SICP Exercise 1.10.rb

def A(x,y)
  if y == 0
    0
  elsif x == 0
    2 * y
  elsif y == 1
    2
  else
    A(x - 1, A(x, y - 1))
  end
end

puts A(1, 10) #=> 1024
puts A(2, 4)  #=> 65536
puts A(3, 3)  #=> 65536

def f(n)
  # A(0, n)
  2 * n
end

def g(n)
  # A(1, n)
  2 ** n  
end

class Numeric
  def tetration(x)
    ([self] * x).inject { |acc,exp| acc ** exp }
  end
end

def h(n)
  # A(2, n)
  2.tetration(n)
end