This document discusses error recovery strategies for different parsing techniques used in compiler design. It describes panic mode, phrase-level, and syntactic phase recovery strategies. For predictive parsing, it explains how panic mode works by skipping tokens until a synchronizing token is found. For LR parsing and operator precedence parsing, it also discusses using panic mode and syntactic phase recovery. The document concludes that effective error recovery ensures parsers can continue processing despite errors and improves the developer experience.

1. DR.B.C ROY ENGINEERING COLLEGE, DURGAPUR - 713206
DEPARTMENT OF COMPUTER SCIENCE & ENGINEERING
(AFFLITIATED TO MAULANA ABUL KALAM AZAD UNIVERSITY OF TECHNOLOGY)
CONTINUOUS ASSESSMENT : 2
REPORT
ON
ERROR RECOVERY
STRATEGIES FOR DIFFERENT PARSING TECHNIQUES
SUBMITTED BY:
PRIYANKA MANNA
ROLL NO: 12000121056
PAPER CODE : PCC-CS 501
PAPER NAME : COMPILER DESIGN

2. AGENDA
o INTRODUCTION
o ERROR RECOVERY FOR DIFFERENT
PARSING TECHNIQUES
o CONCLUSION
o REFERENCES

3. INTRODUCTION:
Parsing is known as Syntax Analysis. It contains arranging the tokens as
source code into gramma cal phases that are used by the compiler to
synthesis output generally gramma cal phases of the source code are
defined by parse tree. There are various types of parsing techniques .
A parser should be able to detect and report any error in the program.
It is expected that when an error is encountered, the parser should be
able to handle it and carry on parsing the rest of the input. Mostly it is
expected from the parser to check for errors but errors may be
encountered at various stages of the compilation process. A program
may have the following kinds of errors at various stages.
 Lexical : name of some identifier typed incorrectly
 Syntactical : missing semicolon or unbalanced
parenthesis
 Semantical : incompatible value assignment
 Logical : code not reachable, infinite loop
So, error handling is important for the correct execution of code.

4. ERROR RECOVERY FOR DIFFERENT PARSING
TECHNIQUES:
There are mainly five error recovery strategies, which are as follows
 Panic mode
 Phrase level recovery
 Error production
 Global correction
 Symbol table
ERROR RECOVERY IN PREDICTIVE PARSING:
o PANIC-MODE :
Panic-mode error recovery says that all the input symbols
are skipped until a synchronizing token is found from the
string.
In this recovery method, we use FOLLOW symbols as
synchronizing tokens and the “synch” in the predictive
parsing table to indicate synchronizing tokens obtained
from the nonterminal’s FOLLOW sets.
The terminal symbols which lie in follow set of non-
terminal they can be used as a synchronizing token set for
that of non-terminal. In simple panic-mode error recovery
for the LL(1)parsing.

5. Selection of synchronizing set:
There are two ways of selecting synchronizing set:
1. Place all symbols in FOLLOW(A) into the synchronizing
set for nonterminal A. We can continue parsing until we
skip tokens of an element of FOLLOW(A) is seen and pop
it from the stack specifically.
We might add keywords that begin statements to the
synchronizing sets for the non-terminals generating
expressions.
2. If a nonterminal can generate the empty string, then
the production deriving ε can be used as a default. This
may produce some delay in error detection, but errors
cannot be missed. This approach reduces the number of
non-terminals that have to be considered during error
recovery.
PHRASE-LEVEL RECOVERY:
Phrase level recovery is implemented by filling in the
blank entries in the predictive parsing table with pointers
to error routines. At each unfilled entry in the parsing
table, it is filled by a pointer to a special error routine that
will take care of that error case specifically. These error
routines can be of different types like :
 change, insert, or delete input symbols.
 issue appropriate error messages.
 pop items from the stack.
ERROR RECOVERY IN LR PARSING:
LR Parser Basically Uses the Mentioned Two Techniques to
Detect Errors:
1. Syntactic Phase recovery
2. Panic mode recovery

6. o SYNTACTIC PHASE RECOVERY:
Syntactic Phase Recovery Follows the Given Steps:
1.Programmer mistakes that call error procedures in the
parser table are determined based on the language.
2.Creating error procedures that can alter the top of the
stack and/or certain symbols on input in a way that is
acceptable for table error entries.
o PANIC-MODE :
This approach involves removing consecutive characters
from the input one by one until a set of synchronized
tokens is obtained. Delimiters such as or are synchronizing
tokens. The benefit is that it is simple to implement and
ensures that you do not end up in an infinite loop. The
drawback is that a significant quantity of data is skipped
without being checked for additional problems.
Panic mode recovery follows the given steps:
1. Scan the stack until you find a state ‘a’ with a goto() on a
certain non-terminal ‘B’ (by removing states from the
stack).
2. Until a symbol ‘b’ that can follow ‘B’ is identified, zero or
more input symbols are rejected.
ERROR RECOVERY IN OPERATOR PRECEDENCE
PARSING:
o PANIC MODE RECOVERY:
When a syntax error is detected, the parser enters a panic
mode by discarding input tokens until it finds a
synchronization point. In operator precedence parsing,
synchronization points are often tokens with lower
precedence or tokens that signal the end of an expression
(e.g., a semicolon).

7. Implementation:
Identify tokens that can serve as synchronization points for
your grammar and error recovery. When an error is
encountered, skip tokens until a synchronization point is found
and resume parsing from there.
o Phrase-Level Recovery:
Instead of skipping tokens, the parser can attempt to insert
or replace tokens to match the expected grammar
production. For example, if an operator is missing, the
parser can insert it or suggest it as an error correction.
Implementation:
Analyze the grammar rules and operator precedence levels
to determine which tokens or constructs are likely to be
missing or incorrect. When an error is detected, insert or
replace tokens accordingly and continue parsing.

8. CONCLUSION:
Error recovery strategies play a vital role in parsing techniques used
in programming language processing and compila on. These
strategies help parsers gracefully handle syntax errors in source code
and provide meaningful feedback to developers. The choice of error
recovery strategy depends on the parsing technique being used and
the language's specific requirements. Some common strategies
include panic mode, phrase-level recovery, backtracking, custom
error messages, and seman c ac ons.
Effec ve error recovery not only ensures that parsers can con nue
processing input despite errors but also aids in improving the overall
developer experience by providing clear and ac onable error
messages. Regardless of the parsing technique employed, a well-
designed error recovery mechanism contributes to more robust
compilers and interpreters, helping developers iden fy and correct
issues in their code efficiently.

9. REFERENCES:
 h ps://www.geeksforgeeks.org/error-recovery-in-predic ve-
parsing/
 h ps://www.geeksforgeeks.org/error-recovery-in-lr-parsing/
 h ps://www.tutorialspoint.com/compiler_design/compiler
design_error_recovery.html
 h ps://gtu-paper-solu on.com/Paper-Solu on/CompilerDesign-
2170701/Winter-2018/Ques on-2-b
 h ps://edurev.in/t/248392/Error-detec on-Recovery-in-Compiler