November 2013 - Beginners

Constrained polymorphic functions as natural transformations
by Matt R 05 Jan '14

05 Jan '14

I read this excellent blog article on natural transformations[1], which helped me gain one intuition as to what they are: "functions that don't make too many assumptions about their arguments". In the article, natural transformations make "no assumptions at all" about their arguments. However, it's often the case that a Haskell function will require a type argument constrained by a type class, e.g. show :: Show a => a -> String. I wondered if it was possible to characterise these functions using category theory too. Below is my attempt, and I'm hoping people can tell me whether I'm in the right ballpark (or point me in the direction of a better ball-park...) For a particular type class T, we can form a subcategory Hask_T of Hask consisting of all the instances of the type class as objects, with the arrows just those functions that "commute" with the signature of the type class. For example, in Hask_Show, the arrows would be all the functions f: A -> B such that: show == show . f Or for Data.Monoid, all the monoid homomorphisms f: mappend (f x) (f y) == f (mappend x y) mempty == f mempty In general, for any type class, we can formulate two functors f and g and a function sig :: f a -> g a that captures the operations in the type class, and then the condition is that: fmap f . sig == sig . fmap f. Then we have that the polymorphic functions with a type class constraint of T are just the same thing as natural transformations in this subcategory Hask_T. Is the above correct / along the right lines? Thanks, -- Matt [1] "You Could Have Defined Natural Transformations", http://blog.sigfpe.com/2008/05/you-could-have-defined-natural.html

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Equality instance for lists
by Patrick Browne 01 Dec '13

01 Dec '13

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Simple Moving Average of a list of real numbers
by Alexandr M 27 Nov '13

27 Nov '13

Hello ! Could anybody explain me how to calculate simple moving average of a list ? I have found several examples in internet but I completely don't understand how it works. Basically it's necessary to iterate over the list of real numbers and calculate average values over last n items in the list. I just can't imagine how to do it without loops. -- Best regards, Alex

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IPerf in Haskell doesn't perform
by Thomas Bach 26 Nov '13

26 Nov '13

Hi list, I had to do a little benchmarking of a network connection between two servers and I thought “Well, Haskell seemed pretty awesome, let's use it to accomplish the task.” So I spent a couple of days with the Haskell library and finally got the following (working!) code (see below). Now my questions: 1) Although being a beautiful language, my code seems to be ugly. Any hints to improve this? My problem is (I think) that I simply cannot get rid of the IO Monad and the only way I know how to deal with this is by using lots of 'do's which doesn't help at all to get rid of the monads. (Probably the imperative background problem…) :) 2) The program I wrote doesn't perform. It sended around 33 MBytes in 10 seconds over a connection where iperf (the one packaged by Ubuntu, written in C) was able send 650 MBytes. Where can I improve performance? I'm especially interested in the low hanging fruits, which are easy to understand as a Haskell novice. ;) 3) When I run this program, I get the following Error (both, on the client, as well as on the server): $./iperf -c localhost "Running as client connecting to localhost" Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize -RTS' to increase it. enlarging the stack space avoids it, but how can I avoid it in the code? I guess that the stack space overflow comes from the recursive call to receiveData on the server and sendIntermediateAndFinalData in the client. I thought that in the way I implemented it, these functions are tailor recursive and therefor not prone to stack overflows!?! Anyway, I didn't check that, so maybe the error is somewhere else… Regards, Thomas Bach. PS: This is the code pasted “As Quotation”. I couldn't find another way to get the code into Thunderbird w/o messing up line breaks and the like. I additionally added the code as an attachment. I hope this is ok – time to get a new MUA! > import Data.Time (DiffTime, utctDayTime, getCurrentTime) > import Foreign (ForeignPtr, Int64, Ptr, mallocForeignPtr, withForeignPtr) > import Foreign.Storable (sizeOf, peek, poke) > import GHC.IO.Handle (Handle, BufferMode(NoBuffering), hClose, hGetBuf, hSetBuffering, hPutBuf) > import Network (Socket, HostName, PortID(PortNumber), PortNumber, accept, connectTo, listenOn, withSocketsDo) > import System.Console.GetOpt (ArgOrder(Permute), ArgDescr(..), OptDescr(..), getOpt, usageInfo) > import System.Environment (getArgs) > > data Flag = Client String | Server > deriving Show > > -- Command line arguments for server and client mode. > > options :: [OptDescr Flag] > options = > [ Option ['c'] ["client"] (ReqArg Client "HOST") "Connect to host as client." > , Option ['s'] ["server"] (NoArg Server) "Run as server." > ] > > perfOpts :: [String] -> IO [Flag] > perfOpts argv = > case getOpt Permute options argv of > ([], [], []) -> ioError (userError ("At least one Option is needed." ++ usageInfo "" options)) > (o, [], []) -> return o > (_, _, errs) -> ioError (userError (concat errs ++ usageInfo "" options)) > > -- Some constants > > port = PortNumber 8456 > type Trans = Int64 > numBytes = sizeOf (0 :: Trans) > inital = -128 :: Trans > intermediate = 0 :: Trans > final = 127 :: Trans > > makePtr = mallocForeignPtr :: IO (ForeignPtr Trans) > > -- The client connects to the server, initializes the connection > -- by sending initial, sends then for the amount of at least > -- 10 secs as much intermediate (0's) as possible and finally > -- sends final. > > runClient :: String -> IO () > runClient host = do > print ("Running as client connecting to " ++ host) > hdl <- initClient host port > ptr <- makePtr > fillInitial ptr > withForeignPtr ptr (sendBuf hdl) > fillIntermediate ptr > curTime <- fmap utctDayTime getCurrentTime > (sentBytes, time) <- sendIntermediateAndFinal ptr hdl curTime 10 0 > print ("Sent " ++ show sentBytes ++ " Bytes in " ++ show time ++ " seconds.") > hClose hdl > > initClient :: HostName -> PortID -> IO Handle > initClient host port = withSocketsDo $ do > hdl <- connectTo host port > hSetBuffering hdl NoBuffering > return hdl > > sendIntermediateAndFinal :: ForeignPtr Trans -> Handle -> DiffTime -> DiffTime -> Int -> IO (Int, DiffTime) > sendIntermediateAndFinal ptr hdl start duration sent = do > curTime <- fmap utctDayTime getCurrentTime > if (curTime - start) > duration > then do fillFinal ptr > withForeignPtr ptr (sendBuf hdl) > finishedTime <- fmap utctDayTime getCurrentTime > return (sent + numBytes, finishedTime - start) > else do withForeignPtr ptr (sendBuf hdl) > sendIntermediateAndFinal ptr hdl start duration (sent + numBytes) > > sendBuf :: Handle -> Ptr Trans -> IO () > sendBuf hdl buf = hPutBuf hdl buf numBytes > > fillPtr :: Trans -> ForeignPtr Trans -> IO () > fillPtr num ptr = withForeignPtr ptr (\p -> poke p num) > > fillInitial = fillPtr inital > fillIntermediate = fillPtr intermediate > fillFinal = fillPtr final > > -- The server simply accepts connections, receives what it can get > -- and adds up the received bytes and transmission time. > > runServer :: IO () > runServer = do > print "Running as server." > initServer port >>= handleConnection > > initServer :: PortID -> IO Socket > initServer port = withSocketsDo $ listenOn port > > handleConnection :: Socket -> IO () > handleConnection socket = do > (hdl, host, port) <- acceptConnection socket > print ("Connection from " ++ host) > ptr <- makePtr > (num, bytes) <- receive ptr hdl > curTime <- fmap utctDayTime getCurrentTime > if num == inital > then do (received, time) <- receiveData ptr hdl curTime 0 > print ("Received " ++ show received ++ " in " ++ show time ++ " seconds.") > hClose hdl > handleConnection socket > else do print ("ERR: Expected " ++ show inital ++ " got " ++ show num) > hClose hdl > handleConnection socket > > acceptConnection :: Socket -> IO (Handle, HostName, PortNumber) > acceptConnection socket = do > (hdl, host, port) <- accept socket > hSetBuffering hdl NoBuffering > return (hdl, host, port) > > receiveData :: ForeignPtr Trans -> Handle -> DiffTime -> Int -> IO (Int, DiffTime) > receiveData ptr hdl started received = do > (num, bytes) <- receive ptr hdl > if num /= final > then receiveData ptr hdl started (received + bytes) > else do curTime <- fmap utctDayTime getCurrentTime > return (received + bytes, curTime - started) > > receive :: ForeignPtr Trans -> Handle -> IO (Trans, Int) > receive ptr hdl = do > withForeignPtr ptr (\p -> hGetBuf hdl p numBytes) > num <- withForeignPtr ptr peek > return (num, numBytes) > > -- The main routine decides whether to run as server or client. > > main :: IO () > main = do > opts <- getArgs >>= perfOpts > if length opts /= 1 > then ioError (userError ("Too many arguments!" ++ usageInfo "" options)) > else > case head opts of > Server -> runServer > Client host -> runClient host

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'read' throws; why not maybe or either?
by Patrick Redmond 26 Nov '13

26 Nov '13

Is there a variation of 'read' which uses Maybe or Either to handle failure instead of an exception?

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FlexibleInstances
by Emanuel Koczwara 25 Nov '13

25 Nov '13

Hi, I have very simple code: class XmlValue a where toValue :: a -> String instance XmlValue String where toValue _ = "lorem" It gives followingerror: Illegal instance declaration for `XmlValue String' (All instance types must be of the form (T t1 ... tn) where T is not a synonym.Use -XTypeSynonymInstancesif you want to disable this.) So, maybe I'll removethat synonym: instance XmlValue [Char] where toValue _ = "lorem" This gives following error: Illegal instance declaration for `XmlValue [Char]' (All instance types must be of the form (T a1 ... an) where a1 ... an are *distinct type variables*, and each type variable appears at most once in the instance head. Use -XFlexibleInstancesif you want to disable this.) So, maybe I'll try to use that (T a1 ... an)form: instance XmlValue ([] String) where toValue _ = "lorem" Illegal instance declaration for `XmlValue [String]' (All instance types must be of the form (T a1 ... an) where a1 ... an are *distinct type variables*, and each type variable appears at most once in the instance head. Use -XFlexibleInstancesif you want to disable this.) Ok. Maybe I'll try to enable FlexibleInstanceswith {-# LANGUAGE FlexibleInstances #-} Ok, this works, but: http://stackoverflow.com/questions/15285822/cant-make-string-an-instance-of… answer): " However, Haskell98 forbids this type of typeclass in order to keep things simple and to make it harder for people to write overlapping instances like | instance Slang[a] where -- Strings would also fit this definition. slangify list= "some list" "| Ok, that's bad. But I want only [Char], [a] shouldn't work. https://ghc.haskell.org/trac/haskell-prime/wiki/FlexibleInstances says, that I'll get all or nothing. I want to enable "instance [Char] where..." but disable "instance [a] where...". Is it possible? Thanks, Emanuel

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understanding type classes and class constraints
by Robert Krahn 18 Nov '13

18 Nov '13

Hi everyone, I'm currently enjoying to learn Haskell and came upon a problem I don't understand: Motivated by the Learn you a Haskell / yes-no typeclass example I tried to create a simple "Tester" type class: class Tester a where test :: a -> Bool I wasn't to define the rules when test returns True/False for various types, e.g. instance Tester Integer where test 0 = False test _ = True For the Maybe instance I want to delegate to the value of Just and add a class constraint: instance (Tester m) => Tester (Maybe m) where test Nothing = False test (Just x) = test x It compiles nicely and works for Just values test (Just 3) -- True test (Just 0) -- False But test Nothing gives me No instance for (Tester a0) arising from a use of `test' The type variable `a0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance Tester m => Tester (Maybe m) instance Tester Integer In the expression: test Nothing Could you please enlighten me what's going on and how to fix my code? Thank you! Robert

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POSIX signals and deamons
by Emanuel Koczwara 14 Nov '13

14 Nov '13

Hi, I'm trying to create a simple daemon with hdaemonize. The deamon runs ok, but i'm having trouble with signals. Here is my test case: import Control.Monad import Control.Concurrent import System.Posix.Syslog import System.Posix.Signals import System.Posix.Daemonize main = daemonize $ do withSyslog "my-test-daemon" [PID] DAEMON $ syslog Info "daemon started" installHandler sigKILL (Catch killHandler) Nothing forever $ threadDelay 1000 killHandler = withSyslog "my-test-daemon" [PID] DAEMON $ do syslog Info "kill signal received" When I run it: $ ./test $ Everything works great: $ ps aux | grep test emanuel 12790 2.6 0.0 6552 852 ? S 20:57 0:00 ./test and: # tail /var/log/daemon.log Nov 14 20:52:06 emanuel-laptop my-test-daemon[12689]: daemon started when I do: $ kill -9 12790 process no longer exist: $ ps aux | grep test $ but/var/log/daemon.log doesn't change: # tail /var/log/daemon.log Nov 14 20:52:06 emanuel-laptop my-test-daemon[12689]: daemon started It looks like my sigKILL handler isn't called, why? Thanks, Emanuel

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Fake Conferences CSCI and WORLDCOMP of Hamid Arabnia
by abbasakbar＠hushmail.com 12 Nov '13

12 Nov '13

Fake Conferences CSCI and WORLDCOMP of Hamid Arabnia Hamid Arabnia from University of Georgia is well known for his fake WORLDCOMP conferences https://sites.google.com/site/worlddump1 or https://sites.google.com/site/dumpconf Hamid Arabnia understood that he cannot deceive researchers anymore using his WORLDCOMP. Hamid Arabnia (Guru of Fake Conferences and champion of academic scam) has recently started 2014 International Conference on Computational Science and Computational Intelligence (CSCI'14) http://www.americancse.org to deceive researchers further. CSCI'14 is started under the title of “American Council on Science and Education” which is a dummy corporation (does not exist anywhere in the world). Hamid Arabnia buried his name in the list of names of other innocent steering and program committee members of CSCI’14 to avoid any special attention. He knows that if his name is given any special attention then researchers immediately know that the conference is fake due to his “track record” with WORLDCOMP. Hamid Arabnia (the money hungry beast) spoiled the reputations and careers of many authors and committee members involved in his infamous WORLDCOMP for more than a decade and he is now ready to do the same using CSCI. Interestingly, CSCI is scheduled to be held at the same venue where WORLDCOMP was held until 2012. Hamid Arabnia claimed that CSCI proceedings will be published by IEEE but no one knows if IEEE really publishes. Many scholars have already sent emails to IEEE protesting for the unethical behavior of Hamid Arabnia and for the new series of his bogus conferences CSCI. CSCI paper submission deadline will be extended multiple times as usual. Do not spoil your resume by submitting your papers in this bogus conference CSCI which will not be held beyond 2014. Sincerely, Many researchers cheated by Hamid Arabnia conferences

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heterogeneous containers and static typing
by Franco 07 Nov '13

07 Nov '13

Hello, I am developing a small game engine (nothing fancy or professional, it is to test the waters of Haskell). You could say it's a very simple DSL: a user would import a library and use its tool to make a game. The problem I encountered is when defining or getting/setting variables for an instance game. Let me illustrate data GameState = GameState [(String, Int)] This is a very simple and naive gamestate, an association list consisting in (variableName, itsValue). This would mean restricting values to "Int" only, but I can easily enough add any kind of value with: data GameState = GameState [(String, Dynamic)] An example helper function signature would look like getBool :: String -> Either MyError Bool data MyError = NotFound String | TypeMismatch String The problem with this approach is that I loathe picking up those two kind of errors (var not found and type mismatch) at runtime. I would like them to be found at compile time (like "normal" Haskell). I thought that a possible solution was to declare a datatype with the needed variables inside: data MyGameVar = MyGameVar { health :: Int, message :: String } myvars = MyGameVar 1 "Don't forget your umbrella!" This would find at compile time typos in variable names and type errors at compile time, of course. But then I would have to change the GameState to: data GameState = GameState MyGameVar which would be fine if I were developing a *game*, but it is not fine when you develop a *game engine* (that would mean having to rewrite MyGameVar every game you made). So I am left to think if the problem I highlighted is "solvable" or do I have to make compromises? I guess I am not the first to encounter this difficulty, so I am asking here. Any hint or idea is appreciated, thanks! -F

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