From: Joel VanderWerf <vjoel@...>
Date: 2004-08-04T09:12:41+09:00
Subject: Re: Macros in Ruby

Jesse Jones wrote:
> For example, suppose you're writing a game and you have a lot of code
> that selects behavior based on percentages (75% of hits are normal
> blows, 20% are critical hits, and 5% kill your foe out-right for
> example). Even in a language as expressive as Ruby this is a bit
> awkward to write and not especially intentional. But with macros it can
> be written very cleanly:
> 
> percent-case
>    0.75 case
>       normal_hit
>    end
> 
>    0.20 case
>       critical_hit
>    end
> 
>    otherwise
>       kill_foe
>    end
> end

It's not really that hard to do something like this in ruby, but it is a 
little less efficient than it would be with macros.

   class PercentCase
     def initialize block
       @cases = []
       instance_eval(&block)
     end

     def choose
       n = rand(100)
       @cases.each do |percent, block|
         n -= percent
         return block.call if n < 0
       end
       otherwise.call
     end

     def chance percent, &block
       @cases << [percent, block]
     end

     def otherwise(&block)
       @otherwise = block
     end
   end

   def percent_case(&block)
     PercentCase.new(block).choose
   end

   10.times do
     percent_case do
       chance 75 do
         puts "normal_hit"
       end

       chance 20 do
         puts "critical_hit"
       end

       otherwise do
         puts "kill_foe"
       end
     end
   end

To make it more efficient, you could keep the PercentCase object around 
and reuse it each time.