+
+@c ================================================== parse.lac
+@item parse.lac
+@findex %define parse.lac
+@cindex LAC
+@cindex lookahead correction
+
+@itemize
+@item Languages(s): C
+
+@item Purpose: Enable LAC (lookahead correction) to improve
+syntax error handling.
+
+Canonical LR, IELR, and LALR can suffer
+from a couple of problems upon encountering a syntax error. First, the
+parser might perform additional parser stack reductions before
+discovering the syntax error. Such reductions perform user semantic
+actions that are unexpected because they are based on an invalid token,
+and they cause error recovery to begin in a different syntactic context
+than the one in which the invalid token was encountered. Second, when
+verbose error messages are enabled (with @code{%error-verbose} or
+@code{#define YYERROR_VERBOSE}), the expected token list in the syntax
+error message can both contain invalid tokens and omit valid tokens.
+
+The culprits for the above problems are @code{%nonassoc}, default
+reductions in inconsistent states, and parser state merging. Thus,
+IELR and LALR suffer the most. Canonical
+LR can suffer only if @code{%nonassoc} is used or if default
+reductions are enabled for inconsistent states.
+
+LAC is a new mechanism within the parsing algorithm that
+completely solves these problems for canonical LR,
+IELR, and LALR without sacrificing @code{%nonassoc},
+default reductions, or state mering. Conceptually, the mechanism is
+straight-forward. Whenever the parser fetches a new token from the
+scanner so that it can determine the next parser action, it immediately
+suspends normal parsing and performs an exploratory parse using a
+temporary copy of the normal parser state stack. During this
+exploratory parse, the parser does not perform user semantic actions.
+If the exploratory parse reaches a shift action, normal parsing then
+resumes on the normal parser stacks. If the exploratory parse reaches
+an error instead, the parser reports a syntax error. If verbose syntax
+error messages are enabled, the parser must then discover the list of
+expected tokens, so it performs a separate exploratory parse for each
+token in the grammar.
+
+There is one subtlety about the use of LAC. That is, when in a
+consistent parser state with a default reduction, the parser will not
+attempt to fetch a token from the scanner because no lookahead is
+needed to determine the next parser action. Thus, whether default
+reductions are enabled in consistent states (@pxref{%define
+Summary,,lr.default-reductions}) affects how soon the parser detects a
+syntax error: when it @emph{reaches} an erroneous token or when it
+eventually @emph{needs} that token as a lookahead. The latter
+behavior is probably more intuitive, so Bison currently provides no
+way to achieve the former behavior while default reductions are fully
+enabled.
+
+Thus, when LAC is in use, for some fixed decision of whether
+to enable default reductions in consistent states, canonical
+LR and IELR behave exactly the same for both
+syntactically acceptable and syntactically unacceptable input. While
+LALR still does not support the full language-recognition
+power of canonical LR and IELR, LAC at
+least enables LALR's syntax error handling to correctly
+reflect LALR's language-recognition power.
+
+Because LAC requires many parse actions to be performed twice,
+it can have a performance penalty. However, not all parse actions must
+be performed twice. Specifically, during a series of default reductions
+in consistent states and shift actions, the parser never has to initiate
+an exploratory parse. Moreover, the most time-consuming tasks in a
+parse are often the file I/O, the lexical analysis performed by the
+scanner, and the user's semantic actions, but none of these are
+performed during the exploratory parse. Finally, the base of the
+temporary stack used during an exploratory parse is a pointer into the
+normal parser state stack so that the stack is never physically copied.
+In our experience, the performance penalty of LAC has proven
+insignificant for practical grammars.
+
+@item Accepted Values: @code{none}, @code{full}
+
+@item Default Value: @code{none}
+@end itemize
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