From: Stefano Crocco <stefano.crocco@...>
Date: 2008-03-15T16:41:44+09:00
Subject: Re: how to abruptly end a program?

On Saturday 15 March 2008, Tom Cloyd wrote:
> Joel VanderWerf wrote:
> > Tom Cloyd wrote:
> >> Joel VanderWerf wrote:
> >>> Tom Cloyd wrote:
> >>>> I read in <Programming Ruby 2nd ed.> (p. 303) - ""If Ruby comes
> >>>> across a line anywhere in the source containing just “_ _END_ _”,
> >>>> with no leading or trailing whitespace, it treats that line as the
> >>>> end of the program—any subsequent lines will not be treated as
> >>>> program code."
> >>>>
> >>>> That's not what's happening for me. The following -
> >>>> _ _END_ _
> >>>> __END__
> >>>> _END_
> >>>>
> >>>> all produce the same result for me - "undefined local variable or
> >>>> method"
> >>>> That DOES bring the program to an abrupt end, but its rather
> >>>> ungraceful. Is this the intended result - a crash due to an
> >>>> interpreter error?
> >>>
> >>> What the quote (a bit ambiguous perhaps) means is that the __END__
> >>> line and following lines are not treated as part of the program
> >>> text. It has nothing to do with control flow. It's like comment
> >>> block that runs to the end of the file without any explicit
> >>> termination. (It's more than that because you can read it with DATA.)
> >>
> >> OK, but couldn't ANY uninitialized variable achieve the same messy
> >> result? What's the point? This surely can't be the intended result.
> >> Makes no sense to me.
> >
> > __END__ by itself on a line shouldn't exit the program. If you're
> > getting that error, maybe there is an extra char on the line. A space
> > perhaps?
>
> Yes, that would explain it, but I made sure this problem didn't occur
> from the beginning, which is one reason I'm puzzled. I really don't see
> anything wrong with what I'm feeding the interpreter.
>
> So, I did a different test program -
>
> require 'readline'
> puts "test under way\ninput var: "
> #opt = readline( "=--> \n")
> opt = gets.chomp
> puts( opt)
> __END__
> puts( opt)
>
> result:
> test under way
> input var:
> 5
> 5
>
> Here, we don't get an error. We just get ignored.
> This is nuts.
>
> t.

As others said, when ruby sees the __END__ identifier on a line by itself
(without leading or trailing spaces), it treats it, and any text following it, 
approximately as a comment. This means that the code is not parsed (syntax
errors after that line aren't recognized) and, obviously, not executed.
Besides, the constant DATA is set to a File object which contains that text.

This is consistent with the result you get from your piece of code: without
the __END__, 5 would have been written 3 times: the first when you entered it 
from the keyboard, the other two because of the two puts. Since the second
puts is after __END__, instead, you only get it twice: the second puts is
ignored.

Try this, for example:

puts "This is the class of DATA: #{DATA.class}"
puts "These are the contents of DATA:\n\n#{DATA.read}"
__END__
This is not valid ruby code

The output is:
This is the class of DATA: File
These are the contents of DATA:

This is not valid ruby code

Of course, removing the __END__ line causes a syntax error:

./prova.rb:6: syntax error, unexpected kNOT, expecting $end
This is not valid ruby code

Regarding your first piece of code (the one which produces the NoMethodError), 
are you sure the error isn't produced before end?

Stefano