July 2009 - Beginners

Re: Count how often a menu has been selected
by Bernhard Lehnert 25 Jul '09

25 Jul '09

Felipe wrote: > If you want variables in the impure sense, use IORefs. > > import Data.IORef > > ... > > a <- newIORef 0 > onActivateLeaf menuAddOne $ do > modifyIORef a (+1) > print a > Hi Felipe, this works great except for the "print a" part which I replaced by: b <- readIORef a print b I should now have the tools to try to implement my real life problem. Thank you, Bernhard

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Count how often a menu has been selected
by Bernhard Lehnert 25 Jul '09

25 Jul '09

Hi list, using gtk2hs I would like to count how often a Menu has been selected (well, not really, but it kind of breaks down to this): let a = 0 onActivateLeaf menuAddOne $ do let a = a+1 print a This leads to a stack overflow. Obviously a=a+1 is iterated over and over again. But how can I add something each time the menu is activated? (Actually I want to open a FileDialog and add the contents of a file to a list but for now, counting would be great). Thanks to everyone for reading, Bernhard

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haskell variables - or not
by Duke Normandin 25 Jul '09

25 Jul '09

Hello.... I'm 2 days into Haskell. NOT new to Perl, PHP, M(umps), TCL, newLISP. I'm using "YAHT.pdf" as an intro to Haskell. Here's a quote: [quote] ... if you have a value x, you must not think of x as a register, a memory location or anything else of that nature. x is simply a name, just as Hal is my name. You cannot arbitrarily decide to store a different person in my name any more than you can arbitrarily decide to store a different value in x. This means that code that might look like the following C code is invalid (and has no counterpart) in Haskell: int x = 5; [/quote] So is fair to say that an expression in Haskell like: x = 5 is more like a constant in imperative languages - say Perl? E.g. use constant SECONDS_PER_DAY => 86400; -- duke

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Typeclasses and "inheritance"
by Patrick LeBoutillier 24 Jul '09

24 Jul '09

Hi, I'm playing around with typeclasses and trying to a feel on how you implement "inheritance" (not sure if that's the good word here) in Haskell. I have the following code that doesn't compile: class (Show a) => IPHost a where class (Show a) => IPMask a where class IPAddr a where host :: (IPHost b) => a -> b mask :: (IPMask b) => a -> b showIPAddr :: (IPAddr a) => a -> String showIPAddr a = (show . host $ a) ++ "/" ++ (show . mask $ a) ghci says: Net/IP.hs:23:23: Ambiguous type variable `b' in the constraint: `IPHost b' arising from a use of `host' at Net/IP.hs:23:23-26 Probable fix: add a type signature that fixes these type variable(s) Net/IP.hs:23:51: Ambiguous type variable `b1' in the constraint: `IPMask b1' arising from a use of `mask' at Net/IP.hs:23:51-54 Probable fix: add a type signature that fixes these type variable(s) What exactly does this error mean in this case? It's no tclear to me what needs to be made more specific. Am I approaching the problem in the wrong way? Any help is appreciated. Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Question about class declaration
by Rafael Gustavo da Cunha Pereira Pinto 24 Jul '09

24 Jul '09

Hi folks, Once in a while I stumble upon declarations like this: class EditAction e a | e -> a where apply :: a -> e -> a Why is the vertical bar there? I understand that => defines a context, but can't find what is the "|" for. Thanks Rafael Gustavo da Cunha Pereira Pinto

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haskell problem
by B1lal 23 Jul '09

23 Jul '09

im having problem with this Haskell functions female, male :: String -> Bool for finding out the gender for a given name which can be inferred from the title. For example, gender> female "Ms. Green" True gender> female "King Arthur" False gender> male "Mr. Scrooge" True gender> female "Miss Beatrix Potter" True

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Double's
by Thomas Friedrich 22 Jul '09

22 Jul '09

Hi everybody, please have a look at the following lines of code: x :: [Double] x = [2,4,6,8,10] p :: [Double] p = [0.05, 0.2, 0.35, 0.3, 0.1] expectation :: Double expectation = sum $ zipWith (*) x p variance :: Double variance = sum $ zipWith f x p where f i p = (i - expectation)^2 * p when I load this into ghci I get the following: *Main> expectation 6.3999999999999995 *Main> expectation == 6.4 False *Main> variance 4.24 *Main> Why does 'expectation' get evaluated to 6.39999... instead of 6.4?? This is very strange. I have another example which is even more annoying: Have a look at the following code: import qualified Data.Map as M newtype Distribution a = Distribution (M.Map a Double) isDistribution :: Distribution a -> Bool isDistribution (Distribution d) = sum (M.elems d) == 1 uniform :: Ord a => [a] -> Distribution a uniform xs = Distribution (M.fromList (map go xs)) where go i = (i, 1 / fromIntegral (length xs)) When I load this into ghci I get the following: *Probability> isDistribution $ uniform [1..6] False *Probability> (\(Distribution d) -> sum (M.elems d)) $ uniform [1..6] 0.9999999999999999 *Probability> I mean ... hey, what is going on here? Is there any way of getting around this? Cheers, Thomas

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Rank beginner question about debugging
by Ben Wise 22 Jul '09

22 Jul '09

Folks, I'm a rank beginner in Haskell, and though Haskell seems like a great language to start using, I've got a serious concern about debugging. In about 15 years of Lisp experience, then 15 years of C++, I've gotten pretty accustomed to the idea of using a debugger with lots of pre- and post-conditions on functions, breakpoints, stack trace, and variable inspection -- even though it gets tricky with delayed evaluation, macros, etc. in Lisp! So I don't understand how serious programmers can work on large projects in Haskell with nothing but compiler type-checking and QuickCheck. Yet they get it done, so I must be missing something. Things like the Haskell version of Eclipse seem to provide nothing but syntax coloring and pass-through to 'make': no break points, stack trace, or variable inspection that I can find. Is there some serious IDE (even "emacs + gdb" is serious enough for me) I'm missing? Do Haskell shops develop their own home-grown tools, or do they do something completely different? -- Ben P. Wise, PhD GPG: 0xCAF514E1

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dependent types
by pat browne 20 Jul '09

20 Jul '09

{- Hi, Consider the following Haskell data, class and instance definitions The intended semantics are: Two objects are equal if they have the same name. The type of the name depends on the type of the object -} data Object a = Object a class Named o n | o -> n where name :: o -> n instance (Eq n, Named o n) => Eq o where o1 == o2 = name(o1) == name(o2) {- I have three questions about these definitions. 1) Does the above code capture the intended meaning? 2) Does Object a *fit* the Named class?, or do I need a different sort of Object? 3) How would I write an instance of Named that implements the name function correctly. Thomson[1] shows how to do this for non-dependent types [1]Haskell: The Craft of Functional Programming, Second Edition, Page 272 -}

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QuickCheck help
by Patrick LeBoutillier 20 Jul '09

20 Jul '09

Hi, I'm writing a small library to manipulate IPv4 addresses and masks. The core types are as such: data IPv4Host = IPv4Host { hbytes :: Word32 } deriving (Eq) data IPv4Mask = IPv4Mask { mbytes :: Word32 } deriving (Eq) I'm trying to test my library using QuickCheck. My test script looks like this so far: import Test.QuickCheck.Batch import Test.QuickCheck import Net.IPv4 prop_host_read_show :: Int -> Bool prop_host_read_show i = let w = fromIntegral i h = IPv4Host w in read . show $ h == h options = TestOptions { no_of_tests = 100 , length_of_tests = 0 , debug_tests = True } main = do runTests "IPv4Host" options [ run prop_host_read_show ] I'm generating arbitrary Ints that are then converted to Word32 values. The problem is that for 100 tests, all the generated Ints seem to be below +/- 100. How do I get the Ints to be more evenly distributed over the entire 32 bit range? I tried making Word32 and instance if Arbitrary using choose (0, 0xFFFFFFFF) but I got the same results... Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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