Ruby module prepend vs derivation

问题:

What are the differences between:

module Mod   
   def here
     puts 'child'
   end    
end

class A
  prepend Mod
  def here
    puts 'parent'
  end
end

and

class A
   def here
     puts 'parent'
   end
end

class B < A
  def here
    puts 'child'
  end
end

Or another way to put it: is derivating a class the same as prepending a module of the child's code?

回答1:

No, it is not. B can only inherit from one class, but Mod can be prepended to many classes. If you were to call super inside B#here, it would always refer to A#here, but inside of Mod#here, it will refer to the #here instance method of whatever class Mod was prepended to:

module Mod   
  def here
    super + ' Mod'
  end    
end

class A
  prepend Mod
  def here
    'A'
  end
end

class B
  prepend Mod
  def here
    'B'
  end
end

A.new.here
# => 'A Mod'

B.new.here
# => 'B Mod'

and

class A
  def here
    'A'
  end
end

class B
  def here
    'B'
  end
end

class C < A
  def here
    super + ' C'
  end
end

C.new.here
# => 'A C'

class C < B
  def here
    super + ' C'
  end
end
# TypeError: superclass mismatch for class C

回答2:

No, it's totally different.

One can prepend as many modules as he wants.

module A
  def foo; "A" end
end

module B
  def foo; "B" end
end

class C
  prepend A, B   # Prepending is done by this line

  def foo; "C" end
end
### take a look at it!
C.ancestors # => [A, B, C, Object, Kernel, BasicObject]
C.new.foo # => "A"

Ruby implements prepend and inheritance using different ways. prepend is internally achieved by including modules, which causes the surprising ancestors.

here is another question about prepend which may be helpful.