Indeed I used "[" and "]" for the brackets, and ?, for the comma, it is not very consistent 
I guess when sending a chunk I thought we send some content to the browser here, while building the JSON chunks I thought here we are building some IO data.
Also there is the Stream.intersperse/2 function that would make the whole code cleaner when inserting commas between records instead of using take(1) and then drop(1).
The code would look simpler :
defp csv_stream(file) do
headers = headers(file)
file
|> File.stream!()
|> CSV.parse_stream(skip_headers: true)
|> Stream.map(fn x -> Enum.zip(headers, x) |> Map.new() |> Jason.encode_to_iodata!() end)
|> Stream.intersperse(",")
end
But note that for my 50 MB JSON it takes ~4s instead of ~3s. because you send each comma as its own chunk.
