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List combination function?
by Ian Duncan 15 Aug '09

15 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 14 Aug '09

14 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 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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