fold

I was wondering how could I write a function in Haskell that interleaves a list of lists into a single lists, for example, if I had a function called interleavelists :: [[a]] -> [a] it should be able to interleave the elements. Example: [[1,2,3] [4,5,6] [7,8]] --> [1,4,7,2,5,8,3,6] . The lists can be both finite or infinite... Can I use foldr ? Daniel Fischer The quickest way to write it is to use transpose from Data.List . import Data.List interleavelists :: [[a]] -> [a] interleavelists = concat . transpose transpose picks the first element of each non-empty list of its argument, putting them

I wonder why scala.Option doesn't have a method fold like this defined: fold(ifSome: A => B , ifNone: => B) equivalent to map(ifSome).getOrElse(ifNone) Is there no better than using map + getOrElse ? Debilski You can do: opt foldLeft (els) ((x, y) => fun(x)) or (els /: opt) ((x,y) => fun(x)) (Both solutions will evaluate els by value, which might not be what you want. Thanks to Rex Kerr for pointing at it.) Edit: But what you really want is Scalaz’s catamorphism cata (basically a fold which not only handles the Some value but also maps the None part, which is what you described) opt.cata(fun,

I'm looking for tips on how to implement the popular 'paper folding / origami' effect in my iOS project. I'm aware of projects such as: https://github.com/xyfeng/XYOrigami but they only offer the 'animated' effect, with no manual control over the opening animation. I've struggled to dissect that project and come up with what I'm after. To be more exact, I'm looking on how to implement the effect shown here: http://vimeo.com/41495357 where the folding animation is not simply animated open, but the user controls the opening folds. Any help would be much appreciated, thanks in advance! EDIT: Okay

I have some Haskell code that does work correctly on an infinite list, but I do not understand why it can do so successfully. (I modified my original code -- that did not handle infinite lists -- to incorporate something from some other code online, and suddenly I see that it works but don't know why). myAny :: (a -> Bool) -> [a] -> Bool myAny p list = foldr step False list where step item acc = p item || acc My understanding of foldr is that it will loop through every item in the list (and perhaps that understanding is incomplete). If so, it should not matter how the "step" function is

Trying to learn F# but got confused when trying to distinguish between fold and reduce . Fold seems to do the same thing but takes an extra parameter. Is there a legitimate reason for these two functions to exist or they are there to accommodate people with different backgrounds? (E.g.: String and string in C#) Here is code snippet copied from sample: let sumAList list = List.reduce (fun acc elem -> acc + elem) list let sumAFoldingList list = List.fold (fun acc elem -> acc + elem) 0 list printfn "Are these two the same? %A " (sumAList [2; 4; 10] = sumAFoldingList [2; 4; 10]) Fold takes an

This is my take version using foldr : myTake n list = foldr step [] list where step x y | (length y) < n = x : y | otherwise = y main = do print $ myTake 2 [1,2,3,4] The output is not what I expect: [3,4] I then tried to debug by inserting the length of y into itself and the result was: [3,2,1,0] I don't understand why the lengths are inserted in decreasing order. Perhaps something obvious I missed? If you want to implement take using foldr you need to simulate traversing the list from left to right. The point is to make the folding function depend on an extra argument which encodes the logic