Error recovery strategies OR Ad-hoc and systematic methods

1. Panic mode

• This strategy is used by most parsing methods. This is simple to implement.
• In this method on discovering error, the parser discards input symbol one at time. This process is continued until one of a designated set of synchronizing tokens is found. Synchronizing tokens are delimiters such as semicolon or end. These tokens indicate an end of input statement.
• Thus in panic mode recovery a considerable amount of input checking it for additional errors.
• This method guarantees not to go in infinite loop. is skipped without
• If there is less number of errors in the same statement then this strategy is best choice.

2. Phrase level recovery

• In this method, on discovering an error parser performs local correction on remaining input.
• It can replace a prefix of remaining input by some string. This actually helps parser to continue its job.
• The local correction can be replacing comma by semicolon, deletion of semicolons or inserting missing semicolon. This type of local correction is decided by compiler designer.
• While doing the replacement a care should be taken for not going in an infinite loop.
• This method is used in many error-repairing compilers.

3. Error production

• If we have good knowledge of common errors that might be encountered, then we can augment the grammar for the corresponding language with error productions that generate the erroneous constructs.
• If error production is used during parsing, we can generate appropriate error message to indicate the erroneous construct that has been recognized in the input.
• This method is extremely difficult to maintain, because if we change grammar then it becomes necessary to change the corresponding productions.

4. Global correction

• We often want such a compiler that makes very few changes in processing an incorrect input string.
• Given an incorrect input string x and grammar G, the algorithm will find a parse tree for a related string y, such that number of insertions, deletions and changes of token require to transform x into y is as small as possible.
• Such methods increase time and space requirements at parsing time.
• Global production is thus simply a theoretical concept.