January 2011 - Beginners

Optimise mininal date conversion function?
by Patrick LeBoutillier 11 Jan '11

11 Jan '11

Hi, I have this crude function that converts a date in YYYY/MM/DD format to an Int, but it's slow. Does any one have a clue on how to optimize it? date2int :: B.ByteString -> Int date2int b = y*12*31 + (m-1)*31 + (d-1) where y = read . B.unpack . B.take 4 $ b m = read . B.unpack . B.take 2 . B.drop 5 $ b d = read . B.unpack . B.drop 8 $ b Here is the original C++ function that was ported: static unsigned date2int (const char *date){ return atoi(date)*12*31+(atoi(date+5)-1)*31+atoi(date+8)-1; } Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Snap 0.3 - problem with embedding IO operations
by Karol Samborski 10 Jan '11

10 Jan '11

Hi, Recently I decided to port little project from snap 0.2 to 0.3 and I saw there were many changes in style of writing code (I mean heistLocal, render functions, extensions etc.). In snap 0.2 I had some function that was fetching data from database and then did some binding (via bindSplice) and finally disconnect from database. Here is the type of this function: myPageBind :: TempleState Snap -> Snap (TempleState Snap) Since Snap is instance of MonadIO I can use liftIO to embed IO computations but now in the 0.3 version I have to pass this function to heistLocal which type is: heistLocal :: (TemplateState n -> TemplateState n) -> m a -> m a so type of my function should be (in new version I need to have my application monad so I used it from bare template project from snap init) : myPageBind :: TemplateState Application -> TemplateState Application How then can I embed IO computations in that function? I can't use my last function either because I don't how to get TemplateState m now. Any suggestions? I'm sorry for my English and lack of code because I'm in work now so I don't have access to it. Best regards, Karol Samborski

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Is there a simpler way? Building a monad on `IO' Monad
by Arlen Cuss 10 Jan '11

10 Jan '11

Hi all, Thanks for previous help on this list! I really appreciate it. Today I wrote a monad and I'm not sure if I took a complicated way about it. It's essentially a State monad, except with some specialised functions that operate on the `state' if you will (though the state is rarely mutated)—you initialise it with two Handles (e.g. stdin, stdout), and then a set of specialised functions defined `within' the monad will operate on those handles. It saves you from passing the Handles throughout the functions and subcalls. It's quite possible there already exists a monad for this job, or that IO will actually let you do this, but I didn't find it in a bit of searching, and concluded this would be a fun way to solve the problem. If anyone has any advice on shortening the code, or possibly removing the need for it, please let me know! Here's the main monad: > import System.IO > import Control.Applicative > > newtype IODirector a = IODirector { runIODirector :: (Handle,Handle) -> IO (a, (Handle,Handle)) } > > instance Monad IODirector where > return a = IODirector $ \hs -> return (a, hs) > m >>= k = IODirector $ \hs -> do (a, hs) <- runIODirector m hs > runIODirector (k a) hs This is basically the same as State, except we `return' to the IO monad, as is the result of the stateful computation an I/O action, IO (a, (Handle,Handle)). We then have a type-class for the actual I/O we can perform within the monad: > class MonadDirectedIO a where > dPutStr :: String -> a () > dPutStrLn :: String -> a () > > dGetLine :: a String > dGetChar :: a Char ... The functions here continue for all the ones from IO I really wanted to use, and the instance is not surprising: > instance MonadDirectedIO IODirector where > dPutStr s = IODirector $ \hs@(_,hOut) -> do hPutStr hOut s > return ((), hs) > dPutStrLn = dPutStr . (++ "\n") > > dGetLine = IODirector $ \hs@(hIn,_) -> do r <- hGetLine hIn > return (r, hs) > dGetChar = IODirector $ \hs@(hIn,_) -> do r <- hGetChar hIn > return (r, hs) I'm aware I didn't have to put this in a type-class, but it seemed a reasonable thing to do. There was a little plumbing work to `enclose' IO within this monad. My question is - did I do it right? Or is there a simpler way? Of course, if there's already such functionality in the built-in, I'd also be interested to hear ... ;-) Cheers! Arlen

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theStdGen unsafePerformIO
by John Smith 09 Jan '11

09 Jan '11

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.

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Folding an applicative functor, wait,.. what?
by Arlen Cuss 08 Jan '11

08 Jan '11

Hi all, I'm probably in serious trouble here (i.e. I'm toying with things I don't understand); I have some code that I was using like follows, context removed for clarity: anotherFunc :: Int -> String ... map ((++) <$> show <*> anotherFunc) [1..20] This works great - the integers 1..20 are passed to each function and the results are catenated together. What if I'd like to add a third function? So far, the only workable solution I've been able to come up with is this: map ((++) <$> ((++) <$> show <*> anotherFunc) <*> yetAnotherFunc) [1..20] That's one unfortunate expression. I've been messing around with concat (trying to get the results of the functions into a list when all is well and done for concat to finally string together?), but all in all, have not had much luck with seeding the values in that way. I think there is probably an obvious solution I've missed... and hold on a minute! map (mconcat [show, anotherFunc, yetAnotherFunc]) [1..20] I just stumbled upon this. While I'm not 100% confident on what's just happened here, I thought I'd mail this out for everyone to enjoy the learning process that has been this email. This serves as a helpful pointer for me to study both monoids and applicative functors in depth, as the former suddenly leapt out of the recesses of my mind somewhere and solved it. Thanks for listening, and nice to meet you all! Cheers, Arlen

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Compilation fails with Data.Numbers.Primes
by Graham Enos 07 Jan '11

07 Jan '11

Hi all, I'm having trouble compiling (with GHC) a program that uses the Data.Numbers.Primes package (newest, 0.2.0.0) on Mac OS X, 10.6. Any help would be greatly appreciated! It gets to the linking stage, and then complains of undefined symbols: ghc -O2 counting_primes_200.hs Undefined symbols: "_primeszm0zi2zi0zi0_DataziNumbersziPrimes_primeFactorszuprimes1_closure", referenced from: _s1bm_info in counting_primes_200.o _s1bv_info in counting_primes_200.o _Main_main5_info in counting_primes_200.o _Main_main2_srt in counting_primes_200.o _Main_main5_srt in counting_primes_200.o "___stginit_primeszm0zi2zi0zi0_DataziNumbersziPrimes_", referenced from: ___stginit_Main_ in counting_primes_200.o ld: symbol(s) not found collect2: ld returned 1 exit status It's probably not a problem with the program I'm trying to compile, because runhaskell and ghci handle it fine (though I'd be happy to send along the source to anyone interested). It's also not necessarily a broken compiler, since GHC will compile simple throwaway stuff still and I've installed the newest Haskell environment; this problem happened on both my previous GHC installation and the newest. I was wondering if anyone else was having linking problems with the primes package, and what I could do to compile/link correctly. Thanks very much! Best, Graham

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QuickCheck documentation
by Christian Maeder 06 Jan '11

06 Jan '11

Hi, where can I find the best documentation for using QuickCheck? I use version QuickCheck-2.1.1.1 that is in the current Haskell Platform 2010.2.0.0. Thanks Christian (The links to home pages http://www.cse.chalmers.se/~koen http://www.math.chalmers.se/~rjmh/QuickCheck/ are pretty dead.)

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Tying the knot
by Alex Rozenshteyn 05 Jan '11

05 Jan '11

I'm trying to understand the technique referred to as "tying the knot", but documentation on the internet seems to be much sparser and more obtuse than I like. So I'm asking here. As far as I understand, "tying the knot" refers to a way of using laziness to implement something like references in a purely functional way. I'm trying to write a toy simulation: I have a population :: [Person] I want to collect a random subset of possible pairs of distinct people. So I go to each person in the population and select a subset of the people after him/her in the list; these are pairs in which s/he is the first element. I want to then be able to ask for all pairs in which a person is the first or the second element. I could give each person a unique id, but it seems like tying the knot is a valid way to implement this. Please correct me if I am misunderstanding. Thank you. -- Alex R

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Re: [Haskell-beginners] Beginners Digest, Vol 31, Issue 1
by mgd 04 Jan '11

04 Jan '11

Hi. i did what you said ant worked. the Platform Haskell, its very interesting. thanks a lot On 2011/01/01, at 11:00, beginners-request(a)haskell.org wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Re: Beginners Digest, Vol 30, Issue 49 (Miguel Dias) > 2. Re: Beginners Digest, Vol 30, Issue 49 (Brent Yorgey) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 31 Dec 2010 12:33:22 +0000 > From: Miguel Dias <dias.69.652(a)gmail.com> > Subject: Re: [Haskell-beginners] Beginners Digest, Vol 30, Issue 49 > To: beginners(a)haskell.org > Message-ID: <80A0FA05-D6F1-43CB-BD2D-45BFD6DC0B3C(a)gmail.com> > Content-Type: text/plain; charset=us-ascii > > i have the version 6.10.4. > i realized now, that has a new version, i've tried to install it, but it still's in the old version. > maybe i need to install cabal. > i've downloaded of my actual version from the www.haskell.org/ghc > > Miguel > > On Dec 31, 2010, at 11:00 AM, beginners-request(a)haskell.org wrote: > >> Send Beginners mailing list submissions to >> beginners(a)haskell.org >> >> To subscribe or unsubscribe via the World Wide Web, visit >> http://www.haskell.org/mailman/listinfo/beginners >> or, via email, send a message with subject or body 'help' to >> beginners-request(a)haskell.org >> >> You can reach the person managing the list at >> beginners-owner(a)haskell.org >> >> When replying, please edit your Subject line so it is more specific >> than "Re: Contents of Beginners digest..." >> >> >> Today's Topics: >> >> 1. Problem installing lhs2tex (Miguel Dias) >> 2. Re: Problem installing lhs2tex (Brent Yorgey) >> >> >> ---------------------------------------------------------------------- >> >> Message: 1 >> Date: Thu, 30 Dec 2010 19:29:38 +0000 >> From: Miguel Dias <dias.69.652(a)gmail.com> >> Subject: [Haskell-beginners] Problem installing lhs2tex >> To: beginners(a)haskell.org >> Message-ID: >> <AANLkTimbk3ePJ2Sn-ed472bqGaTAnUGyv8abqJAK7v3p(a)mail.gmail.com> >> Content-Type: text/plain; charset="iso-8859-1" >> >> Hi, im trying to install lhs2TeX in Mac OS. >> But i got an error, that i dont know how to fix. >> >> Someone can explain me what i need to do to fix that. >> >> Is this error: >> >> " >> MacBook:lhs2tex-1.16 mgd$ make >> /opt/local/bin/ghc -O --make -o lhs2TeX Main.lhs >> [17 of 19] Compiling FileNameUtils ( FileNameUtils.lhs, FileNameUtils.o ) >> >> FileNameUtils.lhs:15:57: >> Module `System.IO' does not export `hSetEncoding' >> >> FileNameUtils.lhs:15:85: Module `System.IO' does not export `utf8' >> make: *** [lhs2TeX] Error 1 >> >> " >> >> Thanks, >> Miguel >>

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