From: "Guy N. Hurst" <gnhurst@...>
Date: 2002-05-05T04:08:39+09:00
Subject: Object#pred


One way to solve this whole issue is to define the set of values 
we wish to use #pred with. So, given a range from X to Y, a set
of values is generated by #succ in a particular order. We can then
use this resulting set or array as the determining factor when
selecting the predecessor of a given element. Then, it will always
be true that "aString.succ.pred  = aString.pred.succ = aString".

Here is a sample implementation which accepts either
a range or some values to make a range from. It is valid
for any object for which #succ is defined.


class Object
  def pred(*args)
    if Range===args[0]
      arr=args[0].to_a
    else
      begin
        rng=((args[0])..(args[1]))
      rescue ArgumentError
        raise "Invalid Range Arguments"
      end
      arr=rng.to_a
    end
    if (pos=arr.index(self).to_i).zero?
      raise "Predecessor out of Range" 
    end
    arr[pos-1]
  end
end


EXAMPLES:

5.pred(0..9)	# 4
5.pred(0,9)	# 4

-3.pred(-9,9)	# -4

"d".pred("a".."z")	# "c"
"0".pred("\0","\255")	# "/"

"23".pred("\0","\255")		# RuntimeError: Predecessor out of Range
"23".pred("\0","\255\255")	# "22"

"2x3".pred("\0","\255\255\255")	# RuntimeError: Predecessor out of Range
"2x3".pred("0a0","9z9")		# "2x2"


class Class
  alias succ superclass
end

Numeric.pred(Fixnum..Object)	# Fixnum (thanks Christoph ;-)


Guy N. Hurst