From: David Jacobs <developer@...>
Date: 2011-05-12T05:26:56+09:00
Subject: Re: Scope problem (?) in implementing Design Patterns in Ruby

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I think the lookup starts at the innermost scope but since All isn't defined
in Not, it reaches the top-level.

On Wed, May 11, 2011 at 4:23 PM, 7stud -- <bbxx789_05ss@yahoo.com> wrote:

> 7stud -- wrote in post #998081:
> > RichardOnRails wrote in post #998059:
> >>
> >
> > 1) Your case statement syntax doesn't work in ruby 1.9.2:
> >
> > prog.rb:4: syntax error, unexpected ':', expecting keyword_then or ','
> > or ';' or '\n'
> >
> > You have to use 'then' in place of a colon.
> >
> > 2) Next, I get this error:
> >
> > prog.rb:67:in `evaluate': uninitialized constant Not::All (NameError)
> >   from prog.rb:74:in `<main>'
> >
> > which relates to this code:
> >
> > class Not < Expression
> >   def initialize(expression)
> >     @expression = expression
> >   end
> >
> >   def evaluate(dir)
> >     all = All.new.evaluate(dir)
> >     other = @expression.evaluate(dir)
> >     all - other
> >   end
> > end
> >
> > In ruby, constants are looked up like directories and files (or if you
> > prefer constants are 'lexically scoped').  When you
> > are inside the Not class (which is a module), the 'directory' you are in
> > for constant lookups is the 'Not' directory.  When you write All.new,
> > because the name All is not preceded by a directory name, ruby looks in
> > the current 'directory' for the constant All.  The current directory is
> > Not, so ruby is looking for Not::All, i.e. the 'file' All in the
> > 'directory' Not.  However, All is not defined inside Not, so you get an
> > error.  In fact, there is no constant named All defined anywhere in
> > your program, so the error is more serious than a scope problem.
> >
> > If you are inside a class/module and you need to access a class at the
> > top level, you do this:
> >
> > class All
> >   def greet
> >     puts 'hi'
> >   end
> > end
> >
> >
> > class Dog
> >   def do_stuff
> >     ::All.new.greet  #<*****
> >   end
> > end
> >
> > Dog.new.do_stuff  #=>hi
> >
>
> In fact, that is unnecessary:
>
> class All
>  def greet
>    puts 'hi'
>  end
> end
>
>
> class Dog
>  def do_stuff
>    All.new.greet  #<*****
>  end
> end
>
> Dog.new.do_stuff  #=>hi
>
>
> I guess the lookup actually starts at the toplevel.  So your error is a
> result of not defining the constant All at the toplevel, and ruby
> obfuscates the error by telling you that All is not defined inside the
> Not module, giving you the error message: Not::All doesn't exist.
>
> --
> Posted via http://www.ruby-forum.com/.
>
>

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