- Haskell-Cafe - Haskell.org

Drastic Prelude changes imminent
by Augustsson, Lennart 27 Jan '15

27 Jan '15

If you didn't know that, e.g., the type of foldr is changing, check out the thread https://www.haskell.org/pipermail/libraries/2015-January/024777.html. This email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please delete all copies and notify the sender immediately. You may wish to refer to the incorporation details of Standard Chartered PLC, Standard Chartered Bank and their subsidiaries at http://www.standardchartered.com/en/incorporation-details.html Insofar as this communication contains any market commentary, the market commentary has been prepared by a sales and/or trading desk of Standard Chartered Bank or its affiliate. It is not and does not constitute research material, independent research, recommendation or financial advice. Any market commentary is for information purpose only and shall not be relied for any other purpose, and is subject to the relevant disclaimers available at http://wholesalebanking.standardchartered.com/en/utility/Pages/d-mkt.aspx. Please visit http://wholesalebanking.standardchartered.com/en/capabilities/financialmark… for important information with respect to derivative products.

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Multi-pattern LambdaCase?
by Tom Ellis 27 Jan '15

27 Jan '15

Is there a particular reason that LambdaCase does not support muliple patterns? Specifically I am suggesting that \case { pA1 pA2 pA3 -> eA, ... } be sugar for \freshName1 freshName2 freshName3 -> case freshName1 freshName2 freshName3 of { pA1 pA2 pA3 -> eA, ... } Is there some technical reason that prevents it? Tom

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Could cabal-install shows dependencies as dot or some format?
by Magicloud Magiclouds 26 Jan '15

26 Jan '15

Hi, I know ghc-pkg could do that. But that is for packages already installed. I'd like cabal-install to do that when it is trying to resolve dependencies. The reason is that, now I got a "Could not resolve dependencies" error. But I could not simply figure out why based on the output of cabal-install. -- 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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Skolems!
by Tom Murphy 26 Jan '15

26 Jan '15

Like any good mystery, let's start with the body: repro.hs:22:5: Couldn't match type `y0' with `y' because type variable `y' would escape its scope This (rigid, skolem) type variable is bound by a type expected by the context: Foo x y -> Bool The following variables have types that mention y0 b :: Foo x0 y0 -> Bool (bound at repro.hs:18:9) Expected type: Foo x y -> Bool Actual type: Foo x0 y0 -> Bool In the first argument of `h', namely `b' In a stmt of a 'do' block: h b In the expression: do { xs <- return ["test"] :: IO [String]; let a = ... b = ...; h b } Failed, modules loaded: none. So it's the "(rigid, skolem)" error you sometimes happen across. The code that's causing it, though, is pretty unsuspicious: {-# LANGUAGE RankNTypes #-} -- remove this pragma and it typechecks (1): {-# LANGUAGE GADTs #-} data Foo x y = Foo x y main :: IO () main = do xs <- return ["test"] :: IO [String] let -- typechecks (2): -- a = Just "test" :: Maybe String -- typechecks (3): -- a = f ["test"] a = f xs :: Maybe String b = g (a :: Maybe String) :: forall x y. Foo x y -> Bool -- typechecks if it's inlined (4): -- h (g (a :: Maybe String) :: forall x y. Foo x y -> Bool) h b f :: [a] -> Maybe a f _ = Nothing -- doesnt affect typechecking: -- g :: Maybe String -> (Foo x y -> Bool) g :: Maybe String -> (forall x y. Foo x y -> Bool) g (Just _) _ = False g Nothing _ = True -- typechecks (5): -- h :: (Foo x y -> Bool) -> IO () h :: (forall x y. Foo x y -> Bool) -> IO () h = undefined This has been reproduced in ghc 7.6.3, 7.8.3, 7.8.4, and 7.10.1 RC1 Any idea what's going on? Thanks! Tom

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NumberSieves status/maintenance
by David Feuer 26 Jan '15

26 Jan '15

This package was last updated in 2012, and the linked darcs repository is inaccessible. Do you intend to continue to maintain this package? If not, I would be happy to take over. Thanks, David Feuer

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ANN: deepseq-bounded 0.5 -> 0.6
by Andrew Seniuk 25 Jan '15

25 Jan '15

This may affect depending code. Please consult the distro changelog for specific information about how your code might be affected, or better yet, refer to http://www.fremissant.net/deepseq-bounded/transition-5-6-7.html and http://www.fremissant.net/deepseq-bounded/grammar.html This is a transitional version of deepseq-bounded, with stability of the grammar expected in early March for version 0.7. This also causes major version bumps to the seqaid and leaky packages. To update everything, issue cabal update cabal install --reinstall --force-reinstalls seqaid which reinstalls the latest deepseq-bounded in passing. And also if you were to run "seqaid demo" it will use the updated version of leaky, so everything's up to date. The homepage for these projects starts at http://www.fremissant.net/deepseq-bounded-seqaid-leaky.html Kind Regards, Andrew

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Monad layering and DSL
by Gautier DI FOLCO 25 Jan '15

25 Jan '15

Hello all, I'm trying to make an IMAP DSL and I'm face to an annoying issue (all the examples after are selected pieces of this: https://github.com/blackheaven/smoothmail) Here we are, IMAP provide a way to statefully interact with a server. For example: select "A" -- Select folder 'A' searchAll -- Get the list of UID (message identifiers) [1, 2, 3] But select "B" searchAll -- Get [4, 5] I decided to use the Free Monad because it seems well suite for what I'm doing and I end-up with something like that: data ImapF next = Select DirectoryName (Maybe DirectoryDescription -> next) | Noop (DirectoryDescription -> next) | Search MailSearch (Maybe [UID] -> next) -- Functor instance type Imap = Free ImapF searchAll :: Imap (Maybe [UID]) searchAll = liftF $ Search undefined id select :: DirectoryName -> Imap (Maybe DirectoryDescription) select directory = liftF $ Select directory id My main problem is the following: if I do a select of an unknown directory, I should stop the computation. My first thought was Monad Transformers. So here are the types: newtype FreeT f m a = FreeT { runFreeT :: m (FreeF f a (FreeT f m a)) } Such that f is a functor and m a monad. If I do a type Imap = FreeT ImapF Maybe I'll end up with some Maybe (ImapF a) which not expresses that the evaluation can fails but that the expression construction can fails. Then I thought harder and ends up with that: type Imap = FreeT Maybe (Free ImapF) Ok, that seems better, but all the available operations can't fail. I also have had a look at extensible effects and I think I'm stuck with the same issue of my first attempt. My goal is to provide flexibility for those who want to continue the evaluation, but to provide an API with less boilerplate in the default case. If you have any ideas/hints/comments, I'll be happy to read them. If you have any questions, I'll do my best to answer them and to make disappear all doubt and ambiguity. Thanks in advance for your help. Regards.

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Haskell wiki
by Ronald Legere 25 Jan '15

25 Jan '15

Greetings, Does anyone know how to get an account to help contribute to the haskell wiki? Registration has been closed for the last 3 years and there is link to email anyone that I could find, hence this "Broadcast" message. There are dead links and such that I would love to help clean up. Cheers! -- Ron Legere (rjljr2(a)gmail.com) legere et discere

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up-to-date and complete version of http://packdeps.haskellers.com/reverse ?
by Ömer Sinan Ağacan 24 Jan '15

24 Jan '15

Hi all, I was wondering if we have up-to-date and complete version of this http://packdeps.haskellers.com/reverse. Any ideas?

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Happstack: serveDirectory vs serveFile
by Semen Trygubenko / Семен Тригубен ко 24 Jan '15

24 Jan '15

Dear all, I've ran a simple benchmark where two serve functions were side-by-side in an msum and looked like dirs "hehe" $ serveDirectory DisableBrowsing [] "/hehe" dirs "favicon.ico" $ serveFile (asContentType "image/x-icon") "favicon.ico" "hehe" directory also contained file "favicon.ico". This is happstack server version 7.3.9. I've been hammering it with ab and confirmed this puzzling result: it seems that serveFile route is approximately two orders of magnitude slower than serveDirectory route... Will be grateful for any clues as to what I'm doing wrong! Thanks, S. -- Семен Тригубенко http://trygub.com

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Unacceptable argument type in foreign declaration: Ptr ()
by Henk-Jan van Tuyl 23 Jan '15

23 Jan '15

L.S., I am trying to compile a package that hasn't been updated since 2009 and I get a lot of messages like: Unacceptable argument type in foreign declaration: Ptr () When checking declaration: foreign import stdcall safe "dynamic" prim_System_Win32_Com_Automation_TypeLib_setGuid :: Ptr () -> Ptr () -> Ptr GUID -> IO Int32 How can I correct this? Regards, 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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cabal keeps relinking
by Semen Trygubenko / Семен Тригубен ко 23 Jan '15

23 Jan '15

Dear Haskell-Cafe, cabal seems to be re-linking every time I run cabal build Wait was not an issue for small projects but for big ones it is starting to get in the way… How could I persuade cabal to re-link only if there are changes in the source tree and only these exes that require it? Many thanks, Semen PS I googled around and found this https://www.haskell.org/pipermail/libraries/2005-August/004306.html which seems to imply relink is (was?) conditional on multiple exes being present in the project. However, for me it was always relinking regardless. -- Семен Тригубенко http://trygub.com

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Munich Haskell Meeting, 2015-01-26 @ 19:30
by Christian Neukirchen 23 Jan '15

23 Jan '15

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, January 26 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-jan-2015/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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hs-fltk build fails
by Jeremy Henty 23 Jan '15

23 Jan '15

I am building with " cabal sandbox init ; cabal install hs-fltk ". Logs attached. Any advice? Regards, Jeremy Henty

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HDBC installation error
by Arunram A 23 Jan '15

23 Jan '15

Hi, I am trying to install HDBC, using the following command. cabal install hdbc and getting the following error. Preprocessing library HDBC-2.4.0.0... [1 of 8] Compiling Database.HDBC.Locale ( Database\HDBC\Locale.hs, dist\build\Database\HDBC\Locale.o ) [2 of 8] Compiling Database.HDBC.SqlValue ( Database\HDBC\SqlValue.hs, dist\build\Database\HDBC\SqlValue.o ) Database\HDBC\SqlValue.hs:265:29: Ambiguous occurrence `defaultTimeLocale' It could refer to either `Data.Time.defaultTimeLocale', imported from `Data.Time' at Database\HDBC\SqlValue.hs:20:1-16 (and originally defined in `time-1.5:Data.Time.Form at.Locale') or `Database.HDBC.Locale.defaultTimeLocale', imported from `Database.HDBC.Locale' at Database\HDBC\SqlValue.hs:22:30-46 (and originally defined in `System.Locale') Seems to be some kind of import conflict. But, I am not able to identify the problem and resolve the issue. May I get a clarity on what the problem could be and is there a known solution for this? Thanks! Regards, Arunram A.

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Type synonyms considered harmful?
by Julian Ospald 23 Jan '15

23 Jan '15

Hi, I've recently had a discussion about type synonyms and when to use them in haskell. On the one hand, they make reading type signatures easier and tell you what is meant, not just what is inside. On the other hand they also sort of hide what is inside, although you will probably need to know exactly that when using them. This might make reading code for new collaborators more difficult if they have to memorize all type synonyms first. So there are basically a few questions: * What do you think is a good policy for when and how to use them? Or would you say just not use them at all and put equivalent information in parameter documentation? * What are the upsides, downsides, pitfalls and also alternatives? (for completeness, also since https://www.haskell.org/haskellwiki/Type_synonym is a bit sparse) * Can we do something to improve the downsides? Or is there already something? (e.g. editor/IDE that can tell me the underlying type, error messages etc.) -- Regards, Julian Ospald

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[ANNOUNCE] New release of SBV
by Levent Erkok 23 Jan '15

23 Jan '15

I'm happy to announce a new release of SBV (v4.0); a library for seamlessly integrating SMT solvers with Haskell. https://hackage.haskell.org/package/sbv Most of the changes in this release are due to Brian Huffman of Galois; essentially adding capabilities so end-users can define symbolic bit-vector types that are not natively supported by SBV. For instance, a user can define `SWord17` for representing 17-bit words and use them just like other symbolic types natively supported by SBV. This feature allows SBV to take advantage of arbitrary bit-size decision procedures for bit-vector logics as found in modern SMT solvers, which are becoming increasingly more valuable in (semi-)automated verification of software artifacts. (Note that SBV already supports other basic types such as unbounded Integers, IEEE Floats and Doubles, Rationals, Algebraic reals, and traditional machine word sizes such as Word8/Int8; 16; 32; and 64 symbolically.) We plan to also add automatic support for SWordN/SIntN for arbitrary N, once GHC gets proper support for type-level naturals. Thanks to Brian Huffman for his contributions to this release. Bug reports/comments are always welcome. -Levent.

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Overlapping instances with "instance F a => G a"
by Bryan Gardiner 22 Jan '15

22 Jan '15

Hi cafe! Happy New Year. I'm writing a version of the Binary typeclass that encodes values with host endianness, and I have the code at the bottom of the message. HostBinary provides the encoding/decoding interface, and with HostBinaryNum I want to be able to write only e.g. "instance HostBinaryNum Int32" for numeric types. I had the manual HostBinary Word8 and HostBinary Int8 instances before I wrote HostBinaryNum. What I can't see is why I get this error at all: Binary.hs:21:29: Overlapping instances for HostBinary Word8 arising from a use of ‘hget’ Matching instances: instance HostBinary Word8 -- Defined at Binary.hs:16:10 instance HostBinaryNum a => HostBinary a -- Defined at Binary.hs:26:10 In the second argument of ‘fmap’, namely ‘(hget :: Get Word8)’ In the expression: fmap fromIntegral (hget :: Get Word8) In an equation for ‘hget’: hget = fmap fromIntegral (hget :: Get Word8) and also why commenting out either the HostBinary Int8 or the HostBinaryNum a => HostBinary a instances fixes the problem; and yet, the HostBinaryNum Word8 is accepted! Doesn't "HostBinaryNum a => HostBinary a" create a HostBinary instance for all instances of HostBinaryNum only? So why would it cause problems with an Int8 instance, and why isn't the HostBinaryNum Word8 instance needed to trigger a collision with the explicit HostBinary Word8? GHC 7.8.3, if it matters. Thanks for any clarifiation you can provide, Bryan Binary.hs: {-# LANGUAGE FlexibleInstances, UndecidableInstances #-} module Binary where import Data.Binary.Get import Data.Binary.Put import Data.Bits import Data.Int import Data.Word import Foreign.Storable class HostBinary a where hget :: Get a hput :: a -> Put instance HostBinary Word8 where hget = getWord8 hput = putWord8 instance HostBinary Int8 where hget = fmap fromIntegral (hget :: Get Word8) hput = hput . (fromIntegral :: Int8 -> Word8) class (Bits a, Integral a, Storable a) => HostBinaryNum a instance HostBinaryNum a => HostBinary a where hget = getNum hput = putNum --instance HostBinaryNum Word8 getNum :: (Bits a, Integral a, Storable a) => Get a getNum = undefined putNum :: (Bits a, Integral a, Storable a) => a -> Put putNum = undefined

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Help with identity functor print instance and monad transformers
by Cody Goodman 22 Jan '15

22 Jan '15

I understand the error below, but I'm not really sure what an instance of liftIO would look like for the Identity functor. I guess I'm not all to clear on what an identity functor is. I'm a little fuzzy on what an identity monad is. I understand that id gives the identity of something back pretty well though. Can anyone help? If anyone could help me fix the code below as well as explain the questions I have above it would be a great help. import Control.Monad.Trans.State import Control.Monad.IO.Class type MyState = State Int modAndPrintState :: Int -> MyState () modAndPrintState x = do get >>= \i -> do liftIO . print $ i if even x then put (x + i) else put (i * (x + 1) - x) main = undefined -- Prelude Control.Monad.Trans.State> :r -- [1 of 1] Compiling Main ( blah2.hs, interpreted ) -- blah2.hs:9:5: -- No instance for (MonadIO Data.Functor.Identity.Identity) -- arising from a use of `liftIO' -- Possible fix: -- add an instance declaration for -- (MonadIO Data.Functor.Identity.Identity) -- In the first argument of `(.)', namely `liftIO' -- In the expression: liftIO . print -- In a stmt of a 'do' block: liftIO . print $ i -- Failed, modules loaded: none. Thanks!

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Haskell Weekly News: Issue 315
by Daniel Santa Cruz 22 Jan '15

22 Jan '15

Welcome to issue 315 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from January 4 to 17, 2015 Quotes of the Week * mpickering: When is the right time to use Data.ByteString.Char8? edwardk: mpickering: some time around 1980 * tpolecat: any lecture by conal elliott has that flavor. he is the bob ross of functional programming. Top Reddit Stories * Write You a Haskell Domain: dev.stephendiehl.com, Score: 280, Comments: 24 Original: [1] http://goo.gl/isSXWc On Reddit: [2] http://goo.gl/kOevyS * Announcing LTS Haskell 1.0 Domain: fpcomplete.com, Score: 87, Comments: 14 Original: [3] http://goo.gl/QW25bK On Reddit: [4] http://goo.gl/PlWgfm * MSFT open sources production serialization system written partially in Haskell Domain: blog.nullspace.io, Score: 83, Comments: 6 Original: [5] http://goo.gl/SDm7i1 On Reddit: [6] http://goo.gl/wljTKD * haskell-emacs - Write Emacs extensions in Haskell Domain: github.com, Score: 64, Comments: 3 Original: [7] http://goo.gl/NkK9FE On Reddit: [8] http://goo.gl/75zURt * The Type Theory Podcast, Episode 3: Dan Licata on Homotopy Type Theory Domain: typetheorypodcast.com, Score: 58, Comments: 17 Original: [9] http://goo.gl/z30wH5 On Reddit: [10] http://goo.gl/5KDPNu * Cabal 1.22 and cabal-install 1.22 Domain: groups.google.com, Score: 54, Comments: 3 Original: [11] http://goo.gl/WdxPu0 On Reddit: [12] http://goo.gl/ih07XV * [Haskell-cafe] The amount of CPP we have to use is getting out of hand Domain: haskell.org, Score: 51, Comments: 94 Original: [13] http://goo.gl/0OYZ7C On Reddit: [14] http://goo.gl/4QCAIc * Boston Haskell lightning talks: Rust, Phantoms, ATS, Freezing, Music, and Code reuse Domain: self.haskell, Score: 50, Comments: 8 Original: [15] http://goo.gl/TAjkMi On Reddit: [16] http://goo.gl/TAjkMi * Haskell for all: total-1.0.0: Exhaustive pattern matching using traversals, prisms, and lenses Domain: haskellforall.com, Score: 47, Comments: 9 Original: [17] http://goo.gl/TJDioE On Reddit: [18] http://goo.gl/sx7aMu * Mutation analysis for Haskell (alpha) Domain: self.haskell, Score: 46, Comments: 9 Original: [19] http://goo.gl/MByMzL On Reddit: [20] http://goo.gl/MByMzL * Simple Algebraic Data Types by Bartosz Milewski Domain: bartoszmilewski.com, Score: 45, Comments: 1 Original: [21] http://goo.gl/Ns3UjL On Reddit: [22] http://goo.gl/uuqCt2 * The Frank programming language Domain: homepages.inf.ed.ac.uk, Score: 42, Comments: 29 Original: [23] http://goo.gl/msxrp5 On Reddit: [24] http://goo.gl/0NPQYY * GHC Weekly News - 2015/01/07 Domain: ghc.haskell.org, Score: 42, Comments: 3 Original: [25] http://goo.gl/HVsDLU On Reddit: [26] http://goo.gl/JiMf4b * The Rust community grapples with the menace of ORPHAN INSTANCES Domain: discuss.rust-lang.org, Score: 42, Comments: 19 Original: [27] http://goo.gl/HIl68S On Reddit: [28] http://goo.gl/dUWa48 * The state and future of supercompilation in GHC. Domain: self.haskell, Score: 39, Comments: 10 Original: [29] http://goo.gl/e9FDLV On Reddit: [30] http://goo.gl/e9FDLV Top StackOverflow Questions * Implementation of null function votes: 18, answers: 2 Read on SO: [31] http://goo.gl/cVqJa3 * What is the type of this self-applying factorial function? votes: 17, answers: 3 Read on SO: [32] http://goo.gl/vQaeQ0 * Haskell fast concurrent queue votes: 17, answers: 3 Read on SO: [33] http://goo.gl/HdAlul * Ghc: partially compile Haskell code? votes: 13, answers: 1 Read on SO: [34] http://goo.gl/hS2QNQ * What is the type of return 5 in Haskell when no context is given? votes: 12, answers: 1 Read on SO: [35] http://goo.gl/IC08Sk * Is it possible to write fmap for this data type involving a type family? votes: 11, answers: 1 Read on SO: [36] http://goo.gl/6V5Mps * How to get cabal and nix work together votes: 11, answers: 1 Read on SO: [37] http://goo.gl/TOXNDv * What is the most general way to compute the depth of a tree with something like a fold? votes: 8, answers: 2 Read on SO: [38] http://goo.gl/vlcioF * What is exactly an indexed functor in Haskell and what are its usages? votes: 8, answers: 1 Read on SO: [39] http://goo.gl/10PK8U * Functor instance for generic polymorphic ADTs in Haskell? votes: 8, answers: 2 Read on SO: [40] http://goo.gl/Oh0uFp * Finite State Transducers in Haskell? votes: 7, answers: 1 Read on SO: [41] http://goo.gl/gu80ZL * Is it possible to encode a generic “lift” function in Haskell? votes: 7, answers: 2 Read on SO: [42] http://goo.gl/R9B8N5 * Haskell Expression Equivalents votes: 6, answers: 1 Read on SO: [43] http://goo.gl/t54YiS Until next time, [44]+Daniel Santa Cruz References 1. http://dev.stephendiehl.com/fun/ 2. http://www.reddit.com/r/haskell/comments/2regln/write_you_a_haskell/ 3. https://www.fpcomplete.com/blog/2015/01/announcing-lts-haskell-1-0 4. http://www.reddit.com/r/haskell/comments/2rapgt/announcing_lts_haskell_10/ 5. http://blog.nullspace.io/bond-oss.html 6. http://www.reddit.com/r/haskell/comments/2rxkmk/msft_open_sources_productio… 7. https://github.com/knupfer/haskell-emacs 8. http://www.reddit.com/r/haskell/comments/2sdvyu/haskellemacs_write_emacs_ex… 9. http://typetheorypodcast.com/2015/01/episode-3-dan-licata-on-homotopy-type-… 10. http://www.reddit.com/r/haskell/comments/2rnumy/the_type_theory_podcast_epi… 11. https://groups.google.com/forum/#!msg/haskell-cafe/rRbP9oJjOBc/J8lf_sGVl_oJ 12. http://www.reddit.com/r/haskell/comments/2rcuak/cabal_122_and_cabalinstall_… 13. https://www.haskell.org/pipermail/haskell-cafe/2015-January/117683.html 14. http://www.reddit.com/r/haskell/comments/2shv2d/haskellcafe_the_amount_of_c… 15. http://www.reddit.com/r/haskell/comments/2sn8la/boston_haskell_lightning_ta… 16. http://www.reddit.com/r/haskell/comments/2sn8la/boston_haskell_lightning_ta… 17. http://www.haskellforall.com/2015/01/total-100-exhaustive-pattern-matching.… 18. http://www.reddit.com/r/haskell/comments/2s1tfx/haskell_for_all_total100_ex… 19. http://www.reddit.com/r/haskell/comments/2rj5sw/mutation_analysis_for_haske… 20. http://www.reddit.com/r/haskell/comments/2rj5sw/mutation_analysis_for_haske… 21. http://bartoszmilewski.com/2015/01/13/simple-algebraic-data-types/ 22. http://www.reddit.com/r/haskell/comments/2sbl93/simple_algebraic_data_types… 23. http://homepages.inf.ed.ac.uk/slindley/papers/frankly-draft-march2014.pdf 24. http://www.reddit.com/r/haskell/comments/2rao0t/the_frank_programming_langu… 25. https://ghc.haskell.org/trac/ghc/blog/weekly20150107 26. http://www.reddit.com/r/haskell/comments/2rq6zc/ghc_weekly_news_20150107/ 27. http://discuss.rust-lang.org/t/orphan-rules/1322/4 28. http://www.reddit.com/r/haskell/comments/2shyfj/the_rust_community_grapples… 29. http://www.reddit.com/r/haskell/comments/2s97d0/the_state_and_future_of_sup… 30. http://www.reddit.com/r/haskell/comments/2s97d0/the_state_and_future_of_sup… 31. http://stackoverflow.com/questions/27941690/implementation-of-null-function 32. http://stackoverflow.com/questions/27814714/what-is-the-type-of-this-self-a… 33. http://stackoverflow.com/questions/27933941/haskell-fast-concurrent-queue 34. http://stackoverflow.com/questions/27780282/ghc-partially-compile-haskell-c… 35. http://stackoverflow.com/questions/27932794/what-is-the-type-of-return-5-in… 36. http://stackoverflow.com/questions/27936725/is-it-possible-to-write-fmap-fo… 37. http://stackoverflow.com/questions/27968909/how-to-get-cabal-and-nix-work-t… 38. http://stackoverflow.com/questions/27769688/what-is-the-most-general-way-to… 39. http://stackoverflow.com/questions/27771474/what-is-exactly-an-indexed-func… 40. http://stackoverflow.com/questions/27856974/functor-instance-for-generic-po… 41. http://stackoverflow.com/questions/27997155/finite-state-transducers-in-has… 42. http://stackoverflow.com/questions/28003135/is-it-possible-to-encode-a-gene… 43. http://stackoverflow.com/questions/27761821/haskell-expression-equivalents 44. https://plus.google.com/105107667630152149014/about

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cabal-install crashes
by Henk-Jan van Tuyl 21 Jan '15

21 Jan '15

L.S., On my Windows PC, cabal-install crashes each time a file must be downloaded, i.e. when I try to install a package that is not in the cache and when I give command "cabal update". I deleted the file %appdata%\cabal\config to make sure it is not the cause of this crash. I tried several versions of cabal-install, the Haskell Platform I use is 2014.2.0.0. How can I solve this? Regards, 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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StateT and ContT
by Ivan Lazar Miljenovic 20 Jan '15

20 Jan '15

In the transformers library, there are two implementations for callCC for use with StateT (both lazy and strict) [1,2], the second of which is documented to not satisfy the laws of a monad transformer. [1]: http://hackage.haskell.org/package/transformers-0.4.2.0/docs/Control-Monad-… [2]: http://hackage.haskell.org/package/transformers-0.4.2.0/docs/Control-Monad-… However, in mtl the MonadCont class [3] uses the second definition for the StateT instance. Is there a good reason for this, or just to match the behaviour of pre-transformers mtl [4] ? [3]: http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Cont-Class.… [4]: http://hackage.haskell.org/package/mtl-1.1.1.1/docs/src/Control-Monad-State… (I did raise an issue for this on mtl's Github issue tracker, but it hasn't had any responses for two months.) -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

4 5

MonoidHashMap?
by Konstantine Rybnikov 20 Jan '15

20 Jan '15

Hi! I'm working with a lot of HashMap's and it's very frustrating how many times I've "lost" my data because of usage of either a Monoid instance of a HashMap (which is defined as H.union, which, upon collision, takes value from first hm and discards from second), or just using fromList in the wrong place. Whereas the data I'm working is is mostly defined as (Monoid v => HashMap k v), so what I need "by default" is actually something like `H.unionWith (<>)`. What I was wondering is this: is something like MonoidHashMap is desired to be in unordered-containers, or is this use-case only popular in my programs? I'm asking because I have a feeling that this thing might be useful quite a lot for others also. If not -- sorry for bothering :) Cheers!

6 7

garbage collection for a data structure
by Elise Huard 20 Jan '15

20 Jan '15

Hi, I was wondering if there was a way to check whether a particular data structure gets garbage collected in a program. A friendly person pointed me to System.Mem.Weak on the Haskell-Beginner list - however I've been unable to verify how it works, so I'm bumping it to this list. See the following toy program: I was trying to see whether the output would contain "garbage collected". I wondered if performGC is a nudge rather than an immediate "garbage collect now" instruction, and performGC is not actually performed? Or I've misunderstood finalizers in this context and they would not actually be executed when z gets garbage collected? import System.Mem.Weak import System.Mem (performGC) import Control.Concurrent (threadDelay) main :: IO () main = do let x = 5 y = "done" z = 3 a <- mkWeak z x (Just (putStrLn "garbage collected")) performGC threadDelay 20000000 print y Thank you, Elise

3 3

New Functional Programming Job Opportunities
by Functional Jobs 19 Jan '15

19 Jan '15

Here are some functional programming job opportunities that were posted recently: Functional Software Developer at Moixa Technology http://functionaljobs.com/jobs/8778-functional-software-developer-at-moixa-… Cheers, Sean Murphy FunctionalJobs.com

1 0

http-client: proxy environment variable support
by Michael Snoyman 19 Jan '15

19 Jan '15

Neil Mitchell opened an issue[1] for http_proxy and https_proxy environment variable support in http-client. I've written that support, and it's ready to go, but there's an open question: what should the default behavior be? In particular, should environment variables, by default, be checked to determine the proxy, or not? Arguments each way: In favor of using environment variables: * Matches behavior of many other tools and libraries * Allows application users control without requiring a code change from application writers Against using environment variables: * It's a change in behavior vs what http-client does today (though that could certainly be seen as just a missing feature) * Environment variables will implicitly change the behavior of code, which generally speaking can be problematic I'm leaning towards having the default behavior be: * If the user explicitly chooses a proxy setting on the manager, use that * If the user explicitly sets a proxy value on the Request, use that * If the environment variable is set, use that * Otherwise, no proxy In addition to whether we should make this change in behavior, I'm also interested in whether people think this should require a major version bump. [1] https://github.com/snoyberg/http-client/issues/94

2 2

enable-executable-profiling failing with cabal 1.22
by Michael Jones 19 Jan '15

19 Jan '15

Using cabal 1.22 for some reason does not like —enable-executable-profiling. I am working on a rebuilt machine that had 1.18 from the distribution, with 1.22 installed afterwards. I found a little info on people having this problem, but I failed to fix the problem. Note than the library profiling option is accepted. Only the executable fails. Is there any know way to start with platform 2014 with GHC 7.8.3 and install cabal 1.22 and can use this option?

3 2

This code produces an infinite output... why? it's a bug in ghc?
by mariopal 19 Jan '15

19 Jan '15

This code produces an infinite output... why? it's a bug in ghc? > -- Glasgow Haskell Compiler, Version 7.6.3 on Debian Sid > import System.Locale (defaultTimeLocale) > import Data.Time > > > mkDated n = formatTime defaultTimeLocale "%FT%TZ" $ addUTCTime (3600*n) $ UTCTime (fromGregorian 2015 1 1) (timeOfDayToTime $ TimeOfDay 0 0 0) > > main = mapM_ putStrLn $ map mkDated [0..2]

2 1

Code-completion problem with emacs, ghc-mod and auto-complete
by Илья Бирюков 18 Jan '15

18 Jan '15

Hi, Cafe! I'm struggling to configure my emacs for haskell development with ghc-mod and auto-complete. It works well for the most part, but I've run into the issue when completing qualified names. In the following example I don't get any completion after BS: module Playground where import qualified Data.ByteString.Lazy as BL doWork :: BL.By -- no completion after dot Completion for unqualified names and module name in imports works just fine. Maybe someone has run into this issue before? My .emacs: (ac-config-default) (auto-complete-mode t) (add-hook 'haskell-mode-hook 'turn-on-haskell-indentation) (autoload 'ghc-init "ghc" nil t) (add-hook 'haskell-mode-hook (lambda () (ghc-init))) (defun my-ac-haskell-mode () (setq ac-sources (append ac-sources '(ac-source-ghc-mod)))) (add-hook 'haskell-mode-hook 'my-ac-haskell-mode) (add-hook 'haskell-mode-hook 'turn-on-haskell-doc-mode) (add-hook 'haskell-mode-hook 'turn-on-haskell-indent) (ac-define-source ghc-mod '((depends ghc) (candidates . ghc-select-completion-symbol) (symbol . "s") (cache))) -- Regards, Ilya Biryukov.

2 2

haskell.org slow
by Hector Urroz 16 Jan '15

16 Jan '15

Anyone know when haskell.org <http://haskell.org/> will be usable again? Page loads take 20+ seconds. Info at https://status.haskell.org <https://status.haskell.org/> indicates there's a DB problem, but there's no ETA and performance has been bad for over 10 days.

2 1

Number of widgets extant and displayed varying over time? (FRP, reactive-banana)
by Jeffrey Brown 16 Jan '15

16 Jan '15

Dear list, I want to write an application in which the set of widgets in existence, and the subset of them that is displayed, depends on user input. I am using reactive-banana. So far the simplest spec I have imagined for it is this: Initially there is just a text entry box and an empty (hence invisible) collection of labels. The user can do two things: enter the word "add", or enter an integer. Entering the word "add" causes a label to be added to the collection, but not displayed. The labels require no text content. Entering an integer N causes the first N labels to be displayed onscreen. The text entry box remains visible. I am totally baffled. In particular, the Behavior paradigm, though it is elegant and beautiful whenever I study it, I have no idea how to apply. Thank you, Jeff

2 3

Undefined symbols when trying to use SDL-mixer
by Peter Reiner Fels 15 Jan '15

15 Jan '15

I'm trying to use SDL-mixer v0.6.1. When I add a dependency to my cabal file and run cabal repl I get the following error: Loading package SDL-mixer-0.6.1 ... <command line>: can't load .so/.DLL for: /home/prf/haskell/link-problem/.cabal-sandbox/lib/x86_64-linux-ghc-7.8.3/SDL-mi xer-0.6.1/libHSSDL-mixer-0.6.1-ghc7.8.3.so (/home/prf/haskell/link-problem/.cab al-sandbox/lib/x86_64-linux-ghc-7.8.3/SDL-mixer-0.6.1/libHSSDL-mixer-0.6.1-ghc7 .8.3.so: undefined symbol: Mix_FreeChunk) I can see these undefined symbols when I run: nm -D SDL-mixer-0.6.1/libHSSDL-mixer-0.6.1-ghc7.8.3.so | grep Free U Mix_FreeChunk U Mix_FreeMusic Running ldd on this file and grepping for SDL gives libHSSDL-0.6.5-ghc7.8.3.so => /home/prf/haskell/link-problem/.cabal-sandbox/lib /x86_64-linux-ghc-7.8.3/SDL-0.6.5/libHSSDL-0.6.5-ghc7.8.3.so (0x00007fb20b30600 0) libSDL-1.2.so.0 => /usr/lib64/libSDL-1.2.so.0 (0x00007fb20a232000) It seems like a dependency to /usr/lib64/libSDL_mixer.so is missing. Line 1815 of SDL-mixer-0.6.1/configure contains the following definition of SDL_LIBS: SDL_LIBS="-L`$SDL_CONFIG --prefix`/lib -lSDL_mixer" This refers to the 32 bit lib directory, but I'm on a 64 bit system. I unpacked SDL-mixer and changed this line to refer to the 64 bit lib directory. Next I recreated my cabal sandbox from scratch and added the patched version of SDL-mixer as a dependency. When I run cabal repl again I get the same error as mentioned above. What am I doing wrong? How do I get SDL-mixer to build against /usr/lib64/libSDL_mixer.so? Thanks, Peter

1 0

uu-parsinglib - Greedy Parser
by Marco Vassena 15 Jan '15

15 Jan '15

In the uu-parsinglib the choice combinator (<|>) is symmetric and does not commit to any alternative. If the grammar being parsed is ambiguous, the corresponding parser will fail at run time on an ambiguous input. Consider for instance this html parser. pTag = pOpenTag <|> pCloseTag <|> pCommentTag <|> pContent pContent = Content <$> some (satisfy (/= '<')) pHtml = some pTag This parser will fail on "<a>123</a>", because this may be interpreted as: [ [Open a , Content "123", Close a], [Open a, Content "12", Content "3", Close a], [Open a, Content "1", Content "2", Content "3", Close a] ... ] In other parsing libraries such as parsec the choice operator <|> is greedy and commits to the first alternative that makes any progress, so that some is greedy and Content "123" would be parsed. The operator <<|> in uu-parsinglib has the same greedy behaviour. I would like to disambiguate the grammar so that the first result ([Open a, Content "123", Close a]) is selected (longest matching rule). I suppose that this may be done using <<|> to define a greedySome to be used in in pContent, however I am wondering whether there is another way to do this, without using such operator <<|>. Any help is appreciated. All the best, Marco

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1/15/2015 3:46:00 PM
by Diony Rosa 15 Jan '15

15 Jan '15

http://www.1367.co.nz/basjaupc/pebrbhpjthzcrumhnityftkmrmxbjau.myfduxbsyxon… Diony Rosa 1/15/2015 3:46:00 PM

1 0

ParallelListComp strange behavior
by Michael Jones 14 Jan '15

14 Jan '15

I am trying to use ParallelListComp and have code that compiles but only produces one value, but I am not clear why. The code is something like: gen :: (a -> b) -> [MyType] gen f = [MyType (Just []) (Just c) (Just d) Nothing | c <- f | d <- [1,2,3,4] When the gen function is used with take or any other function, only one value is returned. Is there something here that can’t desugar properly to mzip that will still compile? Note I did not test this specific code, I am just writing it here to represent the different pieces of a larger piece of real code which is below. Mike ———— Real Code ———— data Config = Config { onOffControl::Maybe [DEVICE_ON_OFF_CONTROL_BITS] , seqUpPos::[Maybe ([DEVICE_SEQ_UP_POSITION_BITS], Word16)] , seqDownPos::[Maybe ([DEVICE_SEQ_DOWN_POSITION_BITS], Word16)] , tonTimers::[Maybe ([DEVICE_TON_TIMERS_BITS], Word16)] , toffTimers::[Maybe ([DEVICE_TOFF_TIMERS_BITS], Word16)] , vRange::Maybe [DEVICE_V_RANGE_BITS] , vThresh::[Maybe (Word16, Word16)] , rstbConfig::Maybe [DEVICE_RSTB_CONFIG_BITS] , faultResponse::Maybe [DEVICE_FAULT_RESPONSE_BITS] , breakpoint::Maybe ([DEVICE_BREAK_POINT_BITS], Word16) , writeProtect::Maybe ([DEVICE_WRITE_PROTECTION_BITS], Word16) , specialLot::Maybe Word16 , deviceId::Maybe Word16 , statusInfo::Maybe ([DEVICE_STATUS_INFORMATION_BITS]) , monitorStatus::Maybe ([DEVICE_MONITOR_STATUS_BITS]) , monitorStatusHistory::Maybe ([DEVICE_MONITOR_STATUS_HISTORY_BITS]) , monitorBackup::Maybe ([DEVICE_MONITOR_BACKUP_BITS]) , seqPosCount::Maybe ([DEVICE_SEQ_POSITION_COUNT_BITS], Word16) generateConfigs seqValues delayValues threshValues = return [Config (Just []) -- On off ([Just (suPol, suPos),Just (suPol, suPos),Just (suPol, suPos),Just (suPol, suPos),Just (suPol, suPos),Just (suPol, suPos)]) ([Just (sdPol, sdPos),Just (sdPol, sdPos),Just (sdPol, sdPos),Just (sdPol, sdPos),Just (sdPol, sdPos),Just (sdPol, sdPos)]) ([Just ([tonMax], tonDelay),Just ([tonMax], tonDelay),Just ([tonMax], tonDelay),Just ([tonMax], tonDelay),Just ([tonMax], tonDelay),Just ([tonMax], tonDelay)]) ([Just ([toffMax], toffDelay),Just ([toffMax], toffDelay),Just ([toffMax], toffDelay),Just ([toffMax], toffDelay),Just ([toffMax], toffDelay),Just ([toffMax], toffDelay)]) (Just [rng1, rng2, rng3, rng4, rng5, rng6]) ([Just (threshOv, threshUv),Just (threshOv, threshUv),Just (threshOv, threshUv),Just (threshOv, threshUv),Just (threshOv, threshUv),Just (threshOv, threshUv)]) (Just ([rst_en] ++ [rst_time])) (Just ([fault_number] ++ [fault_action] ++ [fault_delay] ++ fault_other ++ [fault_count])) (Just ([], 0)) (Just ([], 0)) (Just 0) Nothing Nothing Nothing Nothing Nothing Nothing | suPol <- [DEVICE_seq_up_position_bits] | suPos <- seqValues | sdPol <- [DEVICE_seq_down_position_bits] | sdPos <- seqValues | tonMax <- DEVICE_ton_timers_bits | tonDelay <- delayValues | toffMax <- DEVICE_toff_timers_bits | toffDelay <- delayValues | rng1 <- DEVICE_v_range_bits1 | rng2 <- DEVICE_v_range_bits2 | rng3 <- DEVICE_v_range_bits3 | rng4 <- DEVICE_v_range_bits4 | rng5 <- DEVICE_v_range_bits5 | rng6 <- DEVICE_v_range_bits6 | threshOv <- threshValues | threshUv <- threshValues | rst_time <- DEVICE_rstb_config_bits_time | rst_en <- DEVICE_rstb_config_bits_en | fault_number <- DEVICE_fault_response_bits_retry_number | fault_other <- [DEVICE_fault_response_bits_other] | fault_count <- DEVICE_fault_response_bits_count | fault_delay <- DEVICE_fault_response_bits_delay | fault_action <- DEVICE_fault_response_bits_action ] generateSeqValues :: IO [Word16] generateSeqValues = do vs <- replicateM 10 (randomRIO (0x0000, 0x03FF)) return vs generateDelayValues :: IO [Word16] generateDelayValues = do vs <- replicateM 10 (randomRIO (0x0000, 0x1FFF)) return vs generateThreshValues :: IO [Word16] generateThreshValues = do vs <- replicateM 10 (randomRIO (0x0000, 0x00FF)) return vs

2 4

Re: [Haskell-cafe] working my way through Data.Type.Equality...my head hurts...
by oleg＠okmij.org 14 Jan '15

14 Jan '15

Richard Eisenberg wrote: > `~` is simply equality on types. The constraint `a ~ b` means that `a` > is the same type as `b`. We could have spelled it `=` if that weren't > used elsewhere. Before `~` was added to GHC, it might have been > implemented like this: > class Equals a b | a -> b, b -> a > instance Equals x x > -- no other instances! Actually, `~' has been implemented, back in 2004, and the implementation quite a bit more involved than the above (although still taking a couple of lines). The `~' constrain was called TypeCast back then, and there is a page about it explaining the benefits and peculiarities of the implementation. http://okmij.org/ftp/Haskell/typecast.html Perhaps the many examples of TypeCast make it less mysterious.

1 0

Deploying Scotty App to AWS
by Stephen Portanova 14 Jan '15

14 Jan '15

I'm having trouble running my Scotty binary on amazon web services... I'm using Ubuntu 15.04, this gist (https://gist.github.com/yantonov/10083524) to install the haskell platform dependencies, and then I do a git pull to my repo and build my binary. Everything compiles, but when I do ./cabal-sandbox/bin/app or ./dist/build/app/app, nothing happens. No error messages or anything - the terminal just goes to the next line and hangs (on my Mac development machine the server starts up fine and prints a startup message). Any ideas or experience with deploying Scotty / Haskell apps would be really appreciated, thanks! - Stephen

5 7

Future of the boxes package--call for ideas
by David Feuer 13 Jan '15

13 Jan '15

I've just taken over maintainership of the boxes package, and will be making a maintenance release shortly (as soon as I figure out how and get added to the maintainers group). The package, however, currently suffers from a paucity of bug reports (no problem) and feature requests (not so great). To keep things lively, I need a bit of help from two groups of people. If you use the package but wish it could do something more for you, I want to know about it. If you considered using the package but rejected it because it couldn't quite handle your job, I want to know about that too. I'd prefer if people would open issues and pull requests at https://github.com/treeowl/boxes/issues but I will also accept requests by email. Thanks, David

1 0

working my way through Data.Type.Equality...my head hurts...
by Nicholls, Mark 13 Jan '15

13 Jan '15

Full code here… https://github.com/goldfirere/nyc-hug-oct2014/blob/master/Equality.hs Lets start at the beginning… Ø data a :~: b where Ø Refl :: a :~: a Looks reasonable….” a :~: b” is inhabited by 1 value…of type “a :~: a” Ø sym :: (a :~: b) -> (b :~: a) Ø sym Refl = Refl hmmm… (a :~: b) implies that a is b so (b :~: a) brilliant Ø -- | Transitivity of equality Ø trans :: (a :~: b) -> (b :~: c) -> (a :~: c) Ø trans Refl Refl = Refl Ø -- | Type-safe cast, using propositional equality Ø castWith :: (a :~: b) -> a -> b Ø castWith Refl x = x fine…. But…. Ø -- | Generalized form of type-safe cast using propositional equality Ø gcastWith :: (a :~: b) -> ((a ~ b) => r) -> r Ø gcastWith Refl x = x I don’t even understand the signature What does “~” mean…(it’s something that comes out in error messages when my types are all messed up)….and then there’s a “=>” going on…in the middle of a signature….I know “=>” in the context of “Num a => a -> a -> a” What would a value of type “((a ~ b) => r)” look like? CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

5 13

--disable-{executable-}profiling cabal-install-1.22.0.0 with ghc-7.6.3
by Ozgur Akgun 13 Jan '15

13 Jan '15

A recent change in cabal-install's command line interface seems not to be compatible with ghc-7.6.3. cabal-install-1.22.0.0 doesn't like --disable-executable-profiling as a command line option any more. (I could only find this related to this change: https://github.com/haskell/cabal/pull/2286) $ cabal install --disable-executable-profiling cabal: unrecognized 'install' option `--disable-executable-profiling' I try --disable-profiling instead. $ cabal install transformers --disable-profiling -j3 Resolving dependencies... Notice: installing into a sandbox located at [snip] Configuring transformers-0.4.2.0... Failed to install transformers-0.4.2.0 Build log [snip]: unrecognized 'configure' option `--disable-profiling' cabal: Error: some packages failed to install: transformers-0.4.2.0 failed during the configure step. The exception was: ExitFailure 1 Using the following versions of cabal and ghc. $ cabal -V cabal-install version 1.22.0.0 using version 1.22.0.0 of the Cabal library $ ghc -V The Glorious Glasgow Haskell Compilation System, version 7.6.3 --disable-profiling works fine with ghc-7.8.3, this is only a problem with 7.6.3. PS: transformers is only a dependency of an executable I am trying to build. But this happens with all the other dependencies as well. -- Ozgur Akgun

3 14

FP Complete is hiring: Software engineer
by Mathieu Boespflug 13 Jan '15

13 Jan '15

Hi list, FP Complete is expanding yet again! We are looking to hire for several new engineers to join our Haskell development team, both to build great new core products and in partnership with our clients to apply Haskell at a large scale. Some of our recently announced core products include the Integrated Analysis Platform, Stackage and LTS Haskell, with much more to come. If you’d like to be part of our team and shape the future of Haskell, please send a resume or CV to jobs+dev(a)fpcomplete.com. Any existing work - either a running website or an open source codebase - which you can include as links be greatly appreciated as well. We will want you to start right away. Depending on your current jurisdiction, this will either be a full-time contractor position, or an employee position. This is a telecommute position: you can work from home or wherever you choose, with little or no travel. Location in North America preferred; but you will work with colleagues who are both on North American and European hours. Activities: * distribute existing and new code over large clusters, * code parallelization and performance tuning, * interface with foreign math and scientific libraries, * relentlessly refactor for composability, testability and clarity, * identify performance bottlenecks and debug known issues, * implementing unit tests, integration tests, acceptance tests, * write clear and concise documentation. Skills: * strong experience writing process driven application code in Haskell. * experience building reusable components/packages/libraries and demonstrated ability to write well structured and well documented code, * ability to evolve and refactor large code bases, * experience working under test driven methodologies, * Ability to interact with a distributed development team, and to communicate clearly on issues, in bug reports and emails. These further skills are a plus: * Bachelor’s or Master’s degree in computer science or mathematics, * experience developing products in a regulated environment (avionics/medical/finance). * experience building scalable server/Web applications, * (web) UI design and implementation experience, * an understanding of the implementation of GHC’s multithreaded runtime, * experience as an architect and/or a creator of technical specs.

1 0

ANNOUNCE: picoparsec-0.1
by Mario Blazevic 13 Jan '15

13 Jan '15

The newly released package picoparsec [1] is a fork of attoparsec that works with a much wider variety of input types than just ByteString and Text. Parsers written with Picoparsec will generally perform a bit slower than Attoparsec on strict ByteString and Text, somewhat faster than Attoparsec on lazy Text, and much faster than Parsec on any of the mutually-supported input types. That being said, the main point of the library is to enable the writing of generic parsers that are not constrained to any single input type. Depending on the parser primitives used, Picoparsec will take any input whose type is an instance of either NullMonoid, LeftGCDMonoid, FactorialMonoid, or TextualMonoid. These are some of the subclasses of Monoid defined in the monoid-subclasses package [2]. Some of the input types that are fully supported out of the box would be: - Text - String - Seq Char - Vector Char - ByteStringUTF8, a thin newtype wrapper around ByteString - Stateful (Map String a) Text, if you need to parse Text and keep track of the the symbol table state - OffsetPositioned String, in case you need to keep track of the current input position - LinePositioned Text, to keep track of the current line and column This release can be seen as the belated completion of the work described in my Haskell Symposium 2013 presentation [3]. The name Picoparsec seemed to fit in just the right place between Parsec and Attoparsec, especially since Nanoparsec [4] has already been taken. [1] http://hackage.haskell.org/package/picoparsec [2] http://hackage.haskell.org/package/monoid-subclasses [3] https://github.com/blamario/monoid-subclasses/wiki/Files/HaskellSymposium20… [4] http://hackage.haskell.org/package/nanoparsec

1 0

Vacancies: 4 PhD students Software Technology Utrecht University
by Johan Jeuring 13 Jan '15

13 Jan '15

============================================================ 4 x PhD position in Software Technology ============================================================ The research group of Software Technology is part of the Software Systems division of in the department of Information and Computer Science at the Utrecht University. We focus our research on functional programming, compiler construction, program analysis, validation, and verification. Financed by the Netherlands Organisation for Scientific Research (NWO), the EU, Technology Foundation STW and Utrecht University we currently have job openings for four PhD researchers (PhD students) in Software Technology. We are looking for PhD students to work on some of the following topics: * Version control of structured data The theory and practice underlying structure-aware version control systems capable of handling more than just text files. * Intelligent tutoring technologies Technologies for tutoring subjects such as functional programming, statistics, algebra, etc. * Serious games Domain-specific languages and technologies for specifying strategies for serious games. * iTasks iTasks is a formalism for specifying distributed tasks. Specify and test properties of iTasks, and give run-time feedback for iTasks. Besides research, the successful candidate will be expected to help supervise MSc students and assist teaching courses. We prefer candidates to start no later than September 2015. --------------------------- What we are looking for --------------------------- The candidate should have an MSc in Computer Science, be highly motivated, speak and write English well. Furthermore the successful candidate should be proficient in reporting scientific findings. Knowledge of and experience with at least some of the following areas (depending on the topic chosen to work on for your PhD) is essential: * functional programming, such as Haskell or ML; * datatype generic programming; * intelligent tutoring systems; * serious games; * strategies, rewriting, parsing; * modern version control systems such as git, mercurial, or darcs. ------------------ What we offer ------------------ The candidate is offered a full-time position for 4 years. A part-time position of at least 0.8 fte may also be possible. Salary starts at 2083 euro and increases to 2664 euro gross per month in the fourth year of the appointment. The salary is supplemented with a holiday bonus of 8% and an end-of-year bonus of 8.3% per year. In addition we offer: a pension scheme, partially paid parental leave, flexible employment conditions. Conditions are based on the Collective Labour Agreement Dutch Universities. The research group will provide the candidate with necessary support on all aspects of the project. ---------------- How to apply ---------------- To apply please attach a letter of motivation, a curriculum vitae, and (email) addresses of two referees. Make sure to also include a transcript of the courses you have followed (at bachelor and master level), with the grades you obtained, and to include a sample of your scientific writing, such as your master thesis. It is possible to apply for this position if you are close to obtaining your Master's. In that case include a letter of your supervisor with an estimate of your progress, and do not forget to include at least a sample of your technical writing skills. Application deadline is March 8, 2015. You can apply online through the University's website: http://tinyurl.com/qhlco6s --------------------------- Additional information --------------------------- If you have any questions regarding these positions, please contact Johan Jeuring +31 (0)640010053 J.T.Jeuring(a)uu.nl Wouter Swierstra +31 (0)30 253 9207 w.s.swierstra(a)uu.nl

1 0

May I have some examples on Kinds and Data.Type.Equality?
by Magicloud Magiclouds 12 Jan '15

12 Jan '15

Hi, I am trying to learn some advanced stuff of Haskell. But haskell wiki seems only giving some concepts and so. May I have some code example? Also other documents are welcomed. Thanks. -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

2 1

Re: [Haskell-cafe] Typesafe API calls
by Fabian Bergmark 12 Jan '15

12 Jan '15

I have uploaded a new version of the library https://hackage.haskell.org/package/apis-0.0.1 I'have included 15 more APIs from Facebook and Dropbox. I have also uploaded documentation to Hackage. I have been working on creating an autotools installation script that builds a makepkg package with the FFI bindings for Python and Java. I have tried it on Arch and it seems to work. If you want to try it out, check out the github repository. Building on top of this, Im working on a library that uses Pipes to create producers (triggers) and consumers (actions), allowing one to create effects (recipes) of the kind, when I am tagged in a photo on Facebook, notify me via email. This very much resembles IFTTT but with static and typesafe compositions. Im experimenting with using my javascript interpreter to allow for more dynamic compositions. 2014-12-18 21:59 GMT+01:00 Jonathan Paugh <jpaugh(a)gmx.us>: > Hello Fabian, > > I have been dreaming of something like this for Haskell since I wrote a > (non type-safe) python library to access any REST API generically[1]. > This comes from a different (and more ambitious) angle than I had imagined. > > I don't have time to test this just now, but I will keep it bookmarked. > Thanks for sharing your hard work! > > Regards, > Jonathan > > [1]: http://github.com/jpaugh/agithub (unmaintained, but maybe interesting) > > On 12/12/2014 07:01 AM, Fabian Bergmark wrote:> I just released a > library on hackage >> (https://hackage.haskell.org/package/apis-0.0.0) The goal of the >> library is to provide typesafe API calls to a large amount of existing >> APIs. By typesafe I mean that, for each API the library provides two >> types, Input and Output, and a function :: Input -> IO Output. >> Internally the library knows how to serialize Input to make an HTTP >> request and how to deserialize the response into Output. >> >> Whats cool about this library is that it uses Template Haskell to >> generate the API interfaces. A combination of Open Data Tables, an >> internal YQL engine and JSON Schemas, all written in Haskell makes >> this possible. >> >> Right now only two APIs are included (Swedish weather data and Swedish >> public announcement), but more APIs are soon to follow. >> >> I have also implemented FFI exports of all types and functions so >> that, in combination with SWIG, this library can be used in almost any >> language. So far I have tried Java and Python with success. >> >> If anyone would like to try it and respond with feedback it would be >> greatly appreciated! If you want any specific API supported, I can try >> to import that right away >> >> Fabian Bergmark >> >

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why can it never go smoothly, language-c-0.4.7 edition
by briand＠aracnet.com 12 Jan '15

12 Jan '15

Hi, setup-Simple-Cabal-1.22.0.0-x86_64-linux-ghc-7.8.4: The program 'happy' is required but it could not be found. so i installed happy. then it wanted alex. which i was also able to install. and then language-c-0.4.7 successfully installed. I have seen this sort of thing many times across all sorts of different packages. So my question is : are these dependency bugs that should be reported to the package maintainer ? or is that certain packages can't be built as a dependency target for some reason ? Thanks, Brian

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ghc-mod split feature
by Tobias Pflug 11 Jan '15

11 Jan '15

Hi, i just wanted to try some of the ghcmod features that I haven't used before in order to hopefully add them to my vim haskell workflow somehow. What I apparently can't get to work is the split command: $ ghc-mod split src/Foo.hs Foo 47 7 which points to exactly the code listed in the ghc-mod help: f :: [a] -> a f x = _ It does however not yield any output at all. Am I missing something obvious here ? I'm using version 5.2.1.1 Thanks, Tobi

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new haskell music track: Mother Is Waiting
by Anton Kholomiov 11 Jan '15

11 Jan '15

Dear Haskellers I'd like to present yet another piece of music made in Haskell. https://soundcloud.com/anton-kho/mother-is-waiting It's very soothing piece that can be used as a helper for dreaming or meditation. It's dedicated to my friend. She's waiting for her second child to be born. This time it contains no samples. All sounds are made out of waves and filters. So I can attach the source code right in the mail. The track relies on new versions of the packages `csound-expression`, `csound-sampler` and `csound-catalog`. So you can do > cabal update > cabal install csound-sampler csound-catalog Ad the we can load the file in the ghci and run the main function. > ghci Mother.hs > main and the Csound should be installed to render the synths. Cheers, Anton

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The amount of CPP we have to use is getting out of hand
by Johan Tibell 09 Jan '15

09 Jan '15

Hi, (This was initially written as a Google+ post, but I'm reposting it here to raise awareness of the issue.) The amount of CPP we have to use in Haskell is getting a bit out of hand. Here are the number of modules, per library, that use CPP for some of the libraries I maintain: containers 18/18 hashable 4/5 unordered-containers 6/9 network 3/7 cassava 4/16 cabal/cabal-install 13/75 cabal/Cabal 7/78 ekg 1/15 If this doesn't look like a lot to you (I hope it does!) consider than some languages don't use CPP at all (e.g. Java). CPP really sucks from a maintenance perspective: * It's not Haskell, but this bizarre string concatenation language. * The code is harder to read, bitrots more easily, and is harder to test. * The code can't be compiled without using Cabal (which generates some of the CPP macros for us.) This hurts e.g. ad-hoc testing/benchmarking. There are a couple of reasons we use CPP, but the main one is breaking changes in GHC and libraries we depend on. We need to reduce these kind of breakages in the future. Dealing with breakages and maintaining the resulting CPP-ed code is costing us time we could spend on other things, such as improving our libraries or writing new ones. I for one would like to get on with writing applications instead of spending time on run-of-the-mill libraries. Often these breaking changes are done in the name of "making things cleaner". Breaking changes, no matter how well-intended, doesn't make code cleaner, it makes it less clean*. Users end up having to use *both* the old "unclean" API *and* the new "clean" API. The right way to move to evolve an new API is to add new functions and data types, not modify old ones, whenever possible. * It takes about 3 major GHC releases (~3 years) before you can remove the CPP, but since new things keep breaking all the time you always have a considerable amount of CPP. -- Johan

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PSA regarding Cabal-1.22 and ghc-mod
by Anthony Cowley 09 Jan '15

09 Jan '15

I was lucky to avoid this myself, but I keep seeing it crop up: ghc-mod doesn't work with GHC < 7.10 and Cabal-1.22. If you are still on GHC 7.8.3 or 7.8.4, you may want to hold off on updating cabal or cabal-install. This isn't really my news to report, but folks need to know. See this issue for more information: https://github.com/kazu-yamamoto/ghc-mod/issues/417#issuecomment-68819938 Anthony

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