Computing Tutorial on Compiler Construction in Haskell Tutorial on the Grammatical Framework Conference organizers: * General Chair: Peter Thiemann, University of Freiburg * Program Chair: Robby Findler, Northwestern University * Local Arrangements Chair: Fritz Henglein, University of Copenhagen * Industrial Relations Chair: Andy Adams-Moran, Galois * Workshop Co-Chairs: Patrik Jansson, Chalmers University of Technology Gabriele Keller, University of New South Wales * Programming Contest Chair: Edwin Brady, University of St. Andrews Kevin Hammond, University of St. Andrews * Publicity Chair: Wouter Swierstra, Utrecht University * Video Chair: Malcolm Wallace, Standard Chartered Bank * Student Research Competition Chair: Doaitse Swierstra, Utrecht University =====================================================================

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not enough fusion?
by Johannes Waldmann 02 Jul '12

02 Jul '12

Dear all, while doing some benchmarking (*) I noticed that function s1 is considerably faster than s2 (but I wanted s2 because it looks more natural) (for n = 10000, s1 takes 20 s, s2 takes 13 s; compiled by ghc-7.4.2 -O2) s1 :: Int -> Int s1 n = sum $ do x <- [ 0 .. n-1 ] return $ sum $ do y <- [ 0 .. n-1 ] return $ gcd x y s2 :: Int -> Int s2 n = sum $ do x <- [ 0 .. n-1 ] y <- [ 0 .. n-1 ] return $ gcd x y I was expecting that in both programs, all lists will be fused away (are they?) so the code generator essentially can produce straightforward assembly code (no allocations, no closures, etc.) For reference, I also wrote the equivalent imperative program (two nested loops, one accumulator for the sum) (with the straightforward recursive gcd) and runtimes are (for same input as above) C/gcc: 7.3 s , Java: 7.7 s, C#/Mono: 8.7 s So, they sort of agree with each other, but disagree with ghc. Where does the factor 2 come from? Lists? Laziness? Does ghc turn the tail recursion (in gcd) into a loop? (gcc does). (I am looking at -ddump-asm but can't quite see through it.) (*) benchmarking to show that today's compilers are clever enough such that the choice of paradigm/language does not really matter for this kind of low-level programming.

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package to expand TH macros?
by Eric 02 Jul '12

02 Jul '12

I seem to remember finding a package a few days ago that would take Haskell source with TH, then run and expand the TH macros in-place to produce equivalent, TH-free Haskell source. I just went through the Hackage package list and didn't find anything like that. Did I imagine it? Or can anyone remind me its name? thanks, --Eric

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ANN: wizards-1.0: A High level, generic library for interrogative user interfaces
by Liam O'Connor 02 Jul '12

02 Jul '12

Hi all, wizards is an Haskell library designed for the quick and painless development of interrogative programs, which revolve around a "dialogue" with the user, who is asked a series of questions in a sequence much like an installation wizard. Everything from interactive system scripts, to installation wizards, to full-blown shells can be implemented with the support of wizards. It is developed transparently on top of a free monad (see Swierstra's excellent paper on this topic at http://www.cs.ru.nl/~W.Swierstra/Publications/DataTypesALaCarte.pdf) which separates out the semantics of the program from the wizards interface. A variety of backends exist, including a full featured backend for Haskeline, a debug-friendly simpler implementation in terms of System.IO primitives, and a completely pure implementation modelled as a function from an input string to output. It is also possible to write your own backends, or extend the existing back-ends with new features. While both built-in IO backends operate on a console, there is no reason why wizards cannot also be used for making GUI wizard interfaces. The library is highly extensible - back-ends can be written or extended with the type system helpfully tracking what features are supported by which back-ends. Installation instructions and some educational examples are at the github page: https://github.com/liamoc/wizards Information on how to write backends or extend backends, as well as structured API documentation is available on Hackage: http://hackage.haskell.org/package/wizards (Or, you can just run cabal haddock to generate the documentation from the source). Regards, Liam O'Connor

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