Well … decision on adding features and enhancements is up to @Elixir-Core-Team. What I can say is that you can already do that in a single pass using reduce option:

# Here is a simplified list
books = [%{id: 1}, %{id: 2}, %{id: 3}, %{id: 4}, %{id: 5}]

# Here is an example comprehension:
book_ids = for book <- books, reduce: "" do
  "" -> book.id
  acc -> "#{acc}.#{book.id}"
end

Helpful resource:
:reduce option in Kernel.SpecialForms.for/1

I think he means why doesn’t join take a function that’ll also map it for you, like into, or Map.new does?

You could write your own Collectable module and use :into to achieve this if you expect to use it often.