While scrolling around, I found that Map.new/2 uses :lists.reverse/1 after the transformation on the enumerable is completed. I wonder if that is actually needed. I benchmarked the implementation against one without :lists.reverse/1 with benchee. The results are that it doesn’t slow down the function, but memory usage increases. I guess one reason to use :lists.reverse/1 is in order to handle duplicate keys just as Map.new/1 does. Is that the reason why :lists.reverse/1 is used?
:lists.reverse/1 is used to make sure that elements are inserted in order.
[1, 2, 2, 3, 4] |> Enum.with_index |> Map.new(&(&1))
The snippet above will cause different results between reversing the acc and not reversing it.
The pattern in new_transform is a common one when iterating over a list tail-recursively:
- take an element from the front of the list
- transform the element
- prepend the element to the accumulator
- make a recursive call
This results in the accumulator getting all the transformed results, but in reverse order.
This is the reason yes.
3 Likes
