From: Robert Klemme <bob.news@...>
Date: 2004-11-06T01:23:42+09:00
Subject: Re: value provided for argument with default value - how to check inside method?


"Jamis Buck" <jgb3@email.byu.edu> schrieb im Newsbeitrag
news:418B9D5A.4090704@email.byu.edu...
> itsme213 wrote:
> > "Robert Klemme" <bob.news@gmx.net> wrote
> >
> >>I'm unsure what you're at.  Normally the idea of having a default
value is
> >>that you don't have to worry about where it came from:
> >>
> >>def foo(x=10)
> >>  puts x
> >>end
> >>
> >>Care to unveil a bit more detail why you would need that piece of
> >>information?
> >
> >
> > In this case, I want to use foo() and foo(x) as getter and setter
> > respectively i.e. some form of overloading.

For getters and setters there's a convention in Ruby already.  IMHO in
this case it's better to stick with foo/foo= instead of creating a new
convention.

There are some solutions for the general case of overloading, see here for
example:
http://www.rubygarden.org/ruby?MethodOverloading

> You can always do something like the following. Create a constant and
> initialize it to a unique object instance. Then, use that instance as
> the default value to your method:
>
>    VALUE_NOT_SPECIFIED = Object.new
>    def foo( x = VALUE_NOT_SPECIFIED )
>      case x
>        when VALUE_NOT_SPECIFIED
>          @x
>        else
>          @x = x
>      end
>    end
>
> Of course, someone could still explicitly pass the argument
> VALUE_NOT_SPECIFIED to the method, but then they would obviously be
> trying pretty hard to do what they aren't supposed to do...

I'm sure we could find a way to make VALUE_NOT_SPECIFIED private.  One can
also do this:

def foo(*a)
  @foo = a[0] unless a.empty?
  @foo
end

But as I said, my main concern is that this introduces a different pattern
for state access while Ruby has such a pattern already.

Kind regards

    robert