Question

I have a method with an optional argument. How can I decide whether the Argument was given or not?

I came up with the following solutions. I am asking this question since I am not entirely satisfied with any of them. Exists there a better one?

nil as default value

def m(a= nil)
    if a.nil?
        ...
    end
end

The drawback with this one is, that it cannot be decided whether no argument or nil was given.

custom NoArgument as default value

class NoArgument
end

def m(a= NoArgument.new)
    if NoArgument === a
        ...
    end
end

Whether nil was given can be decided, but the same problem exists for instances of NoArgument.

Evaluating the size of an ellipsis

def m(*a)
    raise ArgumentError if m.size > 1
    if m.size == 1
        ...
    end
end

In this variant it can be always decided whether the optional argument was given.
However the Proc#arity of this method has changed from 1 to -1 (not true, see the comment). It still has the disadvantage of beeing worse to document and needing to manually raise the ArgumentError.

Answer 1

Jorg W Mittag has the following code snippet that can do what you want:

def foo(bar = (bar_set = true; :baz))
  if bar_set
    # optional argument was supplied
  end
end