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ANNOUNCE: Monad.Reader Issue 17
by Brent Yorgey 09 Jan '11

09 Jan '11

I am pleased to announce that Issue 17 of The Monad.Reader is now available [1]. Issue 17 consists of the following three articles: * List Leads Off with the Letter Lambda by Douglas M. Auclair * The InterleaveT Abstraction: Alternative with Flexible Ordering by Neil Brown * The Reader Monad and Abstraction Elimination by Petr Pudlak Feel free to browse the source files [2]. You can check out the entire repository using darcs: darcs get http://code.haskell.org/~byorgey/TMR/Issue17 Watch for a special announcement regarding Issue 18 soon! [1] http://themonadreader.files.wordpress.com/2011/01/issue17.pdf [2] http://code.haskell.org/~byorgey/TMR/Issue17

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Re: [Haskell-cafe] [Haskell-beginners] theStdGen unsafePerformIO
by Antoine Latter 09 Jan '11

09 Jan '11

You might get more answers to this sort of question on the haskell-cafe list. Even there, I think you might need to ask whoever the authors were for a question like this :-) On Sun, Jan 9, 2011 at 9:55 AM, John Smith <voldermort(a)hotmail.com> wrote: > Why does theStdGen require unsafePerformIO? I recompiled the Random module > with > > theStdGen :: IO (IORef StdGen) > theStdGen = do > rng <- mkStdRNG 0 > newIORef rng > > The implementations of a few functions needed to change slightly (to extract > the IORef from IO), but no other type signatures needed changing, and > nothing blew up. > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners >

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Generalizing catMaybes
by Tony Morris 09 Jan '11

09 Jan '11

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I am wondering if it possible to generalise catMaybes: (Something f, SomethingElse t) => t (f a) -> t a I have being doing some gymnastics with Traversable and Foldable and a couple of other things from category-extras to no avail. Perhaps someone else's brain is molded into an appropriate shape to reveal an answer! - -- Tony Morris http://tmorris.net/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk0n0lwACgkQmnpgrYe6r6155gCeLjYizQ/5w1r2qkecbEqiQqq5 ihIAn1bmmK/qNFxM2sSusqjJu/g2/lH7 =+SdM -----END PGP SIGNATURE-----

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Type System vs Test Driven Development
by Jonathan Geddes 08 Jan '11

08 Jan '11

Cafe, In every language I program in, I try to be as disciplined as possible and use Test-Driven Development. That is, every language except Haskell. There are a few great benefits that come from having a comprehensive test suite with your application: 1. Refactoring is safer/easier 2. You have higher confidence in your code 3. You have a sort of 'beacon' to show where code breakage occurs Admittedly, I don't believe there is any magical benefit that comes from writing your tests before your code. But I find that when I don't write tests first, it is incredibly hard to go back and write them for 'completed' code. But as mentioned, I don't write unit tests in Haskell. Here's why not. When I write Haskell code, I write functions (and monadic actions) that are either a) so trivial that writing any kind of unit/property test seems silly, or are b) composed of other trivial functions using equally-trivial combinators. So, am I missing the benefits of TDD in my Haskell code? Is the refactoring I do in Haskell less safe? I don't think so. I would assert that there is no such thing as refactoring with the style of Haskell I described: the code is already super-factored, so any code reorganization would be better described as "recomposition." When "recomposing" a program, its incredibly rare for the type system to miss an introduced error, in my experience. Am I less confidence in my Haskell code? On the contrary. In general, I feel more confident in Haskell code WITHOUT unit tests than code in other languages WITH unit tests! Finally, am I missing the "error beacon" when things break? Again I feel like the type system has got me covered here. One of the things that immediately appealed to me about Haskell is that the strong type system gives the feeling of writing code against a solid test base. The irony is that the type system (specifically the IO monad) force you to structure code that would be very easy to test because logic code is generally separated from IO code. I explained these thoughts to a fellow programmer who is not familiar with Haskell and his response was essentially that any language that discourages you from writing unit tests is a very poor language. He (mis)quoted: "compilation [is] the weakest form of unit testing" [0]. I vehemently disagreed, stating that invariants embedded in the type system are stronger than any other form of assuring correctness I know of. I know that much of my code could benefit from a property test or two on the more complex parts, but other than that I can't think that unit testing will improve my Haskell code/programming practice. Am I putting too much faith in the type system? [0] http://blog.jayfields.com/2008/02/static-typing-considered-harmful.html

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Is this the "right" way to do polymorphism?
by Daryoush Mehrtash 08 Jan '11

08 Jan '11

I am trying to evaluate the polymorphism technique used in Hackage library. I like to know if the approach taken is "right" or not. The code in question is in the Hoaut package http://hackage.haskell.org/package/hoauth/ As far as I can understand the code wants to have polymorphism on HTTP client such that it can use different underlying HTTP. The package comes with Curl and Debug implementation of its HTTPClient class. My question is with how the polymorphism is used. In function such as "serviceRequest" in the Network.OAuth.Consumer module you have: -- | Performs a signed request with the available token. serviceRequest :: (HttpClient c,MonadIO m) => c -> OAuthRequest -> OAuthMonadT m Response serviceRequest c req = do { result <- lift $ runClient c (unpackRq req) ; case (result) of Right rsp -> return rsp Left err -> fail $ "Failure performing the request. [reason=" ++ err ++"]" } The function expects the HTTPClient implementation to be the first argument to the function. Is that the right thing to do? Or should the instance of the client be a type parameter in the computation (in this case OAuthMonadT)? thanks, Daryoush

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Review request for my encoding function
by C K Kashyap 08 Jan '11

08 Jan '11

Hi, I've written a function to encode a color value of type (Int,Int,Int) into 8,16 or 32 byte ByteString depending on the value of bits per pixel. This is for my VNC server implementation. I'd appreciate some feedback on the Haskellism of the implementation. import Data.Bits import Data.ByteString.Lazy import Data.Binary.Put import Data.Word type Red = Int type Green = Int type Blue = Int type Color = (Red,Green,Blue) encode :: Color -> Int-> Int-> Int-> Int-> Int-> Int-> Int -> ByteString encode (r,g,b) bitsPerPixel redMax greenMax blueMax redShift greenShift blueShift = runPut $ do case bitsPerPixel of 8 -> putWord8 z8 16 -> putWord16be z16 32 -> putWord32be z32 where z8 = (fromIntegral $ nr + ng + nb) :: Word8 z16 = (fromIntegral $ nr + ng + nb) :: Word16 z32 = (fromIntegral $ nr + ng + nb) :: Word32 nr = scale r redMax redShift ng = scale g greenMax greenShift nb = scale b blueMax blueShift scale c cm cs = (c * cm `div` 255) `shift` cs Regards, Kashyap

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problem with haskell-mode mailing list?
by Alex Ott 08 Jan '11

08 Jan '11

Hello all Sorry for writing into this mailing list, but I couldn't get answer from support(a)community.haskell.org address because my mails are returned with errors. And I couldn't found The problem is that if I send mail to haskellmode-emacs(a)projects.haskell.org my mails are not delivered to mailing list with error that google mail server couldn't connect to haskell.org's mail server. If somebody could help me with this problem, please contact me - I'll provide more details on this issue -- With best wishes, Alex Ott, MBA http://alexott.blogspot.com/ http://alexott.net/ http://alexott-ru.blogspot.com/ Skype: alex.ott

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Ur vs Haskell
by Alexander Kjeldaas 08 Jan '11

08 Jan '11

I've briefly gone through the Ur demo at http://impredicative.com/ur/demo/ Ur looks very impressive, so the natural question I'm asking myself is: How does it stack up against haskell frameworks, and why can't Ur be implemented in Haskell? I'm thinking mainly of the safety guarantees, not necessarily the performance guarantees, GC-less execution, or even non-lazy evaluation. And I can't answer those.. any takers? Alexander

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Freeglut
by Eric 08 Jan '11

08 Jan '11

Hi all, I would like to use freeglut instead of GLUT for my Haskell OpenGL program, but when I place the freeglut dll in the program's directory and try to run the program on Windows XP, I get the following error message: user error (unknown GLUT call glutSetOption, check for freeglut) Can anyone help? Eric M.

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Newbie type question for wxHaskell learner
by b1g3ar5 08 Jan '11

08 Jan '11

I've tried to solve this but I am failing. I can do this: p0<-panel nb [] e0<-textCtrl p [text:=my_list!!0] but I want to do this on all of my_list, so I tried: let es = map (\x-> textCtrl (panel nb []) [text:=x]) my_list Now, this won't work because the panel nb [] is IO (Panel()) and the parameter to textCtrl needs to be a Panel() How do I get out of IO? Thanks.

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Exportable and importable instances
by Marco Túlio Gontijo e Silva 07 Jan '11

07 Jan '11

Hi. I'm planning to propose a language extension for Haskell to make it possible to control the visibility of type class instances in module export and import lists as a research project. I'm planning to implement this in GHC. I've already mentioned it in #ghc(a)irc.freenode.net and got interesting comments from tibbe and quicksilver. I'd be glad to receive more feedback on the idea. If you have any comments, please reply to this thread or mail me individually, at your will. Greetings. -- marcot http://marcot.eti.br/ [Flattr=54498]

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UTF-8 BOM
by Tony Morris 07 Jan '11

07 Jan '11

I am reading files with System.IO.readFile. Some of these files start with a UTF-8 Byte Order Marker (0xef 0xbb 0xbf). For some functions that process this String, this causes choking so I drop the BOM as shown below. This feels particularly hacky, but I am not in control of many of these functions (that perhaps could use ByteString with a better solution). I'm wondering if there is a better way of achieving this goal. Thanks for any tips. dropBOM :: String -> String dropBOM [] = [] dropBOM s@(x:xs) = let unicodeMarker = '\65279' -- UTF-8 BOM in if x == unicodeMarker then xs else s readBOMFile :: FilePath -> IO String readBOMFile p = dropBOM `fmap` readFile p -- Tony Morris http://tmorris.net/

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Problem on overlapping instances
by Magicloud Magiclouds 07 Jan '11

07 Jan '11

Hi, I am using Data.Binary which defined "instance Binary a => Binary [a]". Now I need to define "instance Binary [String]" to make something special for string list. How to make it work? I looked into the chapter of overlappinginstances, nothing works. -- 竹密岂妨流水过山高哪阻野云飞

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Unknown symbol `__dso_handle' with Template Haskell and wxHaskell
by Alexander Bau 07 Jan '11

07 Jan '11

Hi, I am using Template Haskell and wxHaskell [1]. When Graphics.UI.WX is loaded during THs code generation, I get the following error: > ... > Loading package wxdirect-0.12.1.3 ... linking ... done. > ghc: /usr/local/lib/wxcore-0.12.1.6/ghc-6.12.3/HSwxcore-0.12.1.6.o: > unknown symbol `__dso_handle' > Loading package wxcore-0.12.1.6 ... > linking ... ghc: unable to load package `wxcore-0.12.1.6' System stats: ghc-6.12.3 template-haskell-2.4.0.1 wx-0.12.1.6 Regards, Alex [1] Example: -- Test.hs ---------------------- {-# LANGUAGE TemplateHaskell #-} module Test2 where import Language.Haskell.TH import qualified Graphics.UI.WX as WX foo :: Q [Dec] foo = do info <- reify ''WX.Color runIO $ print info return [] -- Main.hs ---------------------- {-# LANGUAGE TemplateHaskell #-} module Main where import Test $(foo)

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ghc -O2 and HUnit weirdness
by Joey Hess 07 Jan '11

07 Jan '11

Suppose I'm playing with HUnit for nearly the first time. I might write a test program like this, to see what a test failure looks like in practice: import Test.HUnit import Test.HUnit.Tools main = runVerboseTests $ TestList [ TestCase $ assertBool "should fail" False , TestCase $ assertEqual "should also fail" "foo" "bar"] # runghc test.hs Testing 0 ### Failure in: 0 should fail Testing 1 ### Failure in: 1 should also fail expected: "foo" but got: "bar" Cases: 2 Tried: 2 Errors: 0 Failures: 2 (Counts {cases = 2, tried = 2, errors = 0, failures = 2},0) Good, that works. Then I drop it into a Makefile that already knows how to build haskell programs: # make test ghc -odir build -hidir build -O2 --make test [1 of 1] Compiling Main ( test.hs, build/Main.o ) Linking test ... ./test Testing 0 Testing 1 ### Failure in: 1 should also fail expected: "foo" but got: "bar" Cases: 2 Tried: 2 Errors: 0 Failures: 1 Wait, what?! One of my test cases is impossibly, passing. So, the problem seems to be that ghc -O2 somehow optimises the static assertBool _ True away, in what seems to be a bad way. Remove the -O2 and the test fails as expected. Presumably, although I have not verified, less static boolean values would not trigger the optimisation. Is this a ghc or HUnit bug? (Versions: 6.12.1, 1.2.2.1) -- see shy jo

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implementing RealFloat decodeFloat
by Claude Heiland-Allen 07 Jan '11

07 Jan '11

Hi everyone, I've been working on [0] Haskell bindings for [1] libqd for [2] double-double and quad-double arithmetic, and have been struggling to implement [3] RealFloat, in particular [4] decodeFloat, mostly because of its postcondition but also some issues relating to variable precision: ---8<--- If decodeFloat x yields (m,n), then x is equal in value to m*b^^n, where b is the floating-point radix, and furthermore, either m and n are both zero or else b^(d-1) <= m < b^d, where d is the value of floatDigits x. In particular, decodeFloat 0 = (0,0). ---8<--- (BTW: that should probably really be "... <= abs m < ..."; perhaps this code could be added to the report and/or documentation, if it is correct: validDecode :: RealFloat f => f -> Bool validDecode f = case decodeFloat f of (0,0) -> True (m, e) -> b^(d-1) <= abs m && abs m < b^d where b = floatRadix f d = floatDigits f ) The double-double and quad-double types do not have a fixed precision, though they do have a fixed minimum precision: this means that decodeFloat will (in general) lose some precision. This is because for example in: data DoubleDouble = DoubleDouble !CDouble !CDouble dd = DoubleDouble a b the semantics are that "dd = a + b", with |a|>>|b|; coupled with the IEEE implicit leading 1 bit, this means that there may be large gaps between exponents: for example: "1 + 0.5**100 :: DoubleDouble". So far I've got a "mostly working" implementation thus: decodeFloat !(DoubleDouble a b) = case (decodeFloat a, decodeFloat b) of ((0, 0), (0, 0)) -> (0, 0) ((0, 0), (m, e)) -> (m `shiftL` f, e - f) ((m, e), (0, 0)) -> (m `shiftL` f, e - f) ((m1, e1), (m2, e2)) -> let fixup m e = if m < mMin then fixup (m `shiftL` 1) (e - 1) else if m >= mMax then fixup (m `shiftR` 1) (e + 1) else (m, e) mMin = 1 `shiftL` (ff - 1) mMax = 1 `shiftL` ff ff = floatDigits (0 :: DoubleDouble) g = e1 - e2 - f in fixup ((m1 `shiftL` f) + (m2 `shiftR` g)) (e1 - f) where f = floatDigits (0 :: CDouble) This does meet the postcondition as specified (which leads to breakage in other RealFloat methods), but has a recursion with no termination proof (so far), and is lossy in general: > let check f = uncurry encodeFloat (decodeFloat f) == f > check (1 + 0.5 ** 120 :: DoubleDouble) False It does however seem to meet a weaker condition: > let check2 f = (decodeFloat . (`asTypeOf` f) . uncurry encodeFloat . decodeFloat $ f) == decodeFloat f > check2 (1 + 0.5 ** 120 :: DoubleDouble) True Questions: 1. Is this weaker condition likely to be good enough in practice? 2. Can my implementation of decodeFloat be simplified? Thanks for any insights, Claude [0] http://hackage.haskell.org/package/qd [1] http://crd.lbl.gov/~dhbailey/mpdist/ [2] http://crd.lbl.gov/~dhbailey/dhbpapers/arith15.pdf [3] http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.ht… [4] http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.ht… -- http://claudiusmaximus.goto10.org

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Haddock docs don't list instances
by Omari Norman 06 Jan '11

06 Jan '11

Web versions of Haddock documentation, even this old one, list a class's instances. http://www.haskell.org/ghc/docs/6.2.2/html/libraries/base/Prelude.html#5 My Haddock documentation does not have any class instances at all. This is true both of the documentation that comes with my Linux system (I use Debian) and of the documentation I generate locally using cabal-install or the Setup.hs program in my homemade packages. How do I get Haddock to make the instance lists? Anybody else have this problem--maybe it is a Debian bug? Thanks. --Omari -- Pay a little now, or pay a lot later.

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Polymorphic function over pairs of maybes.
by Edward Amsden 06 Jan '11

06 Jan '11

Hello all: I'd like to right a function that could take a structure with type (random example): (Int, (String, (Int, Int))) and another where each individual value is a Maybe of the corresponding type, for example: (Maybe Int, (Maybe String, (Maybe Int, Maybe Int))) and perform a fromMaybe using the values to produce a new value with the original types. The catch is, I'd like to be able to do that for an n-deep structure with any type, without having to write a separate function for them. I'm not even sure that this is possible, but perhaps someone could explain how it is or isn't? -- Edward Amsden Undergraduate Computer Science Rochester Institute of Technology www.edwardamsden.com

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Odd profiling results
by Erik de Castro Lopo 06 Jan '11

06 Jan '11

Hi all, I do some work on Ben Lippmeier's DDC compiler and when profiling DDC compiling a program with 11 source code files (much like ghc --make), we get a memory usage graph like this: http://www.mega-nerd.com/tmp/ddc-heap-usage-20101231.png We have no particular problem with the 11 peaks (one for each source file) but wonder what the hell is going on in the periods when the memory usage is flat. On a similar run the RTS reports that GC time was less than 10% of total runtime. This profiling information was captured on Debian on x86-64 system where DDC was compiled using ghc-6.12.1 (Debian package). Clues? Thanks, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Re: [Haskell-cafe] What's the best seed for random API ?
by Robert Clausecker 06 Jan '11

06 Jan '11

It probably is, but definitly not portable. Try to find /dev/urandom under Windows.

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What's the best seed for random API ?
by z_axis 06 Jan '11

06 Jan '11

picoSec :: IO Integer picoSec = do t <- ctPicosec `liftM` (getClockTime >>= toCalendarTime) return t rollDice :: Int -> IO Int rollDice n = do ps <- picoSec return $ (take 1 $ randomRs (1,n) $ mkStdGen $ fromInteger ps) !! 0 The above code uses `ctPicosec` as seed. Is it better to use the output of /dev/urandom as seed ? Sincerely! ----- e^(π.i) + 1 = 0 -- View this message in context: http://haskell.1045720.n5.nabble.com/What-s-the-best-seed-for-random-API-tp… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Choosing a type-class instance based on the context
by Cristiano Paris 06 Jan '11

06 Jan '11

Hi all, I was reading: http://www.haskell.org/haskellwiki/GHC/AdvancedOverlap and I became curious. Playing with the code I started to find a way to say: instance Show a => ShowPred a HTrue instead of enumerating all the instances, mirroring those of the Show class: instance ShowPred Int HTrue instance ShowPred Bool HTrue instance ShowPred a flag => ShowPred [a] flag Would that be possible? My opinion is that's not possible but I want to hear what the type-system gurus think about it. Thanks, -- Cristiano GPG Key: 4096R/C17E53C6 2010-02-22 Fingerprint = 4575 4FB5 DC8E 7641 D3D8 8EBE DF59 B4E9 C17E 53C6

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Experiences with Intel's manycore lab?
by Michael Lesniak 05 Jan '11

05 Jan '11

Hello, does anyone have experiences (good/bad) or some good-to-know knowledge about Intel's manycore lab, which supposedly has GHC 6.13 installed[1]? Regards, Michael [1] http://software.intel.com/en-us/blogs/2010/10/14/prerelease-ghc-and-haskell… -- Dipl.-Inf. Michael C. Lesniak University of Kassel Programming Languages / Methodologies Research Group Department of Computer Science and Electrical Engineering Wilhelmshöher Allee 73 34121 Kassel Phone: +49-(0)561-804-6269

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thread safety, IO arrays, and IO refs
by Eric Stansifer 05 Jan '11

05 Jan '11

Hello, I wish to use a mutable array in multiple threads. Can IO arrays be used in any thread, or only the thread they are created in? (So if I create an IO array in one thread, pass it to another via an MVar, can I read / edit it in that other thread?) Similarly about IORefs... can they be used across threads? (Obviously I will need to manage locking myself if I use an IO ref or array.) Are there any good resources on concurrency in Haskell that would answer questions similar to these? I've been learning by reading the ghc library documentation, but of course it's not comprehensive. The online tutorials I've seen aren't so in-depth, either. Thanks, Eric

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[ANNOUNCE] Etage 0.1.7, a general data-flow framework
by Mitar 05 Jan '11

05 Jan '11

Hi! I am glad to announce a new version of Etage, a general data-flow framework. Now it supports also GHC 7.0 and not just GHC head and because of this also Hackage successfully generates documentation so it is easier to understand the framework. http://hackage.haskell.org/package/Etage Feel free to test it and provide any feedback Mitar

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Network.Curl and thread safety
by Iustin Pop 05 Jan '11

05 Jan '11

Hi all, I'm not able to find out how one can use Network.Curl with the threaded runtime safely. I have this simple example: import Network.Curl import Control.Concurrent import Control.Concurrent.MVar getUrl :: (Monad m) => String -> IO (m String) getUrl url = do (code, body) <- curlGetString url [CurlSSLVerifyPeer False, CurlSSLVerifyHost 0, CurlTimeout 15, CurlConnectTimeout 5] return $ case code of CurlOK -> return body _ -> fail $ "Curl error for " ++ url ++ " error " ++ show code runGet :: String -> MVar (Maybe String) -> IO () runGet url mv = do body <- getUrl url putMVar mv body main = withCurlDo $ do let urls = replicate 10 "https://google.com/" mvs <- mapM (\_ -> newEmptyMVar) urls mapM_ (\(mv, url) -> forkIO (runGet url mv)) $ zip mvs urls mapM (\mv -> takeMVar mv >>= print) mvs threadDelay 10000000 When using curl linked with GnuTLS and running it with the multi-threaded runtime, it fails immediately with: $ ./main main: ath.c:193: _gcry_ath_mutex_lock: Assertion `*lock == ((ath_mutex_t) 0)' failed. Aborted (core dumped) Reading the Network.Curl docs, it seems that using withCurlDo should be enough to make this work (although I'm not sure about the description of the function and "no forking or lazy returns"). Using Network.Curl built against the OpenSSL headers does successfully retrieve all URLs, but fails a bit later (during the threadDelay, probably in GC) again with segmentation fault in libcurl.4.so. I've tried changing forkIO to forkOS, but still no luck, with either SSL library. Any ideas what I'm doing wrong? regards, iustin

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: haskell-src-meta 0.3
by Ben Millwood 04 Jan '11

04 Jan '11

It seems I wasn't subscribed to haskell-cafe so this reply didn't get through (sorry for sending this twice, Felipe). I've now subscribed with delivery turned off for -cafe so if you want me to read a response be sure to address me directly :) On Tue, Jan 4, 2011 at 5:08 AM, Ben Millwood <haskell(a)benmachine.co.uk> wrote: > On Tue, Jan 4, 2011 at 4:02 AM, Felipe Almeida Lessa > <felipe.lessa(a)gmail.com> wrote: >> On Tue, Jan 4, 2011 at 1:49 AM, Ben Millwood <haskell(a)benmachine.co.uk> wrote: >>> haskell-src-meta is a package originally written by Matt Morrow to >>> provide a translation from the syntax tree provided by >>> haskell-src-exts to template haskell syntax. Essentially this allows >>> TH code to be parsed from a string, and is used in several quasiquoter >>> libraries. >> >> Are the purposes of haskell-src-meta and haskell-src-exts-qq the same? >> What are the main differences? >> >> Cheers! =) >> >> -- >> Felipe. >> > > If you look at the source for Language.Haskell.Exts.Translate from > haskell-src-exts-qq [1], you can see that it was actually taken from > an earlier version of haskell-src-meta, presumably during the long > period for which HSM was not maintained. I guess you could say that it > inlined the key parts of haskell-src-meta into its codebase, but I > hope that the maintainer can feel more confident about using the > haskell-src-meta package as a dependency now. > > My understanding of the *purpose* of haskell-src-exts-qq is that it is > for easily generating ASTs for use with haskell-src-exts, whereas > haskell-src-meta is for generating ASTs for use with template haskell. > > [1] http://hackage.haskell.org/packages/archive/haskell-src-exts-qq/0.3.0/doc/h… >

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GHC.Paths (libDir)
by Serguey Zefirov 04 Jan '11

04 Jan '11

I figured that out, thank you if you are writing answer. ;)

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GHC.Paths (libDir)
by Serguey Zefirov 04 Jan '11

04 Jan '11

I am looking at GHC API examples page: http://www.haskell.org/haskellwiki/GHC/As_a_library One of examples use "import GHC.Paths ( libDir)" and mentions that it needs "-package ghc-paths" option. I tried the second example with latest Haskell Platform (Windows). I commented out libDir = "/usr..." as it is not applicable to my case and inserted GHC.Paths import. So I run it like "ghci -package ghc -package ghc-paths A.hs" and get an error that "<command line>: cannot satisfy -package ghc-paths". When I do not use -package ghc-paths, it fails to load a module "GHC.Paths". What should I do to obtain libDir?

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How about Haskell Golf just like vimgolf.com
by C K Kashyap 04 Jan '11

04 Jan '11

Hi, First of all, happy new year. I found this site called http://vimgolf.com/ ... the idea there is that people come up with challenges and try to come up with the least number of keystrokes to get it done. Perhaps a similar thing for Haskell would be interesting. Just a thought. Regards, Kashyap

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GHC > 6.8.2 on IA64-Linux platform (Itanium)
by Karel Gardas 04 Jan '11

04 Jan '11

Hello, [sorry for cross-post, I assume Itanium interest is quite rare these days so to grab attention of Itanium/Haskell people I send to both haskell-cafe and ghc list] I'd like to compile more recent than 6.8.2 GHC on itanium-linux system I do have access to, but I'm kind of unlucky with this. I'm trying unregisterised build (of 6.12.3 and 6.10.4) and it always fails with: "inplace/bin/ghc-stage2" -H32m -O -package-name dph-base-0.4.0 -hide-all-packages -i -ilibraries/dph/dph-base/. -ilibraries/dph/dph-base/dist-install/build -ilibraries/dph/dph-base/dist-install/build/autogen -Ilibraries/dph/dph-base/dist-install/build -Ilibraries/dph/dph-base/dist-install/build/autogen -Ilibraries/dph/dph-base/include -optP-include -optPlibraries/dph/dph-base/dist-install/build/autogen/cabal_macros.h -package array-0.3.0.1 -package base-4.2.0.2 -package ghc-prim-0.2.0.0 -package random-1.0.0.2 -Odph -funbox-strict-fields -haddock -fcpr-off -XTypeFamilies -XGADTs -XRankNTypes -XBangPatterns -XMagicHash -XUnboxedTuples -XTypeOperators -O2 -XGenerics -fno-warn-deprecated-flags -Wwarn -odir libraries/dph/dph-base/dist-install/build -hidir libraries/dph/dph-base/dist-install/build -stubdir libraries/dph/dph-base/dist-install/build -hisuf hi -osuf o -hcsuf hc -c libraries/dph/dph-base/./Data/Array/Parallel/Base/Hyperstrict.hs -o libraries/dph/dph-base/dist-install/build/Data/Array/Parallel/Base/Hyperstrict.o ghc-stage2: internal error: evacuate: strange closure type 15 (GHC version 6.12.3 for ia64_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug make[1]: *** [libraries/dph/dph-base/dist-install/build/Data/Array/Parallel/Base/Hyperstrict.o] Aborted make: *** [all] Error 2 I even attempted unoptimized build with mk/build.mk containing this: GhcUnregisterised=YES GhcWithNativeCodeGen=NO SplitObjs=NO SRC_HC_OPTS = -H64m -O0 GhcStage1HcOpts = -O0 GhcStage2HcOpts = -O0 GhcLibHcOpts = -O0 but this time ghc segfaults while compiling: "inplace/bin/ghc-stage2" -H64m -O0 -package-name dph-base-0.4.0 -hide-all-packages -i -ilibraries/dph/dph-base/. -ilibraries/dph/dph-base/dist-install/build -ilibraries/dph/dph-base/dist-install/build/autogen -Ilibraries/dph/dph-base/dist-install/build -Ilibraries/dph/dph-base/dist-install/build/autogen -Ilibraries/dph/dph-base/include -optP-include -optPlibraries/dph/dph-base/dist-install/build/autogen/cabal_macros.h -package array-0.3.0.1 -package base-4.2.0.2 -package ghc-prim-0.2.0.0 -package random-1.0.0.2 -Odph -funbox-strict-fields -haddock -fcpr-off -XTypeFamilies -XGADTs -XRankNTypes -XBangPatterns -XMagicHash -XUnboxedTuples -XTypeOperators -O0 -fno-warn-deprecated-flags -Wwarn -odir libraries/dph/dph-base/dist-install/build -hidir libraries/dph/dph-base/dist-install/build -stubdir libraries/dph/dph-base/dist-install/build -hisuf hi -osuf o -hcsuf hc -c libraries/dph/dph-base/./Data/Array/Parallel/Base/Hyperstrict.hs -o libraries/dph/dph-base/dist-install/build/Data/Array/Parallel/Base/Hyperstrict.o make[1]: *** [libraries/dph/dph-base/dist-install/build/Data/Array/Parallel/Base/Hyperstrict.o] Segmentation fault I've also seen, the last available GHC in Debian for IA64 platform is 6.8.2 -- which I'm using for bootstrap btw so debian will not help me here with some clever patch which would change recent ghc to be buildable... So, I'd like to ask here: is there anybody who was lucky enough to find appropriate set of options or any other magic way how to compile more recent GHC sources to the _stable_ compiler binary on IA64/Linux? If so, please let me know. Thanks a lot, Karel

2 2

What's the motivation for η rules?
by David Sankel 04 Jan '11

04 Jan '11

TIA, David -- David Sankel Sankel Software www.sankelsoftware.com

6 6

Re: [Haskell-cafe] [Haskell] ANNOUNCE: haskell-src-meta 0.3
by Felipe Almeida Lessa 04 Jan '11

04 Jan '11

On Tue, Jan 4, 2011 at 1:49 AM, Ben Millwood <haskell(a)benmachine.co.uk> wrote: > haskell-src-meta is a package originally written by Matt Morrow to > provide a translation from the syntax tree provided by > haskell-src-exts to template haskell syntax. Essentially this allows > TH code to be parsed from a string, and is used in several quasiquoter > libraries. Are the purposes of haskell-src-meta and haskell-src-exts-qq the same? What are the main differences? Cheers! =) -- Felipe.

1 0

Haddock: generate documentation for all functions?
by Edward Amsden 04 Jan '11

04 Jan '11

When using Java, one can tell javadoc to generate documentation for all members of a class, including private and protected members. This is useful for generating internal documentation (for instance). Is there a similar way in Haddock to produce documentation for functions and types that are not exported as a part of a module? -- Edward Amsden Undergraduate Computer Science Rochester Institute of Technology www.edwardamsden.com

3 2

Lazy cons, Stream-Fusion style?
by Stephen Tetley 03 Jan '11

03 Jan '11

Hello all Can a lazy cons be implemented for (infinite) Streams in the Stream-Fusion style? I'm trying to make a Stream library, hopefully efficient enough for audio synthesis in the style of Jerzy Karczmarczuk's Clarion. As performance is important, the obvious model is the Stream-Fusion library, but 'cons' is problematic in this style. With the normal, inductive definition of Streams (vis Wouter Swierstra's Stream library), streams are sufficiently lazy: data Stream a = a :< Stream Using the laziness means that this definition of 'ones' is productive: demo01 = take 10 ones where ones = 1 <:> ones -- (<:>) is stream-cons However with Stream-Fusion, streams are not inductively defined, instead they are unfold-like pair of a stepper function and a start state: -- Some details removed... -- data Stream a = forall st. Stream !(st -> Step a st) !st For infinite Streams the Done constructor can be removed from the Step type, a truly infinite is never done: data Step a s = Yield a !s | Skip !s | Done Cons is strict - (:!:) is the constructor for a Strict pair, but the strictness of the pair is irrelevant here (I think). Even with a lazy pair the definition is too strict to be useful for 'ones' defined above as it won't "stream" (S1 and S2 are the constructors of counting / flag datatype used to encode which state the iteration is in): cons :: a -> Stream a -> Stream a cons w (Stream next0 s0) = Stream next (S2 :!: s0) where {-# INLINE next #-} next (S2 :!: s) = Yield w (S1 :!: s) next (S1 :!: s) = case next0 s of Done -> Done Skip s' -> Skip (S1 :!: s') Yield x s' -> Yield x (S1 :!: s') Here's a running example: > {-# LANGUAGE PackageImports #-} > module SFrec where > import qualified "stream-fusion" Data.Stream as S -- -- The S.append1 construct is used to unwind a Stream -- to a list so it can be printed -- > bad_loopy :: [Int] > bad_loopy = S.append1 (S.take 10 v) [] > where > v = 1 `S.cons` v > good_productive :: [Int] > good_productive = S.append1 (S.take 10 v) [] > where > v = S.repeat 1 Thanks Stephen

4 6

Happstack related problem with templates/class methods
by Markus Barenhoff 03 Jan '11

03 Jan '11

Hi! I'am trying to express a happstack component like the simplified one here: https://gist.github.com/763597 What exactly is the problem for the compiler? Is there a better way to describe such a "abstract" or parametrized component? Regards alios

2 1

A good video on monad
by C K Kashyap 03 Jan '11

03 Jan '11

Hi, I found this nice video on monads (although for clojure). http://www.youtube.com/watch?v=ObR3qi4Guys Question - in the video, if I understood right, the guy implements "return" as a function that takes a value and returns a function (action) that takes nothing and returns that value. And upon executing the action (essentially invoking the function that takes void) you get the value. So, what is the correct way of thinking about the similarity(if any) between a function taking 0 parameters and a single parameter type constructor? Regards, Kashyap

2 2

HaXml installation trouble
by Matthew Fairtlough 03 Jan '11

03 Jan '11

Still very new here. I'm trying to install HaXml and I've updated cabal. I see that a similar issue was posted recently as a bug http://hackage.haskell.org/trac/ghc/ticket/4513 but one that's supposedly fixed; do I need to wait for something or can this be fixed some other way? I think I've posted all necessary version numbers below: ~ $ sudo cabal install HaXml-1.20.2 --global Resolving dependencies... Downloading haskell98-1.1.0.0... Configuring haskell98-1.1.0.0... Preprocessing library haskell98-1.1.0.0... ...info deleted... [24 of 30] Compiling Directory ( Directory.hs, dist/build/Directory.o ) Directory.hs:34:38: Not in scope: `setOwnerReadable' Directory.hs:35:38: Not in scope: `setOwnerWritable' Directory.hs:36:38: Not in scope: `setOwnerExecutable' Directory.hs:37:38: Not in scope: `setOwnerSearchable' Directory.hs:38:57: Not in scope: `emptyPermissions' cabal: Error: some packages failed to install: HaXml-1.20.2 depends on haskell98-1.1.0.0 which failed to install. haskell98-1.1.0.0 failed during the building phase. The exception was: ExitFailure 1 polyparse-1.4 depends on haskell98-1.1.0.0 which failed to install. ~ $ cabal --version cabal-install version 0.8.2 using version 1.8.0.6 of the Cabal library ~ $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.3 ~ $ sw_vers ProductName: Mac OS X ProductVersion: 10.6.5 BuildVersion: 10H574

2 2

GHC ParseTree Module
by Jane Ren 03 Jan '11

03 Jan '11

Hi, Does anyone know what GHC module gets the AST and type info of some source code? This is the GHC module that converts all of Haskell into an AST with a small number of pattern cases, where each AST node is annotated with the Haskell type. Thanks

2 1

Data.Typeable TypeRep Ord instance.
by Serguey Zefirov 02 Jan '11

02 Jan '11

Why TypeRep does have equality and doesn't have ordering? It would be good to have that. Right now when I have to order two type representations I convert them to string and then compare. This is somewhat inefficient and not quite straightforward.

7 10

How to write Manatee application?
by Andy Stewart 02 Jan '11

02 Jan '11

Hi all, How to write Manatee extension? Now have answer: Please look http://www.flickr.com/photos/48809572@N02/5031811365/lightbox/ to understand framework before continue. Template code at http://hackage.haskell.org/package/manatee-template First, i explain Manatee package hierarchy: manatee-core contain DBus protocol detail and toolkit use in other manatee packages. manatee-anything is multi-task search interface for user input. manatee contain daemon process, Daemon process is root process to manage other manatee processes: such as render application process' graphics for sandbox protected, window manage ... etc. Daemon process just do manage job and don't running any special application code in it. Other manatee-* package, such as manatee-editor, manatee-browser...etc running in it's own process, you can write any code to implement your application, just need follow some interface: class Typeable a => PageBuffer a where -- Get buffer name. pageBufferGetName :: a -> IO String -- Set buffer name. pageBufferSetName :: a -> String -> IO () -- DBus client to handle DBus signal pageBufferClient :: a -> Client -- How to create view with buffer for MVC design. pageBufferCreateView :: a -> PagePlugId -> IO PageViewWrap -- Page buffer mode. pageBufferMode :: a -> PageMode -- Get package name to update user's configure. pageBufferPackageName :: a -> IO String class Typeable a => PageView a where -- Buffer for view. pageViewBuffer :: a -> PageBufferWrap -- GtkPlug id to send DBus signal to other process. pageViewPlugId :: a -> TVar PagePlugId -- How to handle key action from daemon process. pageViewHandleKeyAction :: a -> Text -> SerializedEvent -> IO () -- How to do when manatee framework focus on current application. pageViewFocus :: a -> IO () -- Scrolled window to contain application widget. pageViewScrolledWindow :: a -> ScrolledWindow -- How to handle cut action, can ignore. pageViewCut :: a -> IO Bool -- How to handle copy action, can ignore. pageViewCopy :: a -> IO Bool -- How to handle paste action, can ignore. pageViewPaste :: a -> IO Bool -- How to scroll top postion, can ignore. pageViewScrollToTop :: a -> IO () -- How to scroll bottom postion, can ignore. pageViewScrollToBottom :: a -> IO () -- How to scroll vertical page, can ignore. pageViewScrollVerticalPage :: Bool -> a -> IO () -- How to scroll vertical step, can ignore. pageViewScrollVerticalStep :: Bool -> a -> IO () -- How to scroll to left side, can ignore. pageViewScrollToLeft :: a -> IO () -- How to scroll to right side, can ignore. pageViewScrollToRight :: a -> IO () -- How to scroll horizontal page, can ignore. pageViewScrollHorizontalPage:: Bool -> a -> IO () -- How to scroll horizontal step, can ignore. pageViewScrollHorizontalStep:: Bool -> a -> IO () If you have any question, feel free to ask me (lazycat.manatee(a)gmail.com) -- Andy

1 0

OpenCL Raw package
by Pavel Perikov 02 Jan '11

02 Jan '11

Hi list (and hopefully Jefferson Heard!) I'm trying to play with OpenCL Raw package. While it has a great amount of work put into it it's not currently usable. Frankly, it does not even link (due to foreign import typo). It does not compile with GHC 7.0.1, has badly specified dependencies on mtl, wrongly defined data types etc. Anyway, many thanks to Jefferson for doing such body of work. I think having OpenCL package is very handy and opens many possibilities for using haskell in high performance computing. Sadly, OpenCL Raw does not seem to have any maintainer currently. I personally not ready to become a maintainer but I started to accumulate patches and I'm willing to send them somebody. Anyone? I set up mercurial repo at http://bitbucket.org/ppavel/opencl-raw and plan to publish patches there. Anyone interested please don't hesitate to contact me. Pavel

1 0

forcing a type constraint on data
by Mathijs Kwik 02 Jan '11

02 Jan '11

Hi all, I would like to make a more restrictive version of a certain datatype. I tried wrapping it in a newtype, fundeps, type families, really got lost :) What I'm trying to achieve is the following: The original package http://hackage.haskell.org/packages/archive/reaction-logic/2010.11.17/doc/h… has a constructor function (mkReactor) with exactly the constraints I'm after. However, the constraint is just on this constructor function, not on the data itself. All functions I write are inferred as "Reactor (State s) c", while the use of that constructor ensures that s == c. Also, I might switch "State s" for some "StateT s m" later on, so I would really like to be able to use "(MonadState c m) => Reactor m c" in my functions as well, since this is the most generic. Trying that blows up however: Could not deduce (m ~ StateT s Identity) from the context (ValidEvent event, MonadState s m) `m' is a rigid type variable bound by the type signature for `stepper2' at Eventer.hs:77:45 In the first argument of `runState', namely `(insertExternals reactor [Serial event])' In the expression: runState (insertExternals reactor [Serial event]) state In an equation for `stepper2': stepper2 (Just reactor, state) event = runState (insertExternals reactor [Serial event]) state Can someone please explain the cause of this problem? Are there workarounds? Language extensions? Thanks in advance, Mathijs

2 1

Defining a type depending on the word size of platform
by Robert Clausecker 02 Jan '11

02 Jan '11

Hi guys! I'm working on a random generator package and want to ask, whether it's possible to generate a type like Int, that can have different sizes at runtime. Motivation: The package ships to generators, one for 32 (lets name it MyGen32) and one for 64 bits (MyGen64). As it is not good for performance to use the wrong one, I also want to provide a type MyGen, that is either a type synonym for MyGen32 or MyGen64, depending on the platform. Any ideas how to do this at compile time? I want this to compile with Cabal so that anybody can download it as a package. I guess the easiest way would be to use CPP, but is there any flag for the wordsize? Yours, Robert Clausecker

3 2

OpenSSL question
by Charles-Pierre Astolfi 02 Jan '11

02 Jan '11

Hi -cafe, I'm using the OpenSSL package, and I don't see any way to forge my own keys. (http://hackage.haskell.org/packages/archive/HsOpenSSL/0.9/doc/html/OpenSSL-…) For example, in the case of RSA, I'm given the exponent and modulus and I would like to create a public key from that, of type RSAPubKey. Is it possible ? How should I do? I'm not particularly tied to OpenSSL, anything that let me use RSA and DSA would be fine. Regards, -- Cp

3 2

getting last char of String
by Aaron Gray 02 Jan '11

02 Jan '11

Is there an easy Haskell function that gets the last Char of a [Char] or String ? Many thanks in advance, Aaron

8 12

V.I.P.s and the associativity of merge'
by Leon Smith 01 Jan '11

01 Jan '11

Ok, after mulling over the issues that Will Ness has brought up in the last few days [1], I think I have a partial explanation for the apparent tension between Will's observations and Heinrich Apfelmus's Implicit Heaps article [2], which both concern the implementation of mergeAll [3]. The merge' function takes two ordered lists, with the head of the first list less than the head of the second, and merges their contents: merge' [] ys = ys merge' (x:xs) ys = x : merge xs ys The nice thing about this function is we can merge an infinite number of lists by folding right, if assume that the heads of those lists are appropriately ordered. This appears in Richard Bird's code at the end of Melissa O'Neill's "Genuine Sieve of Eratosthenes", though undoubtedly this observation has been independently made by many people. Now, in an ordinary sense, merge' *is* an associative operator, in that given three fully defined ordered lists with ordered heads, merge' xs (merge' ys zs) == merge' (merge' xs ys) zs. This allows us to merge an infinite number of lists using an arbitrary tree of merge' operations, without ever worrying that we will return an incorrect result. (However, we might get stuck in a non-productive loop, for example if we fold left over an infinite list of ordered lists) However, Heinrich's article uses a stronger sense of associativity which includes laziness properties and thus captures some operational characteristics. In this sense, merge' *is not* associative. To remedy this, Heinrich uses VIPs to strengthen the associativity property of merge'. This raises the question, is there some combination of the shape of the merge' tree and some input for which using VIPs dramatically changes the efficiency of a mergeAll operation? I suspect the answer is yes, though I don't know for sure at this point in time. However, I do tacitly believe that the current tree that mergeAll uses doesn't exhibit this property for any input, and so I have simplified the implementations of mergeAll and unionAll in the latest version of data-ordlist-0.4.4 by avoiding the use of VIPs. This has the nice side benefit of modestly improving performance when the elements of the result are highly biased towards the first list. Best, Leon [1] http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/84666 [2] http://apfelmus.nfshost.com/articles/implicit-heaps.html [3] http://hackage.haskell.org/packages/archive/data-ordlist/0.4.4/doc/html/Dat…

3 6

Could someone give me a small code review
by Aaron Gray 01 Jan '11

01 Jan '11

I have attached a tiny parser and lexer. This is really key to getting the lexing correct. The only thing I could not get neatly was to disallow hyphens at the end of identifiers, so have left it off until a good solution arrives. I plan to move over to monads next. Any comments welcome, Aaron

2 2

Haskell Weekly News: Issue 163 - Year End Issue
by Daniel Santa Cruz 31 Dec '10

31 Dec '10

Welcome to issue 163 of the HWN, a newsletter covering developments in the [1]Haskell community. This release of the newsletter contains some summary data for the year 2010. Thanks to everyone involved for making this such a wonderful community! Big cheers to Miran Lipovaa, who got multiple top Reddit stories this year for his work on Learn You a Haskell! Also kudos to Don Stewart, Norman Ramsey, camccann, sepp2k, and KennyTM for taking the time to answer so many questions in StackOverflow. The folks at #haskell never cease to amaze with their friendliness and their helpfulness as well. What did Reddit and StackOverflow think was hot this year? Top Reddit Stories this Year * Galois releases the Haskell Lightweight Virtual Machine (HaLVM): From (galois.com) scored 103 with 26 comments. Read on [2]reddit or the [3]original post. * Try Haskell!: From (self.haskell), scored 105 with 29 comments. Read on [4]reddit or the [5]original post. * GHC 7.0.1 released!: From (haskell.org) scored 100 with 31 comments. Read on [6]reddit or the [7]original post. * Learn You a Haskell: Zippers: From (learnyouahaskell.com) scored 93 with 44 comments. Read on [8]reddit or the [9]original post. * The new LLVM backend to GHC is ready to merge in!: From (haskell.org) scored 89 with 27 comments. Read on [10]reddit or the [11]original post. * Haskell vs. Erlang for bittorent clients: laziness, dynamic typing ..: From (jlouisramblings.blogspot.com) scored 88 with 8 comments. Read on [12]reddit or the [13]original post. * There are more than 4000 people in the Haskell Reddit. Tell us what you're using Haskell for! Work and/or play? What would most help you get the job done?: From (self.haskell), scored 84 with 171 comments. Read on [14]reddit or the [15]original post. * Smoking fast Haskell code using GHC's new LLVM codegen: From (donsbot.wordpress.com) scored 83 with 2 comments. Read on [16]reddit or the [17]original post. * Learn You a Haskell for Great Good! The Book!: From (amazon.com) scored 79 with 29 comments. Read on [18]reddit or the [19]original post. * LYAH: Intro to Monads: From (learnyouahaskell.com) scored 78 with 49 comments. Read on [20]reddit or the [21]original post. Top StackOverflow Answers this Year * [22]Large-scale design in Haskell? | votes: 1 I talk a bit about this in Engineering Large Projects in Haskell and in the Design and Implementation of XMonad. Engineering in the large is about managing complexity. The primary code structuring mechanisms in Haskell for managing complexity are : The type system Use the type system to enforce abstractions, simplifying interactions. Enforce key ... * [23]Curious about the HashTable problem | votes: 1 The problem was that the garbage collector is required to traverse mutable arrays of pointers ("boxed arrays") looking for pointers to data that might be ready to deallocate. Boxed, mutable arrays are the main mechanism for implementing a hashtable, so that particular structure showed up the GC traversal issue. This is common to many languages. The symptom ... * [24]Why are side-effects modeled as monads in Haskell? | votes: 1 Suppose a function has side effects. If we take all the effects it produce as the input and output parameters, then the function is pure to the outside world. So for an impure function f' :: Int -> Int we add the RealWorld to the consideration f :: Int -> RealWorld -> (Int, RealWorld) -- input some states of the whole world, -- modify the ... * [25]Haskell - I am too old for this Švotes: 1 (f . g) x = f (g x) This is true. You concluded from that that (f . g) x y = f (g x y) must also be true, but that is not the case. In fact, the following is true: (f . g) x y = f (g x) y which is not the same. Why is this true? Well (f . g) x y is the same as ((f . g) x) y and since we know that (f . g) x = f (g x) we can reduce that to (f (g x)) ... * [26]Haskell: How is <*> pronounced? | votes: 1 Sorry, I don't really know my math, so I'm curious how to pronounce the functions in the Applicative typeclass Knowing your math, or not, is largely irrelevant here, I think. As you're probably aware, Haskell borrows a few bits of terminology from various fields of abstract math, most notably Category Theory, from whence we get functors and monads. The ... * [27]Haskell Weird Kinds | votes: 1 These are GHC-specific extensions of the Haskell kind system. The Haskell 98 report specifies only a simple kind system: ... type expressions are classified into different kinds, which take one of two possible forms: The symbol * represents the kind of all nullary type constructors. If k1 and k2 are kinds, then k1->k2 is the kind of types ... * [28]Is Haskell really a purely functional language considering unsafePerformIO? | votes: 1 The Languages We Call Haskell unsafePerformIO is part of the Foreign Function Interface specification, not core Haskell 98 specification. It can be used to do local side effects that don't escape some scope, in order to expose a purely functional interface. That is, we use it to hide effects when the type checker can't do it for us (unlike the ST monad, ... * [29]What language to learn after Haskell? | votes: 1 I would like to broaden my knowledge of programming. (...) I thought I would pose the question here, along with several stipulations about the type of language I am looking for. Some are subjective, some are intended to ease the transition from Haskell. Strong type system. (...) It also makes informally reasoning about the correctness of my program ... * [30]Why is writing a compiler in a functional language easier? | votes: 1 Often times a compiler works a lot with trees. The source code is parsed into a syntax tree. That tree might then be transformed into another tree with type annotations to perform type checking. Now you might convert that tree into a tree only containing core language elements (converting syntactic sugar-like notations into an unsugared form). Now you might ... * [31]GHC's RTS options for garbage collection | votes: 1 Generally speaking, garbage collection is a space/time tradeoff. Give the GC more space, and it will take less time. There are (many) other factors in play, cache in particular, but the space/time tradeoff is the most important one. The tradeoff works like this: the program allocates memory until it reaches some limit (decided by the GC's automatic tuning ... About the Haskell Weekly News At this point in time, the HWN has been a bit sporadic. I'm hoping that 2011 will see a much more regular schedule for this channel. I'd like to thank everyone who pitched in and sent stories/comments/quotes. If you have any ideas of how to improve this letter, please don't hesitate to contact me at dstcruz(a)gmail.com. The source code that I use to cullate this newsletter will be made available shortly in github. Here is to wishing you a great 2011! Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://www.reddit.com/r/haskell/comments/ee4ve/galois_releases_the_haskell_… 3. http://www.galois.com/blog/2010/11/30/galois-releases-the-haskell-lightweig… 4. http://www.reddit.com/r/haskell/comments/b58rk/try_haskell/ 5. file://localhost/r/haskell/comments/b58rk/try_haskell/ 6. http://www.reddit.com/r/haskell/comments/e6o0y/ghc_701_released/ 7. http://haskell.org/ghc/download_ghc_7_0_1.html 8. http://www.reddit.com/r/haskell/comments/e0qqz/learn_you_a_haskell_zippers/ 9. http://learnyouahaskell.com/zippers 10. http://www.reddit.com/r/haskell/comments/b3tby/the_new_llvm_backend_to_ghc_… 11. http://www.haskell.org/pipermail/cvs-ghc/2010-February/052606.html 12. http://www.reddit.com/r/haskell/comments/bwrhz/haskell_vs_erlang_for_bittor… 13. http://jlouisramblings.blogspot.com/2010/04/haskell-vs-erlang-for-bittorent… 14. http://www.reddit.com/r/haskell/comments/bhygx/there_are_more_than_4000_peo… 15. file://localhost/r/haskell/comments/bhygx/there_are_more_than_4000_people_in_the_haskell/ 16. http://www.reddit.com/r/haskell/comments/b4tel/smoking_fast_haskell_code_us… 17. http://donsbot.wordpress.com/2010/02/21/smoking-fast-haskell-code-using-ghc… 18. http://www.reddit.com/r/haskell/comments/dook4/learn_you_a_haskell_for_grea… 19. http://www.amazon.com/Learn-You-Haskell-Great-Good/dp/1593272839/ 20. http://www.reddit.com/r/haskell/comments/d51ao/lyah_intro_to_monads/ 21. http://learnyouahaskell.com/monads 22. http://stackoverflow.com/questions/3077866/large-scale-design-in-haskell/30… 23. http://stackoverflow.com/questions/3058529/curious-about-the-hashtable-prob… 24. http://stackoverflow.com/questions/2488646/why-are-side-effects-modeled-as-… 25. http://stackoverflow.com/questions/3884121/haskell-i-am-too-old-for-this/38… 26. http://stackoverflow.com/questions/3242361/haskell-how-is-pronounced/324285… 27. http://stackoverflow.com/questions/3034264/haskell-weird-kinds/3034295#3034… 28. http://stackoverflow.com/questions/3124591/is-haskell-really-a-purely-funct… 29. http://stackoverflow.com/questions/3770774/what-language-to-learn-after-has… 30. http://stackoverflow.com/questions/2906064/why-is-writing-a-compiler-in-a-f… 31. http://stackoverflow.com/questions/3171922/ghcs-rts-options-for-garbage-col…

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Deep concatenation [Was: Incorrectly inferring type [t]]
by oleg＠okmij.org 31 Dec '10

31 Dec '10

William Murphy wrote: > I've spent a lot of time trying to write a version of concat, which > concatenates lists of any "depth": It is a little bit more involved, but quite possible. The code is not much longer than the one you wrote (essentially, three lines: one class and two instance declarations). Here is the complete code: {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-} {-# LANGUAGE TypeFamilies, FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE OverlappingInstances #-} module DeepFlat where class DeepFlat a b | a -> b where dflat :: [a] -> [b] -- If we flatten a list of lists instance DeepFlat a b => DeepFlat [a] b where dflat = concatMap dflat -- If we are given a list of non-lists instance a ~ b => DeepFlat a b where dflat = id test1 = dflat "abracadabra" -- "abracadabra" test2 = dflat ["abra","cadabra"] test3 = dflat [["ab","ra"],["cad","abra"]] test4 = dflat [[["a","b"],["ra"]],[["cad","abra"]]]

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