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
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 javascript:; http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe