July 2009 - Beginners

Circular Linked Lists
by Matthew J. Williams 04 Feb '10

04 Feb '10

Dear All How would one mimic, in Haskell, a C++ circular linked list i.e., where the last element precedes (points to) the first? Sincerely Matthew J. Williams

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Re: [Haskell-beginners] Closure
by Matthew J. Williams 16 Aug '09

16 Aug '09

>Williams<matthewjwilliams1(a)googlemail.com> wrote: > > Good morning > > > > What is a closure and, what purpose does it serve? > >This seems like a good explanation: http://www.haskell.org/haskellwiki/Closure > >/M > >Williams<matthewjwilliams1(a)googlemail.com> wrote: How does a closure differ from a binding? An example or two would be nice. :-) Sincerely, MJW

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Getting the Takusen example code to compile - failed import problem
by Daniel Everett 07 Aug '09

07 Aug '09

I have Takusen 0.8.5 installed and I'm trying to build the code found at http://darcs.haskell.org/takusen/doc/html/Database-Enumerator.html using ghc 6.10.3. This always fails with the error: Could not find module `Database.Oracle.Enumerator' The analogous errors are encountered if I substitute Oracle with Sqlite, Postgresql or ODBC. Any ideas how to resolve this? TIA, Daniel

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MonadRandom or Control.Monad.Random
by Michael P Mossey 01 Aug '09

01 Aug '09

Where can I find MonadRandom or Control.Monad.Random to install? It doesn't seem to be a system library, I can't find it with cabal or Hoogle. Thanks, Mike

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type class question from Ternary Trees
by MH 29 Jul '09

29 Jul '09

I am going over ternary tree library and it all looks clear to me with exception of the method below. Could you please explain to me how to read this: data TernarySet a = Node !a !(TernarySet a) !(TernarySet a) !(TernarySet a) | Null !(TernarySet a) | End deriving (Show, Eq) instance Binary a => Binary (TernarySet a) where ...... skipped get = do ---first call to get tag <- getWord8 case tag of _ | tag < 8 -> do ch <- get --- second call to get l <- if tag `testBit` 2 then get else return End ---third call to get e <- if tag `testBit` 1 then get else return End h <- if tag `testBit` 0 then get else return End return (Node ch l e h) 8 -> return (Null End) .......skipped What I don't understand here is recursive(?) call to 'get'. The first 'get' as I understand is an instance implementation of get from Binary module. What is 'ch <- get' (the second call) and 'get' inside 'if tag `testBit` 2 then get ...' (the third call)? tag <- getWord8 reads word8 from monad, then ch <- get reads what (just a copy of tag???)? Is get inside if ..then else statements is a look ahead get calls, to read next word8? Please provide more details if possible. Thanks a lot. Malik

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Closure
by Matthew J. Williams 29 Jul '09

29 Jul '09

Good morning What is a closure and, what purpose does it serve? Sincerely, MJW

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Q) Using Data.Binary to parse a packet
by Yang, Chul-Woong 29 Jul '09

29 Jul '09

Dear Haskellers. I would like to build simple UDP server application. To parse a packet, I decide to use Data.Binary as followings: data Packet = Packet { foo :: Word16, bar :: Word16, baz :: Word32 } deriving (Show, Eq) instance Binary Packet where get = do foo <- get :: Get Word16 bar <- get :: Get Word16 baz <- get :: Get Word32 return (Packet foo bar baz) -- omitting put because we're now just receiving only serveUDP port handlerfunc = withSocketsDo $ do ... procMessages sock where procMessages sock = do (msg, _, addr) <- recvFrom sock 10240 handlerfunc addr msg procMessages sock myHandler addr msg = do putStrLn $ decode (Data.ByteString.Lazy.Char8.Pack msg) run = serveUDP "8080" myHandler But, running it and sending sample udp packet results in following exception. *Main> run Loading package syb ... linking ... done. Loading package base-3.0.3.1 ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package containers-0.2.0.1 ... linking ... done. Loading package bytestring-0.9.1.4 ... linking ... done. Loading package parsec-2.1.0.1 ... linking ... done. Loading package network-2.2.1 ... linking ... done. Loading package binary-0.5.0.1 ... linking ... done. *** Exception: too few bytes. Failed reading at byte position 9 *Main> Why and what can I do for this? Thanks in advance. Chul-Woong Yang

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Question on monads and laziness
by Lakshmi Narasimhan Vaikuntam 27 Jul '09

27 Jul '09

Hello I am studying Real World Haskell chapter 9. Here is a snippet of code data Info = Info { infoPath :: FilePath , infoPerms :: Maybe Permissions , infoSize :: Maybe Integer , infoModTime :: Maybe ClockTime } deriving (Eq, Ord, Show) getInfo :: FilePath -> IO Info -- file: ch09/ControlledVisit.hs traverse order path = do names <- getUsefulContents path contents <- mapM getInfo (path : map (path </>) names) liftM concat $ forM (order contents) $ \info -> do if isDirectory info && infoPath info /= path then traverse order (infoPath info) else return [info] getUsefulContents :: FilePath -> IO [String] getUsefulContents path = do names <- getDirectoryContents path return (filter (`notElem` [".", ".."]) names) isDirectory :: Info -> Bool isDirectory = maybe False searchable . infoPerms When I read about IO in the previous chapter, I learnt that reading a file can be done lazily. Here my doubt is that whether the expression "order contents" would generate a list of directory contents (held in memory) for use in forM construct. Because in a subsequent para, I find this line. "If we are traversing a directory containing 100,000 files of which we care about three, we'll allocate a 100,000-element list before we have a chance to trim it down to the three we really want" Wouldn't laziness ensure that in the traverse function, when iterating over directory contents using the list generated by "order contents", it will generate just one element at a time and then free the memory for that entry immediately since we are not using the referencing list anymore in the rest of the function. Not sure whether I have understood the concept of laziness w.r.t monads correctly. Please clarify my doubts with regard to the code snippet. Thanks for your time. -- Regards Lakshmi Narasimhan T V

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uses of random number generators
by Michael P Mossey 26 Jul '09

26 Jul '09

Hello, I have an application for random numbers. So far, in my journey with Haskell, I have learned basics and a few things about monads, and I was hoping I could get some guidance how to employ random # gens. I need to run some simulated experiments to calculate some statistics by a Monte Carlo method. There are several components to the experiment, but one component is: - Given a hat with 5 red squares and 5 blue squares, draw one square at a time, note its color, for a total of 8 squares. Remember the order of the 8 squares. Another function will take two inputs each of which is "red" or "blue". If they are the same color, return the other color. Otherwise "toss a coin" to determine which color to return. -- this uses Bool rather than "red" "blue" for obvious reasons. balanceColors :: Bool -> Bool -> Bool balanceColors c1 c2 = if c1 == c2 then not c1 else -- flip coin -- Can I get a few examples or pointers? I believe I will have to run this in a State monad or the IO monad, will I not? Thanks, Mike

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Sidebar to variables
by Duke Normandin 25 Jul '09

25 Jul '09

My recent questions concerning "variables" spawned this observation: I take it that programming a solution using a functional language like Haskell is really about "linking" a series of functions from which a solution is derived. It seems to me that this approach would naturally encourage the use of the "bottom-up" method of program development. I mean, breaking the task down to the smallest possible segments, and then writing a function for each segment. Refactoring the task until the smallest segments are achieved. Very much like what I learned about programming in Forth (they use the term "words" to mean "function") Am I on the right track here, in my view of the "Haskell approach" to programming solutions? -- duke

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