From: Rich Morin <rdm@...>
Date: 2006-10-11T14:40:58+09:00
Subject: ERB as a macro pre-processor?

"The Ruby Way" (pp. 43-45) shows a couple of different ways
to generate methods dynamically (using eval).  Looking at the
examples, I began to wonder about different Ways To Do It.

One possibility that occurred to me is to use ERB as a macro
pre-processor.  The book suggests the following code:

  if platform == Windows
    def my_action
      action1
    end
  if platform == Linux
    def my_action
      action2
    end
  else
    def my_action
      default_action
    end
  end

Although this is approach is direct and simple, it seems like it
wouldn't scale very well, given multiple variations.  However, I
think that the following approach might:


% cat erb_ml
#!/usr/bin/env ruby

  require 'erb'

  def action_D; print "default action\n"; end
  def action_L; print "Linux action\n"; end
  def action_W; print "Windows action\n"; end

  action = { :Linux   => 'action_L',
             :Windows => 'action_W' }

  template = ERB.new <<-EOF
    def my_action
      <%= action[platform] || 'action_D' %>
    end
  EOF

  for platform in [ :Linux, :Windows, :Plan9 ] do
    eval template.result(binding)
    print "#{platform.to_s}: "
    my_action
  end

% erb_ml
Linux: Linux action
Windows: Windows action
Plan9: default action


Obviously, the "action" hash could be replaced by other
code-generation and/or -retrieval methods.  Is anyone
here using ERB in this manner?  Are there any caveats
or alternative approaches that I should be aware of?

-r
-- 
http://www.cfcl.com/rdm            Rich Morin
http://www.cfcl.com/rdm/resume     rdm@cfcl.com
http://www.cfcl.com/rdm/weblog     +1 650-873-7841

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