I am not 100% sure, but it seems to be the right optimization for the case where combining function is (++): prepending singleton lists is a lot cheaper than appending them to the end, in which case you'd get quadratic complexity. Sergey On Thu, Mar 3, 2016 at 7:23 PM, Niklas Hambüchen <mail@nh2.me> wrote:
Does anybody know why for fromListWith, the arguments to the combining function seem flipped?
import Data.Map fromListWith (++) [('a',[1]),('a',[2])]
fromList [('a',[2,1])]
I often use it to group things by some key, e.g.
postsByUserId :: Map Int [Int] postsByUserId = fromListWith (++) [ (userId, [postId]) | (userId, postId) <- posts ]
and regularly get tricked by the postIds being reversed in the result.
This is especially unintuitive to me since:
foldl (++) [] [[1],[2]] [1,2] foldr (++) [] [[1],[2]] [1,2]
Any ideas? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe