From: Joel VanderWerf <vjoel@...>
Date: 2003-01-24T03:36:04+09:00
Subject: emulating C preprocessor's #if

Usually, when writing ruby code, I don't worry about conditional 
compilation, and just use a run-time test. But for some critical code, I 
prefer to factor the conditional out to compile time. Unfortunately, 
this can require either a lot of duplication of source code, or 
awkwardly factoring out sub-methods which are conditionally compiled. 
Sometimes, the following approach is more convenient:

module M
   solaris = (RUBY_PLATFORM =~ /solaris/i)
   linux = (RUBY_PLATFORM =~ /linux/i)
   windows = (RUBY_PLATFORM =~ /win/i)

   module_eval %{
     def self.print_platform
       print "Platform is... "
       #{
         if solaris then %{
           puts "solaris"
         }
         elsif linux then %{
           puts "linux"
         }
         elsif windows then %{
           puts "windows"
         }
         else %{
           puts "unknown"
         }
         end
       }
     end
   }
end

M.print_platform