Haskell got this right. You have foldr (right fold) and foldl' (left fold), and the order of the callback is opposite. If you do a left fold, then the initial accumulator is applied on the left; if you do a right fold, then the initial accumulator is applied on the right.
foldr f z [x1, x2, ..., xn] == x1 `f` (x2 `f` ... (xn `f` z)...)
foldl' f z [x1, x2, ..., xn] == (...((z `f` x1) `f` x2) `f`...) `f` xn
The mnemonic here is that the folding function (aka the callback) replaces the comma.
I find this slightly easier to remember than other languages. In contrast most other languages do not simultaneously provide a left fold and a right fold, so they do not consider this aspect, making things more difficult to remember.
That said I totally agree this requires more brainpower to read and write than map or filter. For this reason I have sometimes refactored code to use foldMap instead of foldr or foldl', so one no longer needs to think of the direction of the fold or the order of arguments.
I find this slightly easier to remember than other languages. In contrast most other languages do not simultaneously provide a left fold and a right fold, so they do not consider this aspect, making things more difficult to remember.
That said I totally agree this requires more brainpower to read and write than map or filter. For this reason I have sometimes refactored code to use foldMap instead of foldr or foldl', so one no longer needs to think of the direction of the fold or the order of arguments.