+For a more detailed exposition of LALR(1) parsers and parser
+generators, @pxref{Bibliography,,DeRemer 1982}.
+
+@node Tuning LR
+@section Tuning LR
+
+The default behavior of Bison's LR-based parsers is chosen mostly for
+historical reasons, but that behavior is often not robust. For example, in
+the previous section, we discussed the mysterious conflicts that can be
+produced by LALR(1), Bison's default parser table construction algorithm.
+Another example is Bison's @code{%define parse.error verbose} directive,
+which instructs the generated parser to produce verbose syntax error
+messages, which can sometimes contain incorrect information.
+
+In this section, we explore several modern features of Bison that allow you
+to tune fundamental aspects of the generated LR-based parsers. Some of
+these features easily eliminate shortcomings like those mentioned above.
+Others can be helpful purely for understanding your parser.
+
+Most of the features discussed in this section are still experimental. More
+user feedback will help to stabilize them.
+
+@menu
+* LR Table Construction:: Choose a different construction algorithm.
+* Default Reductions:: Disable default reductions.
+* LAC:: Correct lookahead sets in the parser states.
+* Unreachable States:: Keep unreachable parser states for debugging.
+@end menu
+
+@node LR Table Construction
+@subsection LR Table Construction
+@cindex Mysterious Conflict
+@cindex LALR
+@cindex IELR
+@cindex canonical LR
+@findex %define lr.type
+
+For historical reasons, Bison constructs LALR(1) parser tables by default.
+However, LALR does not possess the full language-recognition power of LR.
+As a result, the behavior of parsers employing LALR parser tables is often
+mysterious. We presented a simple example of this effect in @ref{Mysterious
+Conflicts}.
+
+As we also demonstrated in that example, the traditional approach to
+eliminating such mysterious behavior is to restructure the grammar.
+Unfortunately, doing so correctly is often difficult. Moreover, merely
+discovering that LALR causes mysterious behavior in your parser can be
+difficult as well.
+
+Fortunately, Bison provides an easy way to eliminate the possibility of such
+mysterious behavior altogether. You simply need to activate a more powerful
+parser table construction algorithm by using the @code{%define lr.type}
+directive.
+
+@deffn {Directive} {%define lr.type @var{TYPE}}
+Specify the type of parser tables within the LR(1) family. The accepted
+values for @var{TYPE} are:
+
+@itemize
+@item @code{lalr} (default)
+@item @code{ielr}
+@item @code{canonical-lr}
+@end itemize
+
+(This feature is experimental. More user feedback will help to stabilize
+it.)
+@end deffn
+
+For example, to activate IELR, you might add the following directive to you
+grammar file:
+
+@example
+%define lr.type ielr
+@end example
+
+@noindent For the example in @ref{Mysterious Conflicts}, the mysterious
+conflict is then eliminated, so there is no need to invest time in
+comprehending the conflict or restructuring the grammar to fix it. If,
+during future development, the grammar evolves such that all mysterious
+behavior would have disappeared using just LALR, you need not fear that
+continuing to use IELR will result in unnecessarily large parser tables.
+That is, IELR generates LALR tables when LALR (using a deterministic parsing
+algorithm) is sufficient to support the full language-recognition power of
+LR. Thus, by enabling IELR at the start of grammar development, you can
+safely and completely eliminate the need to consider LALR's shortcomings.
+
+While IELR is almost always preferable, there are circumstances where LALR
+or the canonical LR parser tables described by Knuth
+(@pxref{Bibliography,,Knuth 1965}) can be useful. Here we summarize the
+relative advantages of each parser table construction algorithm within
+Bison:
+
+@itemize
+@item LALR
+
+There are at least two scenarios where LALR can be worthwhile:
+
+@itemize
+@item GLR without static conflict resolution.
+
+@cindex GLR with LALR
+When employing GLR parsers (@pxref{GLR Parsers}), if you do not resolve any
+conflicts statically (for example, with @code{%left} or @code{%prec}), then
+the parser explores all potential parses of any given input. In this case,
+the choice of parser table construction algorithm is guaranteed not to alter
+the language accepted by the parser. LALR parser tables are the smallest
+parser tables Bison can currently construct, so they may then be preferable.
+Nevertheless, once you begin to resolve conflicts statically, GLR behaves
+more like a deterministic parser in the syntactic contexts where those
+conflicts appear, and so either IELR or canonical LR can then be helpful to
+avoid LALR's mysterious behavior.
+
+@item Malformed grammars.
+
+Occasionally during development, an especially malformed grammar with a
+major recurring flaw may severely impede the IELR or canonical LR parser
+table construction algorithm. LALR can be a quick way to construct parser
+tables in order to investigate such problems while ignoring the more subtle
+differences from IELR and canonical LR.
+@end itemize
+
+@item IELR
+
+IELR (Inadequacy Elimination LR) is a minimal LR algorithm. That is, given
+any grammar (LR or non-LR), parsers using IELR or canonical LR parser tables
+always accept exactly the same set of sentences. However, like LALR, IELR
+merges parser states during parser table construction so that the number of
+parser states is often an order of magnitude less than for canonical LR.
+More importantly, because canonical LR's extra parser states may contain
+duplicate conflicts in the case of non-LR grammars, the number of conflicts
+for IELR is often an order of magnitude less as well. This effect can
+significantly reduce the complexity of developing a grammar.
+
+@item Canonical LR
+
+@cindex delayed syntax error detection
+@cindex LAC
+@findex %nonassoc
+While inefficient, canonical LR parser tables can be an interesting means to
+explore a grammar because they possess a property that IELR and LALR tables
+do not. That is, if @code{%nonassoc} is not used and default reductions are
+left disabled (@pxref{Default Reductions}), then, for every left context of
+every canonical LR state, the set of tokens accepted by that state is
+guaranteed to be the exact set of tokens that is syntactically acceptable in
+that left context. It might then seem that an advantage of canonical LR
+parsers in production is that, under the above constraints, they are
+guaranteed to detect a syntax error as soon as possible without performing
+any unnecessary reductions. However, IELR parsers that use LAC are also
+able to achieve this behavior without sacrificing @code{%nonassoc} or
+default reductions. For details and a few caveats of LAC, @pxref{LAC}.
+@end itemize
+
+For a more detailed exposition of the mysterious behavior in LALR parsers
+and the benefits of IELR, @pxref{Bibliography,,Denny 2008 March}, and
+@ref{Bibliography,,Denny 2010 November}.
+
+@node Default Reductions
+@subsection Default Reductions
+@cindex default reductions
+@findex %define lr.default-reductions
+@findex %nonassoc
+
+After parser table construction, Bison identifies the reduction with the
+largest lookahead set in each parser state. To reduce the size of the
+parser state, traditional Bison behavior is to remove that lookahead set and
+to assign that reduction to be the default parser action. Such a reduction
+is known as a @dfn{default reduction}.
+
+Default reductions affect more than the size of the parser tables. They
+also affect the behavior of the parser:
+
+@itemize
+@item Delayed @code{yylex} invocations.
+
+@cindex delayed yylex invocations
+@cindex consistent states
+@cindex defaulted states
+A @dfn{consistent state} is a state that has only one possible parser
+action. If that action is a reduction and is encoded as a default
+reduction, then that consistent state is called a @dfn{defaulted state}.
+Upon reaching a defaulted state, a Bison-generated parser does not bother to
+invoke @code{yylex} to fetch the next token before performing the reduction.
+In other words, whether default reductions are enabled in consistent states
+determines how soon a Bison-generated parser invokes @code{yylex} for a
+token: immediately when it @emph{reaches} that token in the input or when it
+eventually @emph{needs} that token as a lookahead to determine the next
+parser action. Traditionally, default reductions are enabled, and so the
+parser exhibits the latter behavior.
+
+The presence of defaulted states is an important consideration when
+designing @code{yylex} and the grammar file. That is, if the behavior of
+@code{yylex} can influence or be influenced by the semantic actions
+associated with the reductions in defaulted states, then the delay of the
+next @code{yylex} invocation until after those reductions is significant.
+For example, the semantic actions might pop a scope stack that @code{yylex}
+uses to determine what token to return. Thus, the delay might be necessary
+to ensure that @code{yylex} does not look up the next token in a scope that
+should already be considered closed.
+
+@item Delayed syntax error detection.
+
+@cindex delayed syntax error detection
+When the parser fetches a new token by invoking @code{yylex}, it checks
+whether there is an action for that token in the current parser state. The
+parser detects a syntax error if and only if either (1) there is no action
+for that token or (2) the action for that token is the error action (due to
+the use of @code{%nonassoc}). However, if there is a default reduction in
+that state (which might or might not be a defaulted state), then it is
+impossible for condition 1 to exist. That is, all tokens have an action.
+Thus, the parser sometimes fails to detect the syntax error until it reaches
+a later state.
+
+@cindex LAC
+@c If there's an infinite loop, default reductions can prevent an incorrect
+@c sentence from being rejected.
+While default reductions never cause the parser to accept syntactically
+incorrect sentences, the delay of syntax error detection can have unexpected
+effects on the behavior of the parser. However, the delay can be caused
+anyway by parser state merging and the use of @code{%nonassoc}, and it can
+be fixed by another Bison feature, LAC. We discuss the effects of delayed
+syntax error detection and LAC more in the next section (@pxref{LAC}).
+@end itemize
+
+For canonical LR, the only default reduction that Bison enables by default
+is the accept action, which appears only in the accepting state, which has
+no other action and is thus a defaulted state. However, the default accept
+action does not delay any @code{yylex} invocation or syntax error detection
+because the accept action ends the parse.
+
+For LALR and IELR, Bison enables default reductions in nearly all states by
+default. There are only two exceptions. First, states that have a shift
+action on the @code{error} token do not have default reductions because
+delayed syntax error detection could then prevent the @code{error} token
+from ever being shifted in that state. However, parser state merging can
+cause the same effect anyway, and LAC fixes it in both cases, so future
+versions of Bison might drop this exception when LAC is activated. Second,
+GLR parsers do not record the default reduction as the action on a lookahead
+token for which there is a conflict. The correct action in this case is to
+split the parse instead.
+
+To adjust which states have default reductions enabled, use the
+@code{%define lr.default-reductions} directive.
+
+@deffn {Directive} {%define lr.default-reductions @var{WHERE}}
+Specify the kind of states that are permitted to contain default reductions.
+The accepted values of @var{WHERE} are:
+@itemize
+@item @code{full} (default for LALR and IELR)
+@item @code{consistent}
+@item @code{accepting} (default for canonical LR)
+@end itemize
+
+(The ability to specify where default reductions are permitted is
+experimental. More user feedback will help to stabilize it.)
+@end deffn
+
+@node LAC
+@subsection LAC
+@findex %define parse.lac
+@cindex LAC
+@cindex lookahead correction
+
+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 can 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 (@pxref{Error
+Reporting}), 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 (@pxref{Default Reductions}), and parser state
+merging. Because IELR and LALR merge parser states, they suffer the most.
+Canonical LR can suffer only if @code{%nonassoc} is used or if default
+reductions are enabled for inconsistent states.
+
+LAC (Lookahead Correction) is a new mechanism within the parsing algorithm
+that solves these problems for canonical LR, IELR, and LALR without
+sacrificing @code{%nonassoc}, default reductions, or state merging. You can
+enable LAC with the @code{%define parse.lac} directive.
+
+@deffn {Directive} {%define parse.lac @var{VALUE}}
+Enable LAC to improve syntax error handling.
+@itemize
+@item @code{none} (default)
+@item @code{full}
+@end itemize
+(This feature is experimental. More user feedback will help to stabilize
+it. Moreover, it is currently only available for deterministic parsers in
+C.)
+@end deffn
+
+Conceptually, the LAC 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{Default Reductions}) affects how soon the parser
+detects a syntax error: immediately when it @emph{reaches} an erroneous
+token or when it eventually @emph{needs} that token as a lookahead to
+determine the next parser action. The latter behavior is probably more
+intuitive, so Bison currently provides no way to achieve the former behavior
+while default reductions are enabled in consistent states.
+
+Thus, when LAC is in use, for some fixed decision of whether to enable
+default reductions in consistent states, canonical LR and IELR behave almost
+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.
+
+There are a few caveats to consider when using LAC:
+
+@itemize
+@item Infinite parsing loops.
+
+IELR plus LAC does have one shortcoming relative to canonical LR. Some
+parsers generated by Bison can loop infinitely. LAC does not fix infinite
+parsing loops that occur between encountering a syntax error and detecting
+it, but enabling canonical LR or disabling default reductions sometimes
+does.
+
+@item Verbose error message limitations.
+
+Because of internationalization considerations, Bison-generated parsers
+limit the size of the expected token list they are willing to report in a
+verbose syntax error message. If the number of expected tokens exceeds that
+limit, the list is simply dropped from the message. Enabling LAC can
+increase the size of the list and thus cause the parser to drop it. Of
+course, dropping the list is better than reporting an incorrect list.
+
+@item Performance.
+
+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.
+@end itemize
+
+@node Unreachable States
+@subsection Unreachable States
+@findex %define lr.keep-unreachable-states
+@cindex unreachable states
+
+If there exists no sequence of transitions from the parser's start state to
+some state @var{s}, then Bison considers @var{s} to be an @dfn{unreachable
+state}. A state can become unreachable during conflict resolution if Bison
+disables a shift action leading to it from a predecessor state.
+
+By default, Bison removes unreachable states from the parser after conflict
+resolution because they are useless in the generated parser. However,
+keeping unreachable states is sometimes useful when trying to understand the
+relationship between the parser and the grammar.
+
+@deffn {Directive} {%define lr.keep-unreachable-states @var{VALUE}}
+Request that Bison allow unreachable states to remain in the parser tables.
+@var{VALUE} must be a Boolean. The default is @code{false}.
+@end deffn
+
+There are a few caveats to consider:
+
+@itemize @bullet
+@item Missing or extraneous warnings.
+
+Unreachable states may contain conflicts and may use rules not used in any
+other state. Thus, keeping unreachable states may induce warnings that are
+irrelevant to your parser's behavior, and it may eliminate warnings that are
+relevant. Of course, the change in warnings may actually be relevant to a
+parser table analysis that wants to keep unreachable states, so this
+behavior will likely remain in future Bison releases.
+
+@item Other useless states.
+
+While Bison is able to remove unreachable states, it is not guaranteed to
+remove other kinds of useless states. Specifically, when Bison disables
+reduce actions during conflict resolution, some goto actions may become
+useless, and thus some additional states may become useless. If Bison were
+to compute which goto actions were useless and then disable those actions,
+it could identify such states as unreachable and then remove those states.
+However, Bison does not compute which goto actions are useless.
+@end itemize
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