1.rb

# famous trick from The Ruby Way to detect whether default value was used or not

def foobar(variable=((default=true);:value))
  p variable, default
end

foobar :value # => default = nil
foobar # => default = true

# right way (won't work for nil and false but who cares)

def foobar(variable=nil)
  default = not variable
  variable ||= :value
end


# right way in case somebody cares about nil and false O_O

def foobar(variable=:__default__)
  if variable == :__default__
    default = true
    variable = :value
  end
end

2.rb

# variation of this trick to handle arrays and splatted arrays
def coordinates(x, y=(x,_ = x; _))
  p x, y
end

coordinates [1,2] # => 1, 2
coordinates 1, 2 # => 1, 2

# right way

def coordinates(*args)
  x, y = args.flatten
end