Discuss different error recovery strategies.

Error Recovery Strategies in Syntax Analysis

When a parser detects a syntax error, it must attempt to recover and continue parsing the remaining input to find further errors. There are four main strategies used for error recovery:

1. Panic-Mode Recovery

• The simplest and most widely used method.
• On detecting an error, the parser discards input symbols one by one until it finds a synchronizing token (like ; or }).
• Synchronizing tokens are chosen so that parsing can resume safely.
• Advantages: Simple, fast, and guaranteed to terminate (no infinite loop).
• Drawback: May skip large portions of code and miss additional nearby errors.

2. Phrase-Level Recovery

• The parser makes local corrections to the remaining input.
• Example corrections:
  • Replace , with ;
  • Insert a missing )
  • Delete an extra ;
• Common in error-repairing compilers.
• Advantage: Can fix many syntactic errors.
• Drawback: May fix the error after the actual location, leading to confusion.

3. Error Productions

• Extend the grammar with special rules (error productions) that match common programming mistakes.
• The parser uses these rules to detect and report specific errors.
• Advantage: Gives meaningful diagnostics.
• Drawback: Requires anticipating and coding all possible common errors.

4. Global Correction

• A theoretical approach where the compiler finds the least number of changes (insert, delete, replace tokens) to make the program syntactically correct.
• Advantage: Finds the most optimal correction.
• Drawback: Too slow and costly in practice (not used in real-world compilers).