my_module.rb

require 'yaml'

module MyModule
  module ClassMethods
    def from_yaml(file=nil)
      # ...
    end
  end

  module InstanceMethods
    def to_yaml(file=nil)
      file ||= "tmp/#{name.downcase}.yaml"

      File.open(file, 'w') do |file|
        f.puts YAML.dump(to_hash)
      end
    end

    def to_hash
      # ...
    end
  end

  def self.included(receiver)
    receiver.extend         ClassMethods
    receiver.send :include, InstanceMethods
  end
end

my_module_spec.rb

require 'my_module'

class MyModuleHolder
  include MyModule
end

describe "MyModule" do
  # ...

  context "instance methods" do
    subject { MyModuleHolder.new }

    # ...

    describe "to_yaml" do
      before(:each) do
        @directory = "tmp/specs"
        @file_location = "#{@directory}/test.yaml"

        Dir.mkdir(@directory) unless Dir.exists?(@directory)
        File.delete(@file_location) if File.exists?(@file_location)
      end

      after(:each) do
        File.delete(@file_location) if File.exists?(@file_location)
      end

      it "should write to a given file" do
        subject.should_receive(:to_hash) { { 'foo' => 'bar', 'baz' => 'bom' } }
        subject.to_yaml(@file_location)

        File.exists?(@file_location).should == true
      end

      it "should convert the object's hash to yaml" do
        subject.should_receive(:to_hash) { { 'foo' => 'bar', 'baz' => 'bom' } }
        subject.to_yaml(@file_location)

        expected_yaml = <<-YAML
          ---
          foo: bar
          baz: bom
        YAML

        File.read(@file_location).should == expected_yaml
      end
    end
  end
end

Why do these specs fail with "undefined local variable or method 'to_hash' for #MyModuleHolder:0xhexstuff"?

cut and paste fail? "indlue MyModule"

Retyping fail. Good eye. Fixed.