From: Logan Capaldo <logancapaldo@...>
Date: 2006-05-08T09:52:36+09:00
Subject: Re: A major ruby annoyance!

And now I shall go off on a tangent, demonstrating the seeds of  
methods for "declaring variables" in ruby. It's not complete or  
perfect by any means, but I'm sure it can be improved by greater  
coders than I. It makes use of Binding.of_caller which for  
convenience I have stuck in the actual source file. It also makes use  
of an Evil Global Variable�

% cat locals.rb
# File lib/extensions/binding.rb, line 107
def Binding.of_caller(&block)
   old_critical = Thread.critical
   Thread.critical = true
   count = 0
   cc, result, error = Continuation.create(nil, nil)
   error.call if error

   tracer = lambda do |*args|
     type, context = args[0], args[4]
     if type == "return"
       count += 1
       # First this method and then calling one will return --
       # the trace event of the second event gets the context
       # of the method which called the method that called this
       # method.
       if count == 2
	# It would be nice if we could restore the trace_func
	# that was set before we swapped in our own one, but
	# this is impossible without overloading set_trace_func
	# in current Ruby.
	set_trace_func(nil)
	cc.call(eval("binding", context), nil)
       end
     elsif type != "line"
       set_trace_func(nil)
       error_msg = "Binding.of_caller used in non-method context or " +
	"trailing statements of method using it aren't in the block."
       cc.call(nil, lambda { raise(Exception, error_msg ) })
     end
		  end

   unless result
     set_trace_func(tracer)
     return nil
   else
     Thread.critical = old_critical
     yield result
   end
end
# File lib/extensions/continuation.rb, line 61
def Continuation.create(*args, &block)
   cc = nil
   result = callcc { |c|
     cc = c
     block.call(cc) if block and args.empty?
   }
   result ||= args
   return *[cc, *result]
end

def declare_locals(*symbols)
   $__LVARS ||= []
   $__LVARS.concat(symbols)
end

def ensure_locals
   Binding.of_caller do |b|
      lvar_set = eval('local_variables', b)
      undecld_vars = lvar_set - ($__LVARS || [])
      unless undecld_vars.empty?
        warn "Undeclared var(s): #{undecld_vars}"
      end
   end
end

declare_locals 'a', 'b'
a = 1
b = 2
c = 4
puts a + b
ensure_locals

% ruby locals.rb
3
Undeclared var(s): c



Originally I tried to do this with 'set_trace_func' but that doesn't  
apparently have an event for setting a variable.