From: "David A. Black" <dblack@...>
Date: 2001-09-04T03:13:36+09:00
Subject: [ruby-talk:20798] Re: Indirect eval doesn't work??

Hello --

On Tue, 4 Sep 2001, Wai-Sun Chia wrote:

> Dear Rubyists,
> I have this Ruby code:
> 
> ================== foo.rb ====================
> #!/usr/bin/ruby
> 
> code = ""
> File.open ("foo.tmpl" ,"r") { |fd|
>          fd.each { |line|
>                  code << line
>          }
> }
> 
> class Foo
> end
> 
> puts Foo.public_instance_methods.inspect
> print code
> eval code
> puts Foo.public_instance_methods.inspect
> ==============================================
> 
> And this data file:
> 
> ================= foo.tmpl ===================
> class Foo
>          bar = 9999
>          def foo_#{bar}
>                  puts "this is in foo"
>          end
> end
> ==============================================
> 
> When I run it, I get:
> []
> class Foo
>          bar = 9999
>          def foo_#{bar}
>                  puts "this is in foo"
>          end
> end
> 
> ["foo_"]
> 
> 
> Why is it that #{bar} doesn't get extrapolated?

Because the # introduces a comment :-)  Here are two examples which
illustrate (I hope) the role of scope here:

  class A
    a = 9999
    def thing_#{a}   guess what? that's a comment!
    end
  end

  p A.instance_methods  # => ["thing_"]

  class B
    b = 9999
    eval <<-EOE
      def thing_#{b}    # now #{...} will get interpolated
      end
    EOE
  end

  p B.instance_methods  # => ["thing_9999"]

In the first example (A), you've got a bare #, not inside a string.
So it's a comment.

And... if you put the whole thing in a string, and then eval it, the #
is still a comment:

  str = <<'EOS'
    class A
      a = 9999
      def thing_#{a}
      end
    end
    p A.instance_methods
  EOS

  eval str        # => ["thing_"]

That's essentially what happened in your case.  You read the string in
from a file, but that isn't relevant here.

See case B, above.  Another variation on the theme is:

  a = 9999
  str = <<EOS
    class A
      def thing_#{a}  # now this is outer scope's a
      end
    end
  EOS

(Note: double-quoted string in that case.)


David

-- 
David Alan Black
usually dblack@candle.superlink.net
writing from dblack@saturn.superlink.net