- Haskell-Cafe - Haskell.org

The Related monad and constant values in type classes
by Jonas Almström Duregård 18 Feb '10

18 Feb '10

Hi, This literate haskell file was intended to be a quick question about a problem i have been pondering, but it developed into a short presentation instead. What i want to know is if there is already something like this (and suggestions for improvement of course). >{-#LANGUAGE GeneralizedNewtypeDeriving#-} Sometimes i find myself needing to associate a constant with a type or, more precisely, with a type class instance. Something like this would be nice: class Sized a where size :: Int instance Sized Int where size = 32 Of course this will not work since there is no way of knowing which instance i refer to when i use "size". A common work-around is to use a dummy parameter: >class SizedDummy a where > sizeDummy :: a -> Int > >instance SizedDummy Int where > sizeDummy = const 32 The size function is typically passed an undefined value. This is not very pretty, and somewhat unsafe. Another workaround is to define a newtype with a type parameter. >newtype SizeOf a = MkSize {toInt :: Int} >class SizedNewType a where > sizeNewType :: SizeOf a > >instance SizedNewType Int where > sizeNewType = MkSize 32 If we want the size of a pair to be the sum of it's components, something like this is needed: >instance (SizedNewType a, SizedNewType b) => SizedNewType (a,b) where > sizeNewType = sizeNewType' sizeNewType sizeNewType where > sizeNewType' :: SizeOf a -> SizeOf b -> SizeOf (a,b) > sizeNewType' a b = MkSize $ toInt a + toInt b This is way to much code say that "size = size a + size b". A more general solution can be achieved by making "Int" another type variable of "SizeOf". I call the resulting type "Related": >newtype Related a b = Related {unrelated :: b} deriving > (Eq,Ord,Show,Read,Bounded,Enum,Fractional,Num, > Real,Integral,RealFrac,Floating,RealFloat) This type is highly reusable and the GeneralizedNewtypeDeriving language extension is very practical (although the instances could be written manually). It can also be used as an Identity monad: >instance Functor (Related a) where > fmap f (Related a) = Related $ f a > >instance Monad (Related a) where > return = Related > (Related a) >>= f = f a This allows the Sized class and instances to be specified in a slim fashion using a familiar monadic interface: >class Sized a where > size :: Related a Int > >instance Sized Int where > size = return 32 > >instance (Sized a, Sized b) => Sized (a,b) where > size = do > a <- return size :: Sized a => Related (a,b) (Related a Int) > b <- return size :: Sized b => Related (a,b) (Related b Int) > return $ unrelated a + unrelated b This still requires a lot of type signatures, some additional magic is required. It is possible to write general versions of the type signatures above, which allows the following instance definition for (,,): >instance (Sized a, Sized b, Sized c) => Sized (a,b,c) where > size = do > a <- on3 size > b <- on2 size > c <- on1 size > return $ a + b + c With the derivation of Num, this can be done even more compact: >instance (Sized a, Sized b, Sized c, Sized d) => Sized (a,b,c,d) where > size = on1 size + on2 size + on3 size + on4 size The code for the onN functions: >rerelate :: Related a b -> Related c b >rerelate = return . unrelated >on1 :: Related a v -> Related (x a) v >on1 = rerelate >on2 :: Related a v -> Related (x a x0) v >on2 = rerelate >on3 :: Related a v -> Related (x a x0 x1) v >on3 = rerelate >on4 :: Related a v -> Related (x a x0 x1 x2) v >on4 = rerelate Regards, Jonas Almström Duregård

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How can i run darcs in ghci?
by Daryoush Mehrtash 18 Feb '10

18 Feb '10

I am trying to load darc.hs (from darcs 2.3.1, ghci 6.10.4 on Ubuntu) and I get the following :~/darcs-src/darcs-2.3.1/src$ ghci GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> :load darcs Darcs/RepoPath.hs:59:0: error: impossible.h: No such file or directory phase `C pre-processor' failed (exitcode = 1) > What am I doing wrong? Thanks, Daryoush

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CURL and threads
by Eugene Dzhurinsky 18 Feb '10

18 Feb '10

Hello, all! Can somebody please explain, what is the best way of using CURL with several threads? I wrote simple client, which tries to authenticate against HTTP server. With running this client, it starts to eat memory insanely (and I know this code is far, far away of even being close to be called good one ) ========================================================================================= module NTLMTest where import System.IO import Network.Curl import Control.Concurrent import Control.Concurrent.Chan type ResponseState = Either Bool String type RespChannel = Chan ResponseState delay = 500 * 1000 isResponseOk :: String -> CurlResponse -> ResponseState isResponseOk username response = case respCurlCode response of CurlOK -> Left True _ -> Right $ username ++ " => " ++ respStatusLine response ++ " :: " ++ (show . respStatus $ response) checkAuthResponse :: RespChannel -> String -> String -> String -> IO () checkAuthResponse state user passwd url = do response <- curlGetResponse_ url [CurlHttpAuth [HttpAuthAny], CurlUserPwd $ user ++ ":" ++ passwd] writeChan state $ isResponseOk user response threadDelay $ delay runHTTPThread :: RespChannel -> (String,String) -> IO () runHTTPThread state (user,passwd) = checkAuthResponse state user passwd url url = "http://localhost:8082/" credentials = map (\i -> ("user" ++ show i,"123456")) [1..21] main = withCurlDo $ do chan <- newChan :: IO (RespChannel) mapM_ ( \cred -> forkIO $ runHTTPThread chan cred ) credentials dumpChannel chan $ length credentials main where dumpChannel :: RespChannel -> Int -> IO () dumpChannel _chan n | n == 0 = return () | otherwise = do state <- readChan _chan case state of (Left _) -> return () --putStrLn "OK" (Right err) -> putStrLn err dumpChannel _chan $ n-1 ========================================================================================= If I get rid of forkIO - it stops at 40-50 megabytes and don't raise memory usage anymore. Also, I noticed that (either because of buffering, or may be something else) results are appearing on console much slower than if I simply use "wget" with looping in shell script. JMeter also reports awesome speed, so server can authenticate tens of concurrent users per second (thus it's not server or connection bandwidth issue). Hopefully, someone could help me in overcoming my ignorance :) -- Eugene N Dzhurinsky

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ANN: iteratee-parsec 0.0.1
by Maciej Piechotka 18 Feb '10

18 Feb '10

Iteratee-parsec is a library which allows to have a parsec (3) parser in IterateeG monad. It contains 2 implementations: - John Lato's on public domain. It is based on monoid and design with short parsers in mind. - Mine on MIT. It is based on single-linked mutable list. It seems to be significantly faster for larger parsers - at least in some cases - but it requires a monad with references (such as for example IO or ST). Regards

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Re : Friedberg numberings
by Pierre-Etienne Meunier 18 Feb '10

18 Feb '10

Hi, In fact it is not quite hard to see that such a numbering exists : just take any numbering of all Turing machines, say {\phi_i}, then remove all indices i such that there is j<i such that \phi_j = \phi_i (equality is mathematically correctly defined between functions from N to N). Ah, and also, notice that what you call a "programming language" is nothing else than an enumeration of all Turing machines. However, if by "construction" you meant "recursive construction", i.e. for another enumeration (\psi_i), find a Turing machine computing for any i, the index j such that \phi_j=\psi_i, then it is clear that it is not possible : it would allow to decide the equality problem for Turing machines. Hope this helps...

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Friedberg numberings.
by Grigory Sarnitskiy 18 Feb '10

18 Feb '10

Hello! The question is not about Haskell, but I don't know where else to ask. In the book "Computable functions" by Vereshchagin and Shen it is said that "it is possible to invent a programming language such that each programming problem has a unique solution in it". The author claims that this statement is a rewording of the theorem, that there is a universal computable function, such that any computable function has exactly one number. I wonder has such language been actually constructed? the book itself (look at the bottom of p 30 for the statement): http://books.google.ru/books?id=A6uvsks0abgC&pg=PA30#v=onepage&q=&f=false

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[Fwd: Re: [Haskell-cafe] Category Theory woes]
by Mike Pentney 18 Feb '10

18 Feb '10

As well as books and reading material online, nowadays you can also find video lectures...for example, the following was at the top of Googling "category theory video": http://golem.ph.utexas.edu/category/2007/09/the_catsters_on_youtube.html Cheers, Mike. Nick Rudnick wrote: > I haven't seen anybody mentioning «Joy of Cats» by Adámek, Herrlich & > Strecker: > > It is available online, and is very well-equipped with thorough > explanations, examples, exercises & funny illustrations, I would say > best of university lecture style: > http://katmat.math.uni-bremen.de/acc/. (Actually, the name of the book > is a joke on the set theorists' book «Joy of Set», which again is a > joke on «Joy of Sex», which I once found in my parents' bookshelf... ;-)) > > Another alternative: Personally, I had difficulties with the somewhat > arbitrary terminology, at times a hindrance to intuitive understanding > - and found intuitive access by programming examples, and the book was > «Computational Category Theory» by Rydeheart & Burstall, also now > available online at http://www.cs.man.ac.uk/~david/categories/book/, > done with the functional language ML. Later I translated parts of it > to Haskell which was great fun, and the books content is more beginner > level than any other book I've seen yet. > > The is also a programming language project dedicated to category > theory, «Charity», at the university of Calgary: > http://pll.cpsc.ucalgary.ca/charity1/www/home.html. > > Any volunteers in doing a RENAME REFACTORING of category theory > together with me?? ;-)) > > Cheers, > > Nick >

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Re: Pointfree composition for higher arity
by oleg＠okmij.org 18 Feb '10

18 Feb '10

Sean Leather wrote: > (...) :: (c -> d) -> (a -> b -> c) -> a -> b -> d > (...) f g x y = f (g x y) > Does anybody else care about this? What are some alternative > solutions? Here is a different solution: http://okmij.org/ftp/Haskell/polyvariadic.html#polyvar-comp f:: a1->a2-> .... ->cp (where cp is not a function type) g:: cp->d f `mcomp` g:: a1->a2-> .... ->d Now that we know how to generically decide if a type is not a functional type, mcomp can be defined fully generically, for any type cp that is not a function. If there is interest, I can write that code. One particular application of the mcomp combinator is the prod combinator: Given two functions, f:: a1->a2->...->an->c |c,d non-exponential types g:: b1->b2->...->bn->d their product f `prod` g:: a1->a2->...->an->b1->b2->...->bn->(c,d) The number of as and bs is arbitrary. The definition of prod is very simple: > prod = (. ((. (,)) . mcomp)) . mcomp The web page http://okmij.org/ftp/Haskell/polyvariadic.html#categorical-max3 gives a few explanations and further examples.

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What is the meaning of tilde ("~") symbol
by kg 17 Feb '10

17 Feb '10

Hi, I'm reading the following subject : http://www.haskell.org/pipermail/haskell-cafe/2007-July/028227.html In the sample code, we can see : instance ReadAsAnyOf () ex where readAsAnyOf ~() = mzero And, I've search the meaning of the symbol "~", but I've found nothing about this (note that's not easy to search "~" on google ...) Could you give me a link about this, or explain me it ? Thx in advance.

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ANN: concurrent-extra-0.1
by Roel van Dijk 17 Feb '10

17 Feb '10

Hello, We would like to announce the release of concurrent-extra [1]. A library which offers a few extra synchronization primitives. These primitives are found in the standard libraries of languages like Java and Python, but not in Haskell. Quick overview: * Lock: Enforce exclusive access to a resource. Also known as a mutex or a binary semaphore. * RLock: A lock which can be acquired multiple times by the same thread. Also known as a reentrant mutex. * Event: Wake multiple threads by signaling an event. Includes both pessimistic and optimistic versions. * ReadWriteLock: Multiple-reader, single-writer locks. Used to protect shared resources which may be concurrently read, but only sequentially written. * ReadWriteVar: Concurrent read, sequential write variables. Plug & Play: cabal install concurrent-extra Darcs: darcs get http://code.haskell.org/~roelvandijk/code/concurrent-extra/ Thanks to Neil Brown and Simon Marlow for an initial review. Comments are still more than welcome! Regards, Roel & Bas van Dijk [1] http://hackage.haskell.org/package/concurrent-extra

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