I don’t really like introducing general mutability within functions, as others have already said, it creates the issue of “elixir is immutable except…”
Would it instead be possible to introduce some kind of “pinning operator” for comprehensions so that within the scope of the comprehension any assignment/binding of values is actually binding the variable on the outer scope, rather than wholesale mutability within functions?
Thinking out loud:
section_counter = 0
lesson_counter = 0
for section <- sections, ^section_counter, ^lesson_counter do
if section["reset_lesson_position"] do
lesson_counter = 0
end
section_counter = section_counter + 1
lessons =
for lesson <- section["lessons"], ^lesson_counter do
lesson_counter = lesson_counter + 1
Map.put(lesson, "position", lesson_counter)
end
section
|> Map.put("lessons", lessons)
|> Map.put("position", section_counter)
end
if that would work only for those as in that case I would expect to see a new operator which would be part of those constructs instead. I simply don’t like that 
with that type of syntax if it would work within all code blocks (