Moonbase

Ruby has a nice feature where you can override methods on a per-object basis (singleton methods). For example:

foo = "spleen" 
p foo.upcase #=> "SPLEEN" 

def foo.upcase
  "BREAD" 
end

p foo.upcase #=> "BREAD" 

# other objects not affected
p foo.dup.upcase #=> "SPLEEN" 
p "bump".upcase #=> "BUMP" 

# but cloned objects also get the override
p foo.clone.upcase #=> "BREAD"

Maybe, though, sometimes you don’t want an object’s methods to be overridable in this way. Maybe you want the class and only the class to decide. I don’t know why, but let’s say you do.

It’s possible to disable this feature on a per-object basis.

foo = "lung" 
class << foo ; freeze ; end

# DEATH!
def foo.upcase
  "CHEESE" 
end

Attempting to override a method on foo will now throw an exception. Its singleton class is untouchable. It is un-meta-able. Eigen-caltrops, is what this is.