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doc: create a new Tuning LR section in the manual.

Browse Source 
And clean up all other documentation of the features described
there.
* NEWS (2.5): Tweak wording of lr.type and parse.lac entries a
bit, update the cross-references to the manual, and point out that
LAC has caveats.  Don't be so adamant that IELR+LAC=canonical LR.
That is, as the referenced section in the manual documents, LAC
does not fix infinite parsing loops on syntax errors.
* doc/bison.texinfo: Consistently drop the "(1)" suffix from LALR,
IELR, and LR in @cindex.
(%define Summary): Condense the entries for lr.default-reductions,
lr.keep-unreachable-states, lr.type, and parse.lac into brief
summaries, and cross-reference the appropriate subsections of
Tuning LR.  For parse.lac, mention that it's only implemented for
deterministic parsers in C.  In parse.error entry, mention LAC,
and add cross-reference to the LAC section.
(Error Reporting): When mentioning parse.error, mention LAC, and
add cross-reference to the LAC section.
(Tuning LR): New section with an extended version of the
documentation removed from %define Summary.  Change all
cross-references in the manual to point here instead of there.
(Calc++ Parser): When mentioning parse.error, mention LAC, and add
cross-reference to the LAC section.
(Table of Symbols): In %error-verbose entry, add cross-reference
to Error Reporting.
(Glossary): Capitalize entry titles consistently.  Add definitions
for "defaulted state" and "unreachable state".  Expand IELR
acronym in IELR's entry.
(cherry picked from commit 6f04ee6c78)

Conflicts:

	doc/bison.texinfo
This commit is contained in:
Joel E. Denny
2011-02-21 19:09:24 -05:00
parent 5e528941f4
commit 7fceb615a5
3 changed files with 545 additions and 316 deletions
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31
ChangeLog
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@@ -1,3 +1,34 @@
2011-03-06 Joel E. Denny <joeldenny@joeldenny.org>
doc: create a new Tuning LR section in the manual.
And clean up all other documentation of the features described
there.
* NEWS (2.5): Tweak wording of lr.type and parse.lac entries a
bit, update the cross-references to the manual, and point out that
LAC has caveats. Don't be so adamant that IELR+LAC=canonical LR.
That is, as the referenced section in the manual documents, LAC
does not fix infinite parsing loops on syntax errors.
* doc/bison.texinfo: Consistently drop the "(1)" suffix from LALR,
IELR, and LR in @cindex.
(%define Summary): Condense the entries for lr.default-reductions,
lr.keep-unreachable-states, lr.type, and parse.lac into brief
summaries, and cross-reference the appropriate subsections of
Tuning LR. For parse.lac, mention that it's only implemented for
deterministic parsers in C. In parse.error entry, mention LAC,
and add cross-reference to the LAC section.
(Error Reporting): When mentioning parse.error, mention LAC, and
add cross-reference to the LAC section.
(Tuning LR): New section with an extended version of the
documentation removed from %define Summary. Change all
cross-references in the manual to point here instead of there.
(Calc++ Parser): When mentioning parse.error, mention LAC, and add
cross-reference to the LAC section.
(Table of Symbols): In %error-verbose entry, add cross-reference
to Error Reporting.
(Glossary): Capitalize entry titles consistently. Add definitions
for "defaulted state" and "unreachable state". Expand IELR
acronym in IELR's entry.
2011-02-20 Joel E. Denny <joeldenny@joeldenny.org>
doc: add bibliography to manual.

44
NEWS
View File

@@ -116,27 +116,27 @@ Bison News
%define lr.type ielr
%define lr.type canonical-lr
The default reduction optimization in the parser tables can also be
adjusted using `%define lr.default-reductions'. See the documentation
for `%define lr.type' and `%define lr.default-reductions' in the
section `Bison Declaration Summary' in the Bison manual for the
details.
The default-reduction optimization in the parser tables can also be
adjusted using `%define lr.default-reductions'. For details on both
of these features, see the new section `Tuning LR' in the Bison
manual.
These features are experimental. More user feedback will help to
stabilize them.
** LAC (lookahead correction) for syntax error handling:
** LAC (Lookahead Correction) for 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
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 (with %error-verbose or `#define
YYERROR_VERBOSE'), the expected token list in the syntax error
message can both contain invalid tokens and omit valid tokens.
verbose error messages are enabled (with %error-verbose or the
obsolete `#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 %nonassoc, default
reductions in inconsistent states, and parser state merging. Thus,
@@ -144,11 +144,11 @@ Bison News
%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 %nonassoc, default reductions, or state mering. When
LAC is in use, canonical LR and IELR behave exactly the same for
both syntactically acceptable and syntactically unacceptable input.
LAC is a new mechanism within the parsing algorithm that solves
these problems for canonical LR, IELR, and LALR without sacrificing
%nonassoc, default reductions, or state merging. When LAC is in
use, 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
@@ -159,8 +159,8 @@ Bison News
%define parse.lac full
See the documentation for `%define parse.lac' in the section `Bison
Declaration Summary' in the Bison manual for additional details.
See the new section `LAC' in the Bison manual for additional
details including a few caveats.
LAC is an experimental feature. More user feedback will help to
stabilize it.
@@ -314,11 +314,11 @@ Bison News
** Verbose syntax error message fixes:
When %error-verbose or `#define YYERROR_VERBOSE' is specified,
syntax error messages produced by the generated parser include the
unexpected token as well as a list of expected tokens. The effect
of %nonassoc on these verbose messages has been corrected in two
ways, but a complete fix requires LAC, described above:
When %error-verbose or the obsolete `#define YYERROR_VERBOSE' is
specified, syntax error messages produced by the generated parser
include the unexpected token as well as a list of expected tokens.
The effect of %nonassoc on these verbose messages has been corrected
in two ways, but a more complete fix requires LAC, described above:
*** When %nonassoc is used, there can exist parser states that accept no
tokens, and so the parser does not always require a lookahead token

786
doc/bison.texinfo
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@@ -266,6 +266,7 @@ The Bison Parser Algorithm
* Parser States:: The parser is a finite-state-machine with stack.
* Reduce/Reduce:: When two rules are applicable in the same situation.
* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
* Tuning LR:: How to tune fundamental aspects of LR-based parsing.
* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted. How to avoid it.
@@ -277,6 +278,13 @@ Operator Precedence
* Precedence Examples:: How these features are used in the previous example.
* How Precedence:: How they work.
Tuning LR
* 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.
Handling Context Dependencies
* Semantic Tokens:: Token parsing can depend on the semantic context.
@@ -473,21 +481,19 @@ order to specify the language Algol 60. Any grammar expressed in
BNF is a context-free grammar. The input to Bison is
essentially machine-readable BNF.
@cindex LALR(1) grammars
@cindex IELR(1) grammars
@cindex LR(1) grammars
There are various important subclasses of context-free grammars.
Although it can handle almost all context-free grammars, Bison is
optimized for what are called LR(1) grammars.
In brief, in these grammars, it must be possible to tell how to parse
any portion of an input string with just a single token of lookahead.
For historical reasons, Bison by default is limited by the additional
restrictions of LALR(1), which is hard to explain simply.
@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}, for
more information on this.
As an experimental feature, you can escape these additional restrictions by
requesting IELR(1) or canonical LR(1) parser tables.
@xref{%define Summary,,lr.type}, to learn how.
@cindex LALR grammars
@cindex IELR grammars
@cindex LR grammars
There are various important subclasses of context-free grammars. Although
it can handle almost all context-free grammars, Bison is optimized for what
are called LR(1) grammars. In brief, in these grammars, it must be possible
to tell how to parse any portion of an input string with just a single token
of lookahead. For historical reasons, Bison by default is limited by the
additional restrictions of LALR(1), which is hard to explain simply.
@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}, for more
information on this. As an experimental feature, you can escape these
additional restrictions by requesting IELR(1) or canonical LR(1) parser
tables. @xref{LR Table Construction}, to learn how.
@cindex GLR parsing
@cindex generalized LR (GLR) parsing
@@ -5352,65 +5358,17 @@ Boolean.
@c ================================================== lr.default-reductions
@item lr.default-reductions
@cindex default reductions
@findex %define lr.default-reductions
@cindex delayed syntax errors
@cindex syntax errors delayed
@cindex LAC
@findex %nonassoc
@itemize @bullet
@item Language(s): all
@item Purpose: Specify the kind of states that are permitted to
contain default reductions.
That is, in such a state, Bison selects the reduction with the largest
lookahead set to be the default parser action and then removes that
lookahead set.
(The ability to specify where default reductions should be used is
experimental.
More user feedback will help to stabilize it.)
@item Accepted Values:
@itemize
@item @code{all}.
This is the traditional Bison behavior. The main advantage is a
significant decrease in the size of the parser tables. The
disadvantage is that, when the generated parser encounters a
syntactically unacceptable token, the parser might then perform
unnecessary default reductions before it can detect the syntax error.
Such delayed syntax error detection is usually inherent in LALR and
IELR parser tables anyway due to LR state merging (@pxref{%define
Summary,,lr.type}). Furthermore, the use of @code{%nonassoc} can
contribute to delayed syntax error detection even in the case of
canonical LR. As an experimental feature, delayed syntax error
detection can be overcome in all cases by enabling LAC (@pxref{%define
Summary,,parse.lac}, for details, including a discussion of the
effects of delayed syntax error detection).
@item @code{consistent}.
@cindex consistent states
A consistent state is a state that has only one possible action.
If that action is a reduction, then the parser does not need to request
a lookahead token from the scanner before performing that action.
However, the parser recognizes the ability to ignore the lookahead token
in this way only when such a reduction is encoded as a default
reduction.
Thus, if default reductions are permitted only in consistent states,
then a canonical LR parser that does not employ
@code{%nonassoc} detects a syntax error as soon as it @emph{needs} the
syntactically unacceptable token from the scanner.
@item @code{accepting}.
@cindex accepting state
In the accepting state, the default reduction is actually the accept
action.
In this case, a canonical LR parser that does not employ
@code{%nonassoc} detects a syntax error as soon as it @emph{reaches} the
syntactically unacceptable token in the input.
That is, it does not perform any extra reductions.
@end itemize
contain default reductions. @xref{Default Reductions}. (The ability to
specify where default reductions should be used is experimental. More user
feedback will help to stabilize it.)
@item Accepted Values: @code{all}, @code{consistent}, @code{accepting}
@item Default Value:
@itemize
@item @code{accepting} if @code{lr.type} is @code{canonical-lr}.
@@ -5425,42 +5383,10 @@ That is, it does not perform any extra reductions.
@itemize @bullet
@item Language(s): all
@item Purpose: Request that Bison allow unreachable parser states to
remain in the parser tables.
Bison considers a state to be unreachable if there exists no sequence of
transitions from the start state to that state.
A state can become unreachable during conflict resolution if Bison disables a
shift action leading to it from a predecessor state.
Keeping unreachable states is sometimes useful for analysis purposes, but they
are useless in the generated parser.
remain in the parser tables. @xref{Unreachable States}.
@item Accepted Values: Boolean
@item Default Value: @code{false}
@item Caveats:
@itemize @bullet
@item 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 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
@end itemize
@c lr.keep-unreachable-states
@@ -5468,87 +5394,15 @@ However, Bison does not compute which goto actions are useless.
@item lr.type
@findex %define lr.type
@cindex LALR
@cindex IELR
@cindex LR
@itemize @bullet
@item Language(s): all
@item Purpose: Specify the type of parser tables within the
LR(1) family.
(This feature is experimental.
LR(1) family. @xref{LR Table Construction}. (This feature is experimental.
More user feedback will help to stabilize it.)
@item Accepted Values:
@itemize
@item @code{lalr}.
While Bison generates LALR parser tables by default for
historical reasons, IELR or canonical LR is almost
always preferable for deterministic parsers.
The trouble is that LALR parser tables can suffer from
mysterious conflicts and thus may not accept the full set of sentences
that IELR and canonical LR accept.
@xref{Mystery Conflicts}, for details.
However, there are at least two scenarios where LALR may be
worthwhile:
@itemize
@cindex GLR with LALR
@item 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 use of LALR parser tables is guaranteed not
to alter the language accepted by the parser.
LALR parser tables are the smallest parser tables Bison can
currently generate, so they may be preferable.
Nevertheless, once you begin to resolve conflicts statically,
GLR begins to behave more like a deterministic parser, and so
IELR and canonical LR can be helpful to avoid
LALR's mysterious behavior.
@item Occasionally during development, an especially malformed grammar
with a major recurring flaw may severely impede the IELR or
canonical LR parser table generation algorithm.
LALR can be a quick way to generate parser tables in order to
investigate such problems while ignoring the more subtle differences
from IELR and canonical LR.
@end itemize
@item @code{ielr}.
IELR is a minimal LR algorithm.
That is, given any grammar (LR or non-LR),
IELR and canonical LR always accept exactly the same
set of sentences.
However, as for LALR, the number of parser states is often an
order of magnitude less for IELR 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 can significantly reduce the complexity of developing of a grammar.
@item @code{canonical-lr}.
@cindex delayed syntax errors
@cindex syntax errors delayed
@cindex LAC
@findex %nonassoc
While inefficient, canonical LR parser tables can be an interesting
means to explore a grammar because they have a property that IELR and
LALR tables do not. That is, if @code{%nonassoc} is not used and
default reductions are left disabled (@pxref{%define
Summary,,lr.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 using LAC (@pxref{%define Summary,,parse.lac}) are also able
to achieve this behavior without sacrificing @code{%nonassoc} or
default reductions.
@end itemize
@item Accepted Values: @code{lalr}, @code{ielr}, @code{canonical-lr}
@item Default Value: @code{lalr}
@end itemize
@@ -5596,8 +5450,9 @@ function. @xref{Error Reporting, ,The Error Reporting Function
Error messages passed to @code{yyerror} are simply @w{@code{"syntax
error"}}.
@item @code{verbose}
Error messages report the unexpected token, and possibly the expected
ones.
Error messages report the unexpected token, and possibly the expected ones.
However, this report can often be incorrect when LAC is not enabled
(@pxref{LAC}).
@end itemize
@item Default Value:
@@ -5609,84 +5464,13 @@ ones.
@c ================================================== parse.lac
@item parse.lac
@findex %define parse.lac
@cindex LAC
@cindex lookahead correction
@itemize
@item Languages(s): C
@item Languages(s): C (deterministic parsers only)
@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.
syntax error handling. @xref{LAC}.
@item Accepted Values: @code{none}, @code{full}
@item Default Value: @code{none}
@end itemize
@c parse.lac
@@ -6328,10 +6112,11 @@ receives one argument. For a syntax error, the string is normally
@w{@code{"syntax error"}}.
@findex %define parse.error
If you invoke @samp{%define parse.error verbose} in the Bison
declarations section (@pxref{Bison Declarations, ,The Bison Declarations
Section}), then Bison provides a more verbose and specific error message
string instead of just plain @w{@code{"syntax error"}}.
If you invoke @samp{%define parse.error verbose} in the Bison declarations
section (@pxref{Bison Declarations, ,The Bison Declarations Section}), then
Bison provides a more verbose and specific error message string instead of
just plain @w{@code{"syntax error"}}. However, that message sometimes
contains incorrect information if LAC is not enabled (@pxref{LAC}).
The parser can detect one other kind of error: memory exhaustion. This
can happen when the input contains constructions that are very deeply
@@ -6732,6 +6517,7 @@ This kind of parser is known in the literature as a bottom-up parser.
* Parser States:: The parser is a finite-state-machine with stack.
* Reduce/Reduce:: When two rules are applicable in the same situation.
* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
* Tuning LR:: How to tune fundamental aspects of LR-based parsing.
* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted. How to avoid it.
@end menu
@@ -7301,6 +7087,7 @@ redirects:redirect
@node Mystery Conflicts
@section Mysterious Reduce/Reduce Conflicts
@cindex Mysterious Conflicts
Sometimes reduce/reduce conflicts can occur that don't look warranted.
Here is an example:
@@ -7342,8 +7129,8 @@ of lookahead: when a @code{param_spec} is being read, an @code{ID} is
a @code{name} if a comma or colon follows, or a @code{type} if another
@code{ID} follows. In other words, this grammar is LR(1).
@cindex LR(1)
@cindex LALR(1)
@cindex LR
@cindex LALR
However, for historical reasons, Bison cannot by default handle all
LR(1) grammars.
In this grammar, two contexts, that after an @code{ID} at the beginning
@@ -7358,15 +7145,16 @@ contexts, so it makes a single parser state for them both. Combining
the two contexts causes a conflict later. In parser terminology, this
occurrence means that the grammar is not LALR(1).
For many practical grammars (specifically those that fall into the
non-LR(1) class), the limitations of LALR(1) result in difficulties
beyond just mysterious reduce/reduce conflicts. The best way to fix
all these problems is to select a different parser table generation
algorithm. Either IELR(1) or canonical LR(1) would suffice, but the
former is more efficient and easier to debug during development.
@xref{%define Summary,,lr.type}, for details. (Bison's IELR(1) and
canonical LR(1) implementations are experimental. More user feedback
will help to stabilize them.)
@cindex IELR
@cindex canonical LR
For many practical grammars (specifically those that fall into the non-LR(1)
class), the limitations of LALR(1) result in difficulties beyond just
mysterious reduce/reduce conflicts. The best way to fix all these problems
is to select a different parser table construction algorithm. Either
IELR(1) or canonical LR(1) would suffice, but the former is more efficient
and easier to debug during development. @xref{LR Table Construction}, for
details. (Bison's IELR(1) and canonical LR(1) implementations are
experimental. More user feedback will help to stabilize them.)
If you instead wish to work around LALR(1)'s limitations, you
can often fix a mysterious conflict by identifying the two parser states
@@ -7417,6 +7205,409 @@ return_spec:
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{Mystery
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{Mystery 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{all} (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
FIXME: Because of the exceptions described above, @code{all} is a misnomer.
Rename to @code{full}.
@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
@node Generalized LR Parsing
@section Generalized LR (GLR) Parsing
@cindex GLR parsing
@@ -9500,8 +9691,9 @@ propagated.
@end example
@noindent
Use the following two directives to enable parser tracing and verbose
error messages.
Use the following two directives to enable parser tracing and verbose error
messages. However, verbose error messages can contain incorrect information
(@pxref{LAC}).
@comment file: calc++-parser.yy
@example
@@ -10908,9 +11100,9 @@ Precedence}.
@end deffn
@end ifset
@deffn {Directive} %define @var{define-variable}
@deffnx {Directive} %define @var{define-variable} @var{value}
@deffnx {Directive} %define @var{define-variable} "@var{value}"
@deffn {Directive} %define @var{variable}
@deffnx {Directive} %define @var{variable} @var{value}
@deffnx {Directive} %define @var{variable} "@var{value}"
Define a variable to adjust Bison's behavior. @xref{%define Summary}.
@end deffn
@@ -10952,7 +11144,8 @@ token is reset to the token that originally caused the violation.
@end deffn
@deffn {Directive} %error-verbose
An obsolete directive standing for @samp{%define parse.error verbose}.
An obsolete directive standing for @samp{%define parse.error verbose}
(@pxref{Error Reporting, ,The Error Reporting Function @code{yyerror}}).
@end deffn
@deffn {Directive} %file-prefix "@var{prefix}"
@@ -11305,7 +11498,7 @@ Data type of semantic values; @code{int} by default.
@cindex glossary
@table @asis
@item Accepting State
@item Accepting state
A state whose only action is the accept action.
The accepting state is thus a consistent state.
@xref{Understanding,,}.
@@ -11316,9 +11509,8 @@ by John Backus, and slightly improved by Peter Naur in his 1960-01-02
committee document contributing to what became the Algol 60 report.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
@item Consistent State
A state containing only one possible action. @xref{%define
Summary,,lr.default-reductions}.
@item Consistent state
A state containing only one possible action. @xref{Default Reductions}.
@item Context-free grammars
Grammars specified as rules that can be applied regardless of context.
@@ -11327,12 +11519,15 @@ expression, integers are allowed @emph{anywhere} an expression is
permitted. @xref{Language and Grammar, ,Languages and Context-Free
Grammars}.
@item Default Reduction
@item Default reduction
The reduction that a parser should perform if the current parser state
contains no other action for the lookahead token. In permitted parser
states, Bison declares the reduction with the largest lookahead set to
be the default reduction and removes that lookahead set.
@xref{%define Summary,,lr.default-reductions}.
states, Bison declares the reduction with the largest lookahead set to be
the default reduction and removes that lookahead set. @xref{Default
Reductions}.
@item Defaulted state
A consistent state with a default reduction. @xref{Default Reductions}.
@item Dynamic allocation
Allocation of memory that occurs during execution, rather than at
@@ -11364,17 +11559,16 @@ A language construct that is (in general) grammatically divisible;
for example, `expression' or `declaration' in C@.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
@item IELR(1)
A minimal LR(1) parser table generation algorithm. That is, given any
@item IELR(1) (Inadequacy Elimination LR(1))
A minimal LR(1) parser table construction algorithm. That is, given any
context-free grammar, IELR(1) generates parser tables with the full
language recognition power of canonical LR(1) but with nearly the same
number of parser states as LALR(1). This reduction in parser states
is often an order of magnitude. More importantly, because canonical
LR(1)'s extra parser states may contain duplicate conflicts in the
case of non-LR(1) grammars, the number of conflicts for IELR(1) is
often an order of magnitude less as well. This can significantly
reduce the complexity of developing of a grammar. @xref{%define
Summary,,lr.type}.
language-recognition power of canonical LR(1) but with nearly the same
number of parser states as LALR(1). This reduction in parser states is
often an order of magnitude. More importantly, because canonical LR(1)'s
extra parser states may contain duplicate conflicts in the case of non-LR(1)
grammars, the number of conflicts for IELR(1) is often an order of magnitude
less as well. This can significantly reduce the complexity of developing a
grammar. @xref{LR Table Construction}.
@item Infix operator
An arithmetic operator that is placed between the operands on which it
@@ -11385,12 +11579,11 @@ A continuous flow of data between devices or programs.
@item LAC (Lookahead Correction)
A parsing mechanism that fixes the problem of delayed syntax error
detection, which is caused by LR state merging, default reductions,
and the use of @code{%nonassoc}. Delayed syntax error detection
results in unexpected semantic actions, initiation of error recovery
in the wrong syntactic context, and an incorrect list of expected
tokens in a verbose syntax error message. @xref{%define
Summary,,parse.lac}.
detection, which is caused by LR state merging, default reductions, and the
use of @code{%nonassoc}. Delayed syntax error detection results in
unexpected semantic actions, initiation of error recovery in the wrong
syntactic context, and an incorrect list of expected tokens in a verbose
syntax error message. @xref{LAC}.
@item Language construct
One of the typical usage schemas of the language. For example, one of
@@ -11506,6 +11699,11 @@ the lexical analyzer. @xref{Symbols}.
A grammar symbol that has no rules in the grammar and therefore is
grammatically indivisible. The piece of text it represents is a token.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
@item Unreachable state
A parser state to which there does not exist a sequence of transitions from
the parser's start state. A state can become unreachable during conflict
resolution. @xref{Unreachable States}.
@end table
@node Copying This Manual