From: gotoken@... (GOTO Kentaro)
Date: 2001-01-04T06:33:39+09:00
Subject: [ruby-talk:8596] Re: bug??

In message "[ruby-talk:8592] Re: bug??"
    on 01/01/04, Jean Michel <jmichel@schur.institut.math.jussieu.fr> writes:
>It  would be wonderful  if you also  add a
>similar alternative constructor for Array.

I could list two reasons:
  * Array has ordered index set (i.e. Integer) and substituion to 
    a new position `i' will allocate for all posisions before `i'. 
    So such default mechanism leads to many inefficient code parhaps. 
  * We rarely need default mechanism for Array. 

Btw, you can implement by a Ruby code if you want that: 

  class Array
    class << self
      alias __new__ new
      
      def new(*arg, &callback)
        ary = __new__(*arg)
        ary.instance_eval{@default_proc = callback}
        ary
      end
    end

    attr_reader :default_proc
    alias aref []

    def [] (*arg)
      if arg.size == 1 
        i, = arg
        if default_proc and i.is_a? Integer and i+1 > size
          default_proc.call(i, self)
        end
      else
        aref *arg
      end
    end
  end

  a = Array.new{|i,a| a[i] = ""}
  a[3].concat "a"
  p a                            #=> [nil, nil, nil, "a"]

Now, I can't decide we should fill by nil or "" as the elements before
a[3] above.  Furthermore, any behaviors of like a[0..3] or a[0,3] seem
to be non-trivial if a block is given to Array.new.  It may be the
biggest reason why we consider not to propose that for Array.  After
all, Hash and Array are not so similar object, I thought.

Thanks, 

-- Gotoken