Usually, this ERB template

<% [1,2,3].each do |f| %>
  <%= f %>
<% end %>

will produce the following output

=> "\n    1\n\n    2\n\n    3\n\n"

Sometimes you want to keep your ERB code idented properly, but to skip thise spaces before the actual output.

You can use ERB trim_mode for that.

According to the trim mode value - will remove all spaces before <%-.

Attention only before <%-! So it will not work for <%=.

That’s why the only way to achieve this behaviour, is to add some useless <%- 1 %> expression before output, that you want to trim.

<% [1,2,3].each do |f| %>
    <%- 1 %><%= f %>
<% end %>

and try

irb(main):018:0>'f.txt'), nil, '-').result
=> "\n1\n\n2\n\n3\n\n"

BTW, in rails the default value for ERB trim_mode is -.