How to output string containing new line character in eex template?

How about <%= raw str %>?

It seems that raw is part of phoenix. For my lib it would be not planed to depend on phoenix. I checked the implementation but this gets translated to {:save, str} which is also not plain eex.

raw depends only on Phoenix.HTML, which is a separate library that only performs the HTML-rendering functionality of Phoenix. So there is no need to depend on the whole Phoenix webserver.

If you’re using HTML then this will apply for presenting your data with line breaks read by HTML inside of forms. (How to use the raw specified above^)

You can create new input helpers, editing only slightly what phoenix has in their accepted releases.

defmodule FulfillmentCart.Helpers.FormHelpers do
  alias Phoenix.HTML.Form
  import Phoenix.HTML
  import Phoenix.HTML.Tag

   def textarea(form, field, opts \\ []) do
    opts =
      opts
      |> Keyword.put_new(:id, input_id(form, field))
      |> Keyword.put_new(:name, input_name(form, field))

    {value, opts} = Keyword.pop(opts, :value, input_value(form, field))
    content_tag(:textarea, ["\n", html_escape(value || "")], opts)
  end
end

This is source code copied from [Phoenix Master Branch].

Simply change the way your data is returned to the page from escaped data to raw data, like this:

content_tag(:textarea, ["\n", html_escape(value || "")], opts)
to
content_tag(:textarea, ["\n", value |> raw, opts)

and implement the custom helper into your template just like you would any other textarea

(phoenix_html/lib/phoenix_html/form.ex at main · phoenixframework/phoenix_html · GitHub)