- Beginners - Haskell.org

pattern x:xs
by Luca Ciciriello 12 Aug '11

12 Aug '11

Is it possible to use the pattern (x:xs) using a parallel array [::]? In other words, is there a way to parallelize the code: myfunc :: [:String:] -> [String] myfunc [::] = [] myfunc (x:xs) = … if I try to compile this I get the error: Couldn't match expected type `[:String:]' with actual type `[t0]' In the pattern: x : xs Is there a syntax in order to use the (x:xs) pattern? Thanks in advance Luca.

3 2

Cannot install cairo with cabal...
by Sean Charles 12 Aug '11

12 Aug '11

flushed as in happy, not like a buffer, with my recent OpenGL and haskell hacking I attempted to install cairo to try rendering a cairo generated image into a GL texture but once again, the sometimes obstreperous cabal is making it hard. <rant> I sometimes think that cabal is the "weak" point for newbies like me accepting Haskell in all its goodness... if you can't cleanly and simply install libraries then ultimately you will walk away for something less obstructive... </rant> Can anybody tell me that this means and how to solve it ? Thanks, Sean. ----- scharles@ubuntu:~/stuff/hackinge/objitsu/trunk/haskell/felt/src$ cabal --version cabal-install version 0.10.2 using version 1.10.2.0 of the Cabal library scharles@ubuntu:~/stuff/hackinge/objitsu/trunk/haskell/felt/src$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.3 scharles@ubuntu:~/stuff/hackinge/objitsu/trunk/haskell/felt/src$ ghci --version The Glorious Glasgow Haskell Compilation System, version 6.12.3 ----- /tmp/cairo-0.12.013023/cairo-0.12.0/Gtk2HsSetup.hs:25:2: warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when building (prefixed by --ghc-option= when using the 'cabal' command) [1 of 2] Compiling Gtk2HsSetup ( /tmp/cairo-0.12.013023/cairo-0.12.0/Gtk2HsSetup.hs, /tmp/cairo-0.12.013023/cairo-0.12.0/dist/setup/Gtk2HsSetup.o ) /tmp/cairo-0.12.013023/cairo-0.12.0/Gtk2HsSetup.hs:201:69: Couldn't match expected type `PackageDBStack' against inferred type `PackageDB' In the sixth argument of `registerPackage', namely `packageDb' In the expression: registerPackage verbosity installedPkgInfo pkg lbi inplace packageDb In a case alternative: _ | modeGenerateRegFile -> die "Generate Reg File not supported" | modeGenerateRegScript -> die "Generate Reg Script not supported" | otherwise -> registerPackage verbosity installedPkgInfo pkg lbi inplace packageDb cabal: Error: some packages failed to install: cairo-0.12.0 failed during the configure step. The exception was: ExitFailure 1

4 10

Why ShowS?
by Christopher Howard 11 Aug '11

11 Aug '11

Small point I'm confused about: Why is it that in the Numeric module, all the "show" functions return a "ShowS" type (String -> String) instead of just the String? The ShowS type is described as so... "The shows functions return a function that prepends the output String to an existing String. This allows constant-time concatenation of results using function composition." What is meant here by "constant-time concatenation of results using function composition"? Perhaps someone could provide an example? -- frigidcode.com theologia.indicium.us

5 8

compilation error..
by Sunil S Nandihalli 11 Aug '11

11 Aug '11

Hello everybody, when I compile the file https://github.com/sunilnandihalli/is2/blob/master/main.hs I get main.hs:28:64: Could not deduce (Enum (a, t0)) arising from the arithmetic sequence `0 .. ' from the context (Integral a) bound by the type signature for plotAsString :: Integral a => [(a, a)] -> (a, a) -> String at main.hs:(26,1)-(38,41) Possible fix: add (Enum (a, t0)) to the context of the type signature for plotAsString :: Integral a => [(a, a)] -> (a, a) -> String or add an instance declaration for (Enum (a, t0)) In the second argument of `zip', namely `[0 .. ]' In the first argument of `M.fromList', namely `(zip locs [0 .. ])' In the expression: M.fromList (zip locs [0 .. ]) main.hs:28:65: Could not deduce (Num (a, t0)) arising from the literal `0' from the context (Integral a) bound by the type signature for plotAsString :: Integral a => [(a, a)] -> (a, a) -> String at main.hs:(26,1)-(38,41) Possible fix: add (Num (a, t0)) to the context of the type signature for plotAsString :: Integral a => [(a, a)] -> (a, a) -> String or add an instance declaration for (Num (a, t0)) In the expression: 0 In the second argument of `zip', namely `[0 .. ]' In the first argument of `M.fromList', namely `(zip locs [0 .. ])' can somebody help? Thanks, Sunil.

2 2

dph
by Luca Ciciriello 11 Aug '11

11 Aug '11

I'm here again. I'm installed from hackage al dph-<something> packages but still I've problems using elemP and foldP (not found). If I import Data.Array.Parallel.PArr the compiler says: Could not find module `Data.Array.Parallel.PArr' it is a hidden module in the package `dph-par' it is a hidden module in the package `dph-seq' if I use before to build the command "ghc-pkg expose dph-par" and "ghc-pkg expose dph-seq" nothing changes. I'm using GHC 7.2.1 64-bit on MacOS X Lion (Xcode 4.1) Please, please, please, I really need help. Luca

1 0

trouble using the foldl ....
by Sunil S Nandihalli 11 Aug '11

11 Aug '11

Hello everybody, When I compile a file containing ... plotAsString::(Integral a)=>[(a,a)]->a->String plotAsString locs ans = let xs = map fst locs ys = map snd locs locsmap = M.fromList (zip locs [0..]) bufsize=10 min' = ((\x -> x-bufsize) . minimum) max' = ((\x -> x+bufsize) . maximum) (bmin@(x0,y0),bmax@(x1,y1)) = ((min' xs,min' ys),(max' xs,max' ys)) plotString = foldl (\curStr newRowId -> (foldl (\ccStr newColId -> (ccStr++(case (M.member (newRowId,newColId) locsmap) of (Just v) -> ("." ++ (show v) ++ ".") (Nothing) -> " "))) curStr [y0..y1])) "" [x0..x1] in plotString I get the following error.. main.hs:34:107: Couldn't match expected type `Bool' with actual type `Maybe t0' In the pattern: Just v In a case alternative: (Just v) -> ("." ++ (show v) ++ ".") In the second argument of `(++)', namely `(case (M.member (newRowId, newColId) locsmap) of { (Just v) -> ("." ++ (show v) ++ ".") (Nothing) -> " " })' can somebody help? Thanks, Sunil.

2 2

Lang extension
by Luca Ciciriello 10 Aug '11

10 Aug '11

Hi All. I've just installed GHC 7.2.1 And I've some problems with my old code. To use the parallel List [: :] in my code I use {-# LANGUAGE PArr #-}. Now the new compiler tells me the the extension PArr is unsupported. If I remove {-# LANGUAGE PArr #-} the sinter [: compiled by GHC. Wich extension I've to use in order to built my code? Thanks in Advance for any answer (I've installed GHC 7.2.1 on MacOS X Lion, Xcode 4.1) Luca.

3 4

Re: [Haskell-beginners] Lang extension (almost solved)
by Luca Ciciriello 10 Aug '11

10 Aug '11

You are right, my mistake, but this don't solve my problem :-))) Luca On Aug 10, 2011, at 5:37 PM, Colin Adams wrote: > You don't import packages, you import modules. > > On 10 August 2011 16:29, Luca Ciciriello <luca_ciciriello(a)hotmail.com> wrote: > Yes, but in the release note of GHC 7.2.1 there is this sentence: > Most of GHC.PArr has been moved into the dph package. Only the [::] datatype remains. > > My question is which are the packages to import to use dph? > > Luca > > On Aug 10, 2011, at 5:21 PM, Daniel Seidel wrote: > >> On Wed, 2011-08-10 at 17:12 +0200, Luca Ciciriello wrote: >>> Ok, I've replaced {-# LANGUAGE PArr #-} with {-# LANGUAGE ParallelArrays #-}. >>> now my problem is that the compiler is unable to find "elemP" and "foldP" >>> Wich package have I to import? >> >> Hayoo (http://holumbus.fh-wedel.de/hayoo/) says: >> >> GHC.PArr.elemP:: a -> [:a:] -> Bool >> >> Cheers, >> >> Daniel. >> >>> >>> Luca. >>> >>> On Aug 10, 2011, at 5:04 PM, Luca Ciciriello wrote: >>> >>>> Hi All. >>>> I've just installed GHC 7.2.1 And I've some problems with my old code. >>>> >>>> To use the parallel List [: :] in my code I use {-# LANGUAGE PArr #-}. Now the new compiler tells me the the extension PArr is unsupported. If I remove {-# LANGUAGE PArr #-} the sinter [: compiled by GHC. Wich extension I've to use in order to built my code? >>>> >>>> Thanks in Advance for any answer (I've installed GHC 7.2.1 on MacOS X Lion, Xcode 4.1) >>>> >>>> Luca. >>>> >>>> >>>> _______________________________________________ >>>> Beginners mailing list >>>> Beginners(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/beginners >>>> >>> >>> >>> _______________________________________________ >>> Beginners mailing list >>> Beginners(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/beginners >> >> > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > >

2 3

New style exceptions...
by Manfred Lotz 10 Aug '11

10 Aug '11

Hi there, Exception handling in RWH is called old-style and deprecated. I didn't find a tutorial telling how to do the new style. Here I created two examples both dealing with an exception when creating a directory. I would like to know if the way I did is ok or not. Improvements resp. corrections are most welcome First way: <--------------snip------------------------------------> {-# LANGUAGE ScopedTypeVariables #-} module Main where import System.Exit import System.Directory import Control.Exception import Prelude hiding ( catch, readFile ) main :: IO () main = do handle (\(e :: SomeException) -> print $ show e) $ do createDirectoryIfMissing True "/tmr/a/b/c/d" _ <- exitWith $ ExitFailure 1 return () print "Directory created successfully" <--------------snap------------------------------------> Second way: <--------------snip------------------------------------> module Main where import System.Exit import System.Directory import Control.Exception import Prelude hiding ( catch, readFile ) main :: IO () main = do catch (createDirectoryIfMissing True "/tmr/a/b/c/d") exhandler print "Directory created successfully" where exhandler :: SomeException -> IO () exhandler e = do print e _ <- exitWith $ ExitFailure 1 return () <--------------snap------------------------------------> -- Tnanks, Manfred

1 1

What is the use of Data.List.Key merge
by informationen 09 Aug '11

09 Aug '11

Hi, what is the merge function from the Data.List.Key module useful for? Could you give me some concrete examples? Maybe one could add these examples to the module description? Kind regards Chris

2 1