match.rb

module Match
  def self.define_matchers!(m)
    m.constants.each {|c|
      eval "class #{c} < StandardError; end"
    }
  end

  def self.match! obj
    raise self.const_get(obj.class.to_s).new(obj)
  end
end

Match.define_matchers! Object

def foo(arg)
  Match.match! arg
rescue Match::String => s
  p "this is a string"
  p s
rescue Match::Fixnum => i
  p "this is a Fixnum"
  p i
end

foo 1
foo 'bar'

$ ruby p.rb
"this is a Fixnum"
#<Match::Fixnum: 1>
"this is a string"
#<Match::String: bar>