It looks like fromListWith is indeed implemented with a left fold over insertWithKey, with the Map as the accumulator. However, in insertWithKey the value-combiner function has type 'k -> a -> k -> a' which means it can combine in either order (it doesn't have to do with the fold or iteration order at all). The docs say:If the key does exist, the function will insert the pair @(key,f key new_value old_value)@.This doesn't really answer the "why" part of your question, but explains why you get [2, 1] instead of the reverse.
On Thursday, March 3, 2016, 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?
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