Default Parameter -- Dynamic Define, Recursive Call

In method parameter, we can write any kind of assignment statements in Ruby.

The most popular default parameter is literals:

def f(a, b = :aaa)
  ...
end

Rarely we can see method calls in default parameter:

def f(a, b = Array.new)
  ...
end

def g(a, b = 1 + 2)
  ...
end

We can refer the other precedent parameters:

def f(a, b = a * 2)
  b
end
p f(10) #=> 20

We can write more complicated code there.

def g(a, b = (def j; end; 1))
  [a, b]
end

This code works tricky. After g was called without the second argument, method j is defined.

We also can write recursive call there.

def fib(n, x = n == 0 ? 0 : n == 1 ? 1 : fib(n-2) + fib(n-1))
  x
end

p fib(10) #=> 55
p fib(10, :hehehe) #=> :hehehe