What <%- and -%> does on Ruby on Rails?

The Rails documentation only has a very short description for it:

<%- and -%> suppress leading and trailing whitespace, including the trailing newline, and can be used interchangeably with <% and %>.

I use '_' to represent a white space and '\n' for a newline character so that you can see these.

If the rails code is as follows:

source

__<% if something %>\n
____hello\n
__<% end %>\n

This produced following HTML code:

output

__\n
____hello\n
__\n

The leading and trailing whitespaces, including the trailing new line of Rails tags are actually output and produces three lines.

source

__<% if something -%>\n
____hello\n
__<% end %>\n

By changing the first %> with -%>, the trailing newline is omitted and the result looks like as follows. "hello" has six spaces (two spaces from the if line and four of them from the hello line):

output

______hello\n
__\n

By replacing <% with a <%-, it removes two leading whitespaces:

source

__<%- if something -%>\n
____hello\n
__<% end %>\n

output

____hello\n
__\n

By replacing all the <% and %> with <%- and -%>, you can get just the hello line:

source

__<%- if something -%>\n
____hello\n
__<%- end -%>\n

output

____hello\n

However, the sad part is that this won't work is there's any characters in the line.

source

leading_<%- if -%>_trailing

This doesn't trim leading/trailing spaces and produces following result...:

output

leading__trailing

Unless you use <%- and -%> in a pure Rails tag only lines, it doesn't have any meaningful effect.
(11/03/2009)