From: Trans <transfire@...>
Date: 2007-06-23T23:09:40+09:00
Subject: Re: avoiding nil.methodcalls short and cheap



On Jun 23, 9:17 am, Daniel DeLorme <dan...@dan42.com> wrote:
> Thorsten Rossner wrote:
> > Hi,
>
> > I often have to check if a methodcall A is not returning nil before
> > calling another method on the return value of the methodcall A. I'm
> > looking for a short and cheap (avoiding to call method A two times like
> > in this example where xmltag.get_text is method A:
> > textvalue = xmltag.get_text.value if xmltag.get_text
> > )
> > The ideal solution would be a oneliner with only little complexity
> > calling method A just one time. Am I using the wrong approach here
> > anyway?
>
> This the solution I use in *my* code for this common problem:
>    textvalue = xmltag.get_text.ergo.value
>
> This is my personal solution and I've never seen anyone use something
> like this, so it may not be a "common and accepted idiom" but it works
> well for me. YMMV
>
> code for "ergo":
>
> class NilClass
>    def ergo
>      @blackhole ||= Object.new.instance_eval do
>        class << self
>          for m in public_instance_methods
>            undef_method(m.to_sym) unless m =~ /^__.*__$/
>          end
>        end
>        def method_missing(*args); nil; end
>        self
>      end
>      @blackhole unless block_given?
>    end
> end
>
> class Object
>    def ergo
>      if block_given?
>        yield(self)
>      else
>        self
>      end
>    end
> end

Interesting... I like this. Nice use of fluent / magic dot notation.
This is a good use case for Functor too (see Facets):

  class NilClass
    def ergo
      @blackhole ||= Functor.new{ nil }
      @blackhole unless block_given?
    end
  end

One question, I'm not sure what you expect to be returned if a block
is given.

Besides that one issue, I'd like to add this to Facets and credit you.

T.