August 2006 - Haskell-Cafe

Re: [Haskell-cafe] state and exception or types again...
by Andrea Rossato 02 Sep '06

02 Sep '06

Il Wed, Aug 30, 2006 at 11:24:45AM +0100, Brian Hulley ebbe a scrivere: > Thanks, glad to be of help. I met Haskell a couple of months ago, when I switched my window manager to Ion. Tuomo Valkonen, its developer, uses darcs. Moreover he develops a small PIM, riot, written in Haskell. I wanted to play around with it but the source code was literally unreadable, for me. I could not understand a single line. I do some php. Lately I discovered Javascript and Lua, and I fell in love with their object-oriented capabilities. I'm not a real coder (though I develop a wiki in php), but I like getting to know new programming practices. I've been intrigue with functional programming for quite sometime but I just knew a bit of Scheme, because it's used as a scripting language in LilyPond (a package for writing music sheets). So I decided to spend my August holidays to study Haskell. I started reading some tutorial but could not understand what was going on. I then thought to take the long way: I watched the Abelson and Sussman's lectures and used ghci to (sort of) follow what they were doing. Coming from dynamic typed languages (I know very little C) the type system was horrible. Monads seemed mysterious objects. I've read in the "Yet Another..." that the Haskell community is very supportive. With your help I have now a different perspective. Playing with Haskell is like completing a puzzle, whose pieces' shapes are made up with rules that, at first, you seem not be able to grasp. You keep pushing the last piece, and it just doesn't fit in. Then you realize that shapes are actually types. And when you start understanding the rules of type construction and type matching, now you can recognize the shapes of the pieces you are playing with. And start making rational guesses on where they should go. Now, when I see "Compiling ... Ok, modules loaded: " I feel like I ended up solving the puzzle. It's an amazing pleasure... Haskell and functional programming. Without you I couldn't do it. Thank you so much! The tutorial will have this outline: first we build a monad adding output, exception, and state. Then we use monad transformer to take out state and output and add debug, doing lifting, put(ing) and get(ing) by hand, to understand the central role of type matching/construction. We end up with the following code, that should clearly show all the previous (hand made) steps should lead. No lambda calculus inhere! Once again, thanks! Andrea -----The Final Evaluator---- module MyStateT where import Control.Monad.State hiding (State) data Term = Con Int | Add Term Term deriving (Show) type IOStack = [Output] type Output = String type Debug = [String] data EvalST = State {getIOS :: IOStack, getDebug :: Debug, getCount:: Int} deriving(Show) type Exception = String data MT a = Fail Exception | Done {unpackDone :: a } deriving (Show) type Eval s a = StateT s MT a instance Monad MT where return a = Done a m >>= f = case m of Fail e -> Fail e Done a -> f a instance Functor MT where fmap _ (Fail a) = Fail a fmap f (Done a) = Done (f a) emptyState = State [] [] 0 stopExecT exc = lift $ Fail exc catchT e = do st <- get let s = getCount st let es = getDebug st let o = getIOS st let exc = "Debug msg at Iteration " ++ show s ++ ": " ++ e put $ State o (exc:es) s printT :: Output -> Eval EvalST () printT o = do st <- get let s = getCount st let e = getDebug st let os = getIOS st let out = show s ++ " - " ++ o put $ State (out:os) e s incTcounter :: Eval EvalST () incTcounter = do st <- get let s = getCount st let e = getDebug st let o = getIOS st put $ State o e (s+1) evalT :: Term -> Eval EvalST Int evalT (Con a) = do incTcounter printT (formatLine (Con a) a) return a evalT (Add t u) = do a <- evalT t b <- evalT u incTcounter let out = formatLine (Add t u) (a + b) printT out case (a+b) of 42 -> do catchT "The Ultimate Answer Has Been Computed!! Now I'm tired!" return (a+b) 11 -> stopExecT "11.... I do not like this number!" otherwise -> return (a + b) formatLine :: Term -> Int -> Output formatLine t a = "eval (" ++ show t ++ ") <= " ++ show a printAll :: [String] -> IO () printAll [] = return () printAll (a:xs) = do print a printAll xs eval :: Term -> IO () eval exp = case execStateT (evalT exp) emptyState of Fail e -> print e Done (State a b c ) -> do printAll $ reverse a print $ show $ unpackDone $ evalStateT (evalT exp) emptyState case b of [] -> print $ "Iterations: " ++ show c _ -> do printAll $ reverse b print $ "Iterations: " ++ show c {- Some runs: *MyStateT> eval (Add (Add (Add (Add (Con 40) (Con 2)) (Add (Con 12) (Con 3))) (Con 3)) (Con 10)) "1 - eval (Con 40) <= 40" "2 - eval (Con 2) <= 2" "3 - eval (Add (Con 40) (Con 2)) <= 42" "4 - eval (Con 12) <= 12" "5 - eval (Con 3) <= 3" "6 - eval (Add (Con 12) (Con 3)) <= 15" "7 - eval (Add (Add (Con 40) (Con 2)) (Add (Con 12) (Con 3))) <= 57" "8 - eval (Con 3) <= 3" "9 - eval (Add (Add (Add (Con 40) (Con 2)) (Add (Con 12) (Con 3))) (Con 3)) <= 60" "10 - eval (Con 10) <= 10" "11 - eval (Add (Add (Add (Add (Con 40) (Con 2)) (Add (Con 12) (Con 3))) (Con 3)) (Con 10)) <= 70" "70" "Debug msg at Iteration 3: The Ultimate Answer Has Been Computed!! Now I'm tired!" "Iterations: 11" *MyStateT> *MyStateT> eval (Add (Add (Add (Add (Con 10) (Con 2)) (Add (Con 12) (Con 3))) (Add (Con 5) (Con 2))) (Con 2)) "1 - eval (Con 10) <= 10" "2 - eval (Con 2) <= 2" "3 - eval (Add (Con 10) (Con 2)) <= 12" "4 - eval (Con 12) <= 12" "5 - eval (Con 3) <= 3" "6 - eval (Add (Con 12) (Con 3)) <= 15" "7 - eval (Add (Add (Con 10) (Con 2)) (Add (Con 12) (Con 3))) <= 27" "8 - eval (Con 5) <= 5" "9 - eval (Con 2) <= 2" "10 - eval (Add (Con 5) (Con 2)) <= 7" "11 - eval (Add (Add (Add (Con 10) (Con 2)) (Add (Con 12) (Con 3))) (Add (Con 5) (Con 2))) <= 34" "12 - eval (Con 2) <= 2" "13 - eval (Add (Add (Add (Add (Con 10) (Con 2)) (Add (Con 12) (Con 3))) (Add (Con 5) (Con 2))) (Con 2)) <= 36" "36" "Iterations: 13" *MyStateT> *MyStateT> eval (Add (Con 5) (Con 6)) "11.... I do not like this number!" *MyStateT> -}

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data structures question
by Tamas K Papp 01 Sep '06

01 Sep '06

Hi, Having read some tutorials, I would like to start using Haskell "for real", but I have some questions about data structures. The mathematical description of the problem is the following: assume there is a function V(a,b,theta), where a and b can have two values, High or Low, and theta is a number between zero and n (n is given). The range of V is the real numbers. Then there is an algorithm (called value iteration, but that's not important) that takes V and produces a function of the same type, called V'. The algorithm uses a mapping that is not elementwise, ie more than the corresponding values of V are needed to compute a particular V'(a,b,theta) -- things like V(other a,b,theta) and V(a,b,theta+1), where data State = Low | High other :: State -> State other High = Low other Low = High Question 1: V can be represented as a 3-dimensional array, where the first two indices are of type State, the third is Int (<= n). What data structure do you suggest in Haskell to store V? Is there a multidimensional array or something like this? Let's call this structure TypeV. Question 2: I would like to write valueit :: TypeV -> TypeV valueit V = mapondescartesproduct [Low,High] [Low,High] [0..n] mapV where -- mapV would calculate the new V' using V -- mapV :: State -> State -> Int -> Double to fill the new data structure. How to do this sensibly? Thanks, Tamas

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HaXml question
by Tim Newsham 01 Sep '06

01 Sep '06

I thought this one would be easy but I'm starting to think its not. I am playing with HaXml and I want to transform an XML tree into another tree. The transforms are simple enough, but the kicker is that I want them to be "stateful." In this example, the state is a random number generator. So the transformation depends on the current node and the random number generator state. I want to transform every node in the tree. My feeble attempt at this is given in: http://www.thenewsh.com/~newsham/x/tweaker.hs What I see when I run it is that the value of "p" in the "tweak" function is identical each time. Indeed, it seems "g" itself is the same each time "tweak" is invoked (I wrote a variant of the program that used sequential integers instead of generators to verify this). So here's what I think is going on. You can probably skip this section because I am probably off base anyway... I guess "keep" is being applied to one node, and returning a list of 1 node, and this is zipped with my infinite list of generators and as a result I get the same generator each time. If this is the case I guess foldXml is not the right tool for this job. Perhaps I want a newly-written foldXml that works with LabelFilters rather than CFilters? Ok.. so what's really going on here. Can I do what I want to do? Whats the right tool to transform every node in the tree? Is there a more general approach to doing stateful transformations? Thanks in advance for letting me waste your time. P.S. any stylistic advice, or alternate approaches also welcome. The objective here is to learn... Tim Newsham http://www.thenewsh.com/~newsham/

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[Parsec] A combinator to match between M and N times?
by Stephane Bortzmeyer 31 Aug '06

31 Aug '06

Parsec provides "count n p" to run the parser p exactly n times. I'm looking for a combinator "countBetween m n p" which will run the parser between m and n times. It does not exist in Parsec. Much to my surprise, it seems quite difficult to write it myself and, until now, I failed (the best result I had was with the "option" combinator, which unfortunately requires a dummy value, returned when the parser fails). Does anyone has a solution? Preferrably one I can understand, which means not yet with liftM :-)

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Re: [Haskell-cafe] monads once again: a newbie perspective
by Andrea Rossato 31 Aug '06

31 Aug '06

Il Thu, Aug 31, 2006 at 02:39:59PM +0100, Simon Peyton-Jones ebbe a scrivere: > Andrea > > Don't forget to link to it from here! > http://haskell.org/haskellwiki/Books_and_tutorials#Using_monads Simon, I'll do. But now the text is far from being complete: there's only the code... (the most difficult part, for me at least!). Thanks for your kind attention. Andrea

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Haskell for hp-ux (ia64)
by Green Bryan - bgreen 31 Aug '06

31 Aug '06

Does anyone know of a binary for hp-ux (ia64)? I have permission from my company to try and go forward with using Haskell for development with my team. But, we are running hp-ux on a 16 processor(ia64) box and I can't find a build for it. B Green ************************************************************************* The information contained in this communication is confidential, is intended only for the use of the recipient named above, and may be legally privileged. If the reader of this message is not the intended recipient, you are hereby notified that any dissemination, distribution or copying of this communication is strictly prohibited. If you have received this communication in error, please resend this communication to the sender and delete the original message or any copy of it from your computer system. Thank you. *************************************************************************

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Galois Connections seeking a developer
by Isaac Jones 31 Aug '06

31 Aug '06

Galois is seeking a full-time candidate for software development and systems integration in the field of high assurance computing. A successful candidate should have a good understanding of the inner workings of databases, good development skills in a number of languages, including at least one functional language (preferably Haskell), and web development. The candidate should have excellent Linux and Unix skills. If the candidate does not know Haskell, they should be good at learning new programming languages, and can reasonably expect to be fluent in Haskell within a few months. Tasks: * Database analysis * Python and PHP web development * Learning and adapting Linux-related technologies including Xen, SELinux, and Knoppix * Creating or modifying Debian packages * Haskell development Knowledge: * Databases implementation internals * Web development, Services-Oriented Architectures * Fluent in Haskell or other functional languages * Grounding in computer security * XML * Linux and Unix Nice-to-have: * Extreme Programming (XP) development experience * Experience deploying software products * Open source software development experience * Ability to get US security clearance Education: * Masters degree or equivilent experience Please respond with a resume and a cover letter explaining your fit to jobs(a)galois.com. Feel free to forward to interested parties. peace, isaac

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haskell -> fpga
by Matthew Pocock 30 Aug '06

30 Aug '06

Hi, Does anybody know of any compilers that can compile portions of a haskell program to an FPGA? I have found a few domain-specific languages implemented in haskell for FPGAs, but so far no back-ends for compilers like ghc. Thanks, Matthew

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monadplus as monoid, safe?
by Nicolas Frisby 30 Aug '06

30 Aug '06

I'm using the following code in some of my projects: > newtype MonadPlusAsMonoid a = MPM { unMPM :: a } > instance MonadPlus m => Monoid (MonadPlusAsMonoid (m a)) where > mempty = MPM mzero > MPM l `mappend` MPM r = MPM (l `mplus` r) Is there some sort of pitfall I'm not considering? It seems fine to me, but I ask because I couldn't find this instance in the darcs repo for the next base library package. I expected to find it there because it has similar instances for numbers as monoids (e.g. Sum and Product from Data.Monoid). Thanks, Nick

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How to force HTML (rather than XML) output from Haskell XML Toolbox?
by Dimitry Golubovsky 30 Aug '06

30 Aug '06

Hi, I am developing a code that reads a HTML page (template), modifies it (adds some stuff) and then writes the output file which is to be HTML, not XML. I use hxt-5.3 (but I may upgrade if necessary). It is desired to preserve the original HTML template pieces maximally close to the original. But output appears to be closer to XML rather than HTML. The code I use: ==================== let attrs = [(a_parse_html, "1"), (a_output_xml, "0"), (a_encoding, utf8)] (res, err, rc) <- getXmlDocument attrs (fromJust $ tmplPath dopt) putXmlDocument attrs (fromJust $ outPath dopt) res ===================== for this input HTML file: ===================== <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <title>Empty Page Template for Haskell Applications</title> <meta http-equiv="content-type" content="text/html; charset=UTF-8"> </head> <body> </body> </html> ===================== outputs: ===================== <html> <head> <title>Empty Page Template for Haskell Applications</title> <meta http-equiv="content-type" content="text/html; charset=UTF-8"/> </head> <body> </body> </html> ===================== which is close but not exactly as wanted. The <meta> (and I expect other tags like <br> will have the same) tag has closing slash as it would have in XML (XHTML), but not in HTML. Also the DOCTYPE has been lost. Is this something hxt is not able to provide, or just a bug fixed since 5.3? Or should I write my own output code (because I am guessing, hxt functionality regarding manipulation over XML trees is sufficient for what I need)? Or should I use another package? -- Dimitry Golubovsky Anywhere on the Web

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