RSpec::Matchers.define(:parse) do |input, opts|
  match do |parser|
    begin
      @result = parser.parse(input)
      @block ? 
        @block.call(@result) : 
        (@as == @result || @as.nil?)
    rescue Parslet::ParseFailed
      @trace = parser.error_tree.ascii_tree if opts && opts[:trace]
      false
    end
  end

  failure_message_for_should do |is|
    if @block
      "expected output of parsing #{input.inspect}" <<
      " with #{is.inspect} to meet block conditions, but it didn't"
    else
      "expected " << 
        (@as ? 
          "output of parsing #{input.inspect}"<<
          " with #{is.inspect} to equal #{@as.inspect}, but was #{@result.inspect}" : 
          "#{is.inspect} to be able to parse #{input.inspect}") << 
        (@trace ? 
          "\n"+@trace : 
          '')
    end
  end

  failure_message_for_should_not do |is|
    if @block
      "expected output of parsing #{input.inspect} with #{is.inspect} not to meet block conditions, but it did"
    else
      "expected " << 
        (@as ? 
          "output of parsing #{input.inspect}"<<
          " with #{is.inspect} not to equal #{@as.inspect}" :
          
          "#{is.inspect} to not parse #{input.inspect}, but it did")
    end
  end

  # NOTE: This has a nodoc tag since the rdoc parser puts this into 
  # Object, a thing I would never allow. 
  def as(expected_output = nil, &block) # :nodoc: 
    @as = expected_output
    @block = block
    self
  end
end