From: Logan Capaldo <logancapaldo@...>
Date: 2006-10-11T23:04:48+09:00
Subject: Re: ERB as a macro pre-processor?

On Wed, Oct 11, 2006 at 02:40:58PM +0900, Rich Morin wrote:
> "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?
Bah I say! How does that saying go, "Replace conditionals with
polymorphism"?

class Platform
end

class Linux < Platform
  def define_methods
    def a
      puts "linux"
    end
  end
end

class Windows < Platform
  def define_methods
    def a
      puts "windows"
    end
  end
end

class Platform
  def self.get_platform
    case RUBY_PLATFORM
      when /linux/
        Linux
      when /win32/
        Windows
     end
  end
end

platform = Platform.get_platform.new
platform.define_methods