- Beginners - Haskell.org

Circular programming
by Maciej Piechotka 16 Aug '09

16 Aug '09

I'm not understending circular programming. What it is used for? Where feedback variable came from (yes - I know it's wrong question as Haskell is lazy)? I've read http://www.haskell.org/haskellwiki/Circular_programming but I didn't understend the concept and pattern. Regards

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List combination function?
by Ian Duncan 16 Aug '09

16 Aug '09

Hello all, I'm trying to build a function that takes a string such as "123" and gives me permutations including permutations with lesser list lengths. I'm not sure how to phrase it, but here is what the output could look like: foo "123" => ["123","213","321","231","312","132", "12", "13", "21", "23", "31", "32", "1", "2", "3", ""] The ordering doesn't matter, and that null list at the end doesn't particularly matter, but I don't really know the mathematical phrasing of what I'm asking for. I'm trying to build a scrabble helper that can find the optimal score given a set of letters to work with. Thanks for your help, Ian Duncan

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monad transformers
by Michael P Mossey 15 Aug '09

15 Aug '09

In Martin Grabmuller's tutorial "Monad Transformers Step by Step", found here http://user.cs.tu-berlin.de/~magr/pub/Transformers.pdf he gives an example of composing ErrorT, StateT, ReaderT, and WriterT. Early in the paper, where he composes just ErrorT and ReaderT type Eval3 a = ReaderT Env (ErrorT String Identity) a he uses 'ask' and 'throwError' in some example code. I notice that he doesn't have to lift throwError into the ErrorT monad. Why is this? Do I misunderstand something about monad transformers? Is it a convenience definition of throwError? Same thing the rest of the paper. I don't see anywhere he lifts anything. Thanks, Mike

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some terminology
by Michael P Mossey 13 Aug '09

13 Aug '09

I was looking at some code, saw a variable x, and said to myself, "Ah that variable is a monad." Then I realized "Monad" is the name of a type class. So maybe x should be called "an instance of a Monad." I think the word "instance" in this case is OO-like; but in Haskell "instance" refers to a type that is an instance of a type class. Or maybe it can refer to both? And Monad is a type class, not a type. Maybe I need the phrase "monadic type" to refer to an instance of a type class. So maybe x is just "a variable of a monadic type"? Thanks, Mike

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function parameter types after flip
by Robert Ziemba 13 Aug '09

13 Aug '09

Can anyone help me understand the behavior of the following expressions in ghci? I was trying to construct a flipped function 'elem' that would accept a list of characters first but I ran into this problem with the type signature. Thank you. Prelude Data.List Data.Char> :t elem elem :: (Eq a) => a -> [a] -> Bool -- OK Prelude Data.List Data.Char> :t (flip elem) (flip elem) :: (Eq a) => [a] -> a -> Bool -- OK this is what I want Prelude Data.List Data.Char> let fElem = (flip elem) Prelude Data.List Data.Char> :t fElem fElem :: [()] -> () -> Bool -- ?? Function will not accept a [Char]

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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 11 Aug '09

11 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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