Enum.map_join with a comprehension

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

Yes I’m a big fan of reduce in comprehensions.
Was just wondering if a :join option would be useful or software bloat

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

There is always that question.

[%{id: 1}, %{id: 2}, %{id: 3}, %{id: 4}, %{id: 5}] 
|> Enum.map_join(",", fn %{id: id} -> id end)

"1,2,3,4,5"

As the for comprehension is seen by some as a mapper, I can see how there could be a :join variant for for comprehensions.

ps. Now I have written this example I wonder why map_join is a separate function instead of Enum.join/3

Enum.map_join is able to map & join in a single pass, whereas calling Enum.map then Enum.join would require two successive loops.

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

My guess would be because there already is a joiner optional argument, adding an extra optional mapper argument would have made it awkward to use?

join(enumerable, joiner \\ "", mapper \\ fn x -> x end)