Syntax error handling

1

• Syntax error handling
• Errors can occur at many levels
• lexical unknown operator
• syntactic unbalanced parentheses
• semantic variable never declared
• runtime reference a NULL pointer
• Goals of error-handling in a parser
• To detect and report the presence of errors
• To recover from an error and detect subsequent
  errors
• To not slow down the processing of correct
  programs

2
Error recovery strategies

• Panic mode recovery
• On discovering an error, discard input symbols
  one at a time until one of a designated set of
  synchronizing token is found.
• Phrase-level recovery
• On discovering an error, perform a local fix to
  allow the parser to continue.

3

• Error recovery in predictive parsing
• Recovery in a non-recursive predictive parser is
  easier than in a recursive descent parser.
• Panic mode recovery
• If a terminal on stack, pop the terminal.
• If a non-terminal on stack, shift the input until
  the terminal can expand.
• Phrase-level recovery
• Carefully filling in the blank entries about what
  to do.

4

• Error recover in LR parsing
• Canonical LR parsers never make extra reductions
  when recognizing an error.
• SLR and LALR may make extra reductions, but will
  never shift an erroneous input symbol on the
  stack.
• Panic mode recovery
• Scan down stack until a state representing a
  major program construct is found. Input symbols
  are discarded until one is found that is in the
  follow of the nonterminal. Trying to isolate the
  phrase containing the error.
• Phrase level recovery
• Implement an error recovery routine for each
  error entry in the table.

5

• Writing a parser with YACC (Yet Another Compiler
  Compiler).
• Generates LALR parsers
• Work with lex. YACC calls yylex to get next
  token.
• YACC and lex must agree on the values for each
  token.
• Produce y.tab.c file by yacc yaccfile, which
  contains a routine yyparse().
• yyparse() returns 0 if the program is ok,
  non-zero otherwise
• YACC file format
• declarations
• translation rules
• supporting C-routines

6

• The declarations part specifies tokens,
  non-terminals symbols, other C constructs.
• To specify token AAA BBB
• token AAA BBB
• To assign a token number to a token (needed when
  using lex), a nonnegative integer followed
  immediately to the first appearance of the token
• token EOFnumber 0
• token SEMInumber 101
• Non-terminals do not need to be declared unless
  you want to associated it with a type (will be
  discussed later).

7

• Translations rules specify the grammar
  productions
• exp exp PLUSnumber exp
• exp MINUSnumber exp
• exp TIMESnumber exp
• exp DIVIDEnumber exp
• LPARENnumber exp RPARENnumber
• ICONSTnumber
• exp exp PLUSnumber exp
• exp exp MINUSnumber exp

8

• Yacc environment
• Yacc processes the specification file and produce
  a y.tab.c file.
• An integer function yyparse() is produced by
  Yacc.
• Calls yylex() to get tokens.
• Return non-zero when an error is found.
• Return 0 if the program is accepted.
• Need main() and and yyerror() functions.
• Example
• yyerror(str)
• char str
• printf("yyerror s at line d\n", str,
  yyline)
• main()
• if (!yyparse()) printf("accept\n")
• else printf("reject\n")

9

• YACC builds a LALR parser for the grammar.
• May have shift/reduce and reduce/reduce conflicts
  if there are problems with the grammar.
• Default conflict resolution
• shift/reduce --gt shift
• reduce/reduce --gt first production in the state
• should always avoid reduce/reduce conflicts
• yacc -v .y will generate a report in file
  y.output.
• See example1.y
• The programmer MUST resolve all conflicts (unless
  you really know what you are doing).
• modify the grammar. See example2.y
• Use precedence and associativity of operators.

10

• Use precedence and associativity of operators.
• Using keywords left, right, nonassoc in the
  declarations section.
• All tokens on the same line are the same
  precedence level and associativity.
• The lines are listed in order of increasing
  precedence.
• left PLUSnumber, MINUSnumber
• left TIMESnumber, DIVIDEnumber
• See example3.y

11

• Symbol attributes
• Each symbol can be associated with some
  attributes.
• Data structure of the attributes can be specified
  in the union in the declarations. (see
  example4.y).
• union
• int semantic_value
• token ltsemantic_valuegt ICONSTnumber
  119
• type ltsemantic_valuegt exp
• type ltsemantic_valuegt term
• type ltsemantic_valuegt item
• Semantic actions associate with productions can
  be specified

12

• Semantic actions
• Semantic actions associate with productions can
  be specified.
• item LPARENnumber exp RPARENnumber
• 2
• ICONSTnumber
• 1
• is the attribute associated with the left
  handside of the production
• 1 is the attribute associated with the first
  symbol in the right handside, 2 for the second
  symbol,
• An action can be in anyway in the production, it
  is also counted as a symbol.
• Checkout example5.y for examples with multiple
  types associated with different symbol.