Plough => Ruby

Journey through ruby

An Interesting Ruby Method

In this post, I will talk about a ruby method that I had no idea existed which might come handy in debugging your code.

The method I am talking about is set_trace_func. It’s part of the Kernel class and it does what it says. Allows one to set the method tracing on a method. The following snippet explains it further. Here, we have a class, TestingSetProcFunc:

    class TestingSetProcFunc
      def traceThisMethod
        source = 1
        target = 2

To inject set_trace_func in, it needs to be called before the method is invoked, like this:

    puts "Event    File:Line                Id          Binding        Classname"
    set_trace_func proc { |event, file, line, id, binding, classname|
      printf "%8s %s:%-2d %20s %12s %8s\n", event, file, line, id, binding, classname

It takes a proc with upto 6 arguments. These are the events with their brief descriptions:

  • c-call (call a C-language routine)
  • c-return (return from a C-language routine)
  • call (call a Ruby method)
  • class (start a class or module definition)
  • end (finish a class or module definition)
  • line (execute code on a new line)
  • raise (raise an exception)
  • return (return from a Ruby method).

Now, to run this we just need to call the method and look at the output:

    tracer =

This is what the output will look like when you run it on the command line:

Event    File:Line                Id          Binding        Classname
c-return test.rb:11       set_trace_func #   Kernel
    line test.rb:13                      #         
  c-call test.rb:13                  new #    Class
  c-call test.rb:13           initialize # BasicObject
c-return test.rb:13           initialize # BasicObject
c-return test.rb:13                  new #    Class
    line test.rb:14                      #         
    call test.rb:2       traceThisMethod # TestingSetProcFunc
    line test.rb:3       traceThisMethod # TestingSetProcFunc
    line test.rb:4       traceThisMethod # TestingSetProcFunc
  return test.rb:5       traceThisMethod # TestingSetProcFunc    

This output gives one a good idea about the work that is happening in the background. The weird looking Binding instance is the context at that particular place in the code. I have just scratched the surface and I am sure there’s a lot one can do with this method. Have a look at the ruby’s test for set_trace_func for more useful ways of employing it.