August 2009 - Beginners

brief example of ZipList?
by Michael P Mossey 11 Aug '09

11 Aug '09

Can someone give me a brief example using ZipList? I want to do something like z1 :: [Int -> Int] z1 = [succ,succ] z2 :: [Int] z2 = [1,2] z3 = z1 <*> z2 But don't know to get it to regard these as ZipLists and not regular lists. This is purely for learning purposes. No application in mind. -Mike

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some insights into functional programming
by Michael Mossey 11 Aug '09

11 Aug '09

I'm starting to figure out a few things that I didn't "get" about functional programming and monads. I wanted to explain them. I'm not looking for a particular response to this post, other than any elaboration that seems natural. There is an exercise here working with the trivial monad W: <http://blog.sigfpe.com/2007/04/trivial-monad.html> Write a function g :: W a -> W a -> W a such that g (W x) (W y) = W (x+y) except don't use pattern matching, but >>= instead. The answer is g mx my = mx >>= (\x -> my >>= \y -> W (x+y)) There are a couple things here that threw me off. One is that I didn't expect 'my' to be available inside the first lambda. I somehow thought of lambda as isolated, sealed-off from the rest of the universe. But they aren't. I believe this is the concept of closures, or related to it? Secondly, I didn't expect >>= to be available inside the lambda. This is related to the mistaken conception of >>= as a procedural statement rather than an expression. In Python, where I have previously encountered lambdas, no statements are allowed inside lambdas. Of course, >>= is actually an expression and you can put any expression to the right of a lambda ->. Maybe these are typical beginner misconceptions, or maybe they have more to do with coming from Python and complete beginners actually find it more natural. Mike

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creating a complement for a given given test generator.
by Srikanth K 10 Aug '09

10 Aug '09

Hi, I am trying to use the quickcheck to generate some test-data to test an api, along the lines of http://www.haskell.org/haskellwiki/QuickCheck_as_Test_Set_Generator. For the sake of example, I choose the following function to test data Result = Valid | Invalid api_under_test :: (Int,Int) -> Result api_under_test (x,y) | (x == 1) = Valid | otherwise = Invalid I had the following valid-generator which worked quite easily(trivial) validCombinations= [ (1,1), (1,2) ] validGen = elements validCombinations prop_valid_api_under_test = forAll validGen $ \xs -> (api_under_test xs) == (Valid) Now, I want to have a complement to state: forall tuples not in validCombinations, the api_under_test must return "Invalid". (i.e.) prop_invalid_api_under_test = forAll invalidGen $ \xs -> (api_under_test xs) == (Invalid) However, inspite of all googling, and reading the various docs including quickcheck, I am at loss on how I can elegantly define the "invalidGen" generator. One possible way I can think is to have a customized generator, that would generate two random numbers and then look up the tuple generated against the validCombinations list. However, I feel there just might be a better way of solving this. Any suggestion on how I should be trying to solve this. - Srikanth

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unit tests
by Michael Mossey 09 Aug '09

09 Aug '09

As I write Haskell code I find myself needing to write test code. If this test code were organized and documented, I could probably call it a set of unit tests. I'm trying to find some convention for naming functions and variables used in unit testing, deciding on location (in same module or a second module? same directory or sub-directory?), etc. Are there any Haskell community conventions? Thanks, Mike

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fmap fmap
by Michael P Mossey 09 Aug '09

09 Aug '09

How does one write fmap (fmap (*2)) xs without parenthesis? (Using . and $ instead.) I don't really understand . and $ well enough I guess. I tried a bunch of stuff but nothing worked. -Mike

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Does this function already exist in one of the standard modules?
by I. J. Kennedy 09 Aug '09

09 Aug '09

Does this function already exist in one of the standard modules? It computes f(f(f(...f(x))). I suspect it is a common function, but I can't find it, even using Hoogle. applyMany :: Int -> (a -> a) -> a -> a applyMany 0 f x = x applyMany n f x = applyMany (n-1) f (f x)

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Getting the Takusen example code to compile - failed import problem
by Daniel Everett 07 Aug '09

07 Aug '09

I have Takusen 0.8.5 installed and I'm trying to build the code found at http://darcs.haskell.org/takusen/doc/html/Database-Enumerator.html using ghc 6.10.3. This always fails with the error: Could not find module `Database.Oracle.Enumerator' The analogous errors are encountered if I substitute Oracle with Sqlite, Postgresql or ODBC. Any ideas how to resolve this? TIA, Daniel

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am I wording this right?
by Michael P Mossey 04 Aug '09

04 Aug '09

Is this the right way of saying what I'm trying to say? "Functor is a typeclass of type constructors which take one argument." Thanks, Mike

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Trying to read a bitmap with hOpenGL
by builes.adolfo＠googlemail.com 04 Aug '09

04 Aug '09

Hi All, I have been following an OpenGL book which has its examples in cpp, but using haskell instead. I have a problem when trying to read a bitmap, I haven't managed to render the file to the buffer, I did a little parser based on the one in RWH for the bmp file, and apparently everything seems to work well, but when I call drawPixels I don't get a clear image. The code that I'm using is here : http://gist.github.com/160787 What am I doing wrong ? Thanks in advance for any help. abuiles

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Transforming (State s a) to (StateT s m a)
by Toby Murray 04 Aug '09

04 Aug '09

Hi all, Am very new to Haskell, apologies in advance. Suppose I have a transformer A of type State s a. Suppose I have other state transformers of type StateT s m a. These are similar to the first kind but e.g. might throw errors or perform IO or whatever. I can manually convert A to be of the second kind as follows: -- | Used to invoke a "pure" state transformer -- from a context in which other effects may occur. pure :: (Monad m) => State s a -> StateT s m a pure func = StateT (\arg -> (return (runState func arg))) This seems like something one might want to do often. For instance, I might have a state type that I want to operate on, call it System. Then type PureSystemTransformer a = State System a defines a type for "pure" System transformers that return results of type a. However, other system transformers might need to throw errors. So I might define type SystemTransformer a e = StateT System (Either e) a for the type of system transformers that return results of type a or throw errors of type e. 'pure' is then used to convert a PureSystemTransformer to a SystemTransformer. I wonder whether there is something in the standard libraries that achieves the same thing as 'pure' above. i.e. Is there a more standard way to do this? Cheers Toby

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