[Haskell-cafe] Order of Map.fromListWith

[Patrick Redmond]

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 at 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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