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
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]
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