Ruby fun: a one time method override.

hardcoded_swap.rb

require 'minitest/spec'
require 'minitest/autorun'

class String
  def one_time_override(method, &override)
    self.instance_eval do
      def to_s
        self.instance_eval do
          def to_s
            'foo'
          end
        end
        'bar'
      end
    end
  end
end

describe String do
  it "should allow a one-time override of a method" do
    foo = 'foo'
    foo.to_s.must_equal 'foo'
    foo.one_time_override(:to_s) { 'bar' }
    foo.to_s.must_equal 'bar'
    foo.to_s.must_equal 'foo'
  end
end

preserving_original.rb

require 'minitest/spec'
require 'minitest/autorun'

class String
  def one_time_override(method)
    @original = self.method(method)
    self.instance_eval(%Q|def #{method.to_s}; return @original.call if @called; @called = true; "#{yield}"; end|)
  end
end

describe String do
  it "should allow a one-time override of the to_s method" do
    foo = 'foo'
    foo.to_s.must_equal 'foo'
    foo.one_time_override(:to_s) { 'bar' }
    foo.to_s.must_equal 'bar'
    foo.to_s.must_equal 'foo'
  end
  
  it "should allow a one-time override of the downcase method" do
    foo = 'Foo'
    foo.downcase.must_equal 'foo'
    foo.one_time_override(:downcase) { foo.upcase }
    foo.downcase.must_equal 'FOO'
    foo.downcase.must_equal 'foo'
  end
end