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type ambiguity confusion
by Thomas 15 Jun '10

15 Jun '10

Hello all! This is a simplified example: import Control.Exception testHandle :: Integer -> IO (Maybe Integer) testHandle i = handle (\_ -> return Nothing) (return (Just i)) It gives me an "ambiguous type variable" arising from the use of "handle". Ok, I understand that the type variable is ambiguous. And in fact I can resolve this with either wrapping it in a specialised handler: import Control.Exception handleIO :: (IOException -> IO a) -> IO a -> IO a handleIO ec nc = handle ec nc testHandle :: Integer -> IO (Maybe Integer) testHandle i = handleIO (\_ -> return Nothing) (return (Just i)) or resolving it directly: {-# LANGUAGE ScopedTypeVariables #-} import Control.Exception testHandle :: Integer -> IO (Maybe Integer) testHandle i = handle (\ (_ :: IOException) -> return Nothing) (return (Just i)) However: Why does the ambiguity matter in the first place? The corresponding value is never used (\_ -> return Nothing). And there's another doubt: I got this example from a book (Real World Haskell, chapter 9) where actually the first version is used, without resolving the ambiguity in any way. And it does not show up in the errata list. So there's likely something I'm still missing about this issue?!?! Any hints would be really appreciated! Thanks in advance. Thomas

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Meaning of the ! operator
by Nathan Huesken 14 Jun '10

14 Jun '10

Hi, I have often seen in haskell code a "!" in front of variables. In example: data BrickState = Live | Dying !GLfloat deriving (Eq,Show) I never read the meaning of the "!", what does it do? Thanks! Nathan

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hPutChar: invalid argument (Invalid or incomplete multibyte or wide character
by Erik de Castro Lopo 13 Jun '10

13 Jun '10

HI all, I've managed to use the Curl bindings to pull down a web page, and I'm using TagSoup to parse it, but when I try to print the text in a TagText I get hPutChar: invalid argument (Invalid or incomplete multibyte or wide character) The code looks like: parsePage :: String -> IO () parsePage page = do let tags = map deTag $ filter isTagText $ parseTags page mapM_ putStrLn tags where deTag (TagText s) = s deTag x = error $ "Bad Tag '" ++ show x ++ "' in deTag." This is with ghc-6.12.1 on Debian Linux. Any clues appreciated. Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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one element tuple
by bitur mail 13 Jun '10

13 Jun '10

Hello, This is a dumb question but ... I'm just starting with the book Real World Haskell, and in chapter 2 it mentions that "Haskell doesn't have a notion of a one-element tuple". I am confused by this because I type, for example, (143), into the interpreter and it does not complain. What does Haskell consider (143) to be? Thanks :) DG

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Compiling afrob from Yampa
by Nathan Huesken 13 Jun '10

13 Jun '10

Hi, I am trying to learn yampa and downloaded the package, also containing afrob from the homepage. I installed arrows and HGL over cabal. Now I go into the afrob directory and type make, I get: make -C src make[1]: Entering directory `/home/ls/tmp/Yampa-0.9.1/afrob-0.2/src' make -C Common make[2]: Entering directory `/home/ls/tmp/Yampa-0.9.1/afrob-0.2/src/Common' ghc -M -optdep-f -optdep.hs_dependences -optdep--exclude-directory=/usr/local/lib/ghc/imports/arrow:/usr/local/lib/ghc/imports/afrp -fglasgow-exts -O -package lang -i/usr/local/lib/ghc/imports/arrow:/usr/local/lib/ghc/imports/afrp AFrob.hs AFrobDiagnostics.hs AFrobPhysicalDimensions.hs AFrobRobotIO.hs AFrobRobotSimFake.hs || (rm -f .hs_dependences ; exit 1) on the commandline: Warning: -optdep-f is deprecated: Use -dep-makefile instead <command line>: unknown package: lang ghc -c -fglasgow-exts -O -package lang -i/usr/local/lib/ghc/imports/arrow:/usr/local/lib/ghc/imports/afrp -o AFrob.o AFrob.hs <command line>: unknown package: lang make[2]: *** [AFrob.o] Error 1 make[2]: Leaving directory `/home/ls/tmp/Yampa-0.9.1/afrob-0.2/src/Common' make[1]: *** [Common] Error 2 make[1]: Leaving directory `/home/ls/tmp/Yampa-0.9.1/afrob-0.2/src' make: *** [src] Error 2 So I suppose I am missing the "lang" package ?!? How do I get it? Thanks! Nathan

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Yampa, elerea, Peakachu ...
by Nathan Huesken 12 Jun '10

12 Jun '10

Hi, I want to have a look into into FRP for educational purpose and maybe to make games with it (I do not know yet how far I will go). I started to look at yampa (becuase I found that FRAG is using it), and than found out that there is also elerea and Peakachu. And maybe there are more which I just did not find ... Now I am wondering which would be best suited to look into. I did not find enough information on the web to make an informed decision :(. Which would choose for making games? Thanks! Nathan

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DPH
by Luca Ciciriello 10 Jun '10

10 Jun '10

Thanks Stephen and Daniel. Your explanation is clear to me. Luca. _________________________________________________________________ http://clk.atdmt.com/UKM/go/197222280/direct/01/ Do you have a story that started on Hotmail? Tell us now

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DPH
by Luca Ciciriello 10 Jun '10

10 Jun '10

Hi All. I've two question about Data Parallel Haskell. I'm trying some experiment using DPH and I've wrote a little program using the functions lengthP and mapP. I've imported the module GHC.PArr and I've informed the compiler that I want to use some language extension using the line {-# LANGUAGE PArr #-}. All works fine. Looking around I've saw that some code examples use {-# LANGUAGE PArr, ParallelListComp #-} instead. The my first question is: Which is the difference in using or not the extension ParallelListComp? Seems that my program works fine with or without this extension (if I omit the extension PArr I've some compilation errors). The second question is: Have I to use the option -threaded when compile my program? I'm using Haskell Platform (GHC 6.12.1) on MacOS X 10.6.3 Thanks in advance for any answer. Luca. _________________________________________________________________ http://clk.atdmt.com/UKM/go/197222280/direct/01/ Do you have a story that started on Hotmail? Tell us now

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Compiling Test.hs from reactive-fieldtrip
by Nathan Huesken 09 Jun '10

09 Jun '10

Hi, I am trying to compile the Test.hs form reactive-fieltrip. I did: ghc --make Test (22:09) [1 of 3] Compiling FRP.Reactive.FieldTrip.Adapter ( FRP/Reactive/FieldTrip/Adapter.hs, FRP/Reactive/FieldTrip/Adapter.o ) [2 of 3] Compiling FRP.Reactive.FieldTrip ( FRP/Reactive/FieldTrip.hs, FRP/Reactive/FieldTrip.o ) [3 of 3] Compiling Test ( Test.hs, Test.o ) Wonderfull, only it is not giving me an executable, only Test.o. It has a main function, so how can I compile it? Thanks! nathan

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profiling weirdness?
by Thomas 09 Jun '10

09 Jun '10

Hello all! Currently this is more of a curiosity for me rather than a real obstacle to learn Haskell, but I'd appreciate any insight into this behaviour nevertheless. Given the (dumb, I know, but it's for illustration purpose) program: --- import Data.Char type Position = (Char, Integer) sameSquare :: Position -> Position -> Bool sameSquare p1 p2 = ssqHelper p1 p2 ssqHelper :: Position -> Position -> Bool ssqHelper (r1, c1) (r2, c2) | (((c1-1) `div` 3) == ((c2-1) `div` 3) && ((((ord r1) - 65) `div` 3) == (((ord r2) - 65) `div` 3))) = True | otherwise = False main :: IO () main = print (sameSquare ('A', 2) ('B', 2)) --- If I compile this with profiling info and run the profiler all seems well (independently of optimization settings). Now, if I comment the line --ssqHelper :: Position -> Position -> Bool and do the same (compile with profiling info) I can see rather surprising results. While I'd expect exactly one call to ssqHelper for every call to sameSquare this is actually only the case if compiled without optimizations. Using -O2 when compiling I get a full 5 calls of ssqHelper for every call to sameSquare. I am unable to imagine an optimization that would be more efficient with five calls instead of one. And I do not understand either what this has to do with the type annotation. Within such a trivial program the inferred type should be equivalent, shouldn't it? Actually ghci generalizes slightly: Integral instead of Integer. But that's no clue for me either... BTW I'm using ghc 6.10.4. (In case it matters.) Any hint would be appreciated. Thanks, Thomas

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