- Haskell-Cafe - Haskell.org

TVars & throw
by Thomas Conway 08 Mar '07

08 Mar '07

Hi All, Consider the following: foo = do v <- newTVar "hi there!" throwDyn v main = do catchDyn (atomically foo) \v -> do x <- atomically (readTVar v) putStr x I.e. throw information that gets rolled back from inside a transaction, catch it and use it. This looks like bad. I assume it actually works, but should it? T. -- Dr Thomas Conway You are beautiful; but learn to work, drtomc(a)gmail.com for you cannot eat your beauty. -- Congo proverb

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lhs2TeX
by Thomas Conway 07 Mar '07

07 Mar '07

Hi all, I noticed that in the polycode mode, lhs2TeX produces odd looking output for hex constants. E.g. \begin{code} wibble = 0x7F \end{code} It produces a nice little gap between the 0 and the x. Inspecting the latex source, it even goes to some effort to do so: \mathrm{0}\;\Varid{x7F} I couldn't see anything in the manual to show me how. Anyone? -- Dr Thomas Conway You are beautiful; but learn to work, drtomc(a)gmail.com for you cannot eat your beauty. -- Congo proverb

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Usage of . and $
by Jefferson Heard 07 Mar '07

07 Mar '07

Usually, I can do this, but today, my brain is weak, and I'm just trying to get this piece of code out the door. My code looks like this: weight = sum (IntMap.elems (IntMap.intersectionWith (\x y -> x*y) queryVector rationalProjection)) I know that this will work (ignoring indentation): sum $ IntMap.elems $ IntMap.intersectionWith (\x y -> x*y) queryVector rationalProjection But why won't this?: sum . IntMap.elems . IntMap.IntersectionWith ... Is there a difference between the "elegance" of function composition versus application?

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MPTCs and rigid variables
by David House 07 Mar '07

07 Mar '07

class Foo a b | a -> b instance Foo Int String bar :: Foo Int b => b bar = "rargh" Is there any reason why that shouldn't work? GHC gives one of its silly "b is a rigid variable" errors (aside: that's a really confusing error; I'd prefer something like Hugs's "Infered type is not general enough"). -- -David House, dmhouse(a)gmail.com

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ANN: Bugfixes for regex-posix and regex-pcre ByteString use
by Chris Kuklewicz 07 Mar '07

07 Mar '07

I got a bug report from the #haskell IRC channel last night that empty ByteStrings were broken. The new bugfixed versions of the affected packages are: regex-posix-0.92 regex-posix-0.72 regex-pcre-0.92 regex-pcre-0.81 All the above have been uploaded to http://hackage.haskell.org/ and are available via darcs (see below). Details: regex-posix-0.92 replaces the "unstable" regex-posix-0.91 that I announced yesterday. The bug fix also applied to ByteString.Lazy repository: http://darcs.haskell.org/packages/regex-unstable/regex-posix/ regex-posix-0.72 replaces the regex-posix-0.71 that came with GHC 6.6 repository: http://darcs.haskell.org/packages/regex-posix/ The repository version is setup to compile alongside GHC in some way that I cannot easily package, so the tarball on hackage has edited Wrap.hsc and cabal files. regex-pcre-0.92 replaces the "unstable" regex-pcre-0.91 that I announced yesterday. The bug fix also applied to ByteString.Lazy repository: http://darcs.haskell.org/packages/regex-unstable/regex-pcre/ regex-pcre-0.81 replaces the "stable" regex-pcre-0.80 that is compatible with the regex-base-0.71 supplied with GHC or regex-base-0.72 that I announced yesterday. repository: http://darcs.haskell.org/packages/regex-pcre/

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LGPL libraries
by Alex Queiroz 07 Mar '07

07 Mar '07

Hallo, Gtk2Hs and HDBC are both LGPL licensed, but aren't they always static linked? Is there a way to use them in closed-source programs? Cheers, -- -alex http://www.ventonegro.org/

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Haskell File reading
by cornmouse 07 Mar '07

07 Mar '07

I have a txt file, which contains a paragraph. I am trying to read the file, and pass the contents of the file as a string to another function called "createIndex". "createIndex" is a function to generate index of the input string. Below is my code to read the file : main :: IO () main = do putStr "Enter input file name.txt: " filename <- getLine content <- readFile filename createIndex content createIndex :: String -> [ ([Int], String) ] createIndex {- contents of the function... -} I am using Hugs compiler, when i execute, i got an error Type error in generator *** Term : showString content *** Type : [Char] -> [Char] *** Does not match : IO a IO> I know the data type doesn't match. How do i go about this?

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Re: [Haskell] Haddock not being able to read interface file
by Simon Marlow 07 Mar '07

07 Mar '07

Wolfgang Jeltsch wrote: > Am Dienstag, 6. März 2007 12:09 schrieben Sie: >> Wolfgang Jeltsch wrote: >> […] > >>> However, Haddock outputs the following: >>> >>> haddock: Warning: The interface file "base.haddock" could not be >>> read. Maybe it's from a later version of Haddock? >>> >>> This happens with Haddock 0.8 as well as the current development version >>> of Haddock. >> It's possible the interface files are not portable between 32 and 64-bit >> machines, and you're using one generated for a different word size. I >> haven't verified this, but I'll make a note to check for the next version >> of Haddock. I imagine we'll move to using Data.Binary anyway, which should >> fix any non-portability issues. > > Hello Simon, > > I use a 64 bit machine. Was base.haddock on the Haskell website created on a > 32 bit machine? I don't honestly know, it could have been either. Cheers, Simon

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Haskell Cookbook?
by Alexy Khrabrov 07 Mar '07

07 Mar '07

So I'm picking up Haskell bit by bit, and I found the code examples transpiring here most useful. Reflecting why it's harder to pick up Haskell than say Ruby or Python, here's what I found -- those languages deal with a typical domain available to any programmer -- his own computer/system/shell. The artifacts are files, directories, timestamps, etc. The stuff every programmer understands in their sleep. APIs. So I loved the shell-script beautification thread. That's how I learn about new modules and their functions -- as use cases. I also found some pieces of Haskell strewn around people's websites, blogs, forums; even at paste.lisp.org. I'm gathering them for my own purpose and trying to compile and run them. Wouldn't it be nice -- in case it doesn't exist already -- a[n O'Reilly[-like]] Haskell Cookbook? That would be the best way to learn Haskell. I've found a wikibook on Haskell, but I look for a big bag of small, self-contained programs. Perhaps you esteemed veterans can dig your small scripts and paste them into a wiki? Examples needed -- how to connect to a database; to a web service (e.g. Amazon); read a csv file; represent data equivalent to a directory listing; a text file/XML; etc... Cheers, Alexy

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Programming performance
by Tim Newsham 06 Mar '07

06 Mar '07

I'm interested in understanding how fast people can program small utility and experiment type programs in haskell. I'm also interested in seeing what types of solutions people who are experienced and code quickly end up with. Towards this end I set up a small informal experiment on the wiki: http://www.haskell.org/haskellwiki/Programming_performance Hopefully the problem is small, fun and easy without being too unrealistic. and a question about the wiki/page: is there a simple way to host the input data on the wiki? Tim Newsham http://www.thenewsh.com/~newsham/

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