[ruby-talk:21627] Re: Please translate this easy snip of C++ to Ruby

> Dave Thomas wrote:
> 
> >      trace {"a+f"}
> 
> Forgot about blocks going in, but...
> 
> I figured that with 'eval' (and with some shenanigans to transport the 
> scope), we'd have a solution that requires quotes.
> 
> The python fix throws a private exception inside the 'trace', catches it, 
> examines the stack dump, opens the file containing the calling code, reads 
> the line by number, locates the 'trace', and reads what's inside the 
> parens. No quotes required.
> 
> In Ruby can we inspect the current call stack this easy? or maybe easier?

Yes, I think easier. Based on what you said about how it is done in Python
and on Dave's solution, here is a simple, poorly tested approach:

---
def trace_(&b)
	file,line_nr=caller()[0].split(':')
	line_nr=line_nr.to_i
	linecnt=1
	expr=nil
	File.open(file).each_line do |line|
		if linecnt==line_nr
			if line =~ /(^|\W)trace_\s*{\s*(.*?)\s*}/
				expr = $2
			end
			break
		end
		linecnt+=1
	end
	if expr
		puts "#{expr}: #{eval(expr, b).inspect}"
	end
end

a = 1
b = 2

trace_ { a + b };
c='x' * b; trace_{ c*2 }
trace_ { 1+1 }
---

Only one trace_ per line, no multiline blocks, and other limitations that are
going to appear if you use this :)