How to detect variable changes/assignments in ruby

Question

I'm trying to write a debugger in ruby. I want to run some code every time a variable changes. I don't know the variables in advance.

At the moment I'm trying set_trace_func but it seems to execute before each line, rather than after each line eg;

set_trace_func proc { |event, file, line, id, binding, classname|
    if file == "(eval)"
        printf "%8s %s:%-2d %10s %8s\n", event, file, line, id, classname
        args = binding.eval("local_variables").inject({}) do |vars, name|
            value = binding.eval name.to_s
            vars[name] = value unless vars.nil?

            vars
        end
        puts args.inspect
    end
}

eval("blah = 4\nrar = 3\n")

set_trace_func(nil)

When I execute it I get;

n@VirtualBox:~$ ruby blah.rb 
    line (eval):1                     
{:blah=>nil, :rar=>nil}
    line (eval):2                     
{:blah=>4, :rar=>nil}
n@VirtualBox:~$

This is not what I want - it's not showing the assignment to rar. Is there a better way to do this? As a hack, I added one last statement to the eval eg.

eval("blah = 4\nrar = 3\nnil\n")

but that doesn't fix the problem when it comes to loops etc.

I would like to get an output of what line a variable changed on and what it changed to in order.

dbenhur · Accepted Answer · 2012-04-09 13:32:50Z

1

I don't know of a mechanism in pure ruby to help you with writing a debugger and hooking low level execution features such as variable assignment. The ruby-debug is a C extension with deep knowledge and hooks in to the execution engine. You can read it here.

answered Apr 9, 2012 at 13:32

dbenhur

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1 Comment

Anko Over a year ago

Thanks for the response. My c knowledge is a bit out of date and I'm having a lot of trouble getting something useful from that.

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