I found myself wanting a map that looks at neighboring elements. This is
where I used explicit recursion the most. Something like this:
f [] = []
f ((Foo a) : (Bar b) : xs)
| fooBar a b = Foo a : f xs
| otherwise = Bar b : f xs
This is almost a map. A variation is when filtering and you want some
look-ahead to make the filtering decision. There's probably a good way to do
this I'm not aware of.
Johan
On 7/17/07, David F. Place
You hardly ever need to use explicit recursion in Haskell. Every useful way of doing recursion has already been captured in some higher order function. For example here is your subarrays implemented using unfoldr:
subarrays xs = concat $ unfoldr f xs where f [] = Nothing f xs = Just ( [ys | n <- [1..length xs], ys <- [(take n xs)]], tail xs)
On Jul 17, 2007, at 4:26 PM, James Hunt wrote:
Hi,
As a struggling newbie, I've started to try various exercises in order to improve. I decided to try the latest Ruby Quiz (http:// www.rubyquiz.com/quiz131.html) in Haskell. Would someone be kind enough to cast their eye over my code? I get the feeling there's a better way of doing it!
subarrays :: [a] -> [[a]] subarrays [] = [[]] subarrays xs = (sa xs) ++ subarrays (tail xs) where sa xs = [ys | n <- [1..length xs], ys <- [(take n xs)]]
maxsubarrays :: [Integer] -> [Integer] maxsubarrays xs = msa [] (subarrays xs) where msa m [] = m msa m (x:xs) | sum x > sum m = msa x xs | otherwise = msa m xs
--for testing: should return [2, 5, -1, 3] main = maxsubarrays [-1, 2, 5, -1, 3, -2, 1]
I've read tutorials about the syntax of Haskell, but I can't seem to find any that teach you how to really "think" in a Haskell way. Is there anything (books, online tutorials, exercises) that anyone could recommend?
Thanks, James _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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