1. ERROR HANDLING
Lecture on 27/08/2013
PPT: 11CS10037 SAHIL ARORA

2. Syntax Error Handling
Next few lectures will focus on the nature of syntactic errors and general strategies for error-recovery.
If a compiler had to process only correct programs, its design would be extremely simple. However it is expected to Assist the programmer in locating and tracking errors.
One should note that a programming language does not specify how a compiler should respond to errors. It is actually left entirely to the compiler designer. 2

3. Review: Common Programming Errors
Semantic Errors: Type mismatches between operators and operands. Like, a return statements with return type void.
Syntactic Errors: Misplaced semicolons, extra or missing braces.
Lexical errors: Misspellings of keywords, identifiers, and operators.
Logical Errors : Incorrect reasoning or plain carelessness might result in errors like interchangeably using = and == operators.

4. Challenges of Error Handling
Viable-Fixable property: detecting an error as soon as a prefix of the input cannot be completed to form a string of the language. Goals of an Error Handler:
Reporting presence of errors, clearly and accurately.
Recovering from errors quickly enough to detect subsequent errors.
Add minimal overhead to the processing of correct programs.

5. Error recovery There is no universally acceptable method.
The simplest method is for the parser to quit with the appropriate error message, at the first instance of an error.
Problem? Subsequent errors will not be detected.
Solution? If the parser can restore itself to a state where processing can continue, future errors can be detected.
In some cases compiler stops if errors pileup.

6. Error Recovery Strategies
Panic Mode Recovery: The parser discovers an error. It then discards input symbols till a designated set of synchronizing token is found.
Synchronizing tokens selected are such that their role in the program is unambiguous, like Delimiters ; } etc. Advantage? Simple and never goes into an infinite loop.
Drawback: Skips considerable amount of input when checking for additional errors
How long will a tree walk take?
Prove that inorder walk prints in monotonically increasing order

7. Phrase-level Recovery
Local Correction by parser on remaining input, by some string which allows parser to continue.
Replacing comma by semicolon, inserting extra semicolon etc.
Drawbacks: Improper replacement might lead to infinite loops. More importantly, if the error has occurred before the point of detection.
Advantage: It can correct any input string.

8. Error Productions
A method of anticipating common errors that might be encountered.
Augmenting the grammar for the language at hand, with productions that generate erroneous constructs.
Such a parser will detect anticipated errors when an error production is used.
Advantage: Error diagnostics will be readily available for such anticipated errors.

9. Global Corrections
Ideally minimum changes should be made to the input string.
Given an input string x, algorithm finds parse tree for a related string y, such that number of insertions, deletions, and token changes required for converting x to y is minimum.
Drawback: Too costly to implement in terms of both time and space, and only theoretical.
It is however used as a yardstick for evaluating error-recovery techniques.