Did we default or not?

You may sometimes find yourself having to distinguish whether a method attribute was supplied externally or taken from the default specified in the method definition.

Let's say, for example, that you want to warn a user when they have neglected to set an attribute but still continue with execution. Here is a snippet that would accomplish this:

irb(main):060:0> def some_method(first, second=(flag=true; '2nd'))
irb(main):061:1> p "Default value #{second} used for unspecified parameter 'second'" if flag.inspect == "true"
irb(main):062:1> end
=> nil
irb(main):063:0> some_method(1,2)
=> nil
irb(main):064:0> some_method(1,'2nd')
=> nil
irb(main):065:0> some_method(1)
"Default value 2nd used for unspecified parameter 'second'"
=> nil
irb(main):066:0>

Can you work out what is going on in the method parameter declaration?

All that's happening is that the code in the round brackets after the equals sign defines a local variable _flag_, sets its value and returns the default value we want to set it to.

Ruby rocks!