From: Paul Brannan <pbrannan@...>
Date: 2008-04-08T02:34:37+09:00
Subject: Re: Lisper says to rubyist: gimmie your syntax tree

Tracer doesn't inject "trace statements" into the AST.  It calls
set_trace_func, so there's a callback for every event (method call,
method return, new line, etc.).

The performance bottleneck of Tracer is that one of the arguments to the
trace function is a binding; creating this binding is an expensive
operation.  AFAIK the only way to avoid this is to use the C API to call
rb_add_event_hook instead of using set_trace_func from Ruby.  This is
what the ruby-debug gem does, e.g.:

  rdebug --trace test.rb

As for your idea of dynamically modifying program structure at run-time,
I've toyed with this idea, but it's not trivial.  The biggest hurdle is
Ruby's handling of local/dynamic variables.  IMO the AST is too
low-level a program representation to be used for this.  An intermediate
representation that abstracts away some of the details of the AST is I
think what's needed.

Paul