Re: [Haskell-cafe] Is there a nicer way to do this?

6 Jul 2008

      mfeathers:
...
Don Stewart wrote:
...
mfeathers:
...
segment :: Int -> [a] -> [[a]]
segment 0 _ = []
segment _ [] = []
segment n x = (take n x) : segment n (drop n x)
The first set of parens can go,
segment n x = take n x : segment n (drop n x)
...
I did a version of this which used splitAt but I wasn't sure whether it 
was going to buy me anything re performance that would justify its 
ugliness.
Besides,
splitAt n xs =  (take n xs, drop n xs)
Thanks.  That is odd, though.  It makes me wonder what to expect re 
optimization.  Would the compiler/runtime know that splitAt could be 
done in a single pass?
Not with that definition. It would require some moderately unusual fusion
combining the take and drop into a single fold with the (,) on the inside,
rather than on the outside.

However, GHC actually implements splitAt as:

    splitAt (I n) ls
      | n < 0       = ([], ls)
      | otherwise   = splitAt' n ls
        where
            splitAt' :: Int -> [a] -> ([a], [a])
            splitAt' 0 xs     = ([], xs)
            splitAt' _  xs@[] = (xs, xs)
            splitAt' m (x:xs) = (x:xs', xs'')
              where
                (xs', xs'') = splitAt' (m - 1) xs

So there may be some benefit.

-- Don