“If an object quacks like a duck just go ahead and treat it as a duck” – this fundamental approach to ruby objects should take into account not the classes which objects are derived form but rather methods that define the object.

Here is an example of checking an object to see whether it is a string like.

“My string”.respond_to? :to_str  /* returns true
4.respond_to? :to_str /* returns false

Calling on obj.is_a? String tells us that object is derived from the String class but it will not catch other object types not instantiated from String class even though they are intended to be strings. Exception messages is one such example when string representing an exception is derived from Exception class and check for string.is_a? won’t produce expected result. In order to correctly determine whether certain method is of certain type, you need to use .respond_to? Versus .is_a? This is duck typing in it basic form.