Editing LR parser (section)

=== Syntax error recovery ===

LR parsers can generate somewhat helpful error messages for the first syntax error in a program, by simply enumerating all the terminal symbols that could have appeared next instead of the unexpected bad lookahead symbol.  But this does not help the parser work out how to parse the remainder of the input program to look for further, independent errors.  If the parser recovers badly from the first error, it is very likely to mis-parse everything else and produce a cascade of unhelpful spurious error messages.

In the [[yacc]] and bison parser generators, the parser has an ad hoc mechanism to abandon the current statement, discard some parsed phrases and lookahead tokens surrounding the error, and resynchronize the parse at some reliable statement-level delimiter like semicolons or braces.  This often works well for allowing the parser and compiler to look over the rest of the program.

Many syntactic coding errors are simple typos or omissions of a trivial symbol.  Some LR parsers attempt to detect and automatically repair these common cases.  The parser enumerates every possible single-symbol insertion, deletion, or substitution at the error point.  The compiler does a trial parse with each change to see if it worked okay.  (This requires backtracking to snapshots of the parse stack and input stream, normally unneeded by the parser.)  Some best repair is picked.  This gives a very helpful error message and resynchronizes the parse well.  However, the repair is not trustworthy enough to permanently modify the input file.  Repair of syntax errors is easiest to do consistently in parsers (like LR) that have parse tables and an explicit data stack.