- Haskell-Cafe - Haskell.org

DSL
by Закиров Марат 28 Mar '15

28 Mar '15

Hi folks! Are there any examples of Domain Specific Language (DSL) on Haskell? Thank you in advance. -- *Regards, Marat.* *С уважением Марат.*

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I need your feedback (GSoC): Analyzing stock market in Haskell
by CESAR FABIAN ORCCON CHIPANA 27 Mar '15

27 Mar '15

Hello This is my second proposal. It would be really useful for me to read your comments about this proposal. http://cfoch-dev.tumblr.com/post/114754745097/analyzing-stock-market-in-has… Thanks, Fabián Orccón -- ------------------------------------ Fabián Orccón (cfoch) http://cfoch.tumblr.com http://cfoch-art.tumblr.com http://cfoch-dev.tumblr.com

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I need feedback: Create a function which makes a relation between types and functions.
by CESAR FABIAN ORCCON CHIPANA 27 Mar '15

27 Mar '15

Good morning (afternoon, evening) This is one of my proposals. http://cfoch-dev.tumblr.com/post/114756897282/create-a-function-which-makes… I hope you can give me your feedback. Thanks so much. -- ------------------------------------ Fabián Orccón (cfoch) http://cfoch.tumblr.com http://cfoch-art.tumblr.com http://cfoch-dev.tumblr.com

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GSOC 2015 proposal: a standalone functional parser for CommonMark
by Julien Cretel 27 Mar '15

27 Mar '15

Dear Haskell-Cafe, This is very short notice, but here is a link to my proposal for this year's Google Summer of Code: https://gist.github.com/Jubobs/7a9298eeaf02bcefbc35 The goal of the project is to produce a standalone, pure-Haskell library for parsing CommonMark, the updated Markdown specification. Here is a link to the corresponding trac ticket, along with a link to a relevant Google-Groups discussion involving John MacFarlane and Matthew Pickering: https://ghc.haskell.org/trac/summer-of-code/ticket/1660 https://groups.google.com/forum/#!topic/pandoc-discuss/xZrf-dL0ZPs Should you have any comments, suggestions, corrections, etc... before the submission deadline, feel free to chime in. All the best, Julien

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Terra language: Lua+LLVM JIT+multi-stage metaprogramming
by Bulat Ziganshin 27 Mar '15

27 Mar '15

Hello , http://terralang.org/ it doesn't have any relation to haskell, but i found it very interesting - it employs LLVM JIT to produce efficient code on the fly and can save it to .o files, and it's tightly integrated with Lua as metaprogramming language It has predecessors like template haskell, metalua, julia, ocamlp4, but all them lack either convenient multi-stage meta-programming, or efficency of low-level code -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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ListT considered harmful
by Fumiaki Kinoshita 27 Mar '15

27 Mar '15

It is well known that ListT m in transformers is not a monad unless the underlying monad is commutative. The fact that ListT is not a monad transformer is sufficient to remove it from the package, and several packages already defined their own ListT. Oughtn't we deprecate ListT or replace it by a correct one?

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ANNOUNCE: GHC version 7.10.1
by Austin Seipp 27 Mar '15

27 Mar '15

============================================================== The (Interactive) Glasgow Haskell Compiler -- version 7.10.1 ============================================================== The GHC Team is pleased to announce a new major release of GHC. There have been a number of significant changes since the last major release, including: * Several new language features and changes have been implemented: - Applicative is now a superclass of Monad and in the Prelude. - Many prelude combinators have been generalized - Static pointers * GHC now has preliminary and experimental support for DWARF based debugging. * `integer-gmp` has been completely rewritten. * Type-checking plugins can now extend the type checker. * Support for partial type signatures * Many bugfixes and other performance improvements. * Preliminary support for 'backpack' features like signatures. * Typeable is now generated by default for all data types automatically. We've also fixed a handful of issues reported since RC3: - A bug in the call arity analysis that would result in invalid core was fixed (#10176) - A bug in the Win32 package causing it to fail to load was fixed (#10165) - ghc-prim has (correctly) been bumped to version 0.4.0.0, to comply with the PVP. - Several libraries have been bumped to their latest available versions after coordination. The full release notes are here: https://downloads.haskell.org/~ghc/7.10.1/docs/html/users_guide/release-7-1… How to get it ~~~~~~~~~~~~~ The easy way is to go to the web page, which should be self-explanatory: https://www.haskell.org/ghc/ We supply binary builds in the native package format for many platforms, and the source distribution is available from the same place. Packages will appear as they are built - if the package for your system isn't available yet, please try again later. Background ~~~~~~~~~~ Haskell is a standard lazy functional programming language. GHC is a state-of-the-art programming suite for Haskell. Included is an optimising compiler generating good code for a variety of platforms, together with an interactive system for convenient, quick development. The distribution includes space and time profiling facilities, a large collection of libraries, and support for various language extensions, including concurrency, exceptions, and foreign language interfaces (C, whatever). GHC is distributed under a BSD-style open source license. A wide variety of Haskell related resources (tutorials, libraries, specifications, documentation, compilers, interpreters, references, contact information, links to research groups) are available from the Haskell home page (see below). On-line GHC-related resources ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Relevant URLs on the World-Wide Web: GHC home page http://www.haskell.org/ghc/ GHC developers' home page http://ghc.haskell.org/trac/ghc/ Haskell home page http://www.haskell.org/ Supported Platforms ~~~~~~~~~~~~~~~~~~~ The list of platforms we support, and the people responsible for them, is here: https://ghc.haskell.org/trac/ghc/wiki/Platforms https://ghc.haskell.org/trac/ghc/wiki/CodeOwners Ports to other platforms are possible with varying degrees of difficulty. The Building Guide describes how to go about porting to a new platform: https://ghc.haskell.org/trac/ghc/wiki/Building Developers ~~~~~~~~~~ We welcome new contributors. Instructions on accessing our source code repository, and getting started with hacking on GHC, are available from the GHC's developer's site run by Trac: https://ghc.haskell.org/trac/ghc/ Mailing lists ~~~~~~~~~~~~~ We run mailing lists for GHC users and bug reports; to subscribe, use the web interfaces at https://www.haskell.org/mailman/listinfo/glasgow-haskell-users https://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs There are several other haskell and ghc-related mailing lists on www.haskell.org; for the full list, see https://www.haskell.org/mailman/listinfo/ Some GHC developers hang out on #haskell on IRC, too: https://www.haskell.org/haskellwiki/IRC_channel Please report bugs using our bug tracking system. Instructions on reporting bugs can be found here: https://www.haskell.org/ghc/reportabug Hashes & Signatures ~~~~~~~~~~~~~~~~~ On https://downloads.haskell.org/~ghc/7.10.1/ you will find a signed copy of the SHA256 hashes for the tarballs, using my GPG key (0F8F 3AA9 9235 C704 ADA0 B419 B942 AEE5 3B58 D86F). -- Regards, Austin Seipp, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

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Applicative of Applicative
by martin 26 Mar '15

26 Mar '15

Hello all, I've been playing with temporal values, i.e. values which change over time at discrete points in time. I thought it would be good to make it an instance of Applicative and I was pleased with the results. I may be re-inventing some of frp here, but hey. Then I wondered how I would replace one Temporal by another at some point in time. Kind of like switching from summer schedule to winter schedule. Or switching channels on the TV, where each channel is a Temporal Image. It seems that by stacking up Temporals in this way, I could theoretically start with sample values and abstract my way up to a symphony. What is unclear to me is the following: when I have two Temporal Ints, I know what (+) <$> does. It operates on the Ints. But when I have Temporal Temporal Ints, then the function before the <$> would operate on the next level, i.e. it has to be a function which accepts a Temporal. But a Temporal Temporal Int can always be flattened into a Temporal Int. So I may just as well ask to apply a function to the bottom level, namely the Ints. How to I choose how deep down I want to reach? And any other guidance will also be much appreciated. Martin

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Re: [Haskell-cafe] Version constraints and cabal.config files
by Miëtek Bak 26 Mar '15

26 Mar '15

On 2015-03-25, at 12:16, Michael Snoyman <michael(a)fpcomplete.com> wrote: > Trying to understand the problem: currently, with your approach, if the project depends on a library not in the LTS Haskell release, then the cabal-install dependency solver won't be able to find it. Instead, you'd like to be able to use the dependency solver to track down those extra dependencies. Is that correct? > > If so, why not take a multi-pass approach: download the cabal.config from stackage.org (which creates an inclusive snapshot), and then use --dry-run, which will tell you all the packages to be used. You’re correct. However, this will have to be `cabal install --dependencies-only --dry-run`, and not `cabal freeze --dry-run`, because `cabal freeze` always completely ignores any existing version constraints, whether local or global (https://github.com/haskell/cabal/issues/2265) I’m planning to switch away from `cabal freeze` soon, but it’s not going to be a drop-in replacement (https://github.com/mietek/halcyon/issues/52) This is a good moment to carefully consider the meaning of a per-application `cabal.config` file, and whether the `cabal freeze` flavour should be completely replaced by separate constraints files. Perhaps, as mentioned in the original `cabal freeze` proposal, we could also have separate constraints sets for different GHC versions. I’m hoping Cabal developers could chime in on the discussion. -- Miëtek https://mietek.io

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Do we have idiom for lifting a state monad into pair of states?
by Ki Yung Ahn 26 Mar '15

26 Mar '15

Consider you have developed library routines that act on (State s1 a). For some reason, you need more than one state simultaneously. Let's say two side by side for simple example, that is (State (s1,s2) a). To use library functions on one state monad in a two state monad, we need to wrapper that lifts actions of (State s1 a) to (State (s1,s2) a). It is not difficult to write a lifter for such purposes as below. It is kind of like doing liftA in Applicative libarary, but instead of the last argument 'a' but on the fist argument 's' of (State s a). This seems like an idiom that can often come up. So, I tried some searching in some applicative related libraries and monad transformer related libraries but haven't found this idiom yet. If you had a need for idioms like below, what do you call it? Or, is there a way more general way I've missed that makes this a very special case of it. > import Control.Monad.State > import Control.Monad.Error > import Control.Applicative > > -- lift an action over a state into a pair of states > -- > do1st :: State s1 a -> State (s1,s2) a > do1st m1 = do (s1, s2) <- get > let (a, s1') = runState m1 s1 > put (s1',s2) > return a > > do2nd :: State s2 a -> State (s1,s2) a > do2nd m2 = do (s1, s2) <- get > let (a, s2') = runState m2 s2 > put (s1,s2') > return a > > > -- lift an action over a state with error > -- into a pair of states with error > -- > do1 :: Error e => ErrorT e (State s1) a -> ErrorT e (State (s1,s2)) a > do1 m1 = do (s1, s2) <- lift get > let (ma,s1') = (runState . runErrorT) m1 s1 > case ma of > Left e -> throwError e > Right a -> do lift $ put (s1',s2) > return a > > > do2 :: Error e => ErrorT e (State s2) a -> ErrorT e (State (s1,s2)) a > do2 m2 = do (s1, s2) <- lift get > let (ma,s2') = (runState . runErrorT) m2 s2 > case ma of > Left e -> throwError e > Right a -> do lift $ put (s1,s2') > return a

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Re: [Haskell-cafe] Version constraints and cabal.config files
by Miëtek Bak 25 Mar '15

25 Mar '15

On 2015-03-25, at 11:40, Michael Snoyman <michael(a)fpcomplete.com> wrote: > Every Stackage Nightly and LTS Haskell release has a YAML file[1] that provides a lot of information, included the exact versions of packages and package dependencies. The stackage-types[2] package provides the datatypes necessary to parse that YAML file. Putting those two together, it should be possible to write something that looks at a project's list of shallow package dependencies, and construct a list of precisely which package-version combinations are necessary to build the entire project, which I think is what you're saying you get from cabal freeze but not from the current cabal.config on stackage.org. If I understand correctly, the result should be equivalent to configuring `cabal-remote-repo` to point to a particular Stackage snapshot, running `cabal freeze`, and parsing its output. Halcyon already does this — the following command will ignore any constraints declared by the application, and instead use the newest versions of all dependencies available in the specified Cabal repository: ``` $ halcyon install http://github.com/mietek/hello-yesod --ignore-all-constraints --cabal-remote-repo=stackage-lts-latest:http://www.stackage.org/lts ``` However, this means we’re using Stackage in “exclusive” mode. I’d like to support using Stackage in “inclusive” mode, with `cabal-remote-repo` pointing to Hackage, and a Stackage-flavoured `cabal.config`. -- Miëtek https://mietek.io

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Version constraints and cabal.config files
by Miëtek Bak 25 Mar '15

25 Mar '15

On 2015-03-25, at 09:59, Chris Done <chrisdone(a)fpcomplete.com> wrote: >> Halcyon wants a constraints file for a particular app to list all Cabal packages which are the app’s direct or indirect dependencies. > > Why is that? What does it do with the cabal.config file? There are two flavours of `cabal.config` files: (1) `cabal freeze`-flavoured, declaring a complete set of dependencies for a particular application, and (2) the Stackage flavour, declaring a set of packages which are guaranteed to build together. It’s worth noting that the first flavour was originally proposed to be a separate file with a simpler syntax, named `cabal.freeze` (https://github.com/haskell/cabal/issues/1502) Unfortunately, this didn’t make it into cabal-install, due to lack of time (https://github.com/haskell/cabal/pull/1519) Halcyon supports `cabal freeze`-flavoured `cabal.config` files, and also accepts separate constraints files, named `.halcyon/constraints`. The declared version constraints are used to calculate a sandbox hash. When building an application, Halcyon uses the sandbox hash to try and locate a previously-built sandbox archive. If available in local cache or remote storage, this archive is restored, completely side-stepping the need to `cabal install --dependencies-only`. Otherwise, a new sandbox is built, archived, and uploaded to remote storage. Moreover, if a full match for the sandbox hash isn’t located, Halcyon attempts to locate a partial match. Each previously-built sandbox is assigned a score, which reflects the number of required dependencies contained within. Sandboxes with extraneous dependencies are rejected. The highest-scoring partial match is used as a base for building the new sandbox. While inferior to the package-level granularity provided by Nix and proposed for future versions of Cabal, the approach implemented in Halcyon has the benefit of working with regular Cabal packages and repositories, and being used in production since June 2014. -- Miëtek https://mietek.io

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cabal test and tests that overlap in covarage
by Anatoly Yakovenko 25 Mar '15

25 Mar '15

is this a cabal error or hpc error? i see this when running 'cabal test' with tests that overlap in coverage 5 of 5 test suites (5 of 5 test cases) passed. hpc: found 2 instances of RBM.List in ["./.hpc","./dist/hpc/vanilla/mix/proto","./dist/hpc/vanilla/mix/perf-repa-RBM","./dist/hpc/vanilla/mix/test-repa-RBM","./dist/hpc/vanilla/mix/perf-list-RBM","./dist/hpc/vanilla/mix/test-list-RBM","./dist/hpc/vanilla/mix/rbm-0.0"] Thanks, Anatoly

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hpc - ignore some expressions
by Ozgur Akgun 24 Mar '15

24 Mar '15

Hi, I am generating code coverage reports using hpc, it works great. The reports would be much more helpful if I could somehow tell hpc to treat certain expressions as "covered" though. As an example, let's think of "impossible" cases of a function. f x | check x = result x | otherwise = error "impossible" I know there are techniques to get rid of (or minimise the number of) the impossible cases. I also can see how this information can be relevant, i.e. seeing whether `check` always holds or not in the report. However, at least for one version of the report, I want the error call like the above example to be treated as covered. Or maybe a different colour could be used to annotate "ignored"? Did anyone need/want anything like this? Is it possible? My guess is that this is somewhat possible using manually generated .tix files and merging them with the .tix file that is generated in the usual way. However this feels too adhoc, fragile and seems to be too much effort to reach a simple goal. Best, Ozgur

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GSOC 2015 proposal feedback
by Nikita Kartashov 24 Mar '15

24 Mar '15

Hello, everyone! As someone on #haskell-gsoc IRC suggested, I would like to post my proposal. Please, if you have time, read it and send me your thoughts. Thanks! Here is the link: https://gist.github.com/nkartashov/e46fd146b1df2d79aaf3 With regards, Nikita Kartashov

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GSoC Proposal: Concurrent Bag.
by Alex Semin 24 Mar '15

24 Mar '15

Hello everyone! I have written a proposal for this GSoC. It is mostly related to this ticket: https://ghc.haskell.org/trac/summer-of-code/ticket/1608 Here is link to the proposal: https://gist.github.com/Alllex/009d9842f97963aa9955 I will be glad to get any feedback and comments. Regards, Alex Semin

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Getting Haskeline to quit early
by G Philip 23 Mar '15

23 Mar '15

(I posted this to Stack Overflow [1], but no luck there so far.) Hi all, I am trying to use Haskeline [2] to write a program which asks the user a sequence of questions, each one optionally with a default value in [brackets], and reads in their responses. I want the user to be able to 1. Press Enter to submit the [default] value; 2. Type in a string, edit it if needed, and then press Enter to submit this value; 3. Press Ctrl-C to reset all values to the defaults and start over; and, 4. Press Ctrl-D or enter "quit" to quit, in which case all the values which they submitted are lost. I have been able to get points 1-3 working, but I cannot get point 4 to work: pressing Ctrl-D (or entering "quit") just brings up the next prompt instead of making the program quit the questioning. Looking at my program (please see below) I understand why this happens, but I am not able to figure out how to fix this so that Ctrl-D (or "quit") actually makes the questioning stop. How do I fix the program to make this happen? I did see this question [2] over at Stack Overflow which seems to ask something similar, but I could not get much from there; I am not even sure that they are asking the same question as I am. As a secondary question: my current program has quite a few `case` statements which switch on `Maybe` values. In particular, I currently check for `Nothing` two or three levels deep so that I can correctly return a `Nothing` when the user presses Ctrl-D. I have a feeling that this could be simplified using (something like) the monadic `>>=` operator, but I am unable to figure out how to do this in this case. Is my hunch right? Is there a way to do away with all this pattern matching which looks for `Nothing`? Also: please tell me anything else which could improve my code below. I am quite new to this, so it is very likely that I am missing many obvious things here. Thanks in advance! Regards, Philip ---------------- The program --------------- My program asks the user about the composition of a fruit basket. The information associated with a fruit basket consists of the name of the owner of the fruit basket and the names of the different kinds of fruit in the basket. To be able to ask for the latter, I first ask for the _number_ of different kind of fruit in the basket, and then ask for the name of each kind. We start with a default fruit basket whose information is then modified based on what the user tells us. module Main where import System.Console.Haskeline type PersonName = String type FruitName = String data FruitBasket = FruitBasket { ownerName :: PersonName, fruitCount :: Int, fruitNames :: [FruitName] } deriving Show defaultBasket = FruitBasket "Mary" 2 ["Apple", "Peach"] main :: IO () main = do basket <- getBasketData defaultBasket putStrLn $ "Got: " ++ show(basket) -- Prompt the user for information about a fruit basket, and -- return a FruitBasket instance containing this information. The -- first argument is an instance of FruitBasket from which we get -- the default values for the various prompts. The return value -- has a Maybe type because the user may abort the questioning, in -- which case we get nothing from them. getBasketData :: FruitBasket -> IO (Maybe FruitBasket) getBasketData basket = runInputT defaultSettings $ withInterrupt $ getData basket where getData :: FruitBasket -> InputT IO (Maybe FruitBasket) getData initialBasket = handleInterrupt f $ do outputStrLn banner input <- getInputLine $ "Who owns this basket? [" ++ defaultOwner ++ "] : " basket <- case input of Nothing -> return Nothing -- User pressed Ctrl-D with the input being empty Just "" -> return (Just initialBasket) -- User pressed Enter with the input being empty Just "quit" -> return Nothing -- User typed in "quit" and pressed Enter Just newOwner -> return (Just initialBasket{ownerName = newOwner}) input <- getInputLine $ "Number of kinds of fruit in the basket? [" ++ show defaultCount ++ "] : " basket' <- case input of Nothing -> return Nothing Just "" -> return basket Just "quit" -> return Nothing Just count -> return $ updateFruitCount basket (read count) where updateFruitCount Nothing _ = Nothing updateFruitCount (Just realBasket) newCount = Just $ realBasket{fruitCount = newCount} let defaultFruitNames = pruneOrPadNames basket' newNames <- getFruitNames defaultFruitNames 1 case newNames of Nothing -> return (Just defaultBasket) Just newSetOfNames -> return $ updateFruitNames basket' newSetOfNames where updateFruitNames Nothing _ = Nothing updateFruitNames (Just realBasket) realNewNames = Just $ realBasket{fruitNames = realNewNames} where f = (outputStrLn "Press Ctrl-D or enter \"quit\" to quit." >> getData initialBasket) defaultOwner = ownerName initialBasket defaultCount = fruitCount initialBasket banner :: String banner = "Please enter details of the fruit basket below. At each prompt you can do one of the following:\n\ \\t (a) Press Enter to submit the [default] value;\n\ \\t (b) Type in a string, edit it if needed, and then press Enter to submit this value;\n\ \\t (c) Press Ctrl-C to reset all values to the defaults and start over;\n\ \\t (d) Press Ctrl-D or enter \"quit\" to quit; all the values you submitted will be lost." pruneOrPadNames :: Maybe FruitBasket -> Maybe [String] pruneOrPadNames Nothing = Nothing pruneOrPadNames (Just basket) = Just $ pruneOrPad (fruitNames basket) (fruitCount basket) -- When requiredLength is not larger than (length inputList), -- (pruneOrPad inputList requiredLength) is the prefix of -- inputList of length requiredLength. Otherwise, it is inputList -- padded with as many empty strings as required to make the total -- length equal to requiredLength. pruneOrPad :: [String] -> Int -> [String] pruneOrPad inputList requiredLength | requiredLength <= inputLength = take requiredLength inputList | otherwise = inputList ++ (replicate difference "") where inputLength = length inputList difference = requiredLength - inputLength getFruitNames Nothing _ = return Nothing getFruitNames (Just []) _ = return $ Just [""] getFruitNames (Just (name:names)) count = do input <- getInputLine $ "Name of fruit " ++ (show count) ++ " [" ++ name ++ "] : " newNames <- case input of Nothing -> return Nothing Just "" -> do -- Keep the default name for this fruit ... newNames' <- getFruitNames (Just names) (count + 1) case newNames' of Nothing -> return Nothing -- ... unless the user chose to quit -- while entering a name Just [""] -> return $ Just [name] -- At this point names = [] so it is -- already time to stop asking for -- more names. Just furtherNames -> return $ Just (name : furtherNames) Just "quit" -> return Nothing Just name' -> do newNames' <- getFruitNames (Just names) (count + 1) case newNames' of Nothing -> return Nothing Just [""] -> return $ Just [name'] Just furtherNames -> return $ Just (name' : furtherNames) return newNames References ------------- [1] http://stackoverflow.com/questions/29189428/getting-haskeline-to-quit-early [2]: https://hackage.haskell.org/package/haskeline-0.7.1.3/docs/System-Console-H… [3]: http://stackoverflow.com/questions/4771199/haskell-best-practise-early-term…

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ANN: Magic Cookies! Android board game in Haskell
by Ivan Perez 23 Mar '15

23 Mar '15

Dear Café This is a just a quick announcement: we have just released Magic Cookies, an Android board game written in Haskell. It is now available on Google Play [1]. The game uses the Functional Reactive Programming library Yampa and SDL2 for multimedia. If you like the game, please share the link on facebook/twitter/etc and let all of your friends know. We have gone through dozens of iterations during beta testing, but we cannot guarantee that the game works 100% perfectly on all devices. If you find a bug, or anything else you don't like, please do not rate low. Instead, open an issue [2] or send us an email (support(a)keera.co.uk) Suggestions are also welcome. We will continue working to make the game lighter, faster, and more fun. See [3] for more details, and to sign up as a beta-tester for other games we are already working on. We certainly enjoyed making it, and we hope you all enjoy playing it. All the best Ivan, Fass & Jorge [1] https://play.google.com/store/apps/details?id=uk.co.keera.games.magiccookies [2] https://github.com/keera-studios/magic-cookies [3] http://keera.co.uk/blog/2015/03/19/magic-cookies-released-google-play/

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Re: [Haskell-cafe] Cabal and cabal-install minor release (1.22.2.0)
by Miëtek Bak 23 Mar '15

23 Mar '15

Thanks, Ryan. Binaries of cabal-install 1.22.2.0 are now available in Halcyon on the following platforms: - Amazon Linux 2014.09 (x86_64) - Arch Linux (x86_64) - CentOS 6 (i386 and x86_64) - CentOS 7 (x86_64) - Debian 6 (i386 and x86_64) - Debian 7 (i386 and x86_64) - Fedora 19 (i386 and x86_64) - Fedora 20 (i386 and x86_64) - Fedora 21 (x86_64) - openSUSE 13.2 (x86_64) - OS X 10.8 (x86_64) - OS X 10.9 (x86_64) - OS X 10.10 (x86_64) - Red Hat Enterprise Linux 6 (x86_64) - Red Hat Enterprise Linux 7 (x86_64) - SUSE Linux Enterprise Server 12 (x86_64) - Ubuntu 10.04 LTS (i386 and x86_64) - Ubuntu 12.04 LTS (i386 and x86_64) - Ubuntu 14.04 LTS (i386 and x86_64) - Ubuntu 14.10 (i386 and x86_64) See https://github.com/mietek/halcyon/issues/38 for details of cross-platform support. Let me know if I can help with the automation. -- Miëtek On 2015-03-21, at 21:34, Ryan Thomas <ryan(a)ryant.org> wrote: > I have released both Cabal and cabal-install 1.22.2.0 today, > cabal-install has just gone out now. > > These have been published to Hackage and are available on the download > page of haskell.org. > > As a part of this process I have also updated the release > documentation (https://github.com/haskell/cabal/wiki/Making-a-release) > to support the new sftp-only push to haskell.org. > > There are a couple of outstanding items to tie off with this release: > - The {Cabal|cabal-install}-latest symlinks on haskell.org still need > to be updated > - The Windows/OSX/Linux specific binaries need to be built and updated > on the download page; Johan I will probably need some guidance on the > process for this. > > Once I get these items tied off, my main focus will be the automation > of this release process. > > > Cheers, > > ryan > _______________________________________________ > cabal-devel mailing list > cabal-devel(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/cabal-devel

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ANN: Lambdabot 5.0.1
by Bertram Felgenhauer 23 Mar '15

23 Mar '15

Dear Haskell users, I'm pleased to announce a minor release of Lambdabot. Lambdabot is an IRC bot that can also be used offline; it provides tools for querying information about and even for producing Haskell code. To install it, use the following command: cabal install lambdabot --constraint 'transformers installed' (the constraint helps cabal-install to find an install plan) * What's new - lambdabot is now smarter about locating its data files. This means that commands like @src now work out of the box, without having to copy files into ~/.lambdabot/State - lambdabot-5.0.1 uses (and requires) monad-control 1.0 * Development Development of lambdabot happens on github, where you can also find the bugtracker: https://github.com/lambdabot/lambdabot Have fun with lambdabot, Bertram

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time and TimeLocale compatibility
by Tom Ellis 22 Mar '15

22 Mar '15

Hello all, The `formatTime` function from time-1.4 uses the TimeLocale type from old-locale, whereas time-1.5 provides its own definition of the TimeLocale type. I want to provide `formatTime` with the argument `defaultTimeLocale`. 1.4 requires the version in old-locale whereas 1.5 requires the version it defines itself. Is it possible to use this function in a way that is compatible with both 1.4 and 1.5? If time-1.5 had merely reexported the old-locale version, and then (the future) time-1.6 replaced it completely this would have offered a smoother upgrade path, but as far as I can tell nothing like this was done. What are my options? Tom

3 4

Munich Haskell Meeting, 2015-03-25 @ 19:30
by Christian Neukirchen 22 Mar '15

22 Mar '15

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Wednesday, March 25 at Cafe Puck at 19h30. For details see here: http://www.haskell-munich.de/dates If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-mar-2015/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

1 0

GSoC Proposal: Refactor program with HLint suggestions
by Matthew Pickering 22 Mar '15

22 Mar '15

Dear Café, I would appreciate any comments on the proposal I have written for this years GSoC. The proposal focuses on an oft-requested feature, the ability for HLint to perform the refactorings which it suggests. I believe this is now possible with the work that Alan Zimmerman has recently completed on ghc-exactprint. More details can be found here - http://mpickering.github.io/gsoc2015.html (There is also a pdf version available - http://mpickering.github.io/gsoc2015.pdf) Matt

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netwire example error message
by briand＠aracnet.com 22 Mar '15

22 Mar '15

The error message: time.hs:13:8: No instance for (Show e0) arising from a use of ‘testWire’ The type variable ‘e0’ is ambiguous Note: there are several potential instances: instance Show Double -- Defined in ‘GHC.Float’ instance Show Float -- Defined in ‘GHC.Float’ instance (Integral a, Show a) => Show (GHC.Real.Ratio a) -- Defined in ‘GHC.Real’ ...plus 45 others In the expression: testWire clockSession_ pos In an equation for ‘main’: main = testWire clockSession_ pos Failed, modules loaded: none. So here's my type debugging. (netwire sure is convoluted) pos => Wire s e m a Float integral => a -> Wire s e m a a clockSession_ => Session m (Timed NominalDiffTime ()) looks to me like the problem is probably rooted in the interaction of pos with (Timed NominalDiffTime) as it does not seem to have a float instance, but that doesn't seem like it should result in an error about ambiguity. Any help appreciated. Thanks Brian ---- import Prelude hiding ((.)) -- To use (.) in the scope of Categories instead import Control.Wire import FRP.Netwire speed :: Float speed = 0.5 pos :: (HasTime t s, Monad m) => Wire s e m a Float pos = integral 0 . pure speed -- The main function main :: IO () main = testWire clockSession_ pos

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Re: [Haskell-cafe] [Haskell-beginners] Multi-parameter type classes and ambiguous type variables...
by Sumit Sahrawat, Maths & Computing, IIT (BHU) 21 Mar '15

21 Mar '15

This might be better answered at the haskell-cafe. Sending to cafe. On 21 March 2015 at 03:09, Stuart Hungerford <stuart.hungerford(a)gmail.com> wrote: > Hi, > > As a learning exercise I'm modelling some algebraic structures as > Haskell typeclasses. Some of these are multi-parameter type classes. > Here's a very simplified version of the type class relationships: > > class MM a where > one :: a > > class AM a where > zero :: a > > class (AM a, MM a) => SR a > > class (AM a) => AG a where > inv :: a -> a > > class (SR a) => R a where > neg :: a -> a > > class (R r, AG g) => M r g where > sca :: r -> g -> g > > check :: (Eq g, M r g) => g -> Bool > check x = sca one x == x > > The problem I have is that GHC is finding the "r" type variable in the > "check" function ambiguous. Given my still limited Haskell knowledge > I'm not surprised this is happening. What I would like to know is how > experienced Haskellers handle this situation in practice: is there an > idiomatic way of disambiguating "r" or is it a sign of poor type class > design? > > Thanks, > > Stu > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > -- Regards Sumit Sahrawat

3 3

Continuation vs Codensity
by silvio 21 Mar '15

21 Mar '15

I just saw this http://stackoverflow.com/questions/25827227/why-cant-there-be-an-instance-o… Personally, I think the suggested type for callCC newtype ContT m a = ContT { runContT :: forall r. (a -> m r) -> m r } class MonadCont m where callCC :: (forall b. (a -> m b) -> m b) -> m a is much more intuitive and resembles the actual type of ContT much more closely then the current callCC and it can be Fixed. So I was wondering if there was an other reason than convention for not using that type. Silvio

1 0

What's the story of this weird type?
by David Feuer 21 Mar '15

21 Mar '15

I was reading an answer by Luis Casillas on Stack Overflow http://stackoverflow.com/a/23418746/1477667 and he used a somewhat peculiar type, with apparently not-so-useful constraints: newtype f :-> g = Natural { eta :: forall x. (Functor f, Functor g) => f x -> g x } There are a couple things about it that I don't understand. 1. I can construct a :-> from a function that doesn't satisfy the constraints, and I can pattern match to get access to it, but I can't use it. Why am I allowed to construct it at all? data Mb a = N | J a -- Not a Functor instance -- This is accepted (why?) pot = Natural (\case [] -> N (a:_) -> J a) -- And I can even do this: mb :: Functor Mb => Mb Int mb = case pot of Natural f -> f [3] But then of course I can't satisfy the constraint to use it. 2. I would have thought that such a weird/broken thing as this would only be allowed with -XDatatypeContexts, but in fact it is allowed without it. Why? David

2 3

apt-get install haskell-platform fails on ubuntu 14.04
by David Gladstein 20 Mar '15

20 Mar '15

http://askubuntu.com/questions/597278/haskell-platform-unmet-dependencies I'm installing on a newly-built system. I looked at the haskell.org download instructions, and I couldn't find anything that made sense. apt-get install haskell-platform worked about a month ago. Does anyone know what happened?

2 3

Strongly typed data / API specification librares
by Ignas Vyšniauskas 20 Mar '15

20 Mar '15

Hi cafe, Does anyone know of any libraries that allow one to describe data / interfaces (think IDLs), with emphasis on good support for modules, strong typing and extensibility? I am thinking in the lines of Thrift[1], Piqi[2], Swagger[3], but with the core data specification language as decoupled as possible from any kind of implementation related things (such as actual representation of data, or RPC / RESTful bindings). haskell-servant[4] and silk-rest[5] are kind of relevant, but they are very much oriented towards RESTful APIs. Sorry if this is slightly off-topic, but I couldn't think anywhere else to inquire about this. Thanks, Ignas [1]: https://thrift.apache.org/ [2]: http://piqi.org/ [3]: http://swagger.io/ [4]: http://haskell-servant.github.io/ [5]: http://silkapp.github.io/rest/

2 1

Existentials for multi-parameter type classes?
by Lyle Kopnicky 20 Mar '15

20 Mar '15

Hi folks, I know how to write existentially quantified data constructors for single-parameter type classes, but I'm having trouble doing it for multi-parameter type classes. Is it not possible, or am I just using the wrong syntax? I have this class: class Collidable a b where collideWith :: a -> b -> String And I tried to define an existentially quantified data constructor thusly: data Collision = forall a. (forall b. Collidable a b => Collision {collider1 :: a, collider2 :: b}) I get a parse error. So then I tried: data Collision = forall a. forall b. Collidable a b => Collision {collider1 :: a, collider2 :: b} I get a different parse error. Is there any way to make this work? Thanks, Lyle

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can't STOP after interrupt winGHCi or ghci
by Kees Bleijenberg 19 Mar '15

19 Mar '15

Now I realize maybe I didn't make my point clear. What I want is not just interrupting the output of the compiler, but also stop the output of the compiler after interrupting. Van: Haskell-Cafe [mailto:haskell-cafe-bounces@haskell.org] Namens Kees Bleijenberg Verzonden: woensdag 18 maart 2015 11:12 Aan: haskell-cafe(a)haskell.org Onderwerp: [Haskell-cafe] can't nterrupt winGHCi or ghci With a haskell program I generated a very big haskell file. Sometimes the generated haskell is not valid. If I load the genearated file in winGHCi a flood of error messages streams over the screen. If I press ctrl-break WinGHCi stops and says interrupted. If I click Ok the program continues printing error messages. Ctrl-c or just c, doesn't help. So I have to wait until WinGHCi has shown all error messages (can take minutes) or close the WinGHCi window. Ghci has (on Windows) the same behaviour. Is there a special key combination to stop Ghc or Ghci or is this a bug? Kees

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Incremental sequence sorting
by David Feuer 18 Mar '15

18 Mar '15

I had an idea, and it's kind of complicated, so I was wondering if I could get a sanity check from someone before putting *too* much time into it. Specifically, I was thinking about a sort of incremental sorting of sequences (in Data.Sequence). The idea is very closely related to the incremental sorting described by Navarro and Paredes ( http://www.dcc.uchile.cl/~gnavarro/ps/algor09.pdf ). The basic concept: sequences are represented as Hinze-Paterson 2-3 finger trees. They are fairly lazy, so building them from the top down tends to be a good thing. Suppose I have a sequence to be sorted. To start building the result sequence, I need to know which elements go in the first digit and which go in the last digit. To do this, I need two order statistics, starting with one separating the first digit from the rest, then one separating the last digit from the rest. Once I have separated these, I can produce the top of the sequence. The digit separation is the same all down the spine. Within digits, 2-3 trees split similarly. I can get the order statistics using median-of-medians. As Navarro and Paredes (and presumably others) describe, median-of-medians partially sorts the sequence in the process of finding the desired order statistic. Navarro and Paredes use a stack of indices into an array to take advantage of this, but I need things rather more incremental. What I was thinking is that I could use a finger tree of sequences (a sequence of sequences making it easy to find where a particular total length is reached) to separate the different segments from each other. Each time I need to split on a particular order statistic, I can isolate the necessary segment, use median-of-medians (splitting it into pieces), and then append the pieces onto either side as appropriate. Does anyone have a sense of 1. Whether this is sane and/or 2. What the chances are that its performance will be competitive with either of the current Data.Sequence sorting functions and/or 3. How I should actually implement median-of-medians for this particular application (as there seem to be very few Haskell implementations of the algorithm, and I've found no optimized ones)? Thanks, David Feuer

1 0

Proposed significant breaking changes to Win32
by Yitzchak Gale 18 Mar '15

18 Mar '15

There are some discussions going on in this GitHub issue: https://github.com/haskell/win32/issues/24 with proposals for extensive breaking changes to the Win32 library. Well over 100 functions would be involved. These changes would finally fix many long outstanding defects in the API, some of which make it difficult to use important Win32 functions in the way originally intended. It is estimated that about 70 packages on Hackage depend on Win32 and could potentially be affected. If you might be affected, have an opinion, or just are interested, please drop by the GitHub issue. There is also a thread on the libraries mailing list. Thanks, Yitz

1 0

can't nterrupt winGHCi or ghci
by Kees Bleijenberg 18 Mar '15

18 Mar '15

With a haskell program I generated a very big haskell file. Sometimes the generated haskell is not valid. If I load the genearated file in winGHCi a flood of error messages streams over the screen. If I press ctrl-break WinGHCi stops and says interrupted. If I click Ok the program continues printing error messages. Ctrl-c or just c, doesn't help. So I have to wait until WinGHCi has shown all error messages (can take minutes) or close the WinGHCi window. Ghci has (on Windows) the same behaviour. Is there a special key combination to stop Ghc or Ghci or is this a bug? Kees

1 0

[TFP'15] final call for papers - deadline extended march 31 -
by Peter Achten 18 Mar '15

18 Mar '15

----------------------------- L A S T C A L L F O R P A P E R S ----------------------------- ======== TFP 2015 =========== 16th Symposium on Trends in Functional Programming June 3-5, 2015 Inria Sophia Antipolis, France http://tfp2015.inria.fr/ The symposium on Trends in Functional Programming (TFP) is an international forum for researchers with interests in all aspects of functional programming, taking a broad view of current and future trends in the area. It aspires to be a lively environment for presenting the latest research results, and other contributions (see below). Authors of draft papers will be invited to submit revised papers based on the feedback receive at the symposium. A post-symposium refereeing process will then select a subset of these articles for formal publication. The selected revised papers will be published as a Springer Lecture Notes in Computer Science (www.springer.com/lncs) volume. TFP 2015 will be the main event of a pair of functional programming events. TFP 2015 will be accompanied by the International Workshop on Trends in Functional Programming in Education (TFPIE), which will take place on June 2nd. The TFP symposium is the heir of the successful series of Scottish Functional Programming Workshops. Previous TFP symposia were held in * Edinburgh (Scotland) in 2003; * Munich (Germany) in 2004; * Tallinn (Estonia) in 2005; * Nottingham (UK) in 2006; * New York (USA) in 2007; * Nijmegen (The Netherlands) in 2008; * Komarno (Slovakia) in 2009; * Oklahoma (USA) in 2010; * Madrid (Spain) in 2011; * St. Andrews (UK) in 2012; * Provo (Utah, USA) in 2013; * and in Soesterberg (The Netherlands) in 2014. For further general information about TFP please see the TFP homepage. (http://www.tifp.org/) == INVITED SPEAKERS == TFP is pleased to announce talks by the following two invited speakers: * Laurence Rideau is a researcher at INRIA and is interested in the semantics of programming languages , the formal methods, and the verification tools for programs and mathematical proofs. She participated in the beginnings of the Compcert project (certified compiler), and is part of the Component Mathematical team in the MSR-INRIA joint laboratory, who performed the formalization of the Feit-Thompson theorem successfully. Thirty years ago, computers barged in mathematics with the famous proof of the Four Color Theorem. Initially limited to simple calculation, their role is now expanding to the reasoning whose complexity is beyond the capabilities of most humans, as the proof of the classification of finite simple groups. We present our large collaborative adventure around the formalization of the Feit-Thompson theorem (http://en.wikipedia.org/wiki/Feit%E2%80%93Thompson_theorem) that is a first step to the classification of finite groups and that uses a palette of methods and techniques that range from formal logic to software (and mathematics) engineering. * Anil Madhavapeddy == SCOPE == The symposium recognizes that new trends may arise through various routes. As part of the Symposium's focus on trends we therefore identify the following five article categories. High-quality articles are solicited in any of these categories: Research Articles: leading-edge, previously unpublished research work Position Articles: on what new trends should or should not be Project Articles: descriptions of recently started new projects Evaluation Articles: what lessons can be drawn from a finished project Overview Articles: summarizing work with respect to a trendy subject Articles must be original and not simultaneously submitted for publication to any other forum. They may consider any aspect of functional programming: theoretical, implementation-oriented, or experience-oriented. Applications of functional programming techniques to other languages are also within the scope of the symposium. Topics suitable for the symposium include: Functional programming and multicore/manycore computing Functional programming in the cloud High performance functional computing Extra-functional (behavioural) properties of functional programs Dependently typed functional programming Validation and verification of functional programs Debugging and profiling for functional languages Functional programming in different application areas: security, mobility, telecommunications applications, embedded systems, global computing, grids, etc. Interoperability with imperative programming languages Novel memory management techniques Program analysis and transformation techniques Empirical performance studies Abstract/virtual machines and compilers for functional languages (Embedded) domain specific languages New implementation strategies Any new emerging trend in the functional programming area If you are in doubt on whether your article is within the scope of TFP, please contact the TFP 2015 program chair, Manuel Serrano. == BEST PAPER AWARDS == To reward excellent contributions, TFP awards a prize for the best paper accepted for the formal proceedings. TFP traditionally pays special attention to research students, acknowledging that students are almost by definition part of new subject trends. A student paper is one for which the authors state that the paper is mainly the work of students, the students are listed as first authors, and a student would present the paper. A prize for the best student paper is awarded each year. In both cases, it is the PC of TFP that awards the prize. In case the best paper happens to be a student paper, that paper will then receive both prizes. == SPONSORS == TFP is financially supported by Erlang Solutions. == PAPER SUBMISSIONS == Acceptance of articles for presentation at the symposium is based on a lightweight peer review process of extended abstracts (4 to 10 pages in length) or full papers (20 pages). The submission must clearly indicate which category it belongs to: research, position, project, evaluation, or overview paper. It should also indicate which authors are research students, and whether the main author(s) are students. A draft paper for which ALL authors are students will receive additional feedback by one of the PC members shortly after the symposium has taken place. We use EasyChair for the refereeing process. Papers must be submitted at: https://easychair.org/conferences/?conf=tfp2015 Papers must be written in English, and written using the LNCS style. For more information about formatting please consult the Springer LNCS web site: http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0 == IMPORTANT DATES == Submission of draft papers: March 31, 2015 Notification: April 7, 2015 Registration: May 4, 2015 TFP Symposium: June 3-5, 2015 Student papers feedback: June 9, 2015 Submission for formal review: July 1, 2015 Notification of acceptance: September 8, 2015 Camera ready paper: October 8, 2015 == PROGRAM COMMITTEE == Janis Voigtländer University of Bonn, DE Scott Owens University of Kent, UK Neil Sculthorpe Swansea University, UK Colin Runciman University of York, UK Manuel Serrano Inria (PC chair), FR Rinus Plasmeijer University of Nijmegen, NL Tomas Petricek University of Cambridge, UK Marco T. Morazan Seton Hall University, USA Wolfgang De Meuter Vrije Universiteit Brussel, BE Michel Mauny Ensta ParisTech, FR Sam Lindley The University of Edinburgh, UK Daan Leijen Microsoft, USA Jurriaan Hage Utrecht University, NL Andy Gill University of Kansas, USA Thomas Gazagnaire University of Cambrige, UK Lars-Ake Fredlund Universidad Politécnica de Madrid, ES Jean-Christophe Filliatre Université Paris Sud Orsay, FR Marc Feeley Université de Montréal, CA Olaf Chitil University of Kent, UK Edwin Brady University of St Andrews, UK

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Haskell Weekly News
by Kim-Ee Yeoh 18 Mar '15

18 Mar '15

*Top picks:* - Simon Marlow, who built much of GHC's runtime, now works at Facebook. He contributes to Haxl, a tool to "automatically batch and overlap requests for data from multiple data sources in a rule engine for identifying malicious content." In a talk last year at FP Days <http://fpdays.net/2014/sessions/index.php?session=21>, he explains how you too can use Haxl in your projects. The video was just made available at <http://www.infoq.com/presentations/remote-access-haxl> InfoQ <http://www.infoq.com/presentations/remote-access-haxl>. Hat-tip to redditor Pikachut <http://www.reddit.com/r/haskell/comments/2z049t/remote_access_made_easy_and…> for links to slides <https://github.com/simonmar/fpdays14/blob/master/Haxl%20-%20FP%20Days%20201…> and code <https://github.com/simonmar/fpdays14>. - GHC builds can be frightfully slow. Karel Gardas <http://comments.gmane.org/gmane.comp.lang.haskell.ghc.devel/8233> wonders how he can speedup builds on his 32-threaded SPARC T2000. Folks point him toward Andrey Mokhov's ongoing investigation <https://github.com/snowleopard/shaking-up-ghc> of Neil Mitchell's shake <http://community.haskell.org/~ndm/shake/> to replace GHC's current use of GNU make. SPJ hopes that it "will make a big difference." - Back in September last year, Ahmad Fatoum <https://github.com/haskell/win32/issues/24> asks whether we can fix the Win32 FFI so that it supports optional params? Some Win32 functions -- e.g. see FindWindow <https://msdn.microsoft.com/en-us/library/windows/desktop/ms633499%28v=vs.85…> -- accept NULL pointers as invocation of default behavior, currently impossible from Haskell. One option is to make copies of over 100 functions to prevent existing client code from breaking. A more radical one is to extend the existing FFI in-place and alter the type signatures by Maybe-fying the params that are actually optional. The vocal opinion is in favor of the latter, deeper fix. Yitzchak Gale <http://haskell.1045720.n5.nabble.com/Proposed-significant-breaking-changes-…> recently brought up the issue on haskell-libraries. - Maurizio Vitale <http://haskell.1045720.n5.nabble.com/extracting-a-single-declaration-using-…> asks for a way to extract verbatim a single definition from Haskell source. Michael Sloan shows a way to do it <https://gist.github.com/mgsloan/ac77dd33326322fc6ccd> using haskell-src-exts <https://hackage.haskell.org/package/haskell-src-exts>. - Conor McBride <http://www.reddit.com/r/haskell/comments/2y25ly/language_deriveapplicative/…> discusses Naperian / Representable functors as a subclass of Applicative. Elsewhere in the thread, Ed Kmett remarks that "The Log of a coinductive container being an inductive type is a new result for me." - A subreddit called haskelltil <http://www.reddit.com/r/haskelltil/> (TIL = Today I Learned) for little discoveries was announced on haskell reddit <https://pay.reddit.com/r/haskell/comments/2zcnsl/haskelltil_a_subreddit_for…> . *Repo of the week: *Calvin Cheng <http://calvinx.com/about/>, co-founder and CTO at AlgoAccess, releases code <https://github.com/calvinchengx/learnhaskell> and slides <http://www.slideshare.net/calvinchengx/functional-programming-part01> on learning Haskell for the OO programmer. *In your neighborhood:* - Kat Chuang organized an NYC-based study group <http://www.meetup.com/NY-Haskell/events/220997346/> to dissect Nishan Shukla's Haskell Data Analysis Cookbook <http://haskelldata.com/>. "This is your AHA! <https://github.com/ny-haskell/Action-Haskell-Analysis> Haskell Moment." - Redditor netroby <http://www.reddit.com/r/haskell/comments/2y4e0r/haskell_chinese_user_group_…> launched a QQ instant messaging group <http://en.wikipedia.org/wiki/Tencent_QQ>, code number 424801832, to talk Haskell. *Quote of the week:* Dimitri DeFigueiredo <https://mail.haskell.org/pipermail/beginners/2015-March/014900.html> says about the haskell-beginners mailing list, "This is the friendliest mailing list I have ever subscribed to. The people in this list are so nice that even if Haskell were not so wonderfully elegant, I would like to learn it just to be able to chat and work with them." *Acknowledgments* Thanks to Henk-Jan van Tuyl for help with the Quotes. Thanks to Gershom Bazerman, Roman Cheplyaka, and Artyom for offering assistance on web hosting HWN. *Letter to the Editor:* Can I just say that I'm very much in favour of the new format. There's interesting content that can't just be gained from occasionally checking reddit, there's an actual comment on each one rather than just the headline, and most importantly the quote is still there ;) Sorry if this is adding to a deluge of email that you're now getting because of this. Yours, Anon *Response:* Dear Anon, Glad you're enjoying HWN as much as I do creating and publishing it. I love the quotes too, and I depend on my readers to email me what they'd like to share with Team Haskell. (Sneak peak at the top-secret contingency plan: If I don't have quotes for the week, I ransack the archives. Shhh!! Don't tell anyone.) Yours is the only thank you I received. Appreciation like yours is what keeps HWN going, so let me thank you in return for sharing the love, for I remain Yours editorially, Kim-Ee Yeoh p.s. Stay tuned for an upcoming editorial on what the future holds for HWN. -- Kim-Ee

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extracting a single declaration using haskell-src-ext
by Maurizio Vitale 18 Mar '15

18 Mar '15

G'day! Is there an easy way to extract a single declaration from an Haskell source file and print it with the original formatting? [I could live with pretty printing, but the default prettyPrint gives me a very strange formatting where something like: main = do putStrLn "Hello" becomes: main = do putStrLn "Hello"] The closest I've got is to parse the file with comments and then get inside Module ...decls and do printExact with an empty comment list for the decl I'm interested in. This gives me the original formatting and empty lines where comments were that I can then remove. But if there was anything already out there that just take a String for the toplevel I'm interested in and convert that only, I'd love to know. Thanks, Maurizio

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Full page Haskell 2010 report request
by Christopher Done 17 Mar '15

17 Mar '15

Hi, has anyone managed to produce a single-page HTML export of the Haskell 2010 report? Here's one I made by munging together the multi-page one: https://www.haskell.org/report/mono/2010#x8-440003.12 (this link is currently unpublished on the home page). But it's incomplete due to link references and things. I'd like a proper output but in the interest of my mental health prefer not to touch LaTeX except by proxy through other poor souls. Anyone managed to do this? A full, linkable page (don't worry about the section/para links, I added them with a few lines of JS, I can do it again) is really conducive to discussion of Haskell's finer points. Ciao!

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Whole program analysis as a GHC plugin
by David Darais 17 Mar '15

17 Mar '15

Hello cafe, I'm currently implementing a whole-program analysis for Haskell programs using the GHC api. I have a GHC plugin written and I'm successfully slurping in programs and processing them with my analysis tools. I'm getting stuck when my tool encounters an identifier that lives in another module. I'd like to unfold the identifier and retrieve its source code, allowing me to do whole-program analysis. Assuming I have the source code of the whole program, and that I get to (re)compile everything with my plugin enabled, I'm wondering: 1) Does the GHC api already support a nice way of doing this? 2) I see that there is support for unfolding identifiers if they've been specially marked in .hi files. Is there a way to mark everything to support unfolding, and would this give me the information I'm looking for? 3) Does anybody have experience with implementing a whole-program analysis for GHC as a plugin, and if so, how did you go about it? Many Thanks, David

2 1

Haskell development role at Standard Chartered in London
by Don Stewart 16 Mar '15

16 Mar '15

The Strats team at Standard Chartered has an open position for a typed functional programming developer to join the team in London. This is a “front office” finance role – meaning you will work on the trading floor, directly with traders, building software to automate their work and improve their efficiency. The role is highly development focused, and you will use Haskell for almost all tasks: data analysis, market data publishing, database access, web services, desktop GUIs, large parallel tasks, quantitative models, solvers, everything. This is a fast paced role – code you write today will be deployed within hours to hundreds of users and has to work. More details are in the full posting here: https://donsbot.wordpress.com/2015/03/16/haskell-development-role-in-strats…

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Re: [Haskell-cafe] [Haskell-beginners] testing IO code
by Sumit Sahrawat, Maths & Computing, IIT (BHU) 16 Mar '15

16 Mar '15

On 16 March 2015 at 19:51, Maurizio Vitale <mrz.vtl(a)gmail.com> wrote: > suppose I have a restricted IO monad, RIO that only exposes readFile. > and then I have a monad SIO that will eventually provide a virtual file > system from a map path->content, also with a readFile function returning > SIO(String). > > What is the way to write a function parseFile that can operate in both > monads so that I can use SIO for testing? should I define a third monad > CompileMonad that has instances for both RIO and SIO and then having > parseFile :: CompileMonad ast? > You might be able to do something like, class MonadIO m => ProvidesReadFile m where readFile :: FilePath -> m String instance ProvidesReadFile RIO where readFile = readFileRIO -- the RIO specific readFile instance ProvidesReadFile SIO where readFile = readFileSIO -- the SIO specific readFile parseFile :: ProvidesReadFile m => FilePath -> m ast parseFile = do f <- readFile let ast = parse f -- the pure parser return ast -- works for both monads > Thanks, > > Maurizio > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > This is more suitable for the haskell-cafe. I am posting it there so that more people might comment on it. HTH. -- Regards Sumit Sahrawat

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New Functional Programming Job Opportunities
by Functional Jobs 16 Mar '15

16 Mar '15

Here are some functional programming job opportunities that were posted recently: Software Engineer at Xledger http://functionaljobs.com/jobs/8797-software-engineer-at-xledger Lead front end developer at Anchor Systems http://functionaljobs.com/jobs/8796-lead-front-end-developer-at-anchor-syst… Cheers, Sean Murphy FunctionalJobs.com

1 0

Distributing Haskell on a cluster
by felipe zapata 16 Mar '15

16 Mar '15

Hi all, I have posted the following question on stackoverflow, but so far I have not received an answer. http://stackoverflow.com/questions/29039815/distributing-haskell-on-a-clust… I have a piece of code that process files, processFiles :: [FilePath] -> (FilePath -> IO ()) -> IO () This function spawns an async process that execute an IO action. This IO action must be submitted to a cluster through a job scheduling system (e.g Slurm). Because I must use the job scheduling system, it's not possible to use cloudHaskell to distribute the closure. Instead the program writes a new *Main.hs* containing the desired computations, that is copy to the cluster node together with all the modules that main depends on and then it is executed remotely with "runhaskell Main.hs [opts]". Then the async process should ask periodically to the job scheduling system (using *threadDelay*) if the job is done. Is there a way to avoid creating a new Main? Can I serialize the IO action and execute it somehow in the node? Best, Felipe

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is build breakage data about the main Haskell platform projects available?
by Johan Larson 15 Mar '15

15 Mar '15

Dan Luu recently dug up some build performance data about various open source projects, and found that many such projects have broken builds more than half the time. http://danluu.com/broken-builds/ Unfortunately Luu's data set doesn't include Haskell. Does anyone have the facts on this? (A brief search didn't turn up anything.) -- Johan Larson -- Toronto, Canada

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package ParsecToken
by Kattamuri Ekanadham 15 Mar '15

15 Mar '15

I am new to the community and learning Parsec, reading the tutorial "Parsec, a fast combinator Parse" by Daan Leijen. In the tutorial, the examples import the package called "ParsecToken" and I could not find this package any where (I found many variants like Text.Parsec.Token, Text.ParseCombinators.Parsec.Token etc.) So I was unable to execute those examples given there. Q1. How do I find the package name for "ParsecToken" so that I can do cabal import? Thanks.

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Wrong .hsc file paths on Windows
by Marcin Mrotek 15 Mar '15

15 Mar '15

Hello, I was trying to port some of my code to Windows. I've installed GHC with MinGHC (https://github.com/fpco/minghc) Unfortunately some packages, namely zlib and network, won't install, with the same error: getModificationTime cannot locate a file like CodecCompressionZlibStream.hsc or NetworkSocketTypes.hsc. With zlib, I figured out I just need to change mentions of "Codec/Compression/(...)" adding two slashes instead of one. Now with network I'm stuck, I can't find where the path was set at all. Is there some step I should have done prior to installation that would get the paths right? Best regards, Marcin Mrotek

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π-day
by Henk-Jan van Tuyl 14 Mar '15

14 Mar '15

Happy super pi day everybody! Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Re: [Haskell-cafe] install 7.8.4 64 bits on arch linux
by Lars Hupel 14 Mar '15

14 Mar '15

> sudo pacman -S ghc > And I get the same errormessage. > Even sudo pacman -R ghc gives the same error. Odd. Before installing 'ghc', please try this: $ sudo pacman -Rucs ghc haddock haskell-ghc-paths '-Rucs' will recursively remove all existing dependencies. Please double-check the list of packages pacman prints, because they will all be removed. Cheers Lars PS: Please CC the list when you reply to messages. There might be more competent people than me lurking there :-)

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install 7.8.4 64 bits on arch linux
by Kees Bleijenberg 13 Mar '15

13 Mar '15

I want to upgrade from 7.8.3 32 bits to 7.8.4 64 bits on Arch Linux. So I removed ghc, cabal etc and followed the instruction at: http://sapiengames.com/2014/04/05/install-haskell-platform-arch-linux/ Added the lines [haskell-core] Server = http://xsounds.org/~haskell/core/$arch to pacman.conf added the pacman-key as described updated pacman and finally the fun parts should start.. $ sudo pacman -S ghc cabal-install happy alex looking for inter-conflicts... error: failed to prepare transaction (could not satisfy dependencies) :: haddock: requires ghc=7.8.3-2 :: haskell-ghc-paths: requires ghc=7.8.3-1 What is wrong and what can I do? Kees

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ANNOUNCE: Euterpea 1.0.0
by Daniel Winograd-Cort 13 Mar '15

13 Mar '15

We are happy to announce the official release of Euterpea! Euterpea is a library for computer music research, education, and development, providing both note-level and signal-level abstractions. It is a descendant of Haskore and HasSound and is intended for both educational purposes as well as serious computer music applications. Euterpea is suitable for high-level music representation, algorithmic composition, and analysis; mid-level concepts such as MIDI; and low-level audio processing, sound synthesis, and instrument design. It also includes an extensible system for building musical user interfaces using UISF, a new AFRP UI library. Euterpea's performance is sufficient for most real-time midi applications and some basic real-time audio synthesis. Try it out with cabal install Euterpea You can find more at... euterpea.com - a site for a quick start, tutorials, and working examples haskell.cs.yale.edu/euterpea - more detailed info and help, including an in-progress textbook (Haskell School of Music) hackage.haskell.org/package/Euterpea - the hackage page with some docs

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