- Haskell-Cafe - Haskell.org

yi-editor for emacer
by Junior White 15 Apr '13

15 Apr '13

Hi Cafe, I'm glad to announce my fork of yi-editor. As a emacer, I made yi-editor more emacs like, including the following change: 1. ido-mode like file find and buffer find 2. support more color in vty 3. automatic search tags file in parents's directory 4. change some keymap The following step to have a try: 1. clone : git clone https://github.com/onlyu/yi.git 2. build : ./yi/yi.sh --rebuild 3. run : ./yi/yi.sh Thanks for have a test, and thanks yi-editor, now I combine my favourite editor (emacs) and language (haskell) together. Apologize for my noisy!

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Optimizing queries in Haskell
by Jan Stolarek 15 Apr '13

15 Apr '13

Dear list, list comprehensions and SQL-like generalized comprehensions can be used to write queries in Haskell. Does anyone know of any work focused on optimizing such queries? A quick google didn't show up anything meaningful. Janek

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[Announcement] Sparse matrix Conjugate Gradient Solver
by Levent Erkok 15 Apr '13

15 Apr '13

I'm happy to announce the first release of conjugateGradient, implementing the Conjugate Gradient algorithm for solving linear systems of equations over sparse matrices: http://hackage.haskell.org/package/conjugateGradient The conjugate gradient algorithm only applies to the cases when the input matrix is symmetric and positive definite. See: http://en.wikipedia.org/wiki/Conjugate_gradient_method The advantage of this method is that it can handle really large sparse systems quite well, when other direct methods (such as LU decomposition) are just not practical due to algorithmic complexity. Such large and sparse systems naturally arise in engineering applications, such as in ASIC placement algorithms and when solving partial differential equations. Bug reports, feedback, and improvements are always welcome. -Levent.

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Building the Haskell Platform in Linux
by Daniel Díaz Casanueva 15 Apr '13

15 Apr '13

Hi cafe! Probably you all know how to do this, but I myself found confused when building the Haskell Platform in Linux for my first time. I was using Linux for my first time too! The first problem I encountered was to decide what linux packages install to make the ./configure successful in both GHC binary distribution and the Haskell Platform. So I collected the names of the minimum set of packages that you need to have installed before starting with GHC and the Platform in a small blog post: http://deltadiaz.blogspot.com/2013/04/haskell-platform-from-source-in-linux… If I would have to start all over again, forgetting what I have learned so far, I would fine this list very useful! This thought is what brought me here to make it public. Who knows, maybe I am saving somebody of some google searches and trials and errors. With my best intentions, Daniel Díaz. -- E-mail sent by Daniel Díaz Casanueva let f x = x in x

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multivariate normal distribution in Haskell?
by Bas de Haas 14 Apr '13

14 Apr '13

Dear List, I’m implementing a probabilistic model for recognising musical chords in Haskell. This model relies on a multivariate normal distribution. I’ve been searching the internet and mainly hackage for a Haskell library to do this for me, but so far I’ve been unsuccessful. What I’m looking for is a Haskell function that does exactly what the mvnpdf function in matlab does: http://www.mathworks.nl/help/stats/multivariate-normal-distribution.html Does anyone know a library that can help me out? Thanks. Kind regards, Bas de Haas -- dr. W. Bas de Haas Department of Information and Computing Sciences Utrecht University E: W.B.deHaas(a)uu.nl T: +31 30 253 5965 I: http://www.uu.nl/staff/WBdeHaas/

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Re: [Haskell-cafe] <package>-inplace is shadowed by package <package>-<hexstring>
by Niklas Hambüchen 14 Apr '13

14 Apr '13

Hi, I just got the same thing with attoparsec GIT: Preprocessing benchmark 'benchmarks' for attoparsec-0.10.4.0... Building benchmark benchmarks... creating dist/build/benchmarks creating dist/build/benchmarks/benchmarks-tmp /home/niklas/opt/haskell-7.4/bin/ghc --make -fbuilding-cabal-package -O -odir dist/build/benchmarks/benchmarks-tmp -hidir dist/build/benchmarks/benchmarks-tmp -stubdir dist/build/benchmarks/benchmarks-tmp -i -idist/build/benchmarks/benchmarks-tmp -ibenchmarks -idist/build/autogen -Idist/build/autogen -Idist/build/benchmarks/benchmarks-tmp -optP-include -optPdist/build/autogen/cabal_macros.h -hide-all-packages -package-conf dist/package.conf.inplace -package-id base-4.5.1.0-66f22db3dfcd87541c9c7e50e7095d26 -package-id bytestring-0.9.2.1-503e91bb155301fdb1956cb5c26ce6e9 -package-id criterion-0.6.2.1-5bc7e5a96c8208948366b8e3ea536e68 -package-id deepseq-1.3.0.0-c26e15897417ecd448742528253d68f6 -package-id parsec-3.1.3-771f99c8818551756b6f2ec3e86ab039 -package-id text-0.11.2.3-db61832d0c4660614c4ceff234ed4abb -package-id attoparsec-0.10.4.0-inplace -XHaskell98 benchmarks/Benchmarks.hs -o dist/build/benchmarks/benchmarks <command line>: cannot satisfy -package-id attoparsec-0.10.4.0-inplace: attoparsec-0.10.4.0-inplace is shadowed by package attoparsec-0.10.4.0-c391286d724823eaea3d4278acc0acc0 (use -v for more information) I have no clue what's going on!

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Invitation to connect on LinkedIn
by Chatura Roche 14 Apr '13

14 Apr '13

LinkedIn ------------ Chatura Roche requested to add you as a connection on LinkedIn: ------------------------------------------ Steve, I'd like to add you to my professional network on LinkedIn. - Chatura Accept invitation from Chatura Roche http://www.linkedin.com/e/-j6322o-hfici7ky-5i/v4h1gXJ8v-XpaSE4KRp1AY1vvMqM3… View profile of Chatura Roche http://www.linkedin.com/e/-j6322o-hfici7ky-5i/rso/115844279/bO7F/name/25907… ------------------------------------------ You are receiving Invitation emails. This email was intended for Steve Severance. Learn why this is included: http://www.linkedin.com/e/-j6322o-hfici7ky-5i/plh/http%3A%2F%2Fhelp%2Elinke… (c) 2012, LinkedIn Corporation. 2029 Stierlin Ct, Mountain View, CA 94043, USA.

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Fwd: [Haskell-beginners] Monad instances and type synonyms
by Christopher Howard 14 Apr '13

14 Apr '13

I asked this question in Haskell-beginners, but I haven't heard anything yet, so I'm forwarding to Cafe. -------- Original Message -------- Subject: [Haskell-beginners] Monad instances and type synonyms Date: Sat, 13 Apr 2013 17:03:57 -0800 From: Christopher Howard <christopher.howard(a)frigidcode.com> Reply-To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners(a)haskell.org> To: Haskell Beginners <beginners(a)haskell.org> I am playing around with some trivial code (for learning purposes) I wanted to take code: -------- -- SaleVariables a concrete type defined early -- `Adjustment' represents adjustment in a price calculation -- Allows functions of type (a -> Adjustment a) to be composed -- with an appropriate composition function type Adjustment a = SaleVariables -> a -------- And put it into code: -------- instance Monad Adjustment where (>>=) = ... return = ... -------- If I try this, I get code: -------- Type synonym `Adjustment' should have 1 argument, but has been given none In the instance declaration for `Monad Adjustment' -------- But if I give an argument, then it doesn't compile either (it becomes a "*" kind). And I didn't want to make the type with a regular "data" declaration either, because then I have to give it a constructor, which doesn't fit with what I want the type to do. -- frigidcode.com

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Re: [Haskell-cafe] Fwd: [Haskell-beginners] Monad instances and type synonyms
by Chris Wong 14 Apr '13

14 Apr '13

On Sun, Apr 14, 2013 at 5:10 PM, Christopher Howard <christopher.howard(a)frigidcode.com> wrote: > type Adjustment a = SaleVariables -> a > > [...] > > instance Monad Adjustment where > > (>>=) = ... > return = ... Essentially, you can't partially apply type synonyms. I don't recall the exact reasoning, but if this sort of thing was allowed it would probably poke funny holes in the type system. Also, Control.Monad.Instances already supplies a Monad instance for functions (r -> a). So even if that did pass, you'd bump into overlapping instances anyway. Chris > -------- > If I try this, I get > > code: > -------- > Type synonym `Adjustment' should have 1 argument, but has been given none > In the instance declaration for `Monad Adjustment' > -------- > > But if I give an argument, then it doesn't compile either (it becomes a > "*" kind). And I didn't want to make the type with a regular "data" > declaration either, because then I have to give it a constructor, which > doesn't fit with what I want the type to do. > > -- > frigidcode.com > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Chris Wong, fixpoint conjurer e: lambda.fairy(a)gmail.com w: http://lfairy.github.io/

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case switch covering all possible constructor but still fails
by Nathan Hüsken 13 Apr '13

13 Apr '13

Hey, I am trying to fix (or at least get my head around) this: https://github.com/valderman/haste-compiler/issues/27 Which is an issue of the haste haskell->javascript compiler. Doing so I encounter this problem which makes me think I am not understanding something fundamental here (and I believe this not to be haste specific)... OK, the problem is that this code: genAlt :: Bool -> JSVar -> JSVar -> StgAlt -> JSGen Config JSAlt genAlt tailpos scrut res (con, args, used, body) = do construct <- case con of DEFAULT -> return Def LitAlt l -> Cond <$> genLit l DataAlt c | Right tag <- genDataConTag c -> return $ Cond tag _ -> error "Bad data constructor tag generated!" Looking at StgAlt, I conclude that con must be of type AltCon, which is defined like this (from CoreSyn.lhs of the ghc source): data AltCon = DataAlt DataCon | LitAlt Literal | DEFAULT deriving (Eq, Ord, Data, Typeable) What I do not understand: The case switch covers all possibilities of AltCon. So how can this happen? Using trace, the con seemed to have the type "GHC.Integer.Type.S#" in the case of the erro. Thanks! Nathan

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NaN as Integer value
by Алексей Егоров 13 Apr '13

13 Apr '13

Hello haskellers, is there a reason why Integer doesn't have 'NaN' as value? I think it would be very convenient to be able to handle (1 `div` 0) as regular NaN value and not as exception. Thanks.

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ANNOUNCE: monoid-subclasses-0.1.1, incremental-parser-0.2.2, and SCC-0.8
by Mario Blažević 13 Apr '13

13 Apr '13

The new package monoid-subclasses [1] exports a number of classes that sit between monoids and groups: ReductiveMonoid, CancellativeMonoid, GCDMonoid, MonoidNull, and FactorialMonoid among others. The package also comes with class instances for all applicable data types from base, vector, containers, bytestring, and text packages. These classes were initially a part of the incremental-parser package [2] but they've moved, so incremental-parser-0.2.2 now depends on monoid-subclasses which has added more class methods and instances. The parsers constructed using the incremental-parser library generally work on any ReductiveMonoid, FactorialMonoid or TextualMonoid. You can thus construct a generic parser that operates on String, ByteString, or Text input, or even on Product Integer or a Map if you are so inclined. The version 0.8 of Streaming Component Combinators (a.k.a. SCC) has also been re-designed from the ground up. In the earlier versions, as in the pipes and conduit packages, the types of streams and stream-processing components were parametrized by the type of the individual items flowing through. All sources and sinks are now parametrized instead by the type of the monoid they produce or consume. As a consequence, SCC coroutines can now communicate in chunks of ByteString or Text instead of just lists, and these chunks can be of arbitrary size. The boundaries of producer's chunks are invisible to the consumer (excepting some low-level functions), which must specify the amount of data it wants to receive using a ReadRequest. This change at the heart of SCC has had a positive impact on outer layers, and especially on the Splitter component type which has become much simpler. [1] http://hackage.haskell.org/package/monoid-subclasses-0.1.1 [2] http://hackage.haskell.org/package/incremental-parser-0.2.2 [3] http://hackage.haskell.org/package/scc-0.8

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ANN: psqueue-benchmarks - benchmarks of priority queue implementations
by Niklas Hambüchen 13 Apr '13

13 Apr '13

(This is a slightly detailed email. If you are the maintainer of one of the packages benchmarked here, you might want to read it though.) Today I was looking for a Priority Queue that also allows a delete operation (some call this a "Priority Search Queue"). I found http://stackoverflow.com/questions/6976559/comparison-of-priority-queue-imp… and after looking at the 10 queue alternatives, I got to the following conclusions: * Only 3 of them allow to delete entries (are p*s*queues) * Most of them are outdated or at least unmaintained for the last 3-5 years * There was an effort to get a priority queue into containers (see the stackoverflow link), but it was not agreed on * Those efforts were driven by Louis Wasserman, who wrote one of the 3 psqueues (queuelike), but now only maintains a non-ps-queue (pqueue) that seems to be the most popular priority queue implementation PSQueue implementations ----------------------- The three packages that are psqueues are: - PSQueue (http://hackage.haskell.org/package/PSQueue-1.1) * original implementation from paper of Ralf Hinze * last upload 2008 * no test suite (but small commented out QC properties), no benchmarks * no code repository - fingertree-psqueue (http://hackage.haskell.org/package/fingertree-psqueue-0.3) * last upload 2011 * no tests, no benchmarks * no code repository - queuelike (http://hackage.haskell.org/package/queuelike-1.0.9) * last upload 2009 * no tests, no benchmarks * no code repository Benchmarks ---------- Unfortunately, none of them had tests, code in repositories or any indication about their real-world performance, so I made some criterion benchmarks. You can find them here: https://github.com/nh2/psqueue-benchmarks Graphs: http://htmlpreview.github.com/?https://raw.github.com/nh2/psqueue-benchmark… Benchmark results ----------------- * PSQueue throws a stack space overflow if you try to put in 100000 Ints * PSQueue suffers from some significant worst case in terms of queue creation, sometimes creating a queue from random numbers just takes 5 times longer (1st graph). This only happens sometimes (despite Criterion) * queuelike creation is instant - it seems to work around my benchmark somehow * converting a queuelike queue to a list surprisingly takes 10 times longer than with the other packages * in terms of average performance, the three are quite close to each other (apart from the point above). queuelike seems fastest, fingertree-psqueue is second and PSQueue slowest, with a difference of +30% to the next one My questions to the maintainers ------------------------------- @Scott: Do you have an idea why PSQueue stack overflows? @Louis: Why did you switch from queuelike to pqueue? Could you put the code up somewhere manageable (repo)? Is it possible to make pqueue a full pSqueue implementation? Niklas

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Will Haskell Platform 2012.4.0.0 uninstall cleanly for the installation of 2013.2.0.0
by KC 12 Apr '13

12 Apr '13

:) -- -- Regards, KC

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Warsaw Haskell Group
by Przemyslaw Kaminski 12 Apr '13

12 Apr '13

Hello! We are organizing a group of Haskell fans from Warsaw, Poland. So far we have set up a community on G+ (https://plus.google.com/u/0/communities/103183708602453146804) a mailing list is on the way. We would like to meet regularly someplace in the capital (we still decide where), and hack on Haskell together as much as we can :) Should there be more people involved, we could arrange regular lectures. Hope to see you there! Warsaw Haskell Group

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unsafeInterleaveST (and IO) is really unsafe [was: meaning of "referential transparency"]
by oleg＠okmij.org 12 Apr '13

12 Apr '13

One may read this message as proving True === False without resorting to IO. In other words, referential transparency, or the substitution of equals for equals, may fail even in expressions of type Bool. This message is intended as an indirect stab at lazy IO. Unfortunately, Lazy IO and even the raw unsafeInterleaveIO, on which it is based, are sometimes recommended, without warnings that usually accompany unsafePerformIO. http://www.haskell.org/pipermail/haskell-cafe/2013-March/107027.html UnsafePerformIO is known to be unsafe, breaking equational reasoning; unsafeInterleaveIO gets a free pass because any computation with it has to be embedded in the IO context in order to be evaluated -- and we can expect anything from IO. But unsafeInterleaveIO has essentially the same code as unsafeInterleaveST: compare unsafeInterleaveST from GHC/ST.lhs with unsafeDupableInterleaveIO from GHC/IO.hs keeping in mind that IO and ST have the same representation, as described in GHC/IO.hs. And unsafeInterleaveST is really unsafe -- not just mildly or somewhat or vaguely unsafe. In breaks equational reasoning, in pure contexts. Here is the evidence. I hope most people believe that the equality on Booleans is symmetric. I mean the function (==) :: Bool -> Bool -> Bool True == True = True False == False = True _ == _ = False or its Prelude implementation. The equality x == y to y == x holds even if either x or y (or both) are undefined. And yet there exists a context that distinguishes x == y from y ==x. That is, there exists bad_ctx :: ((Bool,Bool) -> Bool) -> Bool such that *R> bad_ctx $ \(x,y) -> x == y True *R> bad_ctx $ \(x,y) -> y == x False The function unsafeInterleaveST ought to bear the same stigma as does unsafePerformIO. After all, both masquerade side-effecting computations as pure. Hopefully even lazy IO will get recommended less, and with more caveats. (Lazy IO may be superficially convenient -- so are the absence of typing discipline and the presence of unrestricted mutation, as many people in Python/Ruby/Scheme etc worlds would like to argue.) The complete code: module R where import Control.Monad.ST.Lazy (runST) import Control.Monad.ST.Lazy.Unsafe (unsafeInterleaveST) import Data.STRef.Lazy bad_ctx :: ((Bool,Bool) -> Bool) -> Bool bad_ctx body = body $ runST (do r <- newSTRef False x <- unsafeInterleaveST (writeSTRef r True >> return True) y <- readSTRef r return (x,y)) t1 = bad_ctx $ \(x,y) -> x == y t2 = bad_ctx $ \(x,y) -> y == x

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Web servers: Running them multiple times in a ghci session
by Niklas Hambüchen 12 Apr '13

12 Apr '13

I'm writing a web server app, which I run in ghci: :main localhost 8000 Unfortunately, after Ctrl-C and :reload, running it again: ** Exception: bind: resource busy (Address already in use) This is pretty annoying, because quitting-and-restarting takes a lot of time since I have many dependencies. How do you deal with this? Can you propose some working code that can be wrapped around my main function to make it work? My first idea is running main in a separate process. If you have a working idea, please also post it as an answer on http://stackoverflow.com/questions/15890912/how-do-i-terminate-a-socket-ser… Thanks

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cabal-install 1.16.0.2 on Mac
by Richard A. O'Keefe 12 Apr '13

12 Apr '13

Machine: an Intel Core 2 Duo desktop Mac. OS: Mac OS X 10.7.4 Xcode: 4.6.1 (including command line tools) Haskell: "Haskell Platform 2012.4.0.0 64bit.pkg" downloaded today (GHC 7.4.2) cabal update advised me to install a new cabal-install. m% cabal install cabal-install Resolving dependencies... Downloading Cabal-1.16.0.3... [ 1 of 65] Compiling Distribution.Compat.Exception ( /var/folders/_n/5bc02hb51d361crtq41g_gc4000g8p/T/Cabal-1.16.0.3-85340/Cabal-1.16.0.3/Distribution/Compat/Exception.hs, /var/folders/_n/5bc02hb51d361crtq41g_gc4000g8p/T/Cabal-1.16.0.3-85340/Cabal-1.16.0.3/dist/setup/Distribution/Compat/Exception.o ) <<< snip >>> [52 of 67] Compiling Distribution.Simple.Build.PathsModule ( Distribution/Simple/Build/PathsModule.hs, dist/build/Distribution/Simple/Build/PathsModule.o ) Distribution/Simple/Build/PathsModule.hs:210:19: Warning: Pattern match(es) are non-exhaustive In a case alternative: Patterns not matched: PPC PPC64 Sparc Arm ... [53 of 67] Compiling Distribution.Simple.GHC ( Distribution/Simple/GHC.hs, dist/build/Distribution/Simple/GHC.o ) <<< snip >>> [65 of 65] Compiling Main ( /var/folders/_n/5bc02hb51d361crtq41g_gc4000g8p/T/Cabal-1.16.0.3-85340/Cabal-1.16.0.3/Setup.hs, /var/folders/_n/5bc02hb51d361crtq41g_gc4000g8p/T/Cabal-1.16.0.3-85340/Cabal-1.16.0.3/dist/setup/Main.o ) Linking /var/folders/_n/5bc02hb51d361crtq41g_gc4000g8p/T/Cabal-1.16.0.3-85340/Cabal-1.16.0.3/dist/setup/setup ... Configuring Cabal-1.16.0.3... Building Cabal-1.16.0.3... Preprocessing library Cabal-1.16.0.3... [ 1 of 67] Compiling Paths_Cabal ( dist/build/autogen/Paths_Cabal.hs, dist/build/Paths_Cabal.o ) <<< snip >>> [56 of 65] Compiling Distribution.Client.SetupWrapper ( Distribution/Client/SetupWrapper.hs, dist/build/cabal/cabal-tmp/Distribution/Client/SetupWrapper.o ) Distribution/Client/SetupWrapper.hs:51:12: Warning: In the use of `ghcVerbosityOptions' (imported from Distribution.Simple.GHC): Deprecated: "Use the GhcOptions record instead" [57 of 65] Compiling Distribution.Client.Upload ( Distribution/Client/Upload.hs, dist/build/cabal/cabal-tmp/Distribution/Client/Upload.o ) <<< snip >>> [65 of 65] Compiling Main ( Main.hs, dist/build/cabal/cabal-tmp/Main.o ) Linking dist/build/cabal/cabal ... Installing executable(s) in /home/cshome/o/ok/.cabal/bin /Developer/usr/bin/strip: object: /home/cshome/o/ok/.cabal/bin/cabal malformed object (unknown load command 15) cabal: Error: some packages failed to install: cabal-install-1.16.0.2 failed during the final install step. The exception was: ExitFailure 1 m% file ~/.cabal/bin/cabal /home/cshome/o/ok/.cabal/bin/cabal: Mach-O 64-bit executable x86_64 The strip(1) page ends with this, which may be relevant: LIMITATIONS Not every layout of a Mach-O file can be stripped by this program. But all layouts produced by the Apple compiler system can be stripped. m% otool -l ~/.cabal/bin/cabal <<< snip >>> Load command 14 cmd LC_FUNCTION_STARTS cmdsize 16 dataoff 12743064 datasize 204136 Load command 15 cmd ?(0x00000029) Unknown load command cmdsize 16 00c58f00 00000000 So something is definitely putting something in there that the Xcode 4.6.1 tools do not like.

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exp implementation
by Christopher Howard 11 Apr '13

11 Apr '13

Hi. For my own learning, I wanted to see how the exp function is implemented in GHC. I have GHC 7.4.1 source code open, but I'm having trouble figuring out which file the actual function definition is in. I see expFloat (F# x) = F# (expFloat# x) in libraries/base/GHC/Float.lhs. -- frigidcode.com

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Which generic zipper?
by Luke Evans 11 Apr '13

11 Apr '13

It's sometimes difficult to figure out a good choice when you're searching for a new feature/library (when you go beyond safety of the HP). I'm looking for a generic zipper to (hopefully) navigate over an AST… so a hetrogeneous structure. It's probably sufficient to make this traversable in a fixed way, but a zipper seemed like a good idea. Google says there are lots of options (various papers and their related implementations etc). I see a generic zipper in an 'syz' package that is based on generics, which looks like a nice choice as it's now easy to add the extension for deriving generics on ADTs. However, that package looks pretty old and unmaintained. Does anyone have any suggestions/advice? Cheers -- Luke

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Numerics and Warnings
by Barak A. Pearlmutter 11 Apr '13

11 Apr '13

In fiddling around with some numeric code in Haskell, I noticed some issues. Basically, you get warnings if you write energy mass = mass * c^2 but not if you write energy mass = mass * c * c which seems a bit perverse. Some more examples are below. I understand the inference issues that cause this, but common innocuous cases could---and I would argue, should---be addressed in ad-hoc ways. -- Barak A. Pearlmutter Hamilton Institute & Dept Comp Sci, NUI Maynooth, Co. Kildare, Ireland http://www.bcl.hamilton.ie/~barak/ ---------------- transcript ---------------- $ cat two.hs main :: IO () main = do print 1 print ((2::Float)^3) print 44444444444444444444444444444444 print (555555555555555555555555::Int) -- Each of the four numeric expressions printed above gives a -- compile-time warning. Except the last, which is the only one that is -- actually incorrect. Maybe ghc could eliminate some of these warnings -- by adding extra reasonableness guards. E.g., if the 2nd argument of -- (^) is a constant that fits in an Int, just do that. Basically, if -- used as an argument to a function which has no danger of overflow, -- default to Int without warning. And if a constant doesn't fit in an -- Int, jamming it into one is an error. Or at least a warning! $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.6.2 $ ghc -Wall two.hs [1 of 1] Compiling Main ( two.hs, two.o ) two.hs:3:9: Warning: Defaulting the following constraint(s) to type `Integer' (Num a0) arising from the literal `1' at two.hs:3:9 (Show a0) arising from a use of `print' at two.hs:3:3-7 In the first argument of `print', namely `1' In a stmt of a 'do' block: print 1 In the expression: do { print 1; print ((2 :: Float) ^ 3); print 44444444444444444444444444444444; print (555555555555555555555555 :: Int) } two.hs:4:20: Warning: Defaulting the following constraint(s) to type `Integer' (Integral b0) arising from a use of `^' at two.hs:4:20 (Num b0) arising from the literal `3' at two.hs:4:21 In the first argument of `print', namely `((2 :: Float) ^ 3)' In a stmt of a 'do' block: print ((2 :: Float) ^ 3) In the expression: do { print 1; print ((2 :: Float) ^ 3); print 44444444444444444444444444444444; print (555555555555555555555555 :: Int) } two.hs:5:10: Warning: Defaulting the following constraint(s) to type `Integer' (Num a0) arising from the literal `44444444444444444444444444444444' at two.hs:5:10-41 (Show a0) arising from a use of `print' at two.hs:5:3-7 In the first argument of `print', namely `44444444444444444444444444444444' In a stmt of a 'do' block: print 44444444444444444444444444444444 In the expression: do { print 1; print ((2 :: Float) ^ 3); print 44444444444444444444444444444444; print (555555555555555555555555 :: Int) } Linking two ... $ ./two 1 8.0 44444444444444444444444444444444 -5035712355010463517

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Bug in Network package
by Florian Hofmann 11 Apr '13

11 Apr '13

I might be mistaken, but is there a bug in the Show instance of PortNum? λ PortNum 1 256 λ PortNum 2 512 λ PortNum 3 768 λ let (PortNum x) = PortNum 10 λ x 10 Tested with network-2.4.1.2

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Haskell Weekly News: Issue 265
by Daniel Santa Cruz 11 Apr '13

11 Apr '13

Welcome to issue 265 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of March 31 to April 6, 2013. Quotes of the Week * Jafet: Haskell is an abstract research language used only in academia, education, banking, stock trading, circuit design, embedded systems, cryptography, operating systems research, bioinformatics, phone apps, and web services. * monochrom: primop is where the turtle tower ends and the russian dolls begin! * sclv: haha. i love this stuff but if i bury myself in it my head will start spinning around like exorcist and i will start spitting blood Top Reddit Stories * Haskell beats C using generalised stream fusion - Geoff Mainland, Simon Peyton Jones, Simon Marlow, Roman Leshchinskiy. Submitted to ICFP 2013 [PDF] Domain: research.microsoft.com, Score: 106, Comments: 47 On Reddit: [1] http://goo.gl/Ui7bS Original: [2] http://goo.gl/es51B * Functor is now a superclass of Monad in GHC HEAD Domain: darcs.haskell.org, Score: 87, Comments: 33 On Reddit: [3] http://goo.gl/B1LYf Original: [4] http://goo.gl/JiunP * Yet Another Lambda Blog » Haskell as fast as C: A case study Domain: lambda.jstolarek.com, Score: 61, Comments: 25 On Reddit: [5] http://goo.gl/cemyl Original: [6] http://goo.gl/uCbTd * Beautiful Concurrency - a series of articles by Simon Peyton-Jones - on FP Complete School of Haskell Domain: fpcomplete.com, Score: 60, Comments: 20 On Reddit: [7] http://goo.gl/pWAA4 Original: [8] http://goo.gl/ynFG8 * Web Woes for Haskell Domain: self.haskell, Score: 43, Comments: 42 On Reddit: [9] http://goo.gl/vl13n Original: [10] http://goo.gl/vl13n * Cap'n Proto: faster protocol buffers, written in Haskell & C++ Domain: kentonv.github.com, Score: 42, Comments: 11 On Reddit: [11] http://goo.gl/6Qwza Original: [12] http://goo.gl/94iip * Resource limits for Haskell Domain: blog.ezyang.com, Score: 38, Comments: 1 On Reddit: [13] http://goo.gl/01WPS Original: [14] http://goo.gl/KjwNx * Dependent Types - A New Paradigm? by Paul Callaghan Domain: pragprog.com, Score: 33, Comments: 1 On Reddit: [15] http://goo.gl/Gjs3K Original: [16] http://goo.gl/4wbYa * So I accidentally a profunctors tutorial Domain: fpcomplete.com, Score: 31, Comments: 43 On Reddit: [17] http://goo.gl/QnzoR Original: [18] http://goo.gl/VDlYh * UPenn CIS 194: Intro to Haskell, Instructor: Brent Yorgey Domain: seas.upenn.edu, Score: 31, Comments: 4 On Reddit: [19] http://goo.gl/zXgrM Original: [20] http://goo.gl/OC3x1 * Sorting with Agda Domain: mazzo.li, Score: 30, Comments: 33 On Reddit: [21] http://goo.gl/rkq58 Original: [22] http://goo.gl/8GRCF * Announcing The Pragmatic Haskeller Domain: cakesolutions.net, Score: 29, Comments: 7 On Reddit: [23] http://goo.gl/3ahrb Original: [24] http://goo.gl/AsOKQ * Flavours of free applicative functors Domain: ro-che.info, Score: 27, Comments: 6 On Reddit: [25] http://goo.gl/oE2sk Original: [26] http://goo.gl/sE5VD * Free Applicative Functors (ICFP submission) Domain: paolocapriotti.com, Score: 25, Comments: 11 On Reddit: [27] http://goo.gl/2dB7B Original: [28] http://goo.gl/VWnHG * Handling out of memory? Domain: self.haskell, Score: 25, Comments: 17 On Reddit: [29] http://goo.gl/mGwUt Original: [30] http://goo.gl/mGwUt * hydra-print - visualizing multiple texts streams Domain: parfunk.blogspot.se, Score: 24, Comments: 7 On Reddit: [31] http://goo.gl/uToAZ Original: [32] http://goo.gl/lc70w * Lazy IO breaks equational reasoning Domain: okmij.org, Score: 24, Comments: 124 On Reddit: [33] http://goo.gl/48f0h Original: [34] http://goo.gl/m8Vbr * Haskell on Windows Domain: gundersen.net, Score: 23, Comments: 14 On Reddit: [35] http://goo.gl/oRV4B Original: [36] http://goo.gl/0hOL4 * Constrained Monads (GADTs indexed by Constraint) Domain: ittc.ku.edu, Score: 21, Comments: 7 On Reddit: [37] http://goo.gl/lVRpl Original: [38] http://goo.gl/jtbU3 Top StackOverflow Questions * Simple examples to illustrate Category, Monoid and Monad? votes: 28, answers: 1 Read on SO: [39] http://goo.gl/HPE8k * Which techniques have contributed the most to Haskell's improving performance? [closed] votes: 18, answers: 1 Read on SO: [40] http://goo.gl/tz904 * Why does Parsec not use Control.Applicative operators votes: 13, answers: 1 Read on SO: [41] http://goo.gl/0mR3f * ghc 7.4.2, Dynamically calling modules votes: 10, answers: 1 Read on SO: [42] http://goo.gl/MZD8q * What's wrong with this implementation of quicksort using Arrows? votes: 10, answers: 2 Read on SO: [43] http://goo.gl/zwhfy * Initializing algebraic data type from list votes: 9, answers: 4 Read on SO: [44] http://goo.gl/nbyUP * Scoped type variables require explicit foralls. Why? votes: 9, answers: 1 Read on SO: [45] http://goo.gl/SUlCf * Parsing UnicodeSyntax with haskell-src-exts votes: 8, answers: 1 Read on SO: [46] http://goo.gl/Yi8Vd Until next time, +Daniel Santa Cruz References 1. http://research.microsoft.com/en-us/um/people/simonpj/papers/ndp/haskell-be… 2. http://www.reddit.com/r/haskell/comments/1br0ls/haskell_beats_c_using_gener… 3. http://darcs.haskell.org/libraries/base/GHC/Base.lhs 4. http://www.reddit.com/r/haskell/comments/1bfojn/functor_is_now_a_superclass… 5. http://lambda.jstolarek.com/2013/04/haskell-as-fast-as-c-a-case-study/ 6. http://www.reddit.com/r/haskell/comments/1bikvs/yet_another_lambda_blog_has… 7. https://www.fpcomplete.com/user/simonpj/beautiful-concurrency 8. http://www.reddit.com/r/haskell/comments/1bm3ep/beautiful_concurrency_a_ser… 9. http://www.reddit.com/r/haskell/comments/1bqsvi/web_woes_for_haskell/ 10. http://www.reddit.com/r/haskell/comments/1bqsvi/web_woes_for_haskell/ 11. http://kentonv.github.com/capnproto/install.html 12. http://www.reddit.com/r/haskell/comments/1bjt4f/capn_proto_faster_protocol_… 13. http://blog.ezyang.com/2013/04/resource-limits-for-haskell/ 14. http://www.reddit.com/r/haskell/comments/1bkjj8/resource_limits_for_haskell/ 15. http://pragprog.com/magazines/2013-04/dependent-types 16. http://www.reddit.com/r/haskell/comments/1bmqba/dependent_types_a_new_parad… 17. https://www.fpcomplete.com/user/liyang/profunctors 18. http://www.reddit.com/r/haskell/comments/1bg21c/so_i_accidentally_a_profunc… 19. http://www.seas.upenn.edu/~cis194/lectures.html 20. http://www.reddit.com/r/haskell/comments/1bqax8/upenn_cis_194_intro_to_hask… 21. http://mazzo.li/posts/AgdaSort.html 22. http://www.reddit.com/r/haskell/comments/1biasi/sorting_with_agda/ 23. http://www.cakesolutions.net/teamblogs/2013/04/05/announcing-the-pragmatic-… 24. http://www.reddit.com/r/haskell/comments/1brbac/announcing_the_pragmatic_ha… 25. http://ro-che.info/articles/2013-03-31-flavours-of-free-applicative-functor… 26. http://www.reddit.com/r/haskell/comments/1be2iv/flavours_of_free_applicativ… 27. http://paolocapriotti.com/blog/2013/04/03/free-applicative-functors/ 28. http://www.reddit.com/r/haskell/comments/1bnql3/free_applicative_functors_i… 29. http://www.reddit.com/r/haskell/comments/1bsucw/handling_out_of_memory/ 30. http://www.reddit.com/r/haskell/comments/1bsucw/handling_out_of_memory/ 31. http://parfunk.blogspot.se/2013/04/hydra-print.html 32. http://www.reddit.com/r/haskell/comments/1bsc9u/hydraprint_visualizing_mult… 33. http://okmij.org/ftp/Haskell/index.html#lazyIO-not-True 34. http://www.reddit.com/r/haskell/comments/1bsitm/lazy_io_breaks_equational_r… 35. http://www.gundersen.net/haskell-on-windows/ 36. http://www.reddit.com/r/haskell/comments/1bsm6v/haskell_on_windows/ 37. http://www.ittc.ku.edu/csdl/fpg/papers/Sculthorpe-13-ConstrainedMonad.html 38. http://www.reddit.com/r/haskell/comments/1bktx3/constrained_monads_gadts_in… 39. http://stackoverflow.com/questions/15726733/simple-examples-to-illustrate-c… 40. http://stackoverflow.com/questions/15750998/which-techniques-have-contribut… 41. http://stackoverflow.com/questions/15767978/why-does-parsec-not-use-control… 42. http://stackoverflow.com/questions/15824756/ghc-7-4-2-dynamically-calling-m… 43. http://stackoverflow.com/questions/15849550/whats-wrong-with-this-implement… 44. http://stackoverflow.com/questions/15734573/initializing-algebraic-data-typ… 45. http://stackoverflow.com/questions/15800878/scoped-type-variables-require-e… 46. http://stackoverflow.com/questions/15784076/parsing-unicodesyntax-with-hask… 47. https://plus.google.com/105107667630152149014/about

1 0

Automated Differentiation of Matrices (hmatrix)
by Dominic Steinitz 10 Apr '13

10 Apr '13

Hi Cafe, Suppose I want to find the grad of a function then it's easy I just use http://hackage.haskell.org/package/ad-3.4: import Numeric.AD import Data.Foldable (Foldable) import Data.Traversable (Traversable) data MyMatrix a = MyMatrix (a, a) deriving (Show, Functor, Foldable, Traversable) f :: Floating a => MyMatrix a -> a f (MyMatrix (x, y)) = exp $ negate $ (x^2 + y^2) / 2.0 main :: IO () main = do putStrLn $ show $ f $ MyMatrix (0.0, 0.0) putStrLn $ show $ grad f $ MyMatrix (0.0, 0.0) But now suppose I am doing some matrix calculations http://hackage.haskell.org/package/hmatrix-0.14.1.0 and I want to find the grad of a function of a matrix: import Numeric.AD import Numeric.LinearAlgebra import Data.Foldable (Foldable) import Data.Traversable (Traversable) g :: (Element a, Floating a) => Matrix a -> a g m = exp $ negate $ (x^2 + y^2) / 2.0 where r = (toLists m)!!0 x = r!!0 y = r!!1 main :: IO () main = do putStrLn $ show $ g $ (1 >< 2) ([0.0, 0.0] :: [Double]) putStrLn $ show $ grad g $ (1 >< 2) ([0.0, 0.0] :: [Double]) Then I am in trouble: /Users/dom/Dropbox/Private/Whales/MyAD.hs:24:21: No instance for (Traversable Matrix) arising from a use of `grad' Possible fix: add an instance declaration for (Traversable Matrix) In the expression: grad g In the second argument of `($)', namely `grad g $ (1 >< 2) ([0.0, 0.0] :: [Double])' In the second argument of `($)', namely `show $ grad g $ (1 >< 2) ([0.0, 0.0] :: [Double])' /Users/dom/Dropbox/Private/Whales/MyAD.hs:24:26: Could not deduce (Element (ad-3.4:Numeric.AD.Internal.Types.AD s Double)) arising from a use of `g' from the context (Numeric.AD.Internal.Classes.Mode s) bound by a type expected by the context: Numeric.AD.Internal.Classes.Mode s => Matrix (ad-3.4:Numeric.AD.Internal.Types.AD s Double) -> ad-3.4:Numeric.AD.Internal.Types.AD s Double at /Users/dom/Dropbox/Private/Whales/MyAD.hs:24:21-26 Possible fix: add an instance declaration for (Element (ad-3.4:Numeric.AD.Internal.Types.AD s Double)) In the first argument of `grad', namely `g' In the expression: grad g In the second argument of `($)', namely `grad g $ (1 >< 2) ([0.0, 0.0] :: [Double])' What are my options here? Clearly I can convert my matrix into a list (which is traversable), find the grad and convert it back into a matrix but given I am doing numerical calculations and speed is an important factor, this seems undesirable. I think I would have the same problem with: http://hackage.haskell.org/package/repa http://hackage.haskell.org/package/yarr-1.3.1 although I haven'¯t checked. Thanks, Dominic.

3 5

abs on Float/Doubles
by Levent Erkok 10 Apr '13

10 Apr '13

Hello all, I definitely do not want to create a yet another thread on handling of floating-point values in Haskell. My intention is only to see what can be done to make it more "compilant" with IEEE754. (Also, my interest is not a theoretical one, but rather entirely practical: I'm interested in using SMT solvers for reasoning about Haskell programs, in particular using the new SMT logics that cover IEEE-754 semantics. So, any discrepancy between Haskell and IEEE-754 is a cause for concern as we strive to build powerful tools around the Haskell eco-system.) In IEEE-754, there are two zero values: +0, and -0; inhabiting all floating point types. IEEE-754 requires these two compare equal, but be distinguished when used as arguments to functions as they can produce different results. (Unfortunately the standard is not freely available, but there's a nice discussion in wikipedia: http://en.wikipedia.org/wiki/Negative_zero.) As far as I know, Haskell's floating-point handling does *not* explicitly claim to be IEEE-754 compliant, but I think it's safe to assume that any serious implementation will take advantage of the underlying CPU's floating-point facilities; and thus will use IEEE-754 semantics. In particular, Haskell supports both +0 and -0; and you can distinguish them using the isNegativeZero function of the RealFloat class. Furthermore, the show instance for Float/Double will turn these two values to strings differently, preserving the minus sign if it receives a negative-zero. Comparisons follow the IEEE rules; so all these behaviors seem to be in accordance with IEEE754. Unfortunately, the same isn't true for the abs function. In Haskell, the call: "isNegativeZero (abs (-0::Double))" evaluates to "True". IEEE754 requires abs to always return "+0" in this case. The same also holds for the semantics of the newly proposed SMT-Lib logic for IEEE754: See section 3.3.1 (page 8) of: http://www.philipp.ruemmer.org/publications/smt-fpa.pdf The fix is easy. A one line change in the Double/Float instances of the Num class in the Prelude would guarantee the IEEE754 semantics. The reason I'm not simply posting this as a "library" issue is that it appears the topic came up in stack-overflow a while ago; without any clear resolution: http://stackoverflow.com/questions/10395761/absolute-value-of-negative-zero… It appears that the consensus is that this is a historical definition dating back to the times when IEEE754 itself wasn't quite clear on the topic itself, and "so nobody thought that hard about negative zeroes." (The quote is from a comment from Lennart.) I think it's a safe bet to assume IEEE754 semantics (for better or worse) is here to stay, and it would be nice to reduce any discrepancy we have in the Haskell libraries with respect to it. So, if there's consensus, I'd like to propose a library change to implement the proper IEEE754 semantics for the abs function for Double and Float types. Note that the argument is not really about semantics here, but rather with compliance to the accepted industrial standard. So, it would be best if we kept the discussion purely to the IEEE-compliance here, as opposed to relative merits of the semantics as defined. -Levent.

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SBLP 2013: Final CFP, deadline for Abstracs 19/4
by Andre Rauber Du Bois 10 Apr '13

10 Apr '13

[Apologies if you receive multiple copies of this CFP] ======================================================= CALL FOR PAPERS 17th BRAZILIAN SYMPOSIUM ON PROGRAMMING LANGUAGES Brasília, Distrito Federal, Brazil September 29th to October 4th, 2013 http://cbsoft2013.unb.br/en/sblp-en ======================================================== IMPORTANT DATES Paper abstract submission (15 lines): April 19th Full paper submission: April 26th, 2013 Notification of acceptance: May 31st, 2013 Final papers due: June 28th, 2013 INVITED SPEAKERS * Tim Harris, Oracle Labs * More TBA INTRODUCTION The 17th Brazilian Symposium on Programming Languages, SBLP 2013, will be held in Brasília, Brazil, on September 29th to October 4th, 2013. SBLP provides a venue for researchers and practitioners interested in the fundamental principles and innovations in the design and implementation of programming languages and systems. The symposium will be part of the 4th Brazilian Conference on Software: Theory and Practice, CBSoft 2013, http://cbsoft2013.cic.unb.br/, which will host four traditional, well-established symposia: * XXVII Brazilian Symposium on Software Engineering (SBES) * XVII Brazilian Symposium on Programming Languages (SBLP) * XVI Brazilian Symposium on Formal Methods (SBMF) * VII Brazilian Symposium on Components, Software Architecture and Software Reuse (SBCARS) SBLP 2013 invites authors to contribute with technical papers related (but not limited) to: * Program generation and transformation, including domain-specific languages and model-driven development in the context of programming languages. * Programming paradigms and styles, including functional, object-oriented, aspect-oriented, scripting languages, real-time, service-oriented, multithreaded, parallel, and distributed programming. * Formal semantics and theoretical foundations, including denotational, operational, algebraic and categorical. * Program analysis and verification, including type systems, static analysis and abstract interpretation. * Programming language design and implementation, including new programming models, programming language environments, compilation and interpretation techniques. SUBMISSIONS Contributions should be written in Portuguese or English. We solicit papers that should fall into one of two different categories: full papers, with at most 15 pages, or short papers, with at most 5 pages. Full papers submitted in English will be published in a volume of Lecture Notes in Computer Science (LNCS), by Springer. For this reason, all papers must be prepared using the LNCS template, available at http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0. We encourage the submission of short papers reporting on partial results of on-going master dissertations or doctoral theses. All accepted papers will be published in the conference proceedings distributed in a digital media by the CBSOFT organizers. Submissions should be done using SBLP 2013 installation of the EasyChair conference mangement system at http://www.easychair.org/conferences/?conf=sblp2013. As in previous editions, a journal special issue, with selected papers from accepted contributions, is anticipated. Selected papers from 2003 to 2008 editions of SBLP were published in special issues of the Journal of Universal Computer Science, by Springer. The post-proceedings of SBLP from 2009 to 2012, also with selected papers from the conference proceedings, are being edited as special issues of Science of Computer Programming, published by Elsevier. CBSOFT CHAIRS Genaina Nunes Rodrigues, UnB Rodrigo Bonifácio, UnB Diego Aranha, UnB PROGRAMME CHAIRS André Rauber Du Bois, UFPel Phil Trinder, Glasgow University PROGRAMME COMMITTEE Alberto Pardo, Univ. de La Republica Alex Garcia, IME Alvaro Freitas Moreira, UFRGS André Rauber Du Bois, UFPel (co-chair) Andre Santos, UFPE Carlos Camarao, UFMG Christiano Braga, UFF Edwin Brady, University of St. Andrews Fernando Castor Filho, UFPE Fernando Quintão Pereira, UFMG Francisco Heron de Carvalho Junior, UFC Hans-Wofgang Loidl, Heriot-Watt University Jeremy Singer, Glasgow University Joao Saraiva, Universidade do Minho João F. Ferreira, Teesside University Lucilia Figueiredo, UFOP Luis Soares Barbosa, Univ. do Minho Manuel António Martins, Univ. de Aveiro Marcelo A. Maia, UFU Marcello Bonsangue, Leiden Univ/CWI Marcelo d'Amorim, UFPE Marco Tulio Valente, UFMG Mariza A. S. Bigonha, UFMG Martin A. Musicante, UFRN Noemi Rodriguez, PUC-Rio Peter Mosses, Swansea University Phil Trinder, Glasgow University (co-chair) Qiu Zongyang, Beijing University Rafael Dueire Lins, UFPE Renato Cerqueira, PUC-Rio Ricardo Massa, UFPE Roberto S. Bigonha, UFMG Roberto Ierusalimschy, PUC-Rio Sandro Rigo, UNICAMP Sergio Soares, UFPE Simon Thompson, Univ. of Kent Varmo Vene, Univ. de Tartu

1 0

ghc head creates premature tarball
by Takayuki Muranushi 10 Apr '13

10 Apr '13

Hi all, These few days I've been having trouble in attempt to use GHC head. I follow the following standard steps to create a tarball: $ git clone http://darcs.haskell.org/ghc.git/ $ cd ghc $ # optionally, git checkout <branch name> $ ./sync-all --testsuite get $ cp mk/build.mk.sample mk/build.mk $ ### edit BuildFlavour to quick ### $ ./boot $ ./configure $ make -jN # <N> is the number of cores you have. $ make binary-dist and then sandbox-install the tarball by hsenv. However, the installed ghc exhibits the following strange behaviors: (1) several .so files need to be manually copied to /usr/lib . (2) the installed ghc doesn't find any package, even 'base' or 'ghc-prim' Then carefully looking, I found that `make binary-dist` is outputting some error messages,: > cd bindistprep && "/bin/tar" hcf - -T ../bindist-list | bzip2 -c > ../bindistprep/ghc-7.7.20130409-x86_64-unknown-linux.tar.bz2 > /bin/tar: ghc-7.7.20130409/compiler/stage2/doc: Cannot stat: No such file or directory > /bin/tar: Exiting with failure status due to previous errors The file "./bindist-list" included a filepath that doesn't exist, so that tar command aborted there and every item after that line on the list was excluded from the tarball. So I tested the following trick; $ touch compiler/stage2/doc Then the previous error message didn't show up any more, and the spurious behavior (1) (2) didn't appear any more. I'm wondering if this is an installer bug or I was doing something wrong. I'd appreciate any advice. Best, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

1 0

[Haskell-cafe] algorithm-for-finding-numerical-permutation-given-lexicographic-index
by Lone Wolf 10 Apr '13

10 Apr '13

> > > http://stackoverflow.com/questions/8940470/algorithm-for-finding-numerical-… > > How would you rewrite this into Haskell? The code snippet is in Scala. > > /** > example: index:=15, list:=(1, 2, 3, 4) > */ > def permutationIndex (index: Int, list: List [Int]) : List [Int] = > if (list.isEmpty) list else { > val len = list.size // len = 4 > val max = fac (len) // max = 24 > val divisor = max / len // divisor = 6 > val i = index / divisor // i = 2 > val el = list (i) > el :: permutationIndex (index - divisor * i, list.filter (_ != el)) } > > >

3 3

[ANN] hoodle-publish-0.1 : publish your hoodle files on the web
by Ian-Woo Kim 10 Apr '13

10 Apr '13

Hi, all, I am happy to announce a utility hoodle-publish for publishing hoodle files on the web as a form of pdf files. I am planning to support html publishing later. For those who do not know what hoodle is, hoodle (http://ianwookim.org/hoodle) is a pen notetaking program written entirely in haskell. hoodle-publish is a shell command which takes hoodle files in a directory recursively and transforms them to pdf files preserving hyperlinks. For detailed explanation, please refer to my blog post : http://ianwookim.org/blog/posts/2013/04-09-publish-your-hoodle-files-on-the… A sample hoodle publication is also shown in http://ianwookim.org/sasha/hoodle/main.pdf Enjoy! Ian-Woo

1 0

GSoC Project Proposal: Markdown support for Haddock
by Johan Tibell 10 Apr '13

10 Apr '13

Hi all, Haddock's current markup language leaves something to be desired once you want to write more serious documentation (e.g. several paragraphs of introductory text at the top of the module doc). Several features are lacking (bold text, links that render as text instead of URLs, inline HTML). I suggest that we implement an alternative haddock syntax that's a superset of Markdown. It's a superset in the sense that we still want to support linkifying Haskell identifiers, etc. Modules that want to use the new syntax (which will probably be incompatible with the current syntax) can set: {-# HADDOCK Markdown #-} on top of the source file. Ticket: http://trac.haskell.org/haddock/ticket/244 -- Johan

28 45

Prolog-style patterns
by Jan Stolarek 09 Apr '13

09 Apr '13

Hi all, consider this simple reimplementation of 'elem' function: member :: Eq a => a -> [a] -> Bool member _ [] = False member y (x:xs) | x == y = True | otherwise = member y xs If Haskell allowed to write pattern matching similar to Prolog then we could write this function like this: member :: Eq a => a -> [a] -> Bool member _ [] = False member x (x:_) = True member x (_:xs) = member x xs The meaning of pattern in the second equation is "match this equation if the first argument equals to head of the list". Many times I have found myself instinctively writing patterns in this way, only to get a compilation error. I was thinking about implementing language extension for GHC that would allow to write Prolog-style patterns. Would there be an interest in such an extension? Also, if I missed something obvious please let me know. Janek

15 21

Can any hackage library do JWT signing
by Ryan Newton 09 Apr '13

09 Apr '13

I want to enable a Haskell benchmarking bot that would upload its data to a Google Fusion Table each time it runs. That seems to require the "service-to-service" mode for Google's OAuth2: https://developers.google.com/accounts/docs/OAuth2ServiceAccount There are a bunch of libraries on Hackage for different flavors of OAuth. But I haven't yet found one that handles this signing of JWT headers and claimsets. Is there something I'm missing? Thanks, -Ryan

1 0

Re: [Haskell-cafe] [Caml-list] Vector Fabrics is hiring!
by Alexey Rodriguez 09 Apr '13

09 Apr '13

Hi Phil, I am posting this on behalf of Martijn Rutten. Cheers, Alexey 8-<--------------- Fair point. When we started the company in 2007, the experts in Silicon Valley told us that our ambition to develop tools to create an efficient, parallel embedded system from sequential C and C++ code is the "holy grail" of embedded system design. In the same sentence, they told us it could not be done. It is widely acknowledged as well as debated that there can be an order of magnitude difference between a real good and an average programmer (http://programmers.stackexchange.com/questions/14052/is-there-such-a-thing-…)) Often these talented programmers are called a "rockstar", "free electron" (http://www.randsinrepose.com/archives/2005/03/20/free_electron.html) or just plain "awesome" for their ability to quickly sort out the many trade-offs in software architecture and design. Without these talented programmers to design and implement the algorithms to make sense out of the literally millions of data dependencies that can make or break parallelism, our company's vision would still be just that: a vision. You are right, if the talent comes with a rockstar's ego, forget it. No matter how smart, you are always part of a team. As for craftsmanship, what we did learn is the huge difference between functional programming and imperative programming. Our analysis engine is fully written in OCaml. Obviously that allowed us to focus on the algorithmic problems at hand without ever wasting time in debugging stray pointers (the one week debugging such a Heisenbug was spent in the few hundred lines of C interfacing with OCaml). I am sure I am preaching to the choir here when I say we learned that thinking functionally allowed us to much better reason at the algorithmic level and we generally wrote significantly cleaner and more reliable code. My 2 cents, Martijn Rutten, Co-founder of Vector Fabrics. On Mon, Apr 8, 2013 at 10:09 PM, Phil Tomson <philtomson(a)gmail.com> wrote: > On Sunday, April 7, 2013 10:30:40 PM UTC-7, Stefan Holdermans wrote: > >> >> * Your friends and colleagues describe you as a "rockstar" programmer; >> >> your programming ability is way above average; >> > > I thought we (as in industry) were over this "rockstar programmer" thing? What does a "rockstar" do? S/He breaks guitars when not happy. Rockstars are petulant and hard to work with. We need less rockstars and more craftspeople. > > Just my 2 cents. > > Phil > -- Alexey Rodriguez Yakushev O +31 (0)40 8200960 | D + 31 (0)40 8200974 | F +31 (0)40 8200979 Vonderweg 22, 5616 RM | Eindhoven | The Netherlands www.vectorfabrics.com | alexey(a)vectorfabrics.com On Mon, Apr 8, 2013 at 10:09 PM, Phil Tomson <philtomson(a)gmail.com> wrote: > On Sunday, April 7, 2013 10:30:40 PM UTC-7, Stefan Holdermans wrote: > >> >> * Your friends and colleagues describe you as a "rockstar" programmer; >> >> your programming ability is way above average; >> > > I thought we (as in industry) were over this "rockstar programmer" thing? What does a "rockstar" do? S/He breaks guitars when not happy. Rockstars are petulant and hard to work with. We need less rockstars and more craftspeople. > > Just my 2 cents. > > Phil > -- Alexey Rodriguez Yakushev O +31 (0)40 8200960 | D + 31 (0)40 8200974 | F +31 (0)40 8200979 Vonderweg 22, 5616 RM | Eindhoven | The Netherlands www.vectorfabrics.com | alexey(a)vectorfabrics.com

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[Conduit] weird action of leftover.
by Magicloud Magiclouds 09 Apr '13

09 Apr '13

Say I have code like below. If I comment the leftover in main, I got (Just "L1\n", Just "L2\n", Just "L3\n", Just "L4\n"). But if I did not comment the leftover, then I got (Just "L1\n", Just "L1\n", Just "", Just "L2\n"). Why is not it (Just "L1\n", Just "L1\n", Just "L2\n", Just "L3\n")? takeLine :: (Monad m) => Consumer ByteString m (Maybe ByteString) takeLine = do mBS <- await case mBS of Nothing -> return Nothing Just bs -> case DBS.elemIndex _lf bs of Nothing -> return $ Just bs Just i -> do let (l, ls) = DBS.splitAt (i + 1) bs leftover ls return $ Just l main = do m <- runResourceT $ sourceFile "test.simple" $$ (do a <- takeLine leftover $ fromJust a b <- takeLine c <- takeLine d <- takeLine return (a, b, c, d)) print m -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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[haskell.org Summer of Code 2013] We're In!
by Edward Kmett 08 Apr '13

08 Apr '13

We (haskell.org) have been officially accepted into the Google Summer of Code for 2013. We should show up in the mentoring organization list as soon as I get some information we need to finalize the listing. Shachaf Ben-Kiki has volunteered to help out as our backup org administrator this year. If you are thinking about joining up as a mentor or a student this year, now would be a good time to start brainstorming about project ideas! I'll follow up with more information as soon as the listing goes in. In the meantime there is the #haskell-gsoc channel on irc.freenode.net. Feel free to pester me (or Shachaf) with questions! -Edward Kmett

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llvm-3.0.1.0 installation on Mac
by Luke Evans 08 Apr '13

08 Apr '13

I'm trying to get LLVM 3.0 installed and then have the llvm-3.0.1.0 package install and bind against it with cabal. I'm on a recent 64 bit Mac which shows up in various messages like: "Target platform inferred as: x86_64-apple-darwin" I was advised on the Haskell IRC channel to install a 3.0 LLVM from a location like MacPorts first, then point cabal at the location where LLVM was installed with the appropriate config option. So, I did a "sudo ./port install llvm-3.0", and everything seemed to proceed fine. Then I did: cabal install llvm --configure-option=--with-llvm-prefix=/opt/local/libexec/llvm-3.0 This looks promising (compared to missing the config option), but eventually this happens: Building llvm-base-3.0.1.0... Preprocessing library llvm-base-3.0.1.0... dyld: Library not loaded: /Users/ian/zz64/ghc-7.4.2/libffi/build/inst/lib/libffi.5.dylib Referenced from: /private/var/folders/tz/vlbc03q5333897z201hkmcb40000gn/T/llvm-base-3.0.1.0-3941/llvm-base-3.0.1.0/dist/build/LLVM/FFI/Analysis_hsc_make Reason: image not found running dist/build/LLVM/FFI/Analysis_hsc_make failed (exit code 5) command was: dist/build/LLVM/FFI/Analysis_hsc_make >dist/build/LLVM/FFI/Analysis.hs Failed to install llvm-base-3.0.1.0 I have no idea where "/Users/ian/..." is coming from! My cabal-fu is extremely weak, so I'll plug on and see if I can figure out how cabal's analysis/build might be going wrong here. I'd be very grateful for any clues though! -- Luke

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%==%, :==: and type equality in the singletons library
by Paul Brauner 08 Apr '13

08 Apr '13

Hello, from the output of -ddump-splices I dont think it is the case but I'm asking anyway: is there any way to deduce a ~ b from a :==: b? Given data T = C1 | ... | Cn I can easily derive data EqT :: T -> T -> * where EqT :: a ~ b => EqT a b eqT :: ST a -> ST b -> Maybe (EqT a b) eqT SC1 SC1 = Just EqT ... eqT SCn SCn = Just EqT eqT _ _ = Nothing but this kind of replicates the boilerplate generated by the singletons library for %==%. However I can't see how to leverage %==% to inhabit eqT since I can't deduce a ~ b from a %==% b == STrue. Any idea? If there's no way to write eqT using what singletons generates, wouldn't it make sense for it to generate something that relates :==: and ~ ? Cheers, Paul

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Re: [Haskell-cafe] Prolog-style patterns
by Stephen Tetley 08 Apr '13

08 Apr '13

This is a recurring theme, see also here: http://www.haskell.org/pipermail/haskell-cafe/2009-May/061498.html On 8 April 2013 16:53, Tom Murphy <amindfv(a)gmail.com> wrote: > > Also, for some history, this was discussed a while back: > http://www.mail-archive.com/haskell@haskell.org/msg03721.html >

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My first Haskell program
by John Wood 08 Apr '13

08 Apr '13

Hello, Cafe I'm new to Haskell and the mailing list, and am wondering if I could get some feedback on my first program -- a Markov text generator. The code is posted here:http://codereview.stackexchange.com/questions/24791/haskell-markov-tex… Thanks, John;

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Vector Fabrics is hiring!
by Stefan Holdermans 08 Apr '13

08 Apr '13

[Apologies for multiple postings.] Vector Fabrics is hiring: we are looking for a top-notch programmer to extend our program-analysis and parallelization products. You design and implement algorithms to assist the programmer to create a parallel design from a sequential C or C++ program. You work with our international team of world-class computer scientists and experts in the Haskell / OCaml functional programming languages. Your work is at the forefront of technology, giving you the opportunity to publish your work in major conferences and directly cooperate with processor design companies and domain-specific application vendors. As we are a startup company, you will quickly have a major impact on our products and get to know all aspects of product creation. You will be part of a strongly committed development team and contribute to our agile development process and automated test suites. Interested? Send your CV, GitHub account or other proof of what you can do to jobs(a)vectorfabrics.com. RESPONSIBILITIES * Design and implement software optimization (e.g. parallelization) algorithms for CPUs and GPUs; * Thoroughly test your code, create automated test suites; * Contribute to our agile development planning and process; * Analyze complex customer applications for optimization opportunities and translate this to new analysis algorithms. PROFILE * Your friends and colleagues describe you as a "rockstar" programmer; your programming ability is way above average; * Demonstrable experience in design and implementation of complex software applications; prior experience in functional programming languages is preferred; * You continuously surprise us with your creative yet pragmatic solutions for complex software problems; * You are strongly committed to deliver working software as early as possible; * You work against very high quality standards. Refactoring is your bread and butter, pair-programming is how you prefer to review your code; * Whatever technologies, languages, or development environments you've been using, we expect you have mastered them in depth, and we expect that you will be able to master any technology, language, or development environment that we need in the future; * You are not afraid to get your hands dirty on low-level code, hijack a malloc() call in the standard C library, port the latest gdb debugger to Android on a jail-broken tablet? You get it done. * Excellent command of written and spoken English. EDUCATION MSc, MEng or PhD in Computer Science or significant relevant experience. ABOUT VECTOR FABRICS Vector Fabrics is a high-tech software company, developing tools for embedded multicore programming. Its technology and expertise is getting widespread recognition in the industry as being innovative and unique in their ability to address heterogeneous multicore application-specific silicon platforms. Due to the advanced nature of its tools, Vector Fabrics operates at the forefront of the next generation of embedded platforms for diverse markets ranging from supercomputers to automotive to cell phones. Vector Fabrics puts absolute priority on hiring top class individuals in key positions. Vector Fabrics’ team profile is exceptional and its ambition is to hire only individuals that match or surpass that profile. The company pays top salary and offers a challenging, engaging and stimulating work environment with a high degree of responsibility.

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Compilers book in Haskell
by Andrés Sicard-Ramírez 08 Apr '13

08 Apr '13

Juan, te puede interesar On Sat, Apr 6, 2013 at 5:56 PM, Sergey Bushnyak <sergey.bushnyak(a)sigrlami.eu > wrote: > I will recommend you book "Modern Compiler Design" by Dick Grune and > others. > Besides discussing different topics, authors use Haskell as example for > describing ideas behind compilers for functional language. > -- Andrés

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ANNOUNCE: binary-conduit-1.0
by Alexander V Vershilov 07 Apr '13

07 Apr '13

I'm happy to announce new binary-conduit [1] library, that's a serialization/deserialization library for using binary [2] package. [1] http://hackage.haskell.org/package/binary-conduit [2] http://hackage.haskell.org/package/binary -- Alexander

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ANNOUNCE: antiquoter-0.1.0.0
by L Corbijn 07 Apr '13

07 Apr '13

Hello cafe, I'm happy to announce the release of my first package antiquoter [1], a combinator library for writing quasiquoters and antiquoters. The main aim is to simplify their definitions and reduce copy-and-paste programming. The main feature the current version is trying to solve is code duplication in antiquoters. The problem for writing antiquoters is that it need to be done twice, once for use as an expression and once for use as a pattern. These two implementations are very similar, which leads to copy-and-paste programming. For a good example of what this leads to see the antiquoters of language-c-quote[2], where almost every antiquoter is written twice. A very simple example with the rewritten version is provided in the documentation [3]. Lars [1]:http://hackage.haskell.org/package/antiquoter [2]: http://hackage.haskell.org/packages/archive/language-c-quote/0.7.1/doc/html… [3]: http://hackage.haskell.org/packages/archive/antiquoter/0.1.0.0/doc/html/Lan… [4]:https://github.com/Laar/antiquoter

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Error when building executable with profiling enabled
by Nikolaos Bezirgiannis 07 Apr '13

07 Apr '13

Hello, i have an executable where i want to run profiling on. I do: ghc --make -prof -auto-all -fth Custom.hs but it does not built and the output is: bench\HLogo\Custom\Custom.hs:1:1: Dynamic linking required, but this is a non-standard build (eg. prof). You need to build the program twice: once the normal way, and then in the desired way using -osuf to set the object file suffix. I tried googling on the error with no success. What can I do to compile this file for profiling? Cheers, Nikos

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code-as-config, run-time checks and error locations
by Steffen Schuldenzucker 07 Apr '13

07 Apr '13

Dear Café, I'm working on a EDSL that will include both type checks (at compile time) and semantic checks (at run time). - Semantic properties are known at compile time but feel too complex to me to be encoded in the type system. If one of the runtime checks fails, I'd like to print the location of the error, i.e. not Error: Unknown field `AMOUNT' in table `ENTRIES' (where? why?) but Error: Unknown field `AMOUNT' in table `ENTRIES' Referenced at analysis1.hs:43:7 by `sumByInvoice' which was called at analysis1.hs:66:3 by `main' ENTRIES defined at analysis1.hs:13:8 I'm not yet sure which level of granularity I want for error messages and one can probably get arbitrarily fancy on this. For the moment I think it would be enough to auto-insert the location of calls to a certain set of functions. Any experience on this? Thanks a lot. -- Steffen

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Is there an escape from MonadState+MonadIO+MonadError monad stack?
by Ömer Sinan Ağacan 07 Apr '13

07 Apr '13

Hi, I'm a hobbyist Haskell programmer and my use of Haskell is mostly consists of writing interpreters, simple virtual machines, and type checkers. One thing I'm not happy about my Haskell programs is, almost all of my programs have a monad transformer stack consisting MonadError, MonadIO and MonadState. Let's say I'm writing an interpreter, I certainly need MonadIO to interpret object language's IO functions: printing, reading from file etc. I also need MonadState for, well, states(dynamic environment etc.). MonadError is also required because most computations can fail(unbound variable error, type mismatch etc. it can be also used for implementing exceptions in object language) Same applies for my other applications as well. Type checkers require MonadError(unification error etc.), MonadState(to keep substitutions). MonadIO is not required in this case. But it also required for virtual machines(IOVector for memory -- ST vectors also work, but IO is still required for other stuff - printing, display etc.) I know these are mostly related with my application area, but I still wanted to write this because I may be missing something, or simply doing things wrong. Advices from experienced Haskell programmers would be appreciated, Cheers, Ömer

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Re: [Haskell-cafe] Haskell is a declarative language? Let's see how easy it is to declare types of things.
by wren ng thornton 07 Apr '13

07 Apr '13

On 4/3/13 11:46 PM, Albert Y. C. Lai wrote: > On 13-04-03 07:39 PM, Alexander Solla wrote: >> There's your problem. Mathematicians do this specifically because it is >> helpful. If anything, explicit quantifiers and their interpretations >> are more complicated. People seem to naturally get how scoping works in >> mathematics until they have to figure out free and bound variables. > > Quantifiers are complicated, but I don't see how explicit is more so > than implicit. When the quantifiers are implicit, we can rely on the unique human ability to DWIM. This is a tremendous advantage when first teaching people about mathematical concerns from a logical perspective. However, once people move beyond the basics of quantification (i.e., schemata, rank-1 polymorphism, etc), things get really hairy and this has lead to no end of quibbles in philosophy and semantics, where people seem perversely attached to ill-specified and outdated logics. On the other hand, with explicit quantification you can't rely on DWIM and must teach people all the gritty details up front--- since the application of those details is being made explicit. This is an extremely difficult task for people who are new to symbolic reasoning/manipulation in the first place. In my experience, it's better to get people fluently comfortable with symbolic manipulations before even mentioning quantifiers. -- Live well, ~wren

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meaning of "referential transparency"
by Henning Thielemann 07 Apr '13

07 Apr '13

Can someone enlighten me about the origin of the term "referential transparency"? I can lookup the definition of "referential transparency" in the functional programming sense in the Haskell Wiki and I can lookup the meaning of "reference" and "transparency" in a dictionary, but I don't know why these words were chosen as name for this defined property.

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Fwd: meaning of "referential transparency"
by Alexander Solla 06 Apr '13

06 Apr '13

On Sat, Apr 6, 2013 at 10:43 AM, Henning Thielemann < lemming(a)henning-thielemann.de> wrote: > > Can someone enlighten me about the origin of the term "referential > transparency"? I can lookup the definition of "referential transparency" in > the functional programming sense in the Haskell Wiki and I can lookup the > meaning of "reference" and "transparency" in a dictionary, but I don't know > why these words were chosen as name for this defined property. > Willard V.O. Quine Referential transparency arises in constrast to referential opacity. A context is referentially opaque (reference can't "see" through the context) when the substitution of equal values are not necessarily equal. For example, quotation marks are referentially opaque: "It is necessary that four and four is eight" "The number of planets is eight" If these contexts were referentially transparent, we would find that "It is necessary that the number of planets is eight", a situation which at the very least puts us in a tight ontological position.

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ANNOUNCE: hydra-print-0.1.0.0
by Ryan Newton 06 Apr '13

06 Apr '13

Hello cafe, This is an NCurses front end for visualizing a dynamic collection of text streams (e.g. as produced by "make -j" or "cabal -j"). It just splits the window when more streams appear. http://hackage.haskell.org/package/hydra-print http://parfunk.blogspot.com/2013/04/hydra-print.html Right now I'm using it for the monad-par benchmark script, but I hope to provide a cabal patch soon. Cheers, -Ryan

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