A simple example of RSpec

factorial_spec.rb

class Integer
  def factorial
    raise if self < 0
    self > 0 ? (1..self).reduce(&:*) : 1
  end
end

describe Integer, '#factorial' do
  context "for values larger than 1" do
    example { 2.factorial.should == 2*1 }
    example { 3.factorial.should == 3*2*1 }
    example { 4.factorial.should == 4*3*2*1 }
    example { 8.factorial.should == 8*7*6*5*4*3*2*1 }
  end

  context "for 1" do
    example { 1.factorial.should == 1 }
  end

  context "for 0" do
    example { 0.factorial.should == 1 }
  end

  context "for negative numbers" do
    example { expect { -1.factorial }.to raise_error }
  end
end