Ruby Trivia

wag_the_dog.rb

class Dog
  def wag(tail=my_tail)
    tail
  end
  def my_tail
    :dogs_tail
  end
end

class Cat
  def wag_dog
    Dog.new.wag
  end
  def my_tail
    :cats_tail
  end
end

puts Cat.new.wag_dog          # What is printed here?
                              # Make a prediction before you try it.

It's evaluated in the context of the method. Try this var = yield. :)

Here's another variation that might also cause confusion:

class Dog
  def wag(tail=my_tail)
    tail
  end

  def self.my_tail
    :dogs_tail
  end

  def my_tail
    :fidos_tail
  end
end

puts Dog.new.wag

Since the default value looks like it is called outside of the method it could be mistaken as a class method.

I thought this was confusing at first, until I realised what self would be in the implicit my_tail method call. Now it's hard to see any other result!

class Dog
  def make_noise(sound = (p self; :bark))
    sound
  end
end

class Cat
  def dog_sound
    Dog.new.make_noise
  end
end

Cat.new.dog_sound

class A
  def foo a, b=a
    puts a; puts b
  end
end

class B < A
  def foo a
    super
    puts 'B'
  end
end

m = A.instance_method :foo
puts "invoking on an A"
A.new.foo :bar

puts "*" * 31

puts "invoking on a B"
B.new.foo :blat

puts "*" * 31

puts "binding to a B"
m.bind(B.new).call :baz

A little twist...

class A
  def foo a, b=a
    puts a; puts b
  end
end

class B < A
  def foo a
    super
    puts 'B'
  end
end

m = A.instance_method :foo
puts "invoking on an A"
A.new.foo :bar

puts "*" * 31

puts "invoking on a B"
b=B.new
b.foo :blat

puts "*" * 31

puts "binding to a B"
m.bind(b).call :baz

puts "*" * 31

puts "invoking on a B"
b.foo :blat

I think any confusion stems from the chained Dog.new.wag

If you instead wrote that your cat wagged its dog thusly:

dog = Dog.new
dog.wag

I hope no one would think that the wag method on dog somehow reaches back to it's caller to get a parameter it wasn't passed...

• its

now with lambdas... (ok, I suck at this)

dog = ->{ :dogs_tail }
cat = ->(dog){ dog.call }
devious_kitty = ->(cat,dog) {|cat| cat.call(dog) }
devious_kitty.call(cat,dog)

Putted even more simply it's all about this:

Foo = Class.new do
  def bar(x=self)
    x
  end
end

Foo.new.bar