List is not updated in for loop

In Elixir, for is a function, and you have to reassign the return of for to lessions

lessions = 
    for lession_name <- chapter["lession"] do
        ....
    end

You should learn to use functions from Enum module instead of for

Your code can be rewrite like this

Enum.each(chapters, fn chapter ->
      lessons =
        Enum.reduce(chapter["lessions"], [], fn lession_name, lessions ->
          %HTTPoison.Response{status_code: 200, body: body} = HTTPoison.get!("LESSON URL")

          case String.split(body, ~r/\n-{3,}\n/, parts: 2) do
            [frontmatter, content] ->
              {:ok, lesson} = YamlElixir.read_from_string(frontmatter)
              lesson = Map.put(lesson, "content", content)
              lessons ++ [lesson]

            _ ->
              lessions
          end
        end)

      new_chapter = %{
        name: chapter["name"],
        slug: chapter["slug"],
        course_id: course.id,
        lessons: lessons
      }

      Courses.create_chapter(new_chapter)
    end)

Elixir’s data structures are never updated. They are immutable. If you are from the Java/C# world, consider everything in Elixir acts like strings, when you update it, you always get a modified copy. The original never changes.

Figured it out. The lessons variable is different inside the case and outside the case. This is why it was not updating. I used Enum.map on chapter to create lessons.

Yeah, I guess the answer here is less about immutability than about scoping in elixir.

Yeah, most of the time immutability is not the cause of how things work. BEAM could have mutable data, but as there is no syntaxt nor functions tu mutate data in Elixir, that wouldn’t change how our code work.