Enforce a duck-typing interface with shared examples

shared_example.rb

require "rspec/autorun"

## A puddle of ducks

class AuthenticDuck
  def squeeze
    "quack"
  end
end

class GeezerDuck
  def squeeze
    "OI! I'M QUACKING HERE"
  end
end

class BrokenDuck
  # Oops no squeeze
end

### Shared examples can describe & enforce a consistent interface

RSpec.shared_examples "quacks like a duck" do
  it "can be squeezed" do
    expect(subject).to respond_to :squeeze
    expect(subject.squeeze).to match(/quack/i)
  end
end

RSpec.describe AuthenticDuck do
  include_examples "quacks like a duck"
end

RSpec.describe GeezerDuck do
  include_examples "quacks like a duck"
end

RSpec.describe BrokenDuck do
  include_examples "quacks like a duck"
end

test_output.txt

Randomized with seed 63125
.F.

Failures:

  1) BrokenDuck can be squeezed
     Failure/Error: expect(subject).to respond_to :squeeze
       expected #<BrokenDuck:0x007fe6072c6d08> to respond to :squeeze
     Shared Example Group: "quacks like a duck" called from example.rb:35
     # example.rb:21:in `block (2 levels) in <main>'

Finished in 0.01598 seconds (files took 0.12824 seconds to load)
3 examples, 1 failure

Failed examples:

rspec 'example.rb[3:1]' # BrokenDuck can be squeezed

Randomized with seed 63125