From: Rick DeNatale <rick.denatale@...>
Date: 2009-10-04T04:06:56+09:00
Subject: Re: On Ranges as conditions

On Sat, Oct 3, 2009 at 1:15 PM, Rafael Cunha de Almeida
<none@example.org> wrote:
> Reading a few tutorials I found this piece of code:
>
>        while input = gets
>            input = input.chomp
>            puts input + " triggered" if input =~ /start/ .. input =~ /end/
>        end
>
> I understand what it does. If I type start, end or anything in between
> it prints
> "triggered" after the input. Now, if I'm out of the start .. end block,
> the if
> fails.
>
> What I don't understand is how it does it. For instance, if I use the
> interpreter to do something like this:
>
>        irb(main):038:0> input = 'fooo'
>        => "fooo"
>        irb(main):039:0> puts input + " triggered" if input =~ /start/ .. input
> =~ /end/
>        => nil
>        irb(main):040:0> input = 'start'
>        => "start"
>        irb(main):041:0> puts input + " triggered" if input =~ /start/ .. input
> =~ /end/
>        start triggered
>        => nil
>        irb(main):042:0> input = 'bar'
>        => "bar"
>        irb(main):043:0> puts input + " triggered" if input =~ /start/ .. input
> =~ /end/
>        => nil
>
> it doesn't work as I'd expect. I thought the if clause there would
> somehow set a
> global to say "the first regex was matched" or something like that. But it
> doesn't seem to be the way it works. It only seems to work inside of a while
> loop. I thought it might have something to do with the $_ global. But I
> made a
> test in the interpreter which ruled out that theory:
>
>        irb(main):045:0> $_ = 'foo'
>        => "foo"
>        irb(main):046:0> input = 'foo'
>        => "foo"
>        irb(main):047:0> puts input + " triggered" if input =~ /start/ .. input
> =~ /end/
>        => nil
>        irb(main):048:0> $_ = input = 'start'
>        => "start"
>        irb(main):049:0> puts input + " triggered" if input =~ /start/ .. input
> =~ /end/
>        start triggered
>        => nil
>        irb(main):050:0> $_ = input = 'foo'
>        => "foo"
>        irb(main):051:0> puts input + " triggered" if input =~ /start/ .. input
> =~ /end/
>        => nil
>        irb(main):052:0>
>
> Can anyone help me find out what's going on?

As far as I can tell, from reading the 1.8 parse.c code, the problem
is that conditional ranges really aren't Range instances but are
really a kind of syntactic sugar.  Every time you retype the source
line into irb, you get an entirely new parse, and a new conditional
range in the initial state.


-- 
Rick DeNatale

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