Programming Ruby 3.3

Using instance_eval and class_eval

No matter where you are in a Ruby program, self always has a value determined by your location in the code. Sometimes it’s useful to be able to manage that relationship and change the value of self for a while.

The methods Module#instance_eval, Module#class_eval, and Module#module_eval let you set self to be an arbitrary object, evaluate the code in a block with that object as self, and then reset self:

"cat".instance_eval do
  puts "Upper case = #{upcase}"
  puts "Length is #{self.length}"
end

Produces:

Upper case = CAT
Length is 3

Inside the instance_eval block, the variable self is temporarily given the value of the object that received the instance_eval message.

All the