Enum.map_join with a comprehension

Do you think a join option would be a good idea for comprehensions?
Something like Enum.map_join/2 that could map and join lists in a single pass.

book_ids = for book <- books, join: ",", do: book.id

If it looks like a good idea, I will open a ticket / PR.

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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}"

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 :thinking:

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

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There is always that question.

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


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)