[Haskell-cafe] Order of Map.fromListWith

[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]

Any ideas?