Difference between -%> and %> in rails [duplicate]

Answer 1

UPDATE: This answer was wrong, see https://stackoverflow.com/a/25626629/895245 instead.

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In Ruby 2.1 (not necessarily with Rails), the - removes one trailing newline:

• the newline must be the first char after the >
• no spaces are removed
• only a single newline is removed
• you must pass the '-' option to use it

Examples:

require 'erb'
ERB.new("<%= 'a' %>\nb").result              == "a\nb"  or raise
begin ERB.new("<%= 'a' -%>\nb").result; rescue SyntaxError ; else raise; end
ERB.new("<%= 'a'  %>\nb"  , nil, '-').result == "a\nb"  or raise
ERB.new("<%= 'a' -%>\nb"  , nil, '-').result == 'ab'    or raise
ERB.new("<%= 'a' -%> \nb" , nil, '-').result == "a \nb" or raise
ERB.new("<%= 'a' -%>\n b" , nil, '-').result == 'a b'   or raise
ERB.new("<%= 'a' -%>\n\nb", nil, '-').result == "a\nb"  or raise

Doc: http://ruby-doc.org/stdlib-2.1.1/libdoc/erb/rdoc/ERB.html

Rails 4.1 documents this at http://api.rubyonrails.org/classes/ActionView/Base.html, and appears to:

• use ERB by default at: https://github.com/rails/rails/blob/fcbdac7e82725c388bf5adf56a9a9a16d4efdbe0/actionview/lib/action_view/template/handlers.rb#L10

• set - by default at: https://github.com/rails/rails/blob/fcbdac7e82725c388bf5adf56a9a9a16d4efdbe0/actionview/lib/action_view/template/handlers/erb.rb#L77

However, Rails 4.1 does remove trailing whitespaces as documented while pure ERB does not, so there may be other differences.

Also, it is not removing the leading newlines as documented: it might be a documentation bug. Opened an issue at: https://github.com/rails/rails/issues/16766

Answer 2

I'm pretty sure - before %> is no longer necessary, and should be left out.

At least in Chrome, the generated html looks the same using -%> or %>.

Answer 3

The extra dash makes ERB not output the newline after the closing tag. There's no difference in your example, but if you have something like this:

<div>
  <% if true -%>
  Hi
  <% end -%>
</div>

It'll produce:

<div>
  Hi
</div>

and not this:

<div>

  Hi

</div>

Answer 4

If you use HAML rather than ERB you can do something similar with a less than or greater symbol than after your tag.

> will remove any whitespace around your tag and < will remove any whitespace within it.

.float-left<
  %p
    Lorem ipsum dolor sit amet

is compiled to:

<div class="float-left"><p>
  Lorem ipsum dolor sit amet
</p></div>

And…

%left_tag
%inside>
%right_tag

is compiled to:

<left_tag /><inside /><right_tag />

If you're not using HAML it's definitely worth checking out.