- Haskell-Cafe - Haskell.org

Martin Odersky on "What's wrong with Monads"
by Jonathan Geddes 05 Jul '12

05 Jul '12

Cafe, I was watching a panel on languages[0] recently and Martin Odersky (the creator of Scala) said something about Monads: "What's wrong with Monads is that if you go into a Monad you have to change your whole syntax from scratch. Every single line of your program changes if you get it in or out of a Monad. They're not polymorphic so it's really the old days of Pascal. A monomorphic type system that says 'well that's all I do' ... there's no way to abstract over things. " [0, 53:45] Thoughts? --J Arthur [0] - http://css.dzone.com/articles/you-can-write-large-programs

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Using "syntactic" to implement the lambda calculus
by Alex Rozenshteyn 05 Jul '12

05 Jul '12

(This is intended as a simplification of the problem I actually need to solve.) I'm trying to implement the lambda calculus (with CBV evaluation) using the "syntactic" package, in such a way that a simple extension is also simple to implement. I am stuck on the fact that it seems that the Value type needs to be parametrized over the Expr type and I can't seem to figure out how to do it. I've read this post<http://www.haskell.org/pipermail/haskell-cafe/2011-May/091770.html>from the archives, but I still seem to be missing something. Does anyone have any suggestions? > -- Lambda calculus > type Ident = String > data Value = VLam Ident Expr > data Expr = Var Ident | Lam Ident Expr | App Expr Expr > eval :: Expr -> Value > eval e = > case e of > Var _ -> error "not closed" > Lam i e' -> VLam i e' > App e1 e2 -> > case eval e1 of > Lam i e' -> subst e' (eval e2) i > _ -> error "illegal application" > -- Lambda calculus with integers and addition > data Value = VLam Ident Expr | VInt Integer > data Expr = Var Ident | Lam Ident Expr | App Expr Expr | Plus Expr Expr > eval e = > case e of > ... > Plus e1 e2 -> > case (eval e1, eval e2) of > (VInt x1, VInt x2) -> VInt $ x1 + x2 > _ -> error "illegal addition" -- Alex R

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translation between two flavors of lexically-scoped type variables
by Kangyuan Niu 05 Jul '12

05 Jul '12

The paper "Lexically scoped type variables" by Simon Peyton Jones and Mark Shields describes two ways to give type variables lexical scoping. They state that one of the advantages of the GHC-style type-sharing approach over the SML-style type-lambda approach is that the former allows for existential quantification in addition to universal quantification. As an example, they give this code: data Ap = forall a. Ap [a] ([a] -> Int) The constructor `Ap` has the type: Ap :: forall a. [a] -> ([a] -> Int) -> Ap And one can write a function: revap :: Ap -> Int revap (Ap (xs :: [a]) f) = f ys where ys :: [a] ys = reverse xs with the annotations on `xs` and `ys` being existential instead of universal. But I'm a little confused about *why* type-lambdas don't allow this. Aren't both Haskell and SML translatable into System F, from which type-lambda is directly taken? What does the translation for the above code even look like? Why isn't it possible to write something like: fun 'a revap (Ap (xs : 'a list) f) = f ys where ys :: 'a list ys = reverse xs in SML? -Kangyuan Niu

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existential types and cast
by Corentin Dupont 04 Jul '12

04 Jul '12

Hi all, I read somewhere (here: http://stackoverflow.com/questions/2300275/how-to-unpack-a-haskell-existent…) that it's bad to try to unbox an existential type using a cast. OK, but without I really can't figure out how to do what I want: *data NewPlayer = NewPlayer deriving (Typeable, Eq) data NewRule = NewRule deriving (Typeable, Eq) class (Eq e, Typeable e) => Event e where data EventData e instance Event NewPlayer where data EventData NewPlayer = P Int instance Event NewRule where data EventData NewRule = R Int instance Typeable1 EventData where typeOf1 _ = mkTyConApp (mkTyCon "EventData") [] data EventHandler = forall e . (Event e) => EH e (EventData e -> IO ()) addEvent :: (Event e) => e -> (EventData e -> IO ()) -> [EventHandler] -> [EventHandler] addEvent e h ehs = (EH e h):ehs triggerEvent :: (Event e) => e -> (EventData e) -> [EventHandler] -> IO () triggerEvent e d ehs = do let r = find (\(EH myEvent _) -> cast e == Just myEvent) ehs case r of Nothing -> return () Just (EH _ h) -> case cast h of Just castedH -> castedH d Nothing -> return ()* How to remove the casts from triggerEvent? All that I want is to apply the handler found on the data passed in parameter. I tried to add a function apply in the class, without success: *apply :: (EventData e -> IO ()) -> (EventData e) -> IO () apply = ($)* Thanks! Corentin

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haskell.org website ?
by Qi Qi 04 Jul '12

04 Jul '12

Hi, Haskell.org website seems dropping offline at this moment. Anyone know what's happening to it? Thanks, Qi

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Re: [Haskell-cafe] Martin Odersky on "What's wrong with Monads"
by Facundo Domínguez 04 Jul '12

04 Jul '12

> That precedent supports the view that e.g. a command-line flag > shouldn't affect behavior without the type reflecting it, e.g. by > doing IO, but the de facto use of the unsafe IO trick means not > everyone agrees. For those interested, here's [1] a case where treating command line arguments as top level constants went wrong. Look specifically at the section named "Crime Doesn't Pay". Best, Facundo [1] http://www.aosabook.org/en/ghc.html > Date: Tue, 3 Jul 2012 17:49:48 -0400 > From: Alvaro Gutierrez <radical(a)google.com> > Subject: Re: [Haskell-cafe] Martin Odersky on "What's wrong with > Monads" > To: Dominique Devriese <dominique.devriese(a)cs.kuleuven.be> > Cc: haskell-cafe(a)haskell.org > Message-ID: > <CAC6k+tpuiDrtaC_cCFA9ewawDpG1adaht96rgMc_S5wtVJwLVw(a)mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > > On Thu, Jun 28, 2012 at 2:53 PM, Dominique Devriese > <dominique.devriese(a)cs.kuleuven.be> wrote: >> 2012/6/27 Tillmann Rendel <rendel(a)informatik.uni-marburg.de>: >>> How would you implement this requirement in Haskell without changing the >>> line "amount (Leaf x) = x"? >> >> I may be missing the point here, but having worked on large code bases >> with a wide variety contributors before, I find it very advantageous >> that programmers are prevented from writing an amount function whose >> behaviour depends on command line arguments without at least an >> indication in the type. The fact that the function can not perform >> stuff like that is precisely the guarantee that the Haskell type gives >> me... > > I don't think there's an answer that's uniformly right; it depends on > whether you think of the input to the program, e.g. the environment, > command-line arguments, etc. as 'constant' and in some sense, pure. > The latter are constant in the sense that they never change, but they > are not fixed at compile-time. Other languages effectively treat them > as pure (by passing them directly to main), whereas Haskell chooses > not to, which is probably the reason why getArgs has IO in its type > (something that seems unintuitive at first.) > > That precedent supports the view that e.g. a command-line flag > shouldn't affect behavior without the type reflecting it, e.g. by > doing IO, but the de facto use of the unsafe IO trick means not > everyone agrees. >

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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 02 Jul '12

02 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 02 Jul '12

02 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 01 Jul '12

01 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 01 Jul '12

01 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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ANN: prof2pretty-0.1.0.0
by Claude Heiland-Allen 30 Jun '12

30 Jun '12

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hi all, I wanted to try visual-prof[1] but found it was broken by ghc RTS changes[2]. I ended up writing prof2pretty[3]: http://hackage.haskell.org/package/prof2pretty - --8<-- sccpragmabomb adds SCC pragmas encoding source location. prof2pretty extracts those SCCs from the profiling output to annotate the sources using HTML + CSS + JavaScript. - --8<-- [1] http://hackage.haskell.org/package/visual-prof [2] http://hackage.haskell.org/trac/ghc/ticket/4094 [3] prof2pretty started as a fork of visual-prof but soon there was none of the original code left, though the essential concept remains the same Thanks, Claude -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iQEcBAEBAgAGBQJP7wPyAAoJEHZDo4jueIiW7WsH/3Lmk2k7o+2i5u+Vxn+EZXRX 5XRDXZ8mLm1NBb/OSfRFr6RIyZxwou8JeavZJuhqOBL38KAlppf89OnaJyxUB+ap Cp03ui/1LbihqIbMxupNH03pQwkNR7b2MV2S07c5TOvOJHsksE53zEin/EHQQ6jO 6QxrQdNibjSYlXDNtZCzEj0HWVpSBD4gXdPFv9nFS+RKPjAkgC4gmY4yItYDep60 qboOrxKVhYJdqTmhl+OXhe7LAAJ9Nc3xiheGLbeyTfH2owAnT8X4zOdtssOsPPdm AQBgxKa8p5KckZCKS7Elj8pap9uzUHGc8KuINpjx40b7envHP44t65xngEq1TaU= =znB1 -----END PGP SIGNATURE-----

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Maintainer of pngload listening?
by Daniel Trstenjak 30 Jun '12

30 Jun '12

Hi, Marko Lauronens email address from the package is invalid, so I'm trying to reach him this way. pngload doesn't compile with ghc >= 7.2 because of it's dependency to base and haskell98. Greetings, Daniel

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Slowdown with GHC when using multiple CPUs
by David Powell 29 Jun '12

29 Jun '12

I recently ran into a problem writing a program that I hoped I could speed up by running over multiple CPUs. I want non-haskell users to be able to run the tool and take advantage of multiple CPUs. *But* there is a serious slowdown when the RTS is run with -N and some of the CPUs are already busy. This is already mentioned in the GHC docs, but the problem I experienced was more serious than I expected causing a slowdown of around 2x compared to running with a single CPU. I reproduced the problem with the following code from the haskell wiki: {-# LANGUAGE BangPatterns #-} import Data.Digest.Pure.MD5 import qualified Data.ByteString.Lazy as L import System.Environment import Control.Concurrent import Control.Monad (replicateM_) main = do files <- getArgs str <- newEmptyMVar mapM_ (forkIO . hashAndPrint str) files printNrResults (length files) str printNrResults i var = replicateM_ i (takeMVar var >>= putStrLn) hashAndPrint str f = do bs <- L.readFile f let !h = show $ md5 bs putMVar str (f ++ ": " ++ h) When run on 4 idle CPU cores, I get the following wall clock times: ./run +RTS -N1 : 20.4 sec ./run +RTS -N2 : 11.0 sec ./run +RTS -N4 : 6.7 sec When run on the same 4 core machine, but with 2 cores already busy: ./run +RTS -N1 : 23.5 sec ./run +RTS -N2 : 14.1 sec ./run +RTS -N4 : 57.8 sec <---- Blowout... This is quite a problem in practice when running on a shared server. Is there anything that can be done to address this? (I wrote up a few more details here: http://thunking.drp.id.au/2012/06/slowdown-with-ghc-when-using-multiple.html ) Thanks, -- David Powell

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Help requested: naming things in conduit
by Michael Snoyman 29 Jun '12

29 Jun '12

Hi all, I'm just about ready to make the 0.5 release of conduit. And as usual, I'm running up against the hardest thing in programming: naming things. Here's the crux of the matter: in older versions of conduit, functions would have a type signature of Source, Sink, or Conduit. For example: sourceFile :: MonadResource m => FilePath -> Source m ByteString I think most people can guess at what this function does: it produces a stream of ByteStrings, which are read from the given file. Now the trick: Source (and Sink and Conduit) are all type synonyms wrapping around the same type, Pipe. Ideally, we'd like to be able to reuse functions like sourceFile in other contexts, such as producing a Conduit that calls sourceFile[1]. However, the type synonym Source over-specifies some of the type parameters to Pipe, and therefore `sourceFile` can't be used directly to create a Conduit[2]. To get around this whole problem, I've added a number of type synonyms with rank-2 types, that don't over-specify. You can see the type synonyms here[3], and more explanation of the problem here[4]. So my question is: can anyone come up with better names for these synonyms? Just to summarize here: * All of the generalized types start with a G, e.g., Source becomes GSource. * For Sinks and Conduits, if leftovers are generated, there's an L after the G (e.g., GLSink). * For Sinks and Conduits which consume all of their input and then return the upstream result, we tack on an Inf for Infinite (e.g., GInfConduit, GLInfSink). I think these names are relatively descriptive, and certain `GSink ByteString m Int` is easier to follow than `Pipe l ByteString o u m Int`, but I was wondering if anyone had some better recommendations. Michael [1] For example, maybe we want to produce `conduitFiles :: MonadResource m => Conduit FilePath m ByteString` [2] This problem exists to a smaller extent in conduit 0.4. This is the purpose of the sinkToPipe function. [3] https://github.com/snoyberg/conduit/blob/52d7bc0b551b877de92be4c87f933e3ffb… [4] https://github.com/snoyberg/conduit/blob/a853141d7b9eed047c7cc790979f73a346…

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crash in hs_init_ghc
by A.M. 29 Jun '12

29 Jun '12

Hello, I am trying to load a Haskell module in a C program as a dynamically-loaded plugin via FFI. I am able to compile and link successfully, but I find the following crash. #0 0x00007f16147da9e7 in ?? () from /usr/lib/ghc/libHSrts_debug-ghc7.4.1.so #1 0x00007f16147e0970 in hs_init_ghc () from /usr/lib/ghc/libHSrts_debug-ghc7.4.1.so #2 0x00007f16147e0795 in hs_init () from /usr/lib/ghc/libHSrts_debug-ghc7.4.1.so #3 0x00007f1614a44ece in plhaskell_plmain (fcinfo=<optimized out>) at plhaskellc.c:26 #4 0x0000000000565e1a in ExecMakeFunctionResult () #5 0x00000000005682f2 in ExecProject () #6 0x000000000057849a in ExecResult () #7 0x0000000000561648 in ExecProcNode () #8 0x000000000055ea5a in standard_ExecutorRun () #9 0x0000000000628577 in PortalRunSelect () #10 0x0000000000629900 in PortalRun () #11 0x0000000000625bec in PostgresMain () #12 0x00000000005eb429 in ServerLoop () #13 0x00000000005ebd3c in PostmasterMain () #14 0x0000000000453d20 in main () In the trace above, one can see that I am linking against the debugging Haskell runtime. hs_init_ghc is called with dual NULL arguments. Is that cause for concern? I noticed that RtsStartup.c does some questionable stuff in a module context right away, such as calling "setlocale". Are there other things I need to watch out for? Do I need to build ghc from source in order to debug this further? Any advice would be appreciated. This is on Ubuntu 12.04. Cheers, M

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Network.Curl curiosity
by Michael Orlitzky 29 Jun '12

29 Jun '12

I've been fighting with this silly bug: I have two functions using curl (Network.Curl) for logging in via POST and downloading pages via GET. They do the usual boring stuff; cookies go in a text file whose path is passed in to both functions. They work great individually. But, I'm calling them sequentially: cj <- make_cookie_jar li_result <- log_in cj username password html <- get_page (Just cj) my_article The symptom that I noticed was that the call to get_page wasn't using the cookies. I looked in the cookie file -- they're all there. And when I call get_page from ghci (with the same cookie file), it works. It turns out that between the calls to log_in and get_page, the cookie file is empty. The stupidest possible solution that works is, cj <- make_cookie_jar li_result <- log_in cj username password putStrLn "Waiting one second..." thread_sleep 1 html <- get_page (Just cj) my_article And this program is not at all time sensitive, so that actually works for me. But now I'm curious. Does anyone have an idea what sort of sorcery is going on? Here's the rest of log_in if it's useful. log_in :: FilePath -> String -> String -> IO Bool log_in cookie_jar username password = withCurlDo $ do -- Create a curl instance. curl <- initialize -- Perform the request, and get back a CurlResponse object. -- The cast is needed to specify how we would like our headers -- and body returned (Strings). resp <- do_curl_ curl login_url curl_opts :: IO CurlResponse -- Pull out the response code as a CurlCode. let code = respCurlCode resp case code of CurlOK -> return True error_code -> do hPutStrLn stderr ("HTTP Error: " ++ (show error_code)) -- If an error occurred, we want to dump as much information as -- possible. If this becomes a problem, we can use respGetInfo -- to query the response object for more information return False where post_submit :: String post_submit = submit_field ++ "=Log+In" post_username :: String post_username = username_field ++ "=" ++ username post_password :: String post_password = password_field ++ "=" ++ password post_data :: [String] post_data = [post_username, post_password, post_submit] post_opts :: [CurlOption] post_opts = [ CurlCookieSession True, CurlCookieJar cookie_jar, CurlPost True, CurlPostFields post_data ] curl_opts :: [CurlOption] curl_opts = default_curl_opts ++ post_opts default_curl_opts :: [CurlOption] default_curl_opts = [ -- The Global cache is not thread-friendly. CurlDNSUseGlobalCache False, -- And we don't want to use a DNS cache anyway. CurlDNSCacheTimeout 0, -- Follow redirects. CurlFollowLocation True, -- Give it a little time... CurlTimeout 45, -- For debugging. CurlVerbose True ]

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Re: [Haskell-cafe] Martin Odersky on "What's wrong with Monads"
by Donn Cave 28 Jun '12

28 Jun '12

To be fair, while it's great that Functor et al. are available to help solve problems like that with a Tree data structure, in the present context it mostly shows that a trivial example was used to illustrate the problem. Unless I'm missing how this generic fold functionality solves the more general problem. Quoth Alberto G. Corona, > ... There is no such problem in imperative languages, which work > ever in an hardwired IO monad. Or to look at it from the other direction, the problem with Haskell is that it allows the programmer to write non-IO functions. To the extent that's a real problem, the solution would obviously be to exercise some restraint with the pure code and write more in the IO monad. If you will humor me, we understand that this code is equally satisfactory from a mathematical perspective and there's no crime in it. But does it help with the `flags problem?' Suppose that functions f and g both have IO types; f calls g, and g has optional behavior depending on some IO value computed previously, the flag. The way I understand it, I can accomplish this without passing the flag around: with an IORef. What's less clear to me is whether Haskell really supports the module level accessible IORef value I need. Also known as "top level mutable state." Is the NOINLINE pragma I've been using with GHC, really a sound and reliable practice that we're reasonably confident will be supported by other compilers? It feels to me like I'm pulling a weird trick, to do something that's trivial in other programming languages. Maybe that's part of the problem, that imperative programming receives something a little short of full support in Haskell, inasmuch as basic features like top level mutable state require relatively arcane incantations. Donn

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ghc-7.4 on CentOS-5.8 ?
by Johannes Waldmann 28 Jun '12

28 Jun '12

Dear all, I need a recent ghc on a not-so-recent (?) CentOS. The ghc binary package (7.2 or 7.4) does not work because of a mismatch in the libc version. ghc-7.0 is working but when I use it to compile 7.4, it breaks with some linker error (relocation R_X86_64_PC32 ...) it also suggests "recompile with -fPIC" but I don't see how. (In this particular case, I absolutely cannot change/update the OS.) Thanks, J.W.

5 5

Fwd: Martin Odersky on "What's wrong with Monads"
by Strake 28 Jun '12

28 Jun '12

On 26/06/2012, Nathan Howell <nathan.d.howell(a)gmail.com> wrote: > On Tue, Jun 26, 2012 at 3:19 PM, Tillmann Rendel > <rendel(a)informatik.uni-marburg.de> wrote: >> All fine so far. Now, consider the following additional requirement: "If >> the >> command-line flag --multiply is set, the function amount computes the >> product instead of the sum." >> >> How would you implement this requirement in Haskell without changing the >> line "amount (Leaf x) = x"? > > One option is to encode the desired behavior at the type level. By > extended the data type slightly and adding a Functor instance, > selecting between a product and a sum can be done using their Monoid > newtypes: ... Better yet, use foldMap: > import Data.Monoid > import Data.Foldable > import System.Environment > data Tree a = Leaf a | Branch (Tree a) (Tree a) > instance Functor Tree where > f `fmap` Leaf x = Leaf (f x) > f `fmap` Branch x y = Branch (fmap f x) (fmap f y) > instance Foldable Tree where > foldMap f (Leaf x) = f x > foldMap f (Branch s t) = foldMap f s <> foldMap f t > main :: IO () > main = do > args <- getArgs > let val :: Tree Int > val = Branch (Leaf 8) (Leaf 18) > let getResult :: Tree Int -> Int > getResult = case args of > ["--multiply"] -> getProduct . foldMap Product > _ -> getSum . foldMap Sum > print . getResult $ val Yet better yet: > {-# LANGUAGE DeriveFunctor, DeriveFoldable #-} > import Data.Monoid; > import Data.Foldable; > import System.Environment > data Tree a = Leaf a | Branch (Tree a) (Tree a) > deriving (Functor, Foldable); > ... (^_^) -- Strake

1 0

Data.Bitmap, readBitmap exception
by Alexander Foremny 28 Jun '12

28 Jun '12

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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Re: [Haskell-cafe] Martin Odersky on "What's wrong with Monads"
by Tillmann Rendel 28 Jun '12

28 Jun '12

Hi Rico, Rico Moorman wrote: >> data Tree = Leaf Integer | Branch (Tree Integer) (Tree Integer) >> >> amount:: Tree -> Integer >> amount (Leaf x) = x >> amount (Branch t1 t2) = amountt1 + amountt2 >> >> [...] additional requirement: "If the command-line flag --multiply is set, >> the function amount computes the product instead of the sum." >> >> How would you implement this requirement in Haskell without changing the >> line "amount (Leaf x) = x"? > The (for me at least) most obvious way to do this would be, to make the > operation to be applied to determine the amount (+ or *) an explicit > parameter in the function's definition. > > data Tree a = Leaf a > | Branch (Tree a) (Tree a) > amount :: (a -> a -> a) -> Tree a -> a > amount fun (Leaf x) = x > amount fun (Branch t1 t2) = amount fun t1 `fun` amount fun t2 I agree: This is the most obvious way, and also a very good way. I would probably do it like this. > Which drawbacks do you see besides increased verbosity? Well, you did change the equation "amount (Leaf x) = x" to "amount fun (Leaf x) = x". In a larger example, this means that you need to change many lines of many functions, just to get the the value of fun from the point where it is known to the point where you need it. > [...] I am wondering which ways of doing this in Haskell you mean. I thought of the following three options, but see also Nathan Howells email for another alternative (that is related to my option (1) below): (1) Implicit parameters: {-# LANGUAGE ImplicitParams #-} data Tree = Leaf Integer | Branch Tree Tree amount :: (?fun :: Integer -> Integer -> Integer) => Tree -> Integer amount (Leaf x) = x amount (Branch t1 t2) = ?fun (amount t1) (amount t2) (2) Lexical Scoping: data Tree = Leaf Integer | Branch Tree Tree amount :: (Integer -> Integer -> Integer) -> Tree -> Integer amount fun = amount where { amount (Leaf x) = x ; amount (Branch t1 t2) = fun (amount t1) (amount t2) } (3) UnsafePerformIO: import System.IO.Unsafe (unsafePerformIO) data Tree = Leaf Integer | Branch Tree Tree amount :: Tree -> Integer amount (Leaf x) = x amount (Branch t1 t2) = fun (amount t1) (amount t2) where fun = unsafePerformIO ... I'm not happy with any of these options. Personally, I would probably go ahead and transform the whole program just to get the value of fun to where it is needed. Nevertheless, having actually done this before, I understand why Martin Odersky doesn't like doing it :) Tillmann

4 3

Long-running request/response protocol server using enumerator/iterator/iterIO/pipes/conduits/...
by Nicolas Trangez 28 Jun '12

28 Jun '12

Hello Cafe, Some time ago I tried to implement a network service using iteratee (or enumerator, can't remember), but gave up in the end. More recently I wanted to create something similar (a similar protocol), but failed again. So I'm looking for some example code or something similar (Google only helped slightly). First of all, I don't care which API/library to use, I guess for my purpose all of enumerator, iteratee, iterIO, pipes, conduits,... are OK, so all feedback is welcome. Here's the catch. Most examples out there implement some server which accepts a single client request, interprets it, creates a response, returns this, and closes the connection (or something alike, think HTTP). The protocol I'd like to implement is different: it's long-running using repeated requests & responses on a single client connection. Basically, a client connects and sends some data to the server (where the length of this data is encoded in the header). Now the server reads & parses this (binary) data, sets up some initial state for this client connection (e.g. opening a file handle), and returns a reply. Now the client can send another request, server parses/interprets it using the connection state, sends a reply, and so on. Might sound easy (and actually it's pretty easy in most other languages I know, including an OCaml implementation), yet I fail to figure out how to get this done using some enumerator-style library. Thanks for any help, I'll most likely write up something if I get things working for future reference. Nicolas

5 7

Re: [Haskell-cafe] Data.Bitmap, readBitmap exception
by Alexander Foremny 28 Jun '12

28 Jun '12

Hello Serge, thank you very much for you response. Unfortunately this does not enlighten me. But maybe this is helpful to someone else who could given an answer. Regards, Alexander Foremny 2012/6/28 Serge Le Huitouze <serge.lehuitouze(a)gmail.com>: > Hi Alexander, > >> >> 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: 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. > > Sorry, no personal experience with Data.Bitmap... > > In case it's a hint, decimal 7077888 is 0x006C0000, and 6C is ascii > code for "l". > > HTH (?). > > --Serge

1 0

Template-haskell loading packages
by Christopher Done 28 Jun '12

28 Jun '12

'Ello, I'm using TH in a big project and whenever TH starts-up in the GHC (6.4.2) compilation process it loads a number of packages: [119 o 119] Compiling Main ( src/Main.hs, dist/build/eudl/eudl-tmp/Main.o ) Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package array-0.4.0.0 ... linking ... done. Loading package strict-0.3.2 ... linking ... done. Loading package split-0.1.4.3 ... linking ... done. Loading package safe-0.3.3 ... linking ... done. Loading package bytestring-0.9.2.1 ... linking ... done. Loading package deepseq-1.3.0.0 ... linking ... done. Loading package containers-0.4.2.1 ... linking ... done. Loading package binary-0.5.1.0 ... linking ... done. Loading package cereal-0.3.5.2 ... linking ... done. Loading package entropy-0.2.1 ... linking ... done. Loading package largeword-1.0.1 ... linking ... done. Loading package tagged-0.4.2.1 ... linking ... done. Loading package crypto-api-0.10.2 ... linking ... done. Loading package pureMD5-2.1.0.3 ... linking ... done. Loading package dlist-0.5 ... linking ... done. Loading package old-locale-1.0.0.4 ... linking ... done. Loading package data-default-0.4.0 ... linking ... done. Loading package pretty-1.1.1.0 ... linking ... done. Loading package template-haskell ... linking ... done. Loading package utf8-string-0.3.7 ... linking ... done. Loading package printf-mauke-3000.0.5.0.1 ... linking ... done. Loading package haskell-lexer-1.0 ... linking ... done. Loading package pretty-show-1.2 ... linking ... done. Loading package parallel-3.2.0.3 ... linking ... done. Loading package transformers-0.2.2.0 ... linking ... done. Loading package mtl-2.0.1.0 ... linking ... done. Loading package text-0.11.2.1 ... linking ... done. Loading package parsec-3.1.3 ... linking ... done. Loading package unix-2.5.1.1 ... linking ... done. Loading package network-2.3.0.14 ... linking ... done. Loading package old-time-1.1.0.0 ... linking ... done. Loading package HTTP-4000.2.3 ... linking ... done. Loading package base64-bytestring-0.1.1.1 ... linking ... done. Loading package blaze-builder-0.3.1.0 ... linking ... done. Loading package blaze-html-0.4.3.4 ... linking ... done. Loading package filepath-1.3.0.0 ... linking ... done. Loading package directory-1.1.0.2 ... linking ... done. Loading package syb-0.3.6.1 ... linking ... done. Loading package hs-bibutils-4.12 ... linking ... done. Loading package json-0.5 ... linking ... done. Loading package pandoc-types-1.9.1 ... linking ... done. Loading package time-1.4 ... linking ... done. Loading package xml-1.3.12 ... linking ... done. Loading package citeproc-hs-0.3.4 ... linking ... done. Loading package extensible-exceptions-0.1.1.4 ... linking ... done. Loading package regex-base-0.93.2 ... linking ... done. Loading package regex-pcre-builtin-0.94.2.1.7.7 ... linking ... done. Loading package highlighting-kate-0.5.1 ... linking ... done. Loading package process-1.1.0.1 ... linking ... done. Loading package random-1.0.1.1 ... linking ... done. Loading package tagsoup-0.12.6 ... linking ... done. Loading package temporary-1.1.2.3 ... linking ... done. Loading package texmath-0.6.0.6 ... linking ... done. Loading package digest-0.0.1.1 ... linking ... done. Loading package zlib-0.5.3.3 ... linking ... done. Loading package zip-archive-0.1.1.8 ... linking ... done. Loading package pandoc-1.9.4.1 ... linking ... done. Loading package convertible-1.0.11.1 ... linking ... done. Loading package HDBC-2.3.1.1 ... linking ... done. Loading package HDBC-postgresql-2.3.2.1 ... linking ... done. Loading package haskelldb-2.1.3 ... linking ... done. Loading package haskelldb-hdbc-2.1.0 ... linking ... done. Loading package haskelldb-hdbc-postgresql-2.1.0 ... linking ... done. Loading package QuickCheck-2.4.2 ... linking ... done. Loading package has-0.5.0.1 ... linking ... done. Loading package feed-0.3.8 ... linking ... done. Loading package MonadCatchIO-mtl-0.3.0.4 ... linking ... done. Loading package xhtml-3000.2.1 ... linking ... done. Loading package cgi-3001.1.8.2 ... linking ... done. Loading package fastcgi-3001.0.2.3 ... linking ... done. Loading package ranges-0.2.4 ... linking ... done. Loading package email-validate-0.2.8 ... linking ... done. Loading package digestive-functors-0.2.0.0 ... linking ... done. Loading package digestive-functors-blaze-0.2.1.0 ... linking ... done. Loading package curl-1.3.7 ... linking ... done. Loading package csv-0.1.2 ... linking ... done. Loading package MonadRandom-0.1.6 ... linking ... done. Loading package HUnit-1.2.4.3 ... linking ... done. Loading package hslogger-1.1.5 ... linking ... done. Loading package regex-posix-0.95.2 ... linking ... done. Loading package regex-compat-0.95.1 ... linking ... done. Loading package MissingH-1.1.1.0 ... linking ... done. Loading package ConfigFile-1.1.1 ... linking ... done. The TH is declared in a separate module, e.g. {-# LANGUAGE TemplateHaskell, QuasiQuotes #-} module Test where import Language.Haskell.TH alias :: String -> Name -> Q [Dec] alias name typ = do d <- newtypeD ctx conName [] con instances return [d] where instances = [''Show,''Num,''Eq,''Ord,''Enum,''Real,''Integral] ctx = return [] conName = mkName name con = recC conName [field] field = return (mkName ("un"++name), NotStrict, ConT typ) aliases :: Name -> [String] -> Q [Dec] aliases typ names = fmap concat $ mapM (flip alias typ) names if I put import Test aliases ''Int ["UserId","EventId"] in the Main module of my project and build the project, I get the above things being loaded. I'm not sure why the modules need to be dynamically linked considering they're not actually being used in the TH. And the module is clearly staged separate to the context in which it's used. I get that I can reify names to things, like “alias "Foo" ''SomePackage'sType” but those types exist at compile time, surely it can't require the actual packages? Can anyone explain? Ciao!

1 0

Long-running request/response protocol server ...
by oleg＠okmij.org 28 Jun '12

28 Jun '12

Nicolas Trangez wrote > The protocol I'd like to implement is different: it's long-running using > repeated requests & responses on a single client connection. Basically, > a client connects and sends some data to the server (where the length of > this data is encoded in the header). Now the server reads & parses this > (binary) data, sets up some initial state for this client connection > (e.g. opening a file handle), and returns a reply. Now the client can > send another request, server parses/interprets it using the connection > state, sends a reply, and so on.'' That is very simple to implement in any Iteratee library; I will use IterateeM for concreteness. The desired functionality is already implemented, in decoding of chunk-decoded inputs. Your protocol is almost the same: read a chunk of data (tagged with its size), and do something about it. After the chunk is handled, read another chunk. The iteratee library takes care of errors. In particular, if the request handler finished (normally or with errors) without reading all of the chunk, the rest of the chunk is read nevertheless and disregarded. Otherwise, we deadlock. The complete code with a simple test is included. The test reads three requests, the middle of which causes the request handler to report an error without reading the rest of the request. module SeveralRequests where import IterateeM import Prelude hiding (head, drop, dropWhile, take, break, catch) import Data.Char (isHexDigit, digitToInt, isSpace) import Control.Exception import Control.Monad.Trans -- Tell the iteratee the stream is finished and write the result -- as the reply to the client -- If the iteratee harbors the error, write that too. reply :: MonadIO m => Iteratee el m String -> Iteratee el m () reply r = en_handle show (runI r) >>= check where check (Right x) = liftIO . putStrLn $ "REPLY: " ++ x check (Left x) = liftIO . putStrLn $ "ERROR: " ++ x -- Read several requests and get iter to handle them -- Each request is formatted as a single chunk -- The code is almost identical to IterateeM.enum_chunk_decoded -- The only difference is in the internal function -- read_chunk below. -- After a chunk is handled, the inner iteratee is terminated -- and we process the new chunk with a `fresh' iter. -- If iter can throw async errors, we have to wrap it -- accordingly to convert async errors into Iteratee errors. -- That is trivial. reply_chunk_decoded :: MonadIO m => Enumeratee Char Char m String reply_chunk_decoded iter = read_size where read_size = break (== '\r') >>= checkCRLF iter . check_size checkCRLF iter m = do n <- heads "\r\n" if n == 2 then m else frame_err (exc "Bad Chunk: no CRLF") iter check_size "0" = checkCRLF iter (return iter) check_size str@(_:_) = maybe (frame_err (exc ("Bad chunk size: " ++ str)) iter) read_chunk $ read_hex 0 str check_size _ = frame_err (exc "Error reading chunk size") iter read_chunk size = take size iter >>= \r -> checkCRLF r $ reply r >> reply_chunk_decoded iter read_hex acc "" = Just acc read_hex acc (d:rest) | isHexDigit d = read_hex (16*acc + digitToInt d) rest read_hex acc _ = Nothing exc msg = toException (ErrorCall $ "Chunk decoding exc: " ++ msg) -- If the processing is restarted, we report the frame error to the inner -- Iteratee, and exit frame_err e iter = throwRecoverableErr (exc "Frame error") (\s -> enum_err e iter >>= \i -> return (return i,s)) -- Test -- A simple request_handler iter for handling requests -- If the input starts with 'abc' it reads and returns the rest -- Otherwise, it throws an error, without reading the rest of the input. request_handler :: Monad m => Iteratee Char m String request_handler = do n <- heads "abc" if n == 3 then stream2list else throwErrStr "expected abc" test_request_handler :: IO () test_request_handler = run =<< enum_pure_1chunk input (reply_chunk_decoded request_handler >> return ()) where input = -- first request "6"++crlf++ "abcdef" ++ crlf++ -- second request "8"++crlf++ "xxxdefgh" ++ crlf++ -- third request "5"++crlf++ "abcde" ++ crlf++ "0"++crlf++ crlf crlf = "\r\n" {- *SeveralRequests> test_request_handler REPLY: def ERROR: expected abc REPLY: de -}

1 0

Scribd's AI challenge has Haskell support
by aditya bhargava 28 Jun '12

28 Jun '12

Hey all, I work at scribd.com. A few weeks ago we had released an AI game. It allowed you to use Javascript to program a bot. I've been wanting to add Haskell as a supported language for some time, and it's finally done! I'm pretty excited since I think we are the first big company to do something like this. Supporting Haskell was trivial thanks to GHC 7.4's Safe Haskell extension. I hope more companies support Haskell in the future! Here's the game: *http://www.scribd.com/jobs/botrace* Here's the tutorial I wrote for Haskell: * http://egonschiele.github.com/robot-fruit-hunt/* Here's the Haskell API: * https://github.com/egonSchiele/robot-fruit-hunt/blob/master/haskell/Haskell… * Here's a sandbox that allows you to test bots locally: * https://github.com/egonSchiele/robot-fruit-hunt* Comments and feedback welcome! Adit -- adit.io

1 0

Haskell Weekly News: Issue 233
by Daniel Santa Cruz 28 Jun '12

28 Jun '12

Welcome to issue 233 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 17 to 23, 2012. Quotes of the Week * ciaranm: if it's ugly, it's imperative. if it's incomprehensible, it's functional. * xplat: when life gives you lemons, make lemonads Top Reddit Stories * What are everyone's favorite "aha" moments in learning Haskell? Domain: self.haskell, Score: 93, Comments: 156 On Reddit: [1] http://goo.gl/8YSyY Original: [2] http://goo.gl/8YSyY * Unit testing isn't enough. You need static typing too. Domain: evanfarrer.blogspot.ca, Score: 82, Comments: 22 On Reddit: [3] http://goo.gl/b73gf Original: [4] http://goo.gl/JF4ND * Snap Framework 0.9 Released Domain: snapframework.com, Score: 60, Comments: 7 On Reddit: [5] http://goo.gl/7fP9D Original: [6] http://goo.gl/7SoU6 * Haskell’s record system is a cruel joke Domain: labs.scrive.com, Score: 52, Comments: 100 On Reddit: [7] http://goo.gl/XKIch Original: [8] http://goo.gl/wOvyc * Polymorphic Update with van Laarhoven Lenses Domain: r6.ca, Score: 51, Comments: 9 On Reddit: [9] http://goo.gl/qvpPg Original: [10] http://goo.gl/34TbY * Building a blog with Snap and Heist Domain: bonus500.github.com, Score: 45, Comments: 30 On Reddit: [11] http://goo.gl/UDHCs Original: [12] http://goo.gl/eo9a3 * Totality vs. Turing completeness (McBride)[PDF, 35 slides, ±] Domain: personal.cis.strath.ac.uk, Score: 42, Comments: 22 On Reddit: [13] http://goo.gl/qua0n Original: [14] http://goo.gl/8auaP * GHC's imprecise (non-deterministic) "error" Domain: stackoverflow.com, Score: 41, Comments: On Reddit: [15] http://goo.gl/6WQWx Original: [16] http://goo.gl/MOCeT * Hackage comments — a simple user script Domain: chrisdone.com, Score: 39, Comments: 17 On Reddit: [17] http://goo.gl/t9oBH Original: [18] http://goo.gl/CmXuB * A Notation for Comonads (Orchard, Mycroft) [pdf] Domain: cl.cam.ac.uk, Score: 38, Comments: 48 On Reddit: [19] http://goo.gl/S4lz9 Original: [20] http://goo.gl/uw8ZY * Taming list syntax in Haskell Domain: labs.scrive.com, Score: 36, Comments: 29 On Reddit: [21] http://goo.gl/YLTZL Original: [22] http://goo.gl/UJpcB * Shell scripting with type-safety using Haskell « Linux-Magazin Online Domain: linux-magazin.de, Score: 36, Comments: 32 On Reddit: [23] http://goo.gl/yEMQ0 Original: [24] http://goo.gl/U3zO5 * Roy, A Statically Typed, Functional Language for Javascript. The Functional Web [PDF] Domain: brianmckenna.org, Score: 32, Comments: 9 On Reddit: [25] http://goo.gl/SBhec Original: [26] http://goo.gl/v0iH2 * [ANN] GenCheck - a generalized property-based testing framework Domain: permalink.gmane.org, Score: 30, Comments: On Reddit: [27] http://goo.gl/GCUGJ Original: [28] http://goo.gl/c2Y0s Top StackOverflow Questions * How do exceptions in Haskell work? votes: 53, answers: 1 Read on SO: [29] http://goo.gl/MOCeT * Has anyone ever compiled a list of the imports needed to avoid the “not polymorphic enough” definitions in Haskell's standard libraries? votes: 19, answers: 1 Read on SO: [30] http://goo.gl/kyXde * Why does adding a polymorphic type signature degrade performance? votes: 13, answers: 2 Read on SO: [31] http://goo.gl/Lf18k * To write or not to write `module Main where` in Haskell votes: 10, answers: 1 Read on SO: [32] http://goo.gl/IDSOx * Why aren't FingerTrees used enough to have a stable implementation? votes: 10, answers: 3 Read on SO: [33] http://goo.gl/wzw4e * What is the difference between PArray and [::] in Data Parallel Haskell? votes: 9, answers: 0 Read on SO: [34] http://goo.gl/FpUJX * Why discarded values are () instead of ⊥ in Haskell? votes: 8, answers: 5 Read on SO: [35] http://goo.gl/jIK26 * Is it possible to embed Haskell in a C library opaquely? votes: 8, answers: 1 Read on SO: [36] http://goo.gl/3H5tu * NoMonomorphismRestriction helps preserve sharing? votes: 8, answers: 2 Read on SO: [37] http://goo.gl/FDdeA * Why doesn't Haskell/GHC support record name overloading votes: 7, answers: 2 Read on SO: [38] http://goo.gl/NsHKM Until next time, Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/vi425/what_are_everyones_favorite_… 2. http://www.reddit.com/r/haskell/comments/vi425/what_are_everyones_favorite_… 3. http://evanfarrer.blogspot.ca/2012/06/unit-testing-isnt-enough-you-need.html 4. http://www.reddit.com/r/haskell/comments/vbwrv/unit_testing_isnt_enough_you… 5. http://snapframework.com/blog/2012/06/17/snap-0.9-released 6. http://www.reddit.com/r/haskell/comments/v779m/snap_framework_09_released/ 7. http://labs.scrive.com/2012/06/haskell-lacks-a-serious-record-system/ 8. http://www.reddit.com/r/haskell/comments/vdg55/haskells_record_system_is_a_… 9. http://r6.ca/blog/20120623T104901Z.html 10. http://www.reddit.com/r/haskell/comments/vhbf4/polymorphic_update_with_van_… 11. http://bonus500.github.com/sc2blog/ 12. http://www.reddit.com/r/haskell/comments/vg3q2/building_a_blog_with_snap_an… 13. https://personal.cis.strath.ac.uk/conor.mcbride/pub/Totality.pdf 14. http://www.reddit.com/r/haskell/comments/vazfp/totality_vs_turing_completen… 15. http://stackoverflow.com/questions/11070690/how-do-exceptions-in-haskell-wo… 16. http://www.reddit.com/r/haskell/comments/v6sec/ghcs_imprecise_nondeterminis… 17. http://chrisdone.com/hackage-comments/ 18. http://www.reddit.com/r/haskell/comments/v8gvc/hackage_comments_a_simple_us… 19. http://www.cl.cam.ac.uk/~dao29/drafts/codo-notation-orchard12.pdf 20. http://www.reddit.com/r/haskell/comments/v6ik6/a_notation_for_comonads_orch… 21. http://labs.scrive.com/2012/06/taming-list-syntax-in-haskell/ 22. http://www.reddit.com/r/haskell/comments/v9ilw/taming_list_syntax_in_haskel… 23. http://www.linux-magazin.de/Online-Artikel/Shell-scripting-with-type-safety… 24. http://www.reddit.com/r/haskell/comments/vdrt0/shell_scripting_with_typesaf… 25. http://brianmckenna.org/files/IC-Roy.pdf 26. http://www.reddit.com/r/haskell/comments/vhkgf/roy_a_statically_typed_funct… 27. http://permalink.gmane.org/gmane.comp.lang.haskell.general/19371 28. http://www.reddit.com/r/haskell/comments/va5ao/ann_gencheck_a_generalized_p… 29. http://stackoverflow.com/questions/11070690/how-do-exceptions-in-haskell-wo… 30. http://stackoverflow.com/questions/11076062/has-anyone-ever-compiled-a-list… 31. http://stackoverflow.com/questions/11145503/why-does-adding-a-polymorphic-t… 32. http://stackoverflow.com/questions/11112371/to-write-or-not-to-write-module… 33. http://stackoverflow.com/questions/11151732/why-arent-fingertrees-used-enou… 34. http://stackoverflow.com/questions/11103707/what-is-the-difference-between-… 35. http://stackoverflow.com/questions/11076938/why-discarded-values-are-instea… 36. http://stackoverflow.com/questions/11137443/is-it-possible-to-embed-haskell… 37. http://stackoverflow.com/questions/11145904/nomonomorphismrestriction-helps… 38. http://stackoverflow.com/questions/11100322/why-doesnt-haskell-ghc-support-…

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ANNOUNCE : Leksah 0.12.1.2 (fixes some metadata issues)
by Hamish Mackenzie 27 Jun '12

27 Jun '12

This release has an important bug fix for the metadata download. When metadata was downloaded using libcurl it was not treated as binary data. If you used one of our binary installers or if you built Leksah with the -flibcurl flag then it is likely you have bad metadata files. To fix this please install Leksah 0.12.1.2, then delete your ~/.leksah-0.12/metadata folder to force Leksah to download the metadata files again. The first time you run leksah after doing this it will take a while for leksah to update the metadata (you can track its progress by looking in the metadata directory). If you choose to generate your metadata locally or you used wget to download (the default if you cabal installed leksah) then your metadata should be ok. There are some other bug fixes in this release including one for metadata generation for users with lots of .cabal files (some lazy IO was leaving file handles open). Source is in Hackage and https://github.com/leksah Binary Installers Use "ghc --version" to work out which one you need. OS X http://leksah.org/packages/leksah-0.12.1.2-ghc-7.0.3.dmg http://leksah.org/packages/leksah-0.12.1.2-ghc-7.0.4.dmg http://leksah.org/packages/leksah-0.12.1.2-ghc-7.4.1.dmg Windows http://leksah.org/packages/leksah-0.12.1.2-ghc-6.12.3.exe http://leksah.org/packages/leksah-0.12.1.2-ghc-7.0.3.exe http://leksah.org/packages/leksah-0.12.1.2-ghc-7.0.4.exe http://leksah.org/packages/leksah-0.12.1.2-ghc-7.4.1.exe

4 4

cabal doens't forget old dependencies
by Sjoerd Visscher 27 Jun '12

27 Jun '12

Hi all, I tried to install reactive-banana. This failed due to a dependency conflict, and then I noticed there was a newer version of reactive-banana. So I did cabal update, and tried to install again. But whatever I do, cabal keeps trying to install fclabels, but fclabels is no longer a dependency of reactive-banana-0.6.0.0! How can I let cabal forget this dependency? Thanks, -- Sjoerd Visscher https://github.com/sjoerdvisscher/blog

3 4

Unambiguous choice implementation
by Bartosz Milewski 27 Jun '12

27 Jun '12

I'm reading Conal Elliot's paper, Push-Pull FRP. At some point he needs an unambiguous choice operator, essentially to implement select: a future that waits for one of its future arguments to fire. His implementation of unamb creates two threads racing on a shared MVar. By his own admission, this is very inefficient. My question is, is there a better implementation? -- [:Bartosz:]

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 106, Issue 38
by John Lato 27 Jun '12

27 Jun '12

> Message: 12 > Date: Wed, 27 Jun 2012 00:19:30 +0200 > From: Tillmann Rendel <rendel(a)informatik.uni-marburg.de> > Subject: Re: [Haskell-cafe] Martin Odersky on "What's wrong with > Monads" > Cc: haskell-cafe(a)haskell.org > Message-ID: <4FEA3572.5060200(a)informatik.uni-marburg.de> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > Hi, > > MightyByte wrote: >> Of course every line of your program that uses a Foo will change if you switch >> to IO Foo instead. > > But we often have to also change lines that don't use Foo at all. For > example, here is the type of binary trees of integers: > > data Tree = Leaf Integer | Branch (Tree Integer) (Tree Integer) > > A function to add up all integers in a tree: > > amount:: Tree -> Integer > amount (Leaf x) = x > amount (Branch t1 t2) = amountt1 + amountt2 > > All fine so far. Now, consider the following additional requirement: "If > the command-line flag --multiply is set, the function amount computes > the product instead of the sum." > > In a language with implicit side effects, it is easy to implement this. > We just change the third line of the amount function to check whether to > call (+) or (*). In particular, we would not touch the other two lines. > > How would you implement this requirement in Haskell without changing the > line "amount (Leaf x) = x"? Why would you do that even in an imperative language? The program logic turns into a spaghetti mess, and it's much harder to test. I would write Tree like this: data Tree a = Leaf a | Branch (Tree a) ( Tree a) deriving (Foldable, Show) and instead of an "amount" function I would use a fold. If you like, you can use "ala" from Control.Newtype *Main Data.Foldable Data.Monoid> let t1 = Branch (Leaf 1) (Branch (Leaf 4) (Leaf 5)) :: Tree Int *Main Data.Foldable Data.Monoid> ala Sum foldMap t1 10 *Main Data.Foldable Data.Monoid> ala Product foldMap t1 20 Now the amount calculation can be something like amount :: Num a => Tree a -> IO a amount tree = multFlag >>= \b -> if b then ala Product foldMap tree else ala Sum foldMap tree although I probably wouldn't actually write it unless it was called in more than one place. There are other ways to write it too; the important part is that checking the configuration is completely separate from the tree traversal. Plus, if it changes again (now there's another flag that says to ignore values == n or something, you can use the same fold, just change the function that's passed to it (or the monoid instance if you're using that). Plus, this type of code is much simpler to debug, test, and maintain than imperative-style magic functions. Cheers, John

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Re: [Haskell-cafe] StableNames and monadic functions
by Ismael Figueroa Palet 26 Jun '12

26 Jun '12

Thanks Lorenzo, I'm cc'ing the list with your response also: As you point out, when you do some kind of "let-binding", using the where clause, or explicit let as in: main :: IO () main = do let f1 = (successor :: Int -> State Int Int) let f2 = (successor :: Int -> Maybe Int) b2 <- eq f2 f2 b1 <- eq f1 f1 print (show b1 ++ " " ++ show b2) The behavior is as expected. I guess the binding triggers some internal optimization or gives more information to the type checker; but I'm still not clear why it is required to be done this way -- having to let-bind every function is kind of awkward. I know the details of StableNames are probably implementation-dependent, but I'm still wondering about how to detect / restrict this situation. Thanks 2012/6/26 Lorenzo Bolla <lbolla(a)gmail.com> > From StableName docs: > >> The reverse is not necessarily true: if two stable names are not equal, >> then the objects they name may still be equal. > > > This version works as expected: > > import System.Mem.StableName > import Control.Monad.State > > eq :: a -> b -> IO Bool > eq a b = do > pa <- makeStableName a > pb <- makeStableName b > return (hashStableName pa == hashStableName pb) > > successor :: (Num a, Monad m) => a -> m a > successor n = return (n+1) > > -- main :: IO () > -- main = do > -- b2 <- eq (successor :: Int -> State Int Int) (successor :: Int > -> State Int Int) > -- b1 <- eq (successor :: Int -> Maybe Int) (successor :: Int -> > Maybe Int) > -- print (show b1 ++ " " ++ show b2) > > main :: IO () > main = do > b2 <- eq f2 f2 > b1 <- eq f1 f1 > print (show b1 ++ " " ++ show b2) > where f1 = (successor :: Int -> Maybe Int) > f2 = (successor :: Int -> State Int Int) > > > > hth, > L. > > > > > On Tue, Jun 26, 2012 at 1:15 PM, Ismael Figueroa Palet < > ifigueroap(a)gmail.com> wrote: > >> I'm using StableNames to have a notion of function equality, and I'm >> running into problems when using monadic functions. >> >> Consider the code below, file Test.hs >> >> import System.Mem.StableName >> import Control.Monad.State >> >> eq :: a -> b -> IO Bool >> eq a b = do >> pa <- makeStableName a >> pb <- makeStableName b >> return (hashStableName pa == hashStableName pb) >> >> successor :: (Num a, Monad m) => a -> m a >> successor n = return (n+1) >> >> main :: IO () >> main = do >> b1 <- eq (successor :: Int -> Maybe Int) (successor :: Int -> >> Maybe Int) >> b2 <- eq (successor :: Int -> State Int Int) (successor :: Int -> >> State Int Int) >> print (show b1 ++ " " ++ show b2) >> >> Running the code into ghci the result is "False False". There is some old >> post saying that this is due to the dictionary-passing style for >> typeclasses, and compiling with optimizations improves the situation. >> >> Compiling with ghc --make -O Tests.hs and running the program, the result >> is "True True", which is what I expect. >> However, if I change main to be like the following: >> >> main :: IO () >> main = do >> b2 <- eq (successor :: Int -> State Int Int) (successor :: Int -> >> State Int Int) >> b1 <- eq (successor :: Int -> Maybe Int) (successor :: Int -> >> Maybe Int) >> print (show b1 ++ " " ++ show b2) >> >> i.e. just changing the sequential order, and then compiling again with >> the same command, I get "True False", which is very confusing for me. >> Similar situations happens when using the state monad transformer, and >> manually built variations of it. >> >> It sounds the problem is with hidden closures created somewhere that do >> not point to the same memory locations, so StableNames yields false for >> that cases, but it is not clear to me under what circumstances this >> situation happens. Is there other way to get some approximation of function >> equality? or a way to "configure" the behavior of StableNames in presence >> of class constraints? >> >> I'm using the latests Haskell Platform on OS X Lion, btw. >> >> Thanks >> >> -- >> Ismael >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> > -- Ismael

2 7

HXT: Replace an element with its text
by Michael Orlitzky 26 Jun '12

26 Jun '12

I would like to replace, <body><a href="#">foo</a></body> with, <body>foo</body> using HXT. So far, the closest I've come is to parse the HTML and apply the following stuff: is_link :: (ArrowXml a) => a XmlTree XmlTree is_link = hasName "a" replace_links_with_their_text :: (ArrowXml a) => a XmlTree XmlTree replace_links_with_their_text = processTopDown $ (getText >>> mkText) `when` is_link Unfortunately, this just removes the "a" element and its text entirely. The other-closest solution is, replace_links_with_their_text :: (ArrowXml a) => a XmlTree XmlTree replace_links_with_their_text = processTopDown $ (txt "foo") `when` is_link Of course, I don't want to hard-code the value "foo", and I can't figure out a way to feed the element's text back into 'txt'. Anyone tried this before?

3 5

StableNames and monadic functions
by Ismael Figueroa Palet 26 Jun '12

26 Jun '12

I'm using StableNames to have a notion of function equality, and I'm running into problems when using monadic functions. Consider the code below, file Test.hs import System.Mem.StableName import Control.Monad.State eq :: a -> b -> IO Bool eq a b = do pa <- makeStableName a pb <- makeStableName b return (hashStableName pa == hashStableName pb) successor :: (Num a, Monad m) => a -> m a successor n = return (n+1) main :: IO () main = do b1 <- eq (successor :: Int -> Maybe Int) (successor :: Int -> Maybe Int) b2 <- eq (successor :: Int -> State Int Int) (successor :: Int -> State Int Int) print (show b1 ++ " " ++ show b2) Running the code into ghci the result is "False False". There is some old post saying that this is due to the dictionary-passing style for typeclasses, and compiling with optimizations improves the situation. Compiling with ghc --make -O Tests.hs and running the program, the result is "True True", which is what I expect. However, if I change main to be like the following: main :: IO () main = do b2 <- eq (successor :: Int -> State Int Int) (successor :: Int -> State Int Int) b1 <- eq (successor :: Int -> Maybe Int) (successor :: Int -> Maybe Int) print (show b1 ++ " " ++ show b2) i.e. just changing the sequential order, and then compiling again with the same command, I get "True False", which is very confusing for me. Similar situations happens when using the state monad transformer, and manually built variations of it. It sounds the problem is with hidden closures created somewhere that do not point to the same memory locations, so StableNames yields false for that cases, but it is not clear to me under what circumstances this situation happens. Is there other way to get some approximation of function equality? or a way to "configure" the behavior of StableNames in presence of class constraints? I'm using the latests Haskell Platform on OS X Lion, btw. Thanks -- Ismael

1 0

Hackage 2 maintainership
by Ben Gamari 26 Jun '12

26 Jun '12

Hey all, Those of you who follow the Haskell subreddit no doubt saw today's post regarding the status of Hackage 2. As has been said many times in the past, the primary blocker at this point to the adoption of Hackage 2 appears to be the lack of an administrator. It seems to me this is a poor reason for this effort to be held up. Having taken a bit of time to consider, I would be willing to put in some effort to get things moving and would be willing to maintain the haskell.org Hackage 2.0 instance going forward if necessary. I currently have a running installation on my personal machine and things seem to be working as they should. On the whole, installation was quite trivial, so it seems likely that the project is indeed at a point where it can take real use (although a "logout" option in the web interface would make testing a bit easier). That being said, it would in my opinion be silly to proceed without fixing the Hackage trac. It was taken down earlier this year due to spamming[1] and it seems the recovery project has been orphaned. I would be willing to help with this effort, but it seems like the someone more familiar with the haskel.org infrastructure might be better equipped to handle the situation. It seems that this process will go something like this, 1) Bring Hackage trac back from the dead 2) Bring up a Hackage 2 instance along-side the existing hackage.haskell.org 3) Enlist testers 4) Let things simmer for a few weeks/months ensuring nothing explodes 5) After it's agreed that things are stable, eventually swap the Hackage 1 and 2 instances This will surely be a non-trivial process but I would be willing to move things forward. Cheers, - Ben [1] http://www.haskell.org/pipermail/cabal-devel/2012-January/008427.html

12 17

Munich Haskell Meeting
by Heinrich Hördegen 25 Jun '12

25 Jun '12

Dear all, once again, we want to invite you to our monthly Munich Haskell Meeting at Thu, 28th of June, at 19h30. This time, the venue will be Max Emanuell Brauerei in Munich (http://www.max-emanuel-brauerei.de/) Be aware of that change! Thursday will also be the day of a local football play, at least for those living in Europe. Therefore, I want to reserve a table on Wednesday in order to get a table at all. If you plan to join, please go to: http://www.haskell-munich.de/dates and click the button. Until then, have fun. Heinrich

1 0

Galois is hiring!
by Jason Dagit 25 Jun '12

25 Jun '12

We're looking for functional programmers, formal methods practitioners, and project leads, interested in either short term or permanent employment. For more information, see: http://corp.galois.com/careers Why Galois? At Galois, we believe in seeking out and solving important problems to make the world a better place. This philosophy runs through everything we do. From internal interactions on an individual level to the interesting technical challenges we take on to the way we orient to the external world. We hire the best people, we help them to better understand their own passions, and then together we take on important problems. It just so happens that we have skilled software engineers with a passion for functional programming who take on challenges in crypto, critical systems, secure networking, and so on. For me, the very compelling reasons to work at Galois are: a strong cultural emphasis on freedom to choose your roles within the organization including seeking external funding to work on whatever you want; strong commitment to open source, and; a cultural emphasis on functional programming and formal methods. What a organization believes has a deep and profound impact on how you spend your time at the organization, who the organization attracts, and what impact the organization will have in the world. We have no shortage of interesting problems to tackle in our quest to make the world a better place. Thanks, Jason

1 0

ghci session slows down over time.
by Jonathan Geddes 25 Jun '12

25 Jun '12

Haskell Cafe, I'm seeing crazy amounts of slowdown in a ghci session after just a few executions of :r (reload). Using :set +r (revert top-level bindings) doesn't seem to help. Is it possible that the dynamically-loaded object code is not being garbage collected? Is this a known issue? More importantly, is there a known workaround? Thanks, --J Arthur

3 3

Origin of Delimited Continuation Operators
by aditya siram 25 Jun '12

25 Jun '12

Hi all, I working through a few papers and Oleg's delimited continuation implementation and I'm wondering how the operators (shift, shift0P, control, reset, etc) got their name. -deech

2 2

About using "type" to do type alias.
by Magicloud Magiclouds 25 Jun '12

25 Jun '12

Hi, There was another mail, but the subject might be confusing. So I write this one. The code is here: http://hpaste.org/70414 If I understand correct, generally, I could use 'type' to do alias to save the ugly-long code. Like section 1. This works when I 't [(0, Just "x")]'. But, if I wrote section 2. Then 'start (M.fromList $ zip ord_args) worker' could not be compiled due to the second argument is type of 'M.Map Arg Arg', not 'JobArgs Arg Arg'. What did I miss to make this work? -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

3 9

ICFP Student Research Competition
by Wouter Swierstra 25 Jun '12

25 Jun '12

===================================================================== Student Research Competition Associated with the The 17th ACM SIGPLAN International Conference on Functional Programming (ICFP 2012) and affiliated events http://www.icfpconference.org/icfp2012/src.html Copenhagen, Denmark, Sep 9-15, 2012 ===================================================================== This year ICFP will host a Student Research Competition where undergraduate and postgraduate students can present posters. The SRC at the ICFP 2012 consists of three rounds: - Extended abstract round: All students are encouraged to submit an extended abstract outlining their research (800 words). - Poster session at ICFP 2012: Based on the abstracts, a panel of judges will select the most promising entrants to participate in the poster session which will take place at ICFP. Students who make it to this round will be supported to attend the conference, to a maximum of $500 for travel and housing. If your total costs are higher than these $500 your conference fee may be waived too. In the poster session, students will have the opportunity to present their work to the judges, who will select three finalists in each category* (graduate/undergraduate) to advance to the next round. - ICFP presentation: The next round consists of an oral presentation at the ICFP to compete for the final awards in each category. ** Prizes ** Both the top three graduate and the top three undergraduate contestants will receive prizes of $500, $300, and $200, respectively. All six winners will receive award medals and a two-year complimentary ACM student membership, including a subscription to ACM’s Digital Library. The names of the winners will be posted on the SRC web site. The winners in each category will be invited to participate in the ACM SRC Grand Finals, an on-line round of competitions among the winners of other conference-hosted SRCs. Grand Finalists and their advisors will be invited to the Annual ACM Awards Banquet for an all-expenses-paid trip, where they will be recognized for their accomplishments along with other prestigious ACM award winners, including the winner of the Turing Award (also known as the Nobel Prize of Computing). The top three graduate Grand Finalists will receive an additional $500, $300, and $200. Likewise, the top three undergraduate Grand Finalists will receive an additional $500, $300, and $200. All six Grand Finalists will receive Grand Finalist certificates. ** Eligibility ** The SRC is open to both undergraduate and graduate students. Upon submission, entrants must be enrolled as a student at their university. The abstract must describe the student’s individual research and must be authored solely by the student. If the work is collaborative with others and/or part of a larger group project, the abstract should make clear what the student’s role was and should focus on that portion of the work. The extended abstract must not exceed 800 words and must not be longer than 2 pages. The reference list does not count towards these limits. To submit an abstract, please register through the submission page and follow the instructions. Abstracts submitted after the deadline may be considered at the committee's discretion, but only after decisions have been made on all abstracts submitted before the deadline. If you have any problems, don't hesitate to contact the competition chair. More information about the submission process can be found online at: http://www.icfpconference.org/icfp2012/src.html ** Important Dates ** Deadline for submission: June 29th Notification of acceptance: July 8th ** Selection Committee ** Koen Claessen, Chalmers University of Technology Robby Findler (ICFP Program chair), Northwestern University Ken Friis Larsen, IT University of Copenhangen Jacques Garrigue, Nagoya University Doaitse Swierstra (Chair), Utrecht University

1 0

Request for optimizing help.
by Captain Freako 25 Jun '12

25 Jun '12

Does anyone have any advice for optimizing the code, below? Currently, the profiling results look like this: COST CENTRE MODULE %time %alloc runFilter Filter 90.9 41.0 convT Filter 9.1 58.6 Thanks for any help! -db 11 data FilterState a = FilterState { 12 as :: [a] -- transfer function denominator coefficients 13 , bs :: [a] -- transfer function numerator coefficients 14 , taps :: [a] -- current delay tap stored values 15 } deriving (Show) 16 17 type Kernel a = (a, FilterState a) -> (a, FilterState a) 18 19 -- FILTER KERNELS 20 -- Time domain convolution filter (FIR or IIR), 21 -- expressed in direct form 2 22 convT :: (Fractional a) => Kernel a 23 convT (x, s) = 24 let wk = (x - sum [a * t | (a, t) <- zip (tail $ as s) (taps s)]) / head (as s) 25 newTaps = wk : init (taps s) 26 s' = s {taps = newTaps} 27 y = sum [b * w | (b, w) <- zip (bs s) (wk : taps s)] 28 in (y, s') 29 30 -- FILTER APPLICATION 31 runFilter :: Kernel a -> FilterState a -> [a] -> IO ([a], FilterState a) 32 runFilter f s0 [] = do 33 return ([], s0) 34 runFilter f s0 (x:xs) = do 35 (y, s') <- return (f (x, s0)) 36 (ys, s) <- runFilter f s' xs 37 return (y:ys, s)

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ANNOUNCE : Leksah 0.12.1.3 (fix for GHCi locking up)
by Hamish Mackenzie 25 Jun '12

25 Jun '12

This release fixes a problem where the GHCi integration would break when the first command set was from the debug scratch pane. Source is in Hackage and https://github.com/leksah Binary Installers Use "ghc --version" to work out which one you need. OS X http://leksah.org/packages/leksah-0.12.1.3-ghc-7.0.3.dmg http://leksah.org/packages/leksah-0.12.1.3-ghc-7.0.4.dmg http://leksah.org/packages/leksah-0.12.1.3-ghc-7.4.1.dmg Windows http://leksah.org/packages/leksah-0.12.1.3-ghc-6.12.3.exe http://leksah.org/packages/leksah-0.12.1.3-ghc-7.0.3.exe http://leksah.org/packages/leksah-0.12.1.3-ghc-7.0.4.exe http://leksah.org/packages/leksah-0.12.1.3-ghc-7.4.1.exe

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Why does Enum succ and pred functions throw exception
by Rouan van Dalen 25 Jun '12

25 Jun '12

Hi everyone, Can anyone shed some light on why the succ and pred functions of the Enum typeclass throw exceptions if we go over the upper or lower boundary, and not return Maybe a? I was hoping to have some functions like: safeSucc :: (Enum a) => a -> Maybe a Because the succ and pred functions throw exceptions I can only catch them in the IO monad. This makes it hard to deal with this situation in pure code. Regards Rouan.

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: set-monad
by George Giorgidze 25 Jun '12

25 Jun '12

Hi Dan, Thanks for your feedback and your question regarding the functor laws. Please try your example in GHCi and/or evaluate it by hand. The library does not violate the functor laws. I committed quick check properties for functor laws, as well as, laws for other type classes to the repo. You can give it a try. It is also possible, with a little bit of effort, to prove those properties by hand. Speaking of laws, BTW, your contrived Ord instance violates one of the Ord laws. The documentation for Ord says that: "The Ord class is used for totally ordered datatypes". Your definition violates the antisymmetry law [1]: If a <= b and b <= a then a == b by reporting two elements that are not equal as equal. Cheers, George [1] http://en.wikipedia.org/wiki/Totally_ordered On 16 June 2012 09:47, Dan Burton <danburton.email(a)gmail.com> wrote: > Convenience aside, doesn't the functor instance conceptually violate some > sort of law? > > fmap (const 1) someSet > > The entire shape of the set changes. > > fmap (g . h) = fmap g . fmap h > > This law wouldn't hold given the following contrived ord instance > > data Foo = Foo { a, b :: Int } > instance Ord Foo where > compare = compare `on` a > > Given functions > > h foo = foo { a = 1 } > g foo = foo { a = b foo } > > Does this library address this? If so, how? If not, then you'd best note it > in the docs. > > On Jun 15, 2012 6:42 PM, "George Giorgidze" <giorgidze(a)gmail.com> wrote: > > I would like to announce the first release of the set-monad library. > > On Hackage: http://hackage.haskell.org/package/set-monad > > The set-monad library exports the Set abstract data type and > set-manipulating functions. These functions behave exactly as their > namesakes from the Data.Set module of the containers library. In > addition, the set-monad library extends Data.Set by providing Functor, > Applicative, Alternative, Monad, and MonadPlus instances for sets. > > In other words, you can use the set-monad library as a drop-in > replacement for the Data.Set module of the containers library and, in > addition, you will also get the aforementioned instances which are not > available in the containers package. > > It is not possible to directly implement instances for the > aforementioned standard Haskell type classes for the Set data type > from the containers library. This is because the key operations map > and union, are constrained with Ord as follows. > > map :: (Ord a, Ord b) => (a -> b) -> Set a -> Set b > union :: (Ord a) => Set a -> Set a -> Set a > > The set-monad library provides the type class instances by wrapping > the constrained Set type into a data type that has unconstrained > constructors corresponding to monadic combinators. The data type > constructors that represent monadic combinators are evaluated with a > constrained run function. This elevates the need to use the > constraints in the instance definitions (this is what prevents a > direct definition). The wrapping and unwrapping happens internally in > the library and does not affect its interface. > > For details, see the rather compact definitions of the run function > and type class instances. The left identity and associativity monad > laws play a crucial role in the definition of the run function. The > rest of the code should be self explanatory. > > The technique is not new. This library was inspired by [1]. To my > knowledge, the original, systematic presentation of the idea to > represent monadic combinators as data is given in [2]. There is also a > Haskell library that provides a generic infrastructure for the > aforementioned wrapping and unwrapping [3]. > > The set-monad library is particularly useful for writing set-oriented > code using the do and/or monad comprehension notations. For example, > the following definitions now type check. > > s1 :: Set (Int,Int) > s1 = do a <- fromList [1 .. 4] > b <- fromList [1 .. 4] > return (a,b) > > -- with -XMonadComprehensions > s2 :: Set (Int,Int) > s2 = [ (a,b) | (a,b) <- s1, even a, even b ] > > s3 :: Set Int > s3 = fmap (+1) (fromList [1 .. 4]) > > As noted in [1], the implementation technique can be used for monadic > libraries and EDSLs with restricted types (compiled EDSLs often > restrict the types that they can handle). Haskell's standard monad > type class can be used for restricted monad instances. There is no > need to resort to GHC extensions that rebind the standard monadic > combinators with the library or EDSL specific ones. > > [1] CSDL Blog: The home of applied functional programming at KU. Monad > Reification in Haskell and the Sunroof Javascript compiler. > http://www.ittc.ku.edu/csdlblog/?p=88 > > [2] Chuan-kai Lin. 2006. Programming monads operationally with Unimo. > In Proceedings of the eleventh ACM SIGPLAN International Conference on > Functional Programming (ICFP '06). ACM. > > [3] Heinrich Apfelmus. The operational package. > http://hackage.haskell.org/package/operational > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

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cvs-ghc mailing list silently dropping my mails
by Erik de Castro Lopo 24 Jun '12

24 Jun '12

Hi all, I've been trying to send mail to the cvs-ghc mailng list and the list seems to be silently dropping my emails. I have: a) Made sure I was sending mail with the subscribed from address. b) Unsubscribed and re-subscribed and tried again. All to no avail. Is there a moderation queue on that list that needs to be seen to? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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ANNOUNCE: dbus 0.10
by John Millikin 23 Jun '12

23 Jun '12

This is completing the merger of the "dbus-core" and "dbus-client" packages. The new package does everything they can do, but better. Hackage: http://hackage.haskell.org/package/dbus Homepage: https://john-millikin.com/software/haskell-dbus/ API reference: https://john-millikin.com/software/haskell-dbus/reference/haskell-dbus/0.10/ Examples: Included in the tarball, and also at https://john-millikin.com/branches/haskell-dbus/0.10/head:/examples/ Notable changes: * The module hierarchy was simplified. Most users will only ever need the "DBus" and "DBus.Client" modules. * Exports which were not used or useful have been removed. * Much better documentation -- most exports now have a description, and several have examples. * The source code isn't "literate" any more. Too many people expressed frustration at trying to contribute, so I just converted it all to standard flat .hs files. * Better support for custom socket transports and authentication mechanisms. * Added support for listening for socket connections. This allows users to implement both sides of a peer-peer session. * Various improvements to performance, including moving from "binary" to "cereal". * Various minor improvements to correctness.

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