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The role of the parser and Error recovery strategies ppt in compiler design
- 2. The role of the parser Error Recovery Strategies
- 4. The role of the parser
- 5. Role of the Parser The parser obtains a string of tokens from the lexical analyzer, and verifies that the string can be generated by the grammar for the source language. The parser returns any syntax error for the source language
- 6. Types of parsers The task of the parser is essentially to determine if and how the input can be derived from the start symbol of the grammar. This can be done in essentially two ways a. Top-down parser b. Bottom-up parser
- 7. Top-down parser: Top-down parsers build parse trees from the top (root) to the bottom (leaves). Input to the parser is scanned from left to right. One symbol at a time. Always expand the leftmostnon-terminal
- 8. Bottom-up parser: Bottom-up parsers build parse trees from the leaves and work up to the root. Input to the parser is scanned from left to right. One symbol at a time. And just as efficient
- 9. The role of parser interm repres errors lexical analyzer parser rest of front end symbol table source program parse tree token get next token regular expressions also technically part or parsing includes augmenting info on tokens in source, type checking, semantic analysis uses a grammar to check structure of tokens produces a parse tree syntactic errors and recovery recognize correct syntax report errors
- 10. Parsing Responsibilities Syntax Error Handling: a. Lexical: Misspellings b. Syntactic: Omission, wrong order of tokens c. Semantic: Incompatible types d. Logical: Infinite loop / recursive call
- 12. Error-Recovery Strategies The simplest approach is for the parser to quit with an informative error message when it detects the first error. Panic-mode recovery. Phrase-level recovery. Error-productions. Global-correction.
- 13. Panic-Mode Recovery This is used by most parsing methods. The synchronizing tokens are usually delimiters, such as ; or }. Skips a considerable amount of input without checking for additional errors It has the advantage of simplicity, and is guaranteed not to go into an infinite loop.
- 14. Phrase-Level Recovery Perform local correction on the remaining input; A typical local correction is to replace a comma by asemicolon. Delete an extraneous semicolon. Insert a missing semicolon. Disadvantage in coping with situations in which the actual error has occurred before the point of detection.
- 15. Error Productions Expand the grammar for the language at hand with productions that generatethe erroneous constructs.
- 16. Global Correction Compiler to make as few changes as possible in processing an incorrect input string. Given an incorrect input string x and grammar G, algorithms will find a parse tree for a related string y, such that the number of insertions, deletions, and changes of tokens required to transform x into y is as small as possible. Not implemented.
- 17. Any Question?
- 18. Thank you
Editor's Notes
- #7: Top-down parser: Top-down parsing expands a parse tree from the start symbol to the leaves Always expand the leftmost non-terminal Bottom-up parser: Start at the leaves and grow toward root And just as efficient Builds on ideas in top-down parsing Preferred method in practice Also called LR parsing L means that tokens are read left to right R means that it constructs a rightmost derivation -->
- #11: Majority of error processing occurs during syntax analysis NOTE: Not all errors are identifiable !! Which ones?