From: Trans <transfire@...>
Date: 2005-11-27T07:22:28+09:00
Subject: Re: string range membership

Florian, your double dispatch is interseting. While I still have no
idea if anyone has understand the #cmp solution I've proposed since no
one has commented on it. A fully general solution of #cmp looks
something like this:

  def cmp( other )
    return 0 if self == other
    loop
      before, after = other.succ, self.succ
      return -1 if before == self
      return 1 if after == self
    end
  end

Of course no one would never use this becuase 'other' may not be an
actual member and thus never hit on ==. So the only way to ensure
member comparsion in a fully general way is to have the Range on hand
--hence your double dispatch. A generalized solution would then be:

  def cmp( other, range )
    return 0 if self == other
    arr = range.to_a
    arr.index( self ) <=> arr.index( other )
  end

But this is silly since Range can do this itself, no need to double
dispatch --if #cmp is not defined on the object, Range can always
expand into an array and compare indexes itself. But there's still the
rick of infinite expansions.

Likewise I think the double dispatching within an #element? method is
in the same league. If a #cmp can't be defined and used to determine
membership neither will an #element? method be able to, so Range then
must resort to 'to_a.include?'

Range is better off depending on a comparision method just for it to
ensure compatibility with #succ --which also ensures determination of
memebership with the methods we already have #member? and #include?
--Then they would do exaclty what the documentation says they're
supposed to do, which they actually DO NOT do at the moment.

T.