distinct_method_defined.rb

class Module
  # is there a given distinct method defined on the instance of a class?
  def distinct_method_defined? method_name
    distinct_instance_methods.include? method_name.to_sym
  end
  
  # display the methods - including inherited - that distinguish this class's instances
  def distinct_instance_methods
    exclude = self == Object ? Kernel.public_instance_methods : Object.public_instance_methods
    public_instance_methods - exclude
  end
end

Array.method_defined? :tap
#=> true
Array.method_defined? :flatten
#=> true

Array.distinct_method_defined? :tap
#=> false
Array.distinct_method_defined? :flatten
#=> true

class SubArray < Array
  def foo
    p __method__
  end
end

SubArray.method_defined? :tap
#=> true
SubArray.method_defined? :flatten
#=> true
SubArray.method_defined? :foo
#=> true

SubArray.instance_methods(false).include? :tap
#=> false
SubArray.instance_methods(false).include? :flatten
#=> false
SubArray.instance_methods(false).include? :foo
#=> true

SubArray.distinct_method_defined? :tap
#=> false
SubArray.distinct_method_defined? :flatten
#=> true
SubArray.distinct_method_defined? :foo
#=> true

Object.instance_methods.length
#=> 138
Object.instance_methods(false).length
#=> 32
Object.distinct_instance_methods.length
#=> 64

Kernel.instance_methods.length
#=> 74
Kernel.instance_methods(false).length
#=> 74
Kernel.distinct_instance_methods.length
#=> 0 # has no distinguishing methods