March 2006 - Haskell-Cafe

RE: [Haskell-cafe] Template haskell instance checking
by Simon Peyton-Jones 02 Mar '06

02 Mar '06

Not at the moment, I'm afraid, but it's the kind of question that TH ought to be able to answer. I could offer guidance if someone wanted to implement it. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | heatsink | Sent: 02 March 2006 00:55 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] Template haskell instance checking | | Is there a way, in template haskell, to find out what instance declarations | exist? I'm not trying to accomplish anything specific right now, but I can | see two potential uses for this. One is to emit compile-time error messages | from TH code which would be more informative than the typechecker's output. | The other is to produce TH functions that conditionally generate code | depending on the instances satisfied by their arguments. | | I offer the following as a motivational example. Suppose you want to write | an extensible library to handle files for, e.g., image data: | | class FileFormat a where | formatName :: a -> String | read :: Handle -> IO a | write :: Handle -> a -> IO () | | instance FileFormat TheStandardFormat where .... | | class (FileFormat a, FileFormat b) => Convert a b where | convert :: a -> b | | Now it would be nice to generate instances of (Convert a TheStandardFormat, | Convert TheStandardFormat b) => Convert a b _except_ where there is an | explicit definition for (Convert a b). AFAIK, there's no way to do this at | present, but it seems like something that could be computed in TH. | | --heatsink | | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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module for probability distributions
by Matthias Fischmann 01 Mar '06

01 Mar '06

hei again, (-: I wrote a module for sampling arbitrary probability distribution, so far including normal (gaussian) and uniform. http://www.wiwi.hu-berlin.de/~fis/code/ For those who need something like this: feel free to take it, it's BSD. For those who feel like growing their karma: - There are a few questions in the code (marked with 'XXX'). - There are probably far better way to do this. I am eager to learn. - There is probably a better implementation out there already. Please point me to it. cheers, Matthias

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enforcing strictness on arbitrary subexpressions
by Matthias Fischmann 01 Mar '06

01 Mar '06

hei, I want to force evaluation on an arbitrary expression. What I have found is only strict datatypes (the '!' thing), but not a strictness idiom for arbitrary subexpressions. (I want this for debugging mostly, so if there is a better way to unsafePerformIO something in one piece I would be happy with that, too.) ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ veryLongString = "..." data StrictThingy = StrictThingy ! String main :: IO () main = do hPutStr stdout veryLongString -- lazy veryLongString `seq` hPutStr stdout veryLongString -- still lazy? (StrictThingy veryLongString) `seq` hPutStr stdout veryLongString -- strict (or at least i hope it is) ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Problem with the last line is that it doesn't work for arbitrary types, and it's kind of ugly. What I would like to have is something like: ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ hPutStr stdout ! veryLongString ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ but I guess it's not that easy? Sorry for asking without consulting the list archives, but I promise I'll add it to the faq in the wiki once i understood. (-: I am using ghci and ghc, latest ubuntu package, but I am happy to switch to new versions. thanks, matthias

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Code snippet, a `trace' with line and column numbers
by dons＠cse.unsw.edu.au 01 Mar '06

01 Mar '06

Suggested by a question from sethk on #haskell irc channel. Solves an FAQ where people have often resorted to cpp or m4: a `trace' that prints line numbers. > module Location (trace, assert) where > > import qualified Control.Exception as C (catch) > import System.IO.Unsafe (unsafePerformIO) > import GHC.Base (assert) > import System.IO > > -- An identity function that also prints the current line and column number > trace :: (Bool -> IO () -> IO ()) -> a -> a > trace assrt f = (unsafePerformIO $ C.catch (assrt False $ return ()) printIt) `seq` f > where > printIt e = let (x,_) = break (== ' ') $ show e > in hPutStrLn stderr (x ++ " trace") for example: > import Location > > main = do > let x = trace assert (1+2) > > putStrLn . show $ x Generates: $ ./a.out M.hs:4:18-23: trace 3 This continues a theme I've noticed: catching internal exceptions can yield some interesting results, i.e. with undefined, missing class methods, and here, assertion failures. Hope this little thing is useful. Cheers, Don

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My piece of an IRC bot
by ihope 01 Mar '06

01 Mar '06

Today I started on a simple IRC bot framework thingy. I decided to post the source code here so people can look at it and tell me what the heck I did wrong :-P > module IRC where > > import Control.Monad.State > import System.IO > import Network > > {----------------------}{------------------------------------------------------} > > data IRCMessage = IRCMessage (Maybe String) String [String] > > type IRCBot a = IRCMessage -> State a [IRCMessage] > type IRCConnection a = IRCBot a -> IO () > > ircConnect :: String -> Integer -> a -> IRCConnection a > > ircConnect server port > state bot = withSocketsDo (do > connection <- connectTo server (PortNumber (fromInteger port)) > ircLoop connection state bot) > > ircLoop connection state > bot = do > eof <- hIsEOF connection > if eof then hWaitForInput connection 500 >> return () else do > message <- hGetLine connection > hPutStr connection (fst (runState (bot (ircParseMessage message)) state) >>= > decodeMessage) > ircLoop connection (snd (runState (bot (ircParseMessage message)) state)) > bot > > -- Oh noes. Let's hope we'll never, ever have to read that ;-) > > {-End I/O capable code-}{------------------------------------------------------} > > ircParseMessage x = let rx = filter (/='\r') x > (p:u:us) = words rx in > if (head p) == ':' then IRCMessage (Just p) u > (ircParseParams us) > else IRCMessage Nothing p (ircParseParams (u:us)) > > ircParseParams (x:xs) = if head x == ':' then [unwords (x:xs)] > else x : ircParseParams xs > > decodeMessage (IRCMessage > Nothing command params)= (command ++ ' ' : unwords (init params) ++ ' ' : > (':' : last params)) > > decodeMessage > (IRCMessage (Just > prefix) command params)= ((':' : prefix) ++ ' ' : command ++ ' ' : unwords > (init params) ++ (':' : last params)) > > {-Thus ends the IRCbot-}{------------------------------------------------------} The very simple bot I've been trying to run with it: > module Main where > > import IRC > import Control.Monad.State > > {----------------------}{------------------------------------------------------} > > main = ircConnect "brown.freenode.net" 6667 False bot > > bot _ = State (\x -> if x then ([],True) else ( > [IRCMessage Nothing "NICK" ["EagleBot"], > IRCMessage Nothing "USER" ["EagleBot", "Null", > "Null", "EagleBot"], > IRCMessage Nothing "PRIVMSG" ["NickServ", > "IDENTIFY censored"], > IRCMessage Nothing "JOIN" ["#esoteric"]],True))

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