September 2012 - Beginners

Who discovered the fold operation?
by Costello, Roger L. 09 Sep '12

09 Sep '12

Hi Folks, Scientists look for recurring patterns in nature. Once a pattern is discovered, its essential ingredients are identified and formalized into a law or mathematical equation. Discovery of a pattern is important and those individuals who make such discoveries are rightfully acknowledged in our annals of science. Scientists are not the only ones that look for recurring patterns, so do functional programmers. Once a pattern is discovered, its essential ingredients are identified and formalized into a function. Long ago someone in the functional programming community discovered a pattern that has become known as the fold operation. It was an important discovery, on-par with some scientific discoveries. Who discovered the fold operation? When? /Roger

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lifting to applicative: recomputing an argument each time it is used?
by Iain Nicol 09 Sep '12

09 Sep '12

Hi, I think I'm trying to lift 'Data.List.intersperse' (to applicative or a monad) in such a way that its (first) argument is recomputed each time it is used. I'm hoping that there's a reusable, elegant or abstract, approach for this that I'm unaware of. If that isn't clear, I'm using QuickCheck to generate a "sentence" of a random number of random words, each word separated by a random number of spaces. Importantly, there should be no connection between the number of spaces separating the first and second word, and the number of spaces separating the second and third word, etc. I have code which works (run 'workingExample'), but it's not very elegant---I ended up implementing the 'myIntersperse' function manually. I had tried to write the code by fmap-ing Data.List.intersperse (see 'badExample'), but doing that naïvely has a major problem. With that approach, the number of spaces between each word is correctly random between sentences, but is incorrectly constant within each generated sentence. If anybody knows a trick that I'm missing, that would be great. Thanks. {-# LANGUAGE ScopedTypeVariables #-} import Control.Applicative import Control.Monad import Data.List (intersperse) import Test.QuickCheck -- | Generate a string consisting of one or more space character. spaces :: Gen String spaces = elements [" ", " "] -- Generate a (nonsensical) word. word :: Gen String word = elements ["foo", "bar", "baz", "bert"] workingExample, badExample :: IO () workingExample = sample $ myIntersperse spaces (listOf word) badExample = sample $ intersperse <$> spaces <*> listOf word -- Like a lifted version of 'Data.List.intersperse'. The interspersed -- seperator is generated each time the separator appears, as opposed to -- just once for the whole list. myIntersperse :: Gen a -> Gen [a] -> Gen [a] myIntersperse genSep genList = myIntersperse' genSep =<< genList where myIntersperse' :: forall a . Gen a -> [a] -> Gen [a] myIntersperse' genSep [] = return [] myIntersperse' genSep xs = do let listElementWithSep :: Gen [(a, a)] listElementWithSep = zipWithM (\el sep -> pure (el, sep)) xs =<< (sequence . repeat) genSep init <$> tupleListToList <$> listElementWithSep -- | Removes the tuple structure from a list, preserving the -- inner elements and their order. tupleListToList :: [(a, a)] -> [a] tupleListToList = concat . map (\(x, y) -> [x, y]) -- Iain

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splitAt and negative values
by Stayvoid 06 Sep '12

06 Sep '12

Hello, Could you explain this behaviour? > splitAt (-1) "Foobar" ("","Foobar") > splitAt (-100) "Foobar" ("","Foobar") I don't understand why splitAt (-1) "Foobar" doesn't output this: ("r", "Fooba") or this: ("", "Fooba") How does it work? Python's list slicing looks similar, but results for negative values differ: >>> a = "Foobar" >>> a[:-1] >>> 'Fooba' Thanks

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Error Loading Stdm.lhs in Haskell platform 2012
by Iwan Awaludin 05 Sep '12

05 Sep '12

Dear Sir/Madam I Installed *Haskell Platform 2012.2.0.0 for Windows<http://lambda.haskell.org/platform/download/2012.2.0.0/HaskellPlatform-2012…> *and try to load Stdm.lhs which is downloaded from http://www.dcs.gla.ac.uk/~jtod/discrete-mathematics/Stdm.lhs I have this error: stdm.lhs:1160:14: Parse error in pattern: n + 1 Failed, modules loaded: none. Is there anything I should do to make it right? Thank you. -- Iwan Awaludin

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Re: [Haskell-beginners] [Haskell-cafe] Haskell seems setup for iterative numerics; i.e. a standard example is Newton's method where lazy evaluation ...
by KC 05 Sep '12

05 Sep '12

The REPA package/library doesn't have LU factorization, eigenvalues, etc. On Wed, Sep 5, 2012 at 12:59 PM, Carter Schonwald <carter.schonwald(a)gmail.com> wrote: > Hello KC, > you should check out the Repa library then and see how it works for you. > Cheers > -Carter > > On Wed, Sep 5, 2012 at 12:46 PM, KC <kc1956(a)gmail.com> wrote: >> >> separates control from computation. >> >> It seems as if Haskell would be better for iterative matrix methods >> rather than direct calculation. >> >> -- >> -- >> Regards, >> KC >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- -- Regards, KC

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Haskell seems setup for iterative numerics; i.e. a standard example is Newton's method where lazy evaluation ...
by KC 05 Sep '12

05 Sep '12

separates control from computation. It seems as if Haskell would be better for iterative matrix methods rather than direct calculation. -- -- Regards, KC

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unfamiliar syntax in class definition
by Christopher Howard 05 Sep '12

05 Sep '12

The MonadState class is defined as: quote: -------- class Monad m => MonadState s m | m -> s where ...etc... -------- Please explain the part "| m -> s", that is, the use of the pipe symbol and the right arrow symbol which follow "MonadState s m". -- frigidcode.com indicium.us

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XML output
by Torsten Otto 05 Sep '12

05 Sep '12

Hi, all! I'm struggling with a strange phenomenon. If I read a file containing __ a b c __ the program below works as expected. If I read a svg file __ <?xml version="1.0"?> <svg xmlns="http://www.w3.org/2000/svg"> <rect x="0" y="0" width="180" height="180" fill="white"/> </svg> ___ the first two lines are not processed. Program: __ module SVG() where import IO main :: IO () main = do s <- readFile "sierpinski1.svg" putStrLn s writeFile "sierpinski1_.svg" s __ Results of putStrLn: __ *SVG> main a b c *SVG> main <rect x="0" y="0" width="180" height="180" fill="white"/> </svg> *SVG> __ File Output lists the whole file, though. I don't understand this, why is GHCi stripping the output this way? Thanks, regards, Torsten

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Thread Blocking
by mukesh tiwari 04 Sep '12

04 Sep '12

Hello All I was going trough Real World Haskell and it says "If we try to put a value into an MVar that is already full, our thread is put to sleep until another thread takes the value out". I wrote a simple code to block main import Data.List import Control.Concurrent fun m = do putMVar m 10 return () main = do m <- newEmptyMVar forkIO $ fun m putMVar m 10 return () What I am expecting that main should be blocked at least couple of times but its behaving more deterministically. [mukesh.tiwari@ Programming]$ ghc-7.4.1 -threaded -fforce-recomp Concurrent.hs [1 of 1] Compiling Main ( Concurrent.hs, Concurrent.o ) Linking Concurrent ... [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ I am expecting to get thread blocked indefinitely on MVar at least half the time. Could some one please tell me why this deterministic behavior ? Regards Mukesh Tiwari

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monads / do syntax
by Christopher Howard 04 Sep '12

04 Sep '12

What does the following do expression translate into? (I.e., using >>= operator and lambda functions.) code: -------- h = do a <- (return 1 :: IO Integer) b <- (return 2) return (a + b + 1) -------- -- frigidcode.com indicium.us

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