August 2013 - Beginners

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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How can I keep delimiters in splitRegex?
by S. H. Aegis 29 Aug '13

29 Aug '13

Hi. I'm newbee to Haskell. I want to keep delimiters in splitRegex like 'keepDelimsR'. I try... splitRegex (keepDelimsR (mkRegex "2013[0-9]{11}AH02")) myDataString but It doesn't work. How can I keep delimiters in splitRegex? Have a nice day. Thank you. S. Chang.

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multi-parameter typeclass with default implementation
by TP 29 Aug '13

29 Aug '13

Hi, I struggle with a dummy example using a multi-parameter typeclass containing a default implementation for a function: --------------------------------------- {-# LANGUAGE MultiParamTypeClasses #-} class Foo a b where bar :: a -> Int foobar :: a -> b -> Int foobar avalue bvalue = bar avalue instance Foo Int Int where bar i = 5 main = do print $ bar (4::Int) --------------------------------------- I obtain the following errors. I have tried various things without any success. Any help appreciated! Thanks TP PS: The errors: $ runghc test.hs test.hs:8:28: Could not deduce (Foo a b1) arising from a use of `bar' from the context (Foo a b) bound by the class declaration for `Foo' at test.hs:(3,1)-(8,37) The type variable `b1' is ambiguous Possible fix: add a type signature that fixes these type variable(s) In the expression: bar avalue In an equation for `foobar': foobar avalue bvalue = bar avalue test.hs:16:9: No instance for (Foo Int b0) arising from a use of `bar' The type variable `b0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there is a potential instance available: instance Foo Int Int -- Defined at test.hs:10:10 Possible fix: add an instance declaration for (Foo Int b0) In the second argument of `($)', namely `bar (4 :: Int)' In a stmt of a 'do' block: print $ bar (4 :: Int) In the expression: do { print $ bar (4 :: Int) }

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Haskell mentors/tutors website?
by Kẏra 29 Aug '13

29 Aug '13

Is there an existing website to connect haskell mentors and mentees? I'm thinking of something along the lines of railsmentors (except, of course, with haskell support). I suppose it doesn't have to be Haskell specific, as long as it's a supported language. Codementor.io <http://www.codementor.io/?rid=4506073984>seems like a really cool option along those lines, but it would also be valuable to have a network for volunteer mentors. The closest I have seen is Haskellers, which is not about mentorship, but instead only about hiring and job hunting. P.S. Are there any LGBTQIA/female/disabled people of color on this list who would be willing to teach myself and other folks from the Empowermentors Collective? http://kxra.tumblr.com/post/54139122042/please-signal-boost-the-empowerment… Thanks! Kẏra -- Board of Directors, Free Culture Foundation: www.freeculture.org Web: kxra.info - StatusNet Microblog: http://identi.ca/kxra Email: kxra(a)freeculture.org - SMS: +1.617.340.3661 Jabber/XMPP: kxra(a)riseup.net - IRC: kxra @freenode @oftc @indymedia

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print []
by Lukas Lehner 27 Aug '13

27 Aug '13

Hello, This code runs ok http://lpaste.net/91814 but only thanks to enforcing [()] on line 12. In GHCI *Main> :t (List []) (List []) :: NestedList a and *Main> :t flatten (List []) flatten (List []) :: [a] That means ghc cannot infer the type. Is there a way how to # print flatten (List []) ? Or even more general, print [] without enforcing the type? Thank you Lukas

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Lifting (Either SqlError a) in a monad stack
by Bryan Vicknair 26 Aug '13

26 Aug '13

I'm at the beginning of the monad transformers journey, and this is stumping me. I've read "Monad Transformers Step by Step" which made me confident enough to play with simple stacks, but I'm a bit lost in the following example from work. Please excuse any incorrect transformer terminology. In a WAI web app, I want to chain these two actions inside a monad stack: > safeConnect :: ConnectInfo -> IO (Either SqlError Connection) > safeInsert :: Thing -> Connection -> IO (Either SqlError Id) Here is an example where I would like 'result' to be (Right Id) only if the DB connection *and* the insert were successful, (Left SqlError) otherwise. > add :: Request -> ResourceT IO Response > add _ = do > result <- liftIO $ safeConnect devConnInfo >>= safeInsert thing > case result of > (Left e) -> return $ dbErr e > (Right _) -> return $ postRedirect > where thing = exampleThing The compiler tells me (edited for brevity): Expected type: Either SqlError Connection -> GHC.Types.IO a0 Actual type: Connection -> GHC.Types.IO (Either SqlError Id) In the return type of a call of `safeInsert' In the second argument of `(>>=)', namely `safeInsert thing' How can I chain safeConnect and safeInsert using the (Either SqlError a) monad inside WAI's (ResourceT IO Response) monad stack?

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fonction in a typeclass that does not mention the type variable
by TP 21 Aug '13

21 Aug '13

Hi, I have a simple test case containing a typeclass Foo with one type variable a. The goal is to write once and for all a function symbolToInfixLevel function that combines two other functions defined in the typeclass: ----------------------- class Foo a where symbolToConstructor :: String -> ( a -> a -> a ) infixLevel :: a -> Int symbolToInfixLevel :: String -> Int symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined undefined ----------------------- This yields an error because there is no "a" in the type signature for symbolToInfixLevel: $ runghc test_typeclass_without_typevariable.hs test_typeclass_without_typevariable.hs:1:1: The class method `symbolToInfixLevel' mentions none of the type variables of the class Foo a When checking the class method: symbolToInfixLevel :: String -> Int In the class declaration for `Foo' Now, if I define symbolToInfixLevel out of the typeclass: ----------------------- class Foo a where symbolToConstructor :: String -> ( a -> a -> a ) infixLevel :: a -> Int symbolToInfixLevel :: String -> Int symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined undefined ----------------------- Now, I obtain: $ runghc test_typeclass_without_typevariable.hs test_typeclass_without_typevariable.hs:7:24: No instance for (Foo a0) arising from a use of `infixLevel' In the expression: infixLevel In the expression: infixLevel $ (symbolToConstructor s) undefined undefined In an equation for `symbolToInfixLevel': symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined undefined How to get rid from this situation? Thanks in advance, TP

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cabal package haskore-vintage fails to build
by Dimitri Hendriks 21 Aug '13

21 Aug '13

Hi all, I'm new to this list, and know very little about haskell. I am trying to install the package haskore-vintage on mac osx version 10.6.8, using cabal, but this fails; see the log below. I have ghc version 7.6.3, and cabal-install version 1.16.0.2. ------------------------------------------------------------------------------ $ cabal install haskore-vintage Resolving dependencies... Configuring haskore-vintage-0.1... Building haskore-vintage-0.1... Preprocessing library haskore-vintage-0.1... [ 1 of 16] Compiling Haskore.Monads ( src/Haskore/Monads.hs, dist/build/Haskore/Monads.o ) [ 2 of 16] Compiling Haskore.Utils ( src/Haskore/Utils.hs, dist/build/Haskore/Utils.o ) src/Haskore/Utils.hs:87:23: Not in scope: `catch' src/Haskore/Utils.hs:93:23: Not in scope: `catch' Failed to install haskore-vintage-0.1 cabal: Error: some packages failed to install: haskore-vintage-0.1 failed during the building phase. The exception was: ExitFailure 1 $ ------------------------------------------------------------------------------ Does anyone know how to resolve this? Related problems I found: https://code.google.com/p/leksah/issues/detail?id=272 (no solution given). And here: https://github.com/haskell/cabal/issues/1137 it appears they think it is a combination of Cabal 1.10 / cabal-install 0.10 and GHC 7.6 the causes the problem with "catch", but I use cabal 1.16. Many thanks in advance, Dimitri

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How Haskell Fits Into an Operating System / API Environment
by Philippe Sismondi 20 Aug '13

20 Aug '13

I am interested in tracking down discussions on how Haskell fits into an operating system environment. That is probably not a very clear question, so let me explain. One of the great obstacles I have faced in using two of my favourite languages (Common Lisp and Haskell) for anything practical is that I am constantly frustrated by their relationship with the host OS, or perhaps more accurately with the APIs provided by the OS. For example, I tend to program mostly on OS X. I have a reasonable knowledge of Cocoa, and can get quite a lot done with Objective-C. So, if I want to use Haskell on OS X, I can run off and learn about Haskell's FFI, or some hand-rolled widget package, etc. But here is the problem: Cocoa et al provide an entire ecosystem that includes data structures, error handling, and so on. So, when trying to use Haskell with a GUI, or perhaps to do some audio processing, or other things that are provided by the OS X APIs, I feel that I am layering two quite incompatible worlds together: Haskell and its host environment. I ran into the same dilemma with Common Lisp. Clozure Common Lisp provides an excellent Cocoa binding. But using it feels (to me) like communications between alien races. The OS APIs want to do things the Objective-C way, and that is (clearly) not the Haskell or Lisp way. One soon notices that books on Haskell pay virtually no attention to this issue. At least, the ones I have don't. Maybe what I am really experiencing is that OS X just "wants" to be programmed in its native language. I had something of a better feeling about using F# on Windows, because that architecture does leverage the native stuff much better. The upshot is that I remain skeptical that Haskell is actually a practical language for many kinds of development. Can anyone point me to discussions on this? I may be having a dimwitted interval here, but I don't know what to search to find this. - P -

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