From: Chris Pine <nemo@...>
Date: 2002-12-31T03:13:18+09:00
Subject: Re: capturing puts in mod_ruby

Well, I figured it out.  Just in case anyone is interested, here is the
complete code I'm using.  This executes the code in the ``code'' parameter
(a string), supplying the strings in the ``input'' array as user input when
requested, and it returns the output in a string.  (The variable ``@@INPUT''
is just a string I am using to help me to distinguish output from echoed
input.)

The idea was to create a sufficiently robust execution environment for
running code samples and showing what those code samples do if run on the
command line.  (Of course, "sufficiently" is a moving target... or maybe a
shrinking target.)  I wanted as few side-effects as possible (since I'm
running this in mod_ruby).  Aside from a tutorial, I think this might be
nice to incorporate into a ruby-based wiki... but I don't know much about
$SAFE levels and all that.

It's my dream that someday we'll have a wiki-based documentation of the
built-in classes and standard libraries, along with live code examples, so
that we can all help to keep the docs up-to-date.  (The Pickaxe reference
section, while wonderful, is slowly falling out of date... it would be nice
to have a more-or-less "definitive" place where we could keep up-to-date
docs.)  Anyway, I need to finish this tutorial first...

So here's the code.  It handles errors (including parse errors), and
all-in-all seems to work pretty well.  There are a few sticky areas when
running from mod_ruby... basically, this code acts like you are running it
in mod_ruby (go figure!) instead of on the command line.

  def executeCode (code, input)
    #  Wrap code to catch errors and to stop SystemExit.
    code = <<-END_CODE
      begin
        #{code}
      rescue SystemExit
      rescue Exception => error
        puts error.inspect
      end
    END_CODE

    strIO = StringIO.new

    if !input.empty?
      input = input.join("\n")+"\n"
      input = StringIO.new (input, "r")
      class << strIO; self; end.module_eval do
        ['gets', 'getc', 'read'].each do |meth|
          define_method(meth) do |*params|
            inStr = input.method(meth).call(*params)
            puts @@INPUT+inStr.chomp+(@@INPUT.reverse)  #  Echo input.
            inStr
          end
        end
      end
    end

    #  Pass these methods to strIO:
    kernelMethods = ['puts', 'putc', 'gets']

    #  Swap out Kernel methods...
    kernelMethods.each do |meth|
      Kernel.module_eval "alias __temp__tutorial__#{meth}__ #{meth}"
      Kernel.module_eval do
        define_method(meth) do |*params|
          strIO.method(meth).call(*params)
        end
      end
    end

    begin
      strIO.instance_eval code
    rescue Exception => error  #  Catch parse errors.
      return error.inspect
    end

    #  ...and swap them back in.
    kernelMethods.each do |meth|
      Kernel.module_eval "alias #{meth} __temp__tutorial__#{meth}__"
    end

    strIO.string
  end

Thoughts?  Suggestions?  Errors?

Chris