From: Robert Klemme <bob.news@...>
Date: 2004-11-08T18:28:41+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:418BAC7C.2010201@email.byu.edu...
> Robert Klemme wrote:
>
> > But as I said, my main concern is that this introduces a different
pattern
> > for state access while Ruby has such a pattern already.
>
> It has, but it breaks down in certain situations. Consider:
>
>    class T
>      def initialize( &block )
>        instance_eval &block
>      end
>      def v
>        @v
>      end
>      def v=( x )
>        @v = x
>      end
>    end
>
>    t = T.new do
>      v = 5
>    end
>
> Does T#v= get called? Nope. v is interpreted as a local variable. As it
> should be--I'm not arguing for it to do otherwise. All I'm saying is
> that there are times where it is useful to have the setter be like the
> OP requested. There is even code in Ruby's stdlib that does it that way
> (tk, for instance).

I didn't think of this situation, thanks for the reminder!  For the
initialization I prefer this idiom:

   class T
     DEFAULT_VALUES = {
        :v => 10
     }.freeze

     def initialize( values = {} )
       DEFAULT_VALUES.merge(values).each do |k,v|
         send("#{k}=",v)
       end
     end
     def v
       @v
     end
     def v=( x )
       @v = x
     end
   end

   t = T.new(
     :v => 5
   )

Advantage of this variant over the one with the block is, that you can
easily determine which values were provided by the caller.  That's more
difficult with the block.

> Ruby is known for it's position on TMTOWTDI. Let's not discourage people
> from exploring these other avenues. If an approach turns out to be
> cumbersome, it won't be adopted. If it works well, people will use it.

Of course everybody is free to do what they like.  My point is, that it
might cause hazzle if someone uses a different pattern for something so
basic like attribute assignment where there is a well established
different pattern for this.  While we should not follow conventions
slaverishly, conventions are in fact important and do have their benefits.

Kind regards

    robert