July 2010 - Beginners

doc on accessing C struct binary data
by Sean Perry 01 Aug '10

01 Aug '10

I would like to read C structs that have been written to disk as binary data. Is there a reference on doing this somewhere. The IO is not too hard, but how do I mimic the C struct in Haskell and still honor the exact sizes of the various struct members?

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Parsec and Validation
by Vladimir Solmon 31 Jul '10

31 Jul '10

I'm attempting to use Parsec to write a parser for times and dates. I'm aware of Data.Time.Format, but it doesn't offer the flexibility I need for this project. My current code (see below) uses Control.Monad.guard to validate the numeric values for hours and minutes inside the parser. The problem I'm running into is that checking for errors in the parsing code causes Text.Parsec.Combinator.choice (in the time function below) to fail and return an error if it cannot parse input using the first option (tTimeHourMin), even if the input should match the second option (t24hrClock). I have several questions: Why is choice failing in the time function below? Is there a better way to do numeric validation while parsing? Should I only use Parsec to validate that the input is syntactically correct and do the numeric validation elsewhere? What are some good examples of validating input using Parsec? Here is a bit of the code I'm working with: time :: Parser TimeOfDay time = choice [ tTimeHourMin, t24hrClock ] tTimeHourMin :: Parser TimeOfDay tTimeHourMin = do hour <- range (0, 23) oneOf " :,." min <- range (0, 59) return (TimeOfDay hour min 0) t24hrClock :: Parser TimeOfDay t24hrClock = do (h, m) <- splitAt 2 <$> count 4 digit let hour = read h let min = read m guard (hour >= 0 && hour <= 23 && min >= 0 && min <= 59) <?> printf "24hr time represented as hhmm" return (TimeOfDay hour min 0) range :: (Int,Int) -> Parser Int range (lower, upper) = do t <- read <$> many1 digit guard (t >= lower && t <= upper) <?> printf "integer in range [%d,%d]" lower upper return t Thanks for any suggestions, vladimir

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Announcement: Real World Haskell - Reading Group
by Jeff Greer 31 Jul '10

31 Jul '10

We are organizing a reading group for O'Reilly's - Real World Haskell We will start with Chapter 1 on August 8, 2010 and then cover additional chapters at two week intervals. If interested: http://linuxagora.com This is an all volunteer web site - no advertising, no fees Thank you, Jeff -- http://linuxagora.com

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Re: Announcement: Real World Haskell - Reading Group
by Jeff Greer 31 Jul '10

31 Jul '10

Hi Russel, Thanks for the suggestion to start a Haskell Reading Group mail list. At the current time, I can not fit it into my schedul... full time teaching job, gard school, and family.... I hope some find the reading group useful. -- http://teacherwikiweb.com

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edit-dist
by Andy Larocque 30 Jul '10

30 Jul '10

I am a senior who used to do a lot of programming in imperative languages, and now am new at learning this functional approach. I have been experimenting with one of Simon Thompson's examples from his book "The Craft 2e.." ; the edit-distance problem, and have some questions below I would really appreciate answers to. Here is the code from his book -- -- A type to represent the different sorts of Edit operations. data Edit = Change Char |Copy |Delete |Insert Char |Kill deriving (Eq,Show) -- Transforming one string into another, optimally. transform :: String -> String -> [Edit] transform [ ] [ ] = [] transform xs [ ] = [Kill] transform [ ] ys = map Insert ys transform (x:xs) (y:ys) | x==y = Copy : transform xs ys | otherwise = best [ Delete : transform xs (y:ys) , Insert y : transform (x:xs) ys, Change y : transform xs ys ] -- choose the sequence with the lowest cost. best :: [ [Edit] ] -> [Edit] best [ x ] = x best (x:xs) | cost x <= cost b = x | otherwise = b where b = best xs -- The cost is given by charging one for every operation except copy. cost :: [Edit] -> Int cost = length . filter (/=Copy) ----------------------------------------------------- -- When I run the program as listed above,with the simple strings shown, -- the 'optimal' solution is given by -- transform "AZ" "5Z" == [Change '5',Copy] -- but when I replace 'best' with 'concat' in the transform -- function to see all the possibilities, some strange solutions appear. --concat gave me one long list, which i tried to break into all the possible solutions. - I numbered the 6 ?? possibilities below, and noticed #5 doesn't provide any -- solution at all, and there are 2 'optimal' solutions. ----------------------------------------------------------------- --transform "AZ" "5Z" (using 'concat' instead of 'best' ) -- 1 [Delete,Delete,Insert '5',Insert 'Z', -- 2 Insert '5',Copy, -- 3 Change '5',Insert 'Z', -- 4 Insert '5',Delete,Copy, -- 5 Insert 'Z',Kill, Change 'Z',Kill, -- 6 Change '5',Copy] --------------------------------------- - -First, I really would like to know how to print ALL the possibilities as a list of lists as they are produced -- in order by the various function calls, not one long list as concat does. This would let me see how the -- recursion steps work as well. -- Secondly, what is happening in #5 possibility above ? The program never seems to me to check -- anywhere that (x:xs) has actually become (y:ys). Since "AZ" is really seen as 'A' : 'Z' : [ ], then some -- 'terminal' case involving [ ]'s ends the recursion each time, and that should produce a correct possible sequence? -- It appears this can end up as an error. -- A good explanation will go a long ways in my understanding of this small aspect of Haskell. -- I can only hope one of you has the time and patience to answer this beginner. Thx.

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Foreign function interface?
by skip＠pobox.com 30 Jul '10

30 Jul '10

Still just doinking around with Haskell, trying to get a feel for it. (It still looks pretty obscure to me so far.) I see that Haskell has a foreign function interface and a preprocessor to help generate. I normally work in Python, so am familiar with its concept of extension modules (C code written using Python's C API to go back and forth between Python and underlying C library code), tools to automate that like SWIG and Boost.Python, and Python's ctypes module which uses libffi. What of these tools does Haskell's foreign function interface most closely resemble? Can it interface with C++ as well as C? Thx, -- Skip Montanaro - skip(a)pobox.com - http://www.smontanaro.net/

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(no subject)
by Hongchao Liu 30 Jul '10

30 Jul '10

-- Best wishes, Hongchao Character is the virtue of hard times.

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Re: [Haskell-beginners] Monad transformers and MonadPlus
by Antoine Latter 30 Jul '10

30 Jul '10

You could always add a MondPlus instance to IO directly (or a newtype wrapper). 'mplus' would set up exception handling and 'mzero' would through an exception. You might want to limit it to a subset of exceptions, though. Antoine On Jul 30, 2010 5:09 AM, "Johann Bach" <johann.bach1127(a)gmail.com> wrote: This interesting page in Wikibooks describes how to write MaybeT: http://en.wikibooks.org/wiki/Haskell/Monad_transformers And it makes MaybeT an instance of MonadPlus. Even when used with IO as the inner monad. the constraint on the inner monad is Monad, not Monad plus: instance (Monad m) => MonadPlus (MaybeT m) where Apparently IO is not normally an instance of MonadPlus. It is interesting to be able to combine IO operations with mplus, because the idea of "trying computations until one succeeds" is so common in IO. Then I started wondering if StateT or ErrorT could be used to make IO the inner monad of a MonadPlus instance. Well, StateT is an instance of MonadPlus but with a MonadPlus constraint on the inner monad, so IO won't cut it. This leads me to wonder if there is a way to write mplus and mzero in StateT or ErrorT without a MonadPlus constraint on the inner monad. But if not, what was special about MaybeT as described in Wikibooks? _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners

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Monad transformers and MonadPlus
by Johann Bach 30 Jul '10

30 Jul '10

This interesting page in Wikibooks describes how to write MaybeT: http://en.wikibooks.org/wiki/Haskell/Monad_transformers And it makes MaybeT an instance of MonadPlus. Even when used with IO as the inner monad. the constraint on the inner monad is Monad, not Monad plus: instance (Monad m) => MonadPlus (MaybeT m) where Apparently IO is not normally an instance of MonadPlus. It is interesting to be able to combine IO operations with mplus, because the idea of "trying computations until one succeeds" is so common in IO. Then I started wondering if StateT or ErrorT could be used to make IO the inner monad of a MonadPlus instance. Well, StateT is an instance of MonadPlus but with a MonadPlus constraint on the inner monad, so IO won't cut it. This leads me to wonder if there is a way to write mplus and mzero in StateT or ErrorT without a MonadPlus constraint on the inner monad. But if not, what was special about MaybeT as described in Wikibooks?

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Re: [Haskell-beginners] lazy database queries
by Michael Hendricks 30 Jul '10

30 Jul '10

Thanks Gabríel. That's exactly what I was looking for. I had avoided the docs for System.IO.Unsafe since "unsafe" scared me away :-) I've cc'd the list so that others can find your excellent answer in the archives later. -- Michael On Thu, Jul 29, 2010 at 03:32:25PM +0000, "Gabríel A. Pétursson" wrote: > You might be interested in unsafeInterleaveIO in the module > System.IO.Unsafe > <http://users.skynet.be/jyp/html/base/System-IO-Unsafe.html>. > > On 29.07.2010 14:04, Michael Hendricks wrote: > >I have a data structure roughly like > > > > data Prices = Prices { > > today :: [Price], > > thisYear :: [Price] > > } > > > >Both today and thisYear are initially populated by database queries > >using HDBC. I then have functions which call today and possibly call > >thisYear. thisYear is not called often and the query to retrieve that > >data is very expensive. > > > >I thought I could use HDBC's laziness to postpone actually running the > >slow query until thisYear was required. Attempts with quickQuery > >suggest that the query is executed immediately and the only laziness > >is with fetching the results. > > > >Is it possible to make thisYear a lazy list which only executes the > >query if thisYear's values are required? > > > >Thank you.

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