Ruby's __END__

I recently used Ruby's __END__ keyword in a client project.

__END__ denotes the end of regular source code in a Ruby file; nothing below it will be executed.

It's essentially a one-line block-commenting device, which is how we used it when trying to recreate a successful pattern borrowed from somewhere else in the project. Instead of toggling between files, we copied beneath __END__ the code we wanted to emulate, started building the new method, and cleaned up before committing.

One benefit of this technique is that any code below __END__ is available via the special filehandle DATA:

# foo.rb
puts DATA.map(&:upcase)

__END__
'foo'
'bar'
'baz'

$ ruby foo.rb
'FOO'
'BAR'
'BAZ'

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