- Haskell-Cafe - Haskell.org

[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 31 Aug '06

31 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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state and exception or types again...
by Andrea Rossato 30 Aug '06

30 Aug '06

Hello! Sorry if I keep bothering, but I'm still trying to understand types and monads. Now I'm trying to create a statefull evaluator, with output and exception, but I'm facing a problem I seem not to be able to conceptually solve. Take the code below. Now, in order to get it run (and try to debug) the Eval_SOI type has a Raise constructor that produces the same type of SOIE. Suppose instead it should be constructing something like Raise "something". Moreover, I wrote a second version of >>=, commented out. This is just to help me illustrate to problem I'm facing. Now, >>= is suppose to return Raise if "m" is matched against Raise (second version commented out). If "m" matches SOIE it must return a SOIE only if "f a" does not returns a Raise (output must be concatenated). I seem not to be able to find a way out. Moreover, I cannot understand if a way out can be possibly found. Something suggests me it could be related to that Raise "something". But my feeling is that functional programming could be something out of the reach of my mind... by the way, I teach Law, so perhaps you'll forgive me...;-) If you can help me to understand this problem all I can promise is that I'll mention your help in the tutorial I'm trying to write on "the monadic way"... that seems to lead me nowhere. Thanks for your kind attention. Andrea the code: data Eval_SOI a = Raise { unPackMSOIandRun :: State -> (a, State, Output) } | SOIE { unPackMSOIandRun :: State -> (a, State, Output) } instance Monad Eval_SOI where return a = SOIE (\s -> (a, s, "")) m >>= f = SOIE (\x -> let (a, y, s1) = unPackMSOIandRun m x in case f a of SOIE nextRun -> let (b, z, s2) = nextRun y in (b, z, s1 ++ s2) Raise e1 -> e1 y --Is it right? ) -- (>>=) m f = case m of -- Raise e -> error "ciao" -- why this is not going to happen? -- SOIE a -> SOIE (\x -> -- let (a, y, s1) = unPackMSOIandRun m x in -- let (b, z, s2) = unPackMSOIandRun (f a) y in -- (b, z, s1 ++ s2)) incSOIstate :: Eval_SOI () incSOIstate = SOIE (\s -> ((), s + 1, "")) print_SOI :: Output -> Eval_SOI () print_SOI x = SOIE (\s -> ((),s, x)) raise x e = Raise (\s -> (x,s,e)) eval_SOI :: Term -> Eval_SOI Int eval_SOI (Con a) = do incSOIstate print_SOI (formatLine (Con a) a) return a eval_SOI (Add t u) = do a <- eval_SOI t b <- eval_SOI u incSOIstate print_SOI (formatLine (Add t u) (a + b)) if (a + b) == 42 then raise (a+b) " = The Ultimate Answer!!" else return (a + b) runEval exp = case eval_SOI exp of Raise a -> a 0 SOIE p -> let (result, state, output) = p 0 in (result,state,output) --"Result = " ++ show result ++ " Recursions = " ++ show state ++ " Output = " ++ output --runEval (Add (Con 10) (Add (Con 28) (Add (Con 40) (Con 2))))

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'Compiling' expression graph into Arrows
by Daniel McAllansmith 30 Aug '06

30 Aug '06

Hello, I have a graph of function applications which I would like to 'compile' into an Arrow, specifically the SF Arrow from Yampa. I'd appreciate any advice on how I might go about this. The graphs, of which there will be many, will be constructed at runtime and will be executed for extended periods of time so execution time will dominate compilation time. The graphs might also need to be passed around and executed 'elsewhere', where 'elsewhere' will almost certainly be the same GHC version but maybe on different architectures. The hand-written 'compilation' in the attached file gives a rough idea of what I start with and what I need to end up with. I could just automate the hand-written process by generating the SFs and arrow-syntax blocks, write them into a file then call GHC... seems a bit cowboyish. Or, write the graph into a template file that has Template Haskell to generate the SFs and arrow-syntax blocks then call GHC... almost as cowboyish? Or, fold the graph up using loop and arr... probably the simplest, once I get rid of my sugar dependency and figure out how to use loop that is ;). Or, maybe I can actually get the expression graph into a GHC api representation somehow and let it work marvels of optimisation on the expression before turning it into an arrow. Any idea on the relative merits of these, or other ideas? Can GHC optimise arrow code much during compilation, or will code folded up at runtime using loop/arr be just as efficient? Could GHC condense the functions of multiple graph nodes into a single function for conversion to an SF arrow? Will the necessity of introducing delays rule out using GHC to simplify the raw expression? Thanks Daniel

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state and exception or types again...
by tpledger＠ihug.co.nz 29 Aug '06

29 Aug '06

Andrea Rossato wrote: > > Now I'm trying to create a statefull evaluator, with output and > exception, but I'm facing a problem I seem not to be able to > conceptually solve. If a computation fails in your monad, do you still want to return a value of the result type? I'd expect not, and hence remove the 'a' from the 'Raise' constructor. data Eval_SOI a = Raise (State -> (State, Output)) | SOIE (State -> (a, State, Output)) The above is very similar to using the monad foundation classes: import Control.Monad.Error import Control.Monad.State hiding (State) import Control.Monad.Writer type Eval_SOI = ErrorT String (StateT State (Writer Output)) ...assuming that you're happy to send back a descriptive String when a computation fails. Have a look at the Control.Monad.Error source code, to see how 'instance Error a => Monad (Either a)' is defined. It's the sort of thing you were trying to do in your 'instance Monad Eval_SOI'. Regards, Tom

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