August 2011 - Beginners

QuickCheck with monadic IO
by Hartmut 13 Aug '11

13 Aug '11

Everyone, I want to run an QuickCheck on "a most simple" IO monad. When I compile and run :main, it says: ....................................................................................................+++ OK, passed 100 tests. Well, thats fine, since *1 mapped on any Int gives the same Int back. But it lasts 10 seconds (!) for doing those 100 Tests. Something must be wrong?! I'd appreciate your ideas. Hartmut module MonadicQuickCheck where import IO import Random import Test.QuickCheck import Test.QuickCheck.Monadic main = do quickCheck x x :: Property x = monadicIO $ do a <- pick arbitrary pre $ not(null a) b <- run $ testfunction a assert $ b == a testfunction :: [Int] -> IO [Int] testfunction x = do putStr "." return $ map (*1) x

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Relationship between graphs and queues?
by Christopher Howard 12 Aug '11

12 Aug '11

I working through the practice problems in Davie's book (by myself) and there is one that is not quite clear to me: "Construct an abstract data type for a queue. Attempt to supply an implementation (hidden from the user of the type) which allows access to the head of the queue and the construction of a new queue by adding elements to the end of an existing one. Both these interface functions should operate in time independent of the length of the queue. What implications does the this exercise suggest for search and 'updating' (by copying all unchanged nodes) a general graph?" There are two parts to the problem here: 1) defining the data type and the interface functions, and 2) indicating the implications for general graphs. On the first part I'm a little unclear because I am not sure what he means by "allows access to the head of the queue"; i.e., does he mean to just get the value in the head of the queue, or pop the value off the head of the queue and return the new queue? If the former, a solution is not hard, but if the latter, then I am not sure how to solve this without somehow transversing the queue, which of course is not a time-independent activity. (Even in the implementation for a queue in the Data.Graph.Inductive.Internal.Queue, eventually a "reverse" call must be made.) Anyway, maybe if someone could explain the relationship between this problem and graphs, I'll see the bigger picture and have a clearer view of the problem. -- frigidcode.com theologia.indicium.us

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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.

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

11 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

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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

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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.

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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

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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.

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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.

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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 > >

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