September 2013 - Beginners

hidden packages and ambiguous modules
by Dimitri Hendriks 06 Sep '13

06 Sep '13

Hi, I have ghc version 7.6.3. I am trying to compile some code (not mine) with $ ghc --make Main and get the message: Could not find module `Time' It is a member of the hidden package `haskell98-2.0.0.2'. Then I tried: $ ghc -package haskell98-2.0.0.2 --make Main resulting in Ambiguous module name `Prelude': it was found in multiple packages: base haskell98-2.0.0.2 At this point I am stuck. How do I load the right packages without introducing ambiguities? Thanks! Greetings, Dimitri

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Haskell exectuable size
by Nathan Hüsken 03 Sep '13

03 Sep '13

Hey, I have written a small wxHaskell application. Since wxWidgets seems to be required to be linked dynamicly, I have to add the wxWidgets dlls. Those make a total of about 25MB, wow. Now the compiled exe (compiled with ghc -O2) has size 15MB. In total I am at about 50Mb, which is just to much. Why is the exe so big? Has anyone a suggestion on how to reduce the total size? Thanks! Nathan

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Detect system language
by Nathan Hüsken 03 Sep '13

03 Sep '13

Hey, Is there a cross plarform way to detect the system language during runtime? Seems like something simple, but my googling failed. Regards, Nathan

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getting cabal to work under windows 8
by Nathan Hüsken 03 Sep '13

03 Sep '13

Hey, I have installed haskell-plarform 2013.2.0.0 on windows 8. I have MinGW + Msys isntalled. I am trying to install some package with cabal. So I do: cabal install netwire in the msys console. Now I get: cabal.exe: The program ghc-pkg is required but the version of c:\Program Files (x86)\Haskell Platform\2013.2.0.0\bin\ghc-pkg.exe could not be determined. ghc-pkg.exe exists at the given position. What am I missing? Thanks! Nathan

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typo in tutorial?
by Jason Sachs 02 Sep '13

02 Sep '13

is there a typo in the tutorial? http://www.haskell.org/tutorial/io.html sequence_ :: [IO ()] -> IO () sequence_ [] = return () sequence_ (a:as) = do a sequence as Shouldn't that last line be the following? (if so could someone please report it? i don't know where to report typos/bugs in the tutorial) sequence_ (a:as) = do a sequence_ as

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Pattern match(es) are overlapped ... but I do not see that they do
by Nathan Hüsken 01 Sep '13

01 Sep '13

Hey, I have the following code (which uses wxHaskell): import Graphics.UI.WX import Graphics.UI.WX.Controls import Graphics.UI.WXCore import Graphics.UI.WXCore.Events (...) dealWithUnsavedChanges :: Var ProgramState -> TextCtrl a -> IO Bool dealWithUnsavedChanges state tc = do ProgramState unsavedChanges _ <- varGet state if not unsavedChanges then return True else do res <- messageDialog tc "Unsaved changes ..." "You have unsaved changes, do you want to save them?" (wxYES_NO .+. wxCANCEL .+. wxICON_EXCLAMATION) case res of wxID_CANCEL -> return False wxID_NO -> return True wxID_YES -> do onSave state tc -- check if saving worked ProgramState newUnsavedChanges _ <- varGet state return (not newUnsavedChanges) When I compile it, I get: Main.hs:130:5: Warning: Pattern match(es) are overlapped In a case alternative: wxID_NO -> ... wxID_YES -> ... which is strange. I checked, wxID_NO is defined as 5104 and wxID_YES as 5103. So they are not overlapping. On the other hand, when I replace the wxID_NO/YES/CANCEL with the actual values, the warning vanishes. Any Idea what this could be? Thanks! Nathan

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Early return in IO monad
by Nathan Hüsken 01 Sep '13

01 Sep '13

Hey, Is it somehow possible to return "early" in a do block of the IO monad? The eqivalent do in C: void doBlock() { if (some preCondition) { return; } ... } ??? Thanks! Nathan

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