Handling errors

When parglare encounter a situation in which no SHIFT or REDUCE operation could be performed it will report an error by raising an instance of parglare.ParseError class.

ParseError has the following attributes:

  • file_name - the name of the file being parsed (None if string is parsed),

  • position - an absolute position in the input where the error occured,

  • line/column - line and column where the error occured.

If there is an error in the grammar itself parglare will raise parglare.GrammarError exception.

Error recovery

There are a lot of situations where you would want parser to report all the errors in one go. To do this, parser has to recover from errors, i.e. get to the valid state and continue.

To enable error recovery set error_recovery parameter of parser construction to True. This will enable implicit error recovery strategy that will simply drop characther/object at the place of the error and try to continue. All errors will be collected as an errors list on the parser instance.

Each error is an instance of parglare.Error class. This class has the following attributes:

  • position - an absolute position of the error,
  • length - the length of erroneous part of input,
  • message - the error message,
  • input_str - the input string/list of objects,
  • expected_symbols - A set of grammar symbols expected at the location.

which are supplied to the Error constructor to build a new instance. Either a message or input_str with expected_symbols should be provided. If message is given it is used as a string representation of the error, otherwise the message is constructed based on other parameters.

position can be converted to line, column by calling parglare.pos_to_line_col(input, position).

Custom recovery strategy

To provide a custom strategy for error recovery set error_recovery to a Python function. This function should have the following signature:

def error_recovery_strategy(parser, input, position, expected_symbols):
  • parser - instance of the parser, should be used only for querying the configuration, not for altering the state,
  • input - the input string/list,
  • position - the position in the input where the error has been found,
  • expected_symbols - a set grammar symbols that are expected at this position.

The recovery function should return the tuple (error, position, token). error should be an instance of parglare.Error class or None if no error should be reported. position should be a new position where the parser should continue or None if a new token is introduced. token should be a new token introduced at the given position or None if position is advanced.

So, either position should be advanced if input is skipped, or new token should be returned if a new token is introduced at the position. You can't both advance position and introduce new token at the same time.

The token symbol should be from the expected_symbols for the parser to recover successfully. token should be an instance of parser.Token. This class constructor accepts symbol, value and length. symbol should a grammar symbol from expected_symbols, value should be a matched part of the input (actually, in the context of error recovery this value is a made-up value) and length is the length of the token. For introduced tokens, length should be set to 0.