From: Brian Candler <B.Candler@...>
Date: 2007-03-12T18:43:44+09:00
Subject: Re: #define alternative for Ruby

On Mon, Mar 12, 2007 at 06:00:21PM +0900, Mike wrote:
> I know it but imagine you must do it 100 times?
> I'd rather write:
> set_default(var1, 15)
> set_default(myvar1, "string")
> ...
> set_default(lastvar, myhash)

OK, if you *really* insist:

---- 8< -------------------------------------------------
require 'rubygems'
require 'active_support/binding_of_caller'

def set_default(var, val)
  Binding.of_caller do |binding|
    eval("#{var} ||= #{val}", binding)
  end
end

var1 = nil                  # <<<<<<< A
set_default("var1", 5)
set_default("var1", 15)
puts var1                   # <<<<<<< B    (prints 5)
---- 8< -------------------------------------------------

This works as written; but you'll find you get bitten by the static nature
of local variables if you remove line "A". It then fails.

This is because of the ambiguity in line B: "puts var1" could be referring
to a method call - puts self.var1() - or a local variable. This ambiguity is
resolved at *parse* time, before any code has been executed at all, simply
based on whether a statement of the form "var1 = xxx" has been seen earlier
in the file.

If you remove line A, then line B is already resolved as puts self.var1()
*before* any code has been executed.

Now, macros would be fine here - but only if these macros were expanded at
parse time. That makes them very static, and not very useful in a dynamic
language like Ruby.

As others have said before: provide a *real* example of where you would need
macros, not a noddy "var ||= val" replacement, and we'll show you how
Rubyists would handle this case.

Regards,

Brian.