Making a comprehension level "variable" available to use in another function

Questions / Help
1

I am brand new so sorry if this is obvious but reading through the documentation I’m not able to figure it out. I have a function that returns a list of urls and I want to pattern match that list to a name to be stored in memory then use that name to reference the list in another function but have been unsuccessful so far. Here is the code urls_list is the “variable” name (I know that variable assignment doesn’t exist in elixir I just can’t figure out the right way):

def build_urls() do
      page_num = ["1","2","3","4","5"]
      tail_url = ["/?filter=#link"]
      for page <- page_num, tail <- tail_url do
        urls_list = @endpoint <> page <> tail
      end
    end
  
    def get_body() do
      urls_list
      |> Enum.map(fn url -> handle_response(url) end)
      |> Floki.parse
      |> Floki.find(".review-entry")
      |> Enum.map(fn entry ->
        [{"div", _, [date]}] = Floki.find(entry, "div.italic")
        [{"p", _, [content]}]  = Floki.find(entry, "p.review-content")
        {date, content} 
      end)
    end
2

You can assign the result of a comprehension to a variable:

urls_list = for page <- page_num, tail <- tail_url do
  @endpoint <> page <> tail
end
3

Thank you! So to make sure I understand; would it make most sense to just forgo creating the build_urls() function all together and just place it inside the get_body() function?

4

You can put it in a function if you want, in which case using it would look like:

def build_urls() do
  page_num = ["1","2","3","4","5"]
  tail_url = ["/?filter=#link"]
  for page <- page_num, tail <- tail_url do
    @endpoint <> page <> tail
  end
end

def get_body() do
  build_urls()
  |> Enum.map(fn url -> handle_response(url) end)
  # etc...
end

Values always need to be called from one function or passed in as an argument. You can’t assign a variable in one function and then access in another without passing it in as an argument.

1 Like
5

Assigning urls_list inside a comprehension is extremely misleading :slight_smile:

6

Thanks for this warning Aleksei!

7

Elixir compiler would produce the same warning as well. What happens there, the result of the last expression gets implicitly returned from the function.

def build_urls() do
  page_num = ["1","2","3","4","5"]
  tail_url = ["/?filter=#link"]
  for page <- page_num, tail <- tail_url do
    # urls_list = @endpoint <> page <> tail
    @endpoint <> page <> tail
  end
end

And the result of the comprehension gets returned. The assignment is redundant, I believe it happened to appear in the great answer above due to copy-paste issues.

1 Like
8

Yup that’s my bad, I had accidentally included it from the original when I copied / pasted. Good catch!

2 Likes