David F. Place d at vidplace.com
Tue Jul 17 17:05:56 EDT 2007

```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]
>
> 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
> _______________________________________________