From: Robert Klemme <bob.news@...>
Date: 2003-07-22T14:09:11+09:00
Subject: Case Checks


When I couldn't sleep this morning, this came across my head:

class Check
  def initialize(sym, args)
    @args=args
    instance_eval "def ===(obj); obj.#{sym}(*@args); end"
  end

  class <<self
    def create(sym=nil, *args, &bl)
      if bl
        def bl.===(obj); call(obj); end
        bl
      elsif sym
        new sym, args
      else
        raise "Need either block or symbol with arguments"
      end
    end

    private :new
  end
end


Now you can do:

Negative = Check.create {|x| x.kind_of? Numeric and x<0}
Positive = Check.create {|x| x.kind_of? Numeric and x>0}
Zero     = Check.create {|x| x.kind_of? Numeric and x==0}

# error:
begin
  Foo      = Check.new :foo
  raise "No exception!"
rescue NameError
  # puts "ok"
end

[-2,-10,23,666,10.4,-23.4,"foo"].each do |i|
  case i
    when Negative
      puts "#{i} Neg"
    when Positive
      puts "#{i} Pos"
    when Zero
      puts "#{i} Zero"
    else
      puts "#{i.inspect} not numeric"
  end
end


Empty    = Check.create :empty?

for i in 0..10 do
  a=Array.new( i, "x" )

  case a
    when Empty
      puts "empty"
    else
      puts "filled (#{a.inspect})"
  end
end

    robert