# Encapsulates some object and executes expressions
# in the context of that object tracking the result 
# of the evaluation. As soon as an expression is 
# false, the verification fails.
#
#   verification = Verifier.new('Marcel') do
#     try { size == 6 } # Passes
#     try { size == 5 } # Fails
#     try { raise }     # Doesn't get here
#   end
#
#   verification.verify
#   => false
class Verifier
  class FailedVerification < Exception
  end
 
  class << self
    def verify(object, &block)
      new(object, &block).verify
    end
  end
 
  attr_reader :object, :results, :block
  def initialize(object, &block)
    @object   = object
    @results  = []
    @verified = false
    @block    = block
  end
 
  def verified?
    @verified && results.all?
  end
 
  def verify
    instance_eval(&block)
    @verified = true
  rescue FailedVerification
    false
  end
 
  def try(&block)
    results << object.instance_exec(&block)
    raise FailedVerification if results.last == false
  end
end