Error Recovery

1
Error Recovery

• CS 671
• February 5, 2008

2
Previously

• Top-Down Parsing, e.g. LL(1)
• Bottom-Up Parsing, e.g. LR(1)
• Next Question
• What if the compiler finds a syntactic error?

3
Error Recovery

• What if the compiler finds a syntactic error?
• Ideally, report all errors, not just first
• Approaches
• Local error recovery Find synch point, resume
• Global error recovery Change input, resume

4
Local Error Recovery

• Informally, skip to next synchronizing token
• Example
• exp ? ID
• exp ? exp exp
• exp ? ( exps )
• exps ? exp
• exps ? exps exp
• Skip to next SEMICOLON or RPAREN
• Resume parsing
• How? Introduce new terminal symbols and rules
• exp ? ( error )
• exps ? error exp

5
Caution

• Unfortunately, error states can cause problems
  when semantic actions are attached (unless were
  careful). Consider
• statements statements exp SEMICOLON
• statements error SEMICOLON
• / empty /
• exp increment exp decrement
• ID
• increment LPAREN nestnest1
• increment RPAREN nestnest-1

6
Global Error Repair

• Another way to recover insert/delete tokens
  before the point of the error
• let type a intArray 10 of 0 in
• (real error type should be var)
• Global repair Find smallest set of insertions or
  deletions that would convert string to a
  syntactically correct string (not always at error
  point)
• Outcome type ? var (1 replacement)

7
Burke-Fisher Error Repair

• Tries every possible single-token insertion,
  deletion, replacement up to K tokens before
  error.
• Chooses Whichever replacement, etc. allows it
  to progress farthest after error
• Realistic limit 4 tokens beyond error is good
  enough

error
K
8
Yacc Support for Error Correction

• The value directive (used during insertions)
• value ID (bogus)
• value INT (1)
• value STRING ()
• Programmer specified substitutions
• change EQ -gt ASSIGN
• ASSIGN -gt EQ
• SEMICOLON ELSE -gt ELSE
• -gt RBRACE

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