From: Intransition <transfire@...>
Date: 2011-10-15T01:18:04+09:00
Subject: Re: What’s the best way of checking if an argument has been given or not?



On Oct 14, 12:11 pm, Nikolai Weibull <n...@bitwi.se> wrote:
> On Fri, Oct 14, 2011 at 17:31, Jens Wille <jens.wi...@uni-koeln.de> wrote:
> > class A
>
> >  def b(c = d = true)  # or some other truthy value
> >    if d
> >      puts 'no argument was given'
> >    else
> >      puts c
> >    end
> >  end
>
> > end
>
> > can't remember where i first saw it, but it works perfectly ;)
>
> Yes, of course!  That’s the trick I saw too; checking if the optional
> argument’s default value was evaluated.
>
> Here’s a more general solution that allows you to set d to a value
> independent of the desired default value for the argument:
>
> def m(a = (d = true; nil))
>   # d will be true if argument a wasn’t specified
> end
>
> One easily forgets how expressive Ruby can be.  Or how easily it is to abuse it.
>
> Thanks!

Well shit, that might be my new favorite too.

  def m(a=(na=true))
    p a, na
  end

  > m
  true
  true

  > m('foo')
  "foo"
  nil

So, is there a downside to this?