July 2007 - Haskell-Cafe

Fwd: [Haskell-cafe] HDBC or HSQL
by George Moschovitis 26 Jul '07

26 Jul '07

Ok, I am evaluating HDBC. I wrote a simple test program: import Database.HDBC import Database.HDBC.PostgreSQL main = do db <- connectPostgreSQL "dbname=Test user=postgres password='mypass'" putStrLn "done" when I run it I get the following error: db: exception :: Database.HDBC.SqlError Can I get a more specific error? Is there a way to inspect this exception more deeply? thanks, -g. -- http://www.me.gr http://phidz.com http://blog.gmosx.com http://cull.gr http://www.joy.gr http://nitroproject.org

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expension of fractions
by Tom Pledger 25 Jul '07

25 Jul '07

Arie Groeneveld wrote: : | Looking at the result of my rewriting gives me the idea it isn't | Haskelly enough. | | Anyways, here's my interpretation: | | -- period m/n base = (period length, preperiod digits, period digits) | period :: Integer -> Integer -> Integer -> (Int, ([Integer], [Integer])) : Haskelliness I'd be inclined to try using a Rational (from Data.Ratio) and the properFraction function, instead of numerators and denominators and gcd. You could do away with the parts where you subscript a linked list, if you did something with elem and zip and span instead of findIndex and splitAt. Instead of translating Nothing to -1 and Just i to i, you could use the result of findIndex directly in a case expression. Correctness *Main> period 8 70 10 (6,([0],[5,7,1,4,2,8])) That should be (6,([1],[1,4,2,8,5,7])). Regards, Tom

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Compile-time here document facility
by Dave Bayer 25 Jul '07

25 Jul '07

I couldn't find a compile-time here document facility, so I wrote one using Template Haskell: > module HereDocs(hereDocs) where > > import Control.Exception > import Language.Haskell.TH.Syntax > > getDoc :: String -> [String] -> (String,[String]) > getDoc eof txt = > let (doc,rest) = break (== eof) txt > in (unlines doc, drop 1 rest) > > makeVal :: String -> String -> [Dec] > makeVal var doc = let name = mkName var in > [SigD name (ConT (mkName "String")), > ValD (VarP name) (NormalB (LitE (StringL doc))) []] > > scanSrc :: [Dec] -> [String] -> Q [Dec] > scanSrc vals [] = return vals > scanSrc vals (x:xs) = case words x of > [var, "=", ('<':'<':eof)] -> > let (doc,rest) = getDoc eof xs > val = makeVal var doc > in scanSrc (vals ++ val) rest > _ -> scanSrc vals xs > > hereDocs :: FilePath -> Q [Dec] > hereDocs src = > let fin = catchJust assertions (evaluate src) (return.takeWhile > (/= ':')) > in runIO (fin >>= readFile >>= return . lines) >>= scanSrc [] One binds here documents embedded in comments by writing > import HereDocs > $(hereDocs "Main.hs") As an idiom, one can refer to the current file as follows; the first thing hereDocs does is catch assert errors in order to learn the file name: > import HereDocs > $(hereDocs $ assert False "") Here is an example use: > {-# OPTIONS_GHC -fth -Wall -Werror #-} > > module Main where > > import System > import Control.Exception > > import HereDocs > $(hereDocs $ assert False "") > > {- > ruby = <<RUBY > #!/usr/bin/env ruby > hello = <<EOF > Ruby is not > an acceptable Lisp > EOF > puts hello > RUBY > > lisp = <<LISP > #!/usr/bin/env mzscheme -qr > (display #<<EOF > Lisp is not > an acceptable Haskell > EOF > ) > (newline) > LISP > -} > > exec :: FilePath -> String -> IO ExitCode > exec fout str = do > writeFile fout str > system ("chmod +x " ++ fout ++ "; ./" ++ fout) > > main :: IO ExitCode > main = do > exec "hello.rb" ruby > exec "hello.scm" lisp

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upgrading Text.Printf
by Benjamin Franksen 25 Jul '07

25 Jul '07

Hi, I am using Text.Printf (package base, ghc-6.6.1) and am missing %X formatting. Checking out the darcs repo of the base package I see that it has already been added there. Is there any way to upgrade to the latest version while keeping ghc-6.6.1? I keep hearing Bulat saying that this is not possible, but I wanted to check anyway. Is there, perhaps, a separate package that contains (a newer version of) Text.Printf? Cheers Ben

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Another analogy
by Andrew Coppin 25 Jul '07

25 Jul '07

From the guy who brought you "data in Haskell is like an undead quantum cat", I present the following: "If programming languages were like vehicles, C would be a Ferrari, C++ would be a Porshe, Java would be a BWM and Haskell would be a hovercraft. It doesn't even have WHEELS! There is no steering wheel, no gearbox, no clutch... it doesn't even have BRAKES!!! It completely turns the rules upside down. I mean, it moves by PUSHING AIR. That's just crazy! It even STEERS by pushing air. It sounds so absurd, it couldn't possibly work... ...oh, but it DOES work. Very well, actually. In fact, a hovercraft can do some things that the others can't. It works on water. It can go sideways. It can REALLY turn on the spot." Insert whitty replies here...

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Printing the function result
by Alexteslin 25 Jul '07

25 Jul '07

Hi, I am going through examples from the textbook and trying them out but some don't work. For example: addNum :: Int -> (Int -> Int) addNum n = addN where addN m = n+m This error message i am getting: ERROR - Cannot find "show" function for: *** Expression : addNum 4 *** Of type : Int -> Int Now, the type of show is a->String, but i need some kind of function to print the function as a result. I tried some input output functions but i don't think that is the right way to follow. Can anyone suggest me where to look? Thank you -- View this message in context: http://www.nabble.com/Printing-the-function-result-tf4145303.html#a11790986 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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NDP documentation
by Andrew Coppin 25 Jul '07

25 Jul '07

Actually, I was just reading through all the Data Parallel Haskell and Nested Data Parallelism documentation. It says in several places that "parallel array comprehensions are available since GHC 6.6, but they are broken; please use the development versions; this will be fixed in GHC 6.6.1". Can anybody clarify - was this actually fixed? (And shouldn't somebody update the Wiki?)

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filterFirst
by Alexteslin 25 Jul '07

25 Jul '07

Hi, first I like to thank all of you guys - it really helps! I am still on a same chapter with higher order functions and this function is also confusing. But before i even define this function i am getting the type error - i don't know why? So i wrote the simpler one like: filterAlpha :: (a -> Bool) -> [a] -> [a] filterAlpha f [] = [] filterAlpha f (x:xs) |f x = x : filterAlpha xs |otherwise = filterAlpha xs and i am getting this error message: Type error in application Expression :filterAlpha xs Type : [b] Dous not match : a -> Bool To even my very little knowledge i think that this should work. What am i doing wrong? Thanks again -- View this message in context: http://www.nabble.com/filterFirst-tf4131377.html#a11749336 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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snoc vs cons
by Eric 25 Jul '07

25 Jul '07

Does anyone know of a good article which discusses snoc vs cons lists? E.

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GHC and GLUT
by Paul L 25 Jul '07

25 Jul '07

I wonder if anybody has experience with the GLUT library that GHC currently supports? I was trying to use freeglut because I need the actionOnWindowClose function. GHC documentation seems to indicate it's possible to do this, but my effort isn't successful. I even replaced the system-wide libglut.so with the new libglut.so that comes with freeglut installation, but GHC still won't recognize it's now freeglut and I still can't use the actionOnWindowClose function. This is on Linux, and I've not tried Windows. Do I need to compile GHC myself in order to enable freeglut support? Any help is greatly appreciately! Regards, Paul L

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