Duck Typing

Direct Answer

Duck typing means Ruby code depends on the messages an object can handle rather than requiring one declared class or interface. A method that calls notifier.deliver(message) can work with Email, SMS, or a test double as long as each honours that contract. This gives flexible composition, but missing methods fail at runtime, so clear behaviour contracts and focused tests matter.

Deep Dive & Architecture

The caller defines a protocol implicitly through the messages it sends. If a report exporter calls open, write, and close, any object with compatible implementations can participate. The protocol includes more than method names: argument forms, returned values, side effects, and failure behaviour must also line up.

This is late binding. Ruby selects the method from the runtime receiver, so unrelated classes can provide interchangeable behaviour. Test doubles are simple for the same reason: a fake object needs the small protocol exercised by the test, not the full production class hierarchy.

Flexibility moves some feedback from compile time to execution. A call may fail with NoMethodError, and a method with the right name can still have the wrong semantics. Shared examples or contract tests give every implementation the same behavioural checks. Static tooling or RBS signatures can add earlier feedback without changing Ruby's runtime dispatch.

respond_to? asks whether an object reports a method, including respond_to_missing? for dynamic APIs. It is appropriate when the branch truly supports optional behaviour—for example, call flush only on writers that offer it. For a required capability, calling the method and testing the contract usually makes the failure clearer than scattering defensive checks.

Code Example

class EmailNotifier
  def deliver(message)
    "email: #{message}"
  end
end

class SmsNotifier
  def deliver(message)
    "sms: #{message}"
  end
end

def send_alert(notifier, message)
  notifier.deliver(message)  # no class check
end

send_alert(EmailNotifier.new, 'build failed') # => 'email: build failed'
send_alert(SmsNotifier.new, 'build failed')   # => 'sms: build failed'

Common Mistakes & Pitfalls

Over-checking types with is_a? (use duck typing — call the method), or assuming a method exists without a clear contract.