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Expiring a method. And let’s keep meta out of this.

 class Trial
 def run_me
 def self.run_me; raise Exception, "NO MORE." end
 puts "Your trial period has ended." 
 end
 end
 t = Trial.new
 t.run_me
 #=> Your trial period has ended.
 t.run_me
 #=> (trial):3:in `run_me': NO MORE. (Exception)

The run_me overwrites itself. But notice it uses def self. This overwrites the method in the object, not in the class. So you can create more Trial objects which haven’t self-destructed yet.

Can also be used as a memoization technique. But, rather than caching the data, you just ensure that the code runs only once.

 class Hit
 def initialize(ip)
 @ip = ip
 end
 def country
 def self.country; @country end
 @country = `geoiplookup #{@ip}`.chomp.gsub(/^GeoIP Country Edition: /,"")
 end
 end

I should correct one thing. I’m not actually overwriting the method. Just capping it off with a new singleton method.

For more on singleton methods: an intro and some old thoughts on duck-typed singletons.