bison-3.3.1

[Bruce Dubbs]

New minor version.

[Douglas R. Reno]

Now 3.3.1

[Bruce Dubbs]

Noteworthy changes in release 3.3.1 (2019-01-27) [stable]

• Changes

The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison extensions into errors. It now makes them simple warnings (-Wyacc).

Noteworthy changes in release 3.3 (2019-01-26) [stable]

A new mailing list was created, Bison Announce. It is low traffic, and is only about announcing new releases and important messages (e.g., polls about major decisions to make).

​https://lists.gnu.org/mailman/listinfo/bison-announce

• Backward incompatible changes

Support for DJGPP, which has been unmaintained and untested for years, is removed.

• Deprecated features

A new feature, --update (see below) helps adjusting existing grammars to deprecations.

• Deprecated directives

The %error-verbose directive is deprecated in favor of '%define parse.error verbose' since Bison 3.0, but no warning was issued.

The '%name-prefix "xx"' directive is deprecated in favor of '%define api.prefix {xx}' since Bison 3.0, but no warning was issued. These directives are slightly different, you might need to adjust your code. %name-prefix renames only symbols with external linkage, while api.prefix also renames types and macros, including YYDEBUG, YYTOKENTYPE, yytokentype, YYSTYPE, YYLTYPE, etc.

Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix {xx}' will typically have to update YY_DECL from

#define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)

to

#define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)

• Deprecated %define variable names

The following variables, mostly related to parsers in Java, have been renamed for consistency. Backward compatibility is ensured, but upgrading is recommended.

    abstract           -> api.parser.abstract
    annotations        -> api.parser.annotations
    extends            -> api.parser.extends
    final              -> api.parser.final
    implements         -> api.parser.implements
    parser_class_name  -> api.parser.class
    public             -> api.parser.public
    strictfp           -> api.parser.strictfp

New features

• Generation of fix-its for IDEs/Editors

When given the new option -ffixit (aka -fdiagnostics-parseable-fixits), bison now generates machine readable editing instructions to fix some issues. Currently, this is mostly limited to updating deprecated directives and removing duplicates. For instance:

    $ cat foo.y
    %error-verbose
    %define parser_class_name "Parser"
    %define api.parser.class "Parser"
    %%
    exp:;

See the "fix-it:" lines below:

    $ bison -ffixit foo.y
    foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
     %error-verbose
     ^~~~~~~~~~~~~~
    fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
    foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
     %define parser_class_name "Parser"
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
    foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
     %define api.parser.class "Parser"
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    foo.y:2.1-34:     previous definition
     %define parser_class_name "Parser"
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    fix-it:"foo.y":{3:1-3:34}:""
    foo.y: warning: fix-its can be applied.  Rerun with option '--update'. [-Wother]

This uses the same output format as GCC and Clang.

• Updating grammar files

Fixes can be applied on the fly. The previous example ends with the suggestion to re-run bison with the option -u/--update, which results in a cleaner grammar file.

    $ bison --update foo.y
    [...]
    bison: file 'foo.y' was updated (backup: 'foo.y~')

    $ cat foo.y
    %define parse.error verbose
    %define api.parser.class {Parser}
    %%
    exp:;

• Bison is now relocatable

If you pass '--enable-relocatable' to 'configure', Bison is relocatable.

A relocatable program can be moved or copied to a different location on the file system. It can also be used through mount points for network sharing. It is possible to make symbolic links to the installed and moved programs, and invoke them through the symbolic link.

• %expect and %expect-rr modifiers on individual rules

One can now document (and check) which rules participate in shift/reduce and reduce/reduce conflicts. This is particularly important GLR parsers, where conflicts are a normal occurrence. For example,

      %glr-parser
      %expect 1
      %%

      ...

      argument_list:
        arguments %expect 1
      | arguments ','
      | %empty
      ;

      arguments:
        expression
      | argument_list ',' expression
      ;

      ...

Looking at the output from -v, one can see that the shift-reduce conflict here is due to the fact that the parser does not know whether to reduce arguments to argument_list until it sees the token _after_ the following ','. By marking the rule with %expect 1 (because there is a conflict in one state), we document the source of the 1 overall shift-reduce conflict.

In GLR parsers, we can use %expect-rr in a rule for reduce/reduce conflicts. In this case, we mark each of the conflicting rules. For example,

      %glr-parser
      %expect-rr 1

      %%

      stmt:
        target_list '=' expr ';'
      | expr_list ';'
      ;

      target_list:
        target
      | target ',' target_list
      ;

      target:
        ID %expect-rr 1
      ;

      expr_list:
        expr
      | expr ',' expr_list
      ;

      expr:
        ID %expect-rr 1
      | ...
      ;

In a statement such as

x, y = 3, 4;

the parser must reduce x to a target or an expr, but does not know which until it sees the '='. So we notate the two possible reductions to indicate that each conflicts in one rule.

This feature needs user feedback, and might evolve in the future.

• C++: Actual token constructors

When variants and token constructors are enabled, in addition to the type-safe named token constructors (make_ID, make_INT, etc.), we now generate genuine constructors for symbol_type.

For instance with these declarations

    %token           ':'
       <std::string> ID
       <int>         INT;

you may use these constructors:

    symbol_type (int token, const std::string&);
    symbol_type (int token, const int&);
    symbol_type (int token);

which should be used in a Flex-scanner as follows.

    %%
    [a-z]+   return yy::parser::symbol_type (ID, yytext);
    [0-9]+   return yy::parser::symbol_type (INT, text_to_int (yytext);
    ":"      return yy::parser::symbol_type (’:’);
    <<EOF>>  return yy::parser::symbol_type (0);

Correct matching between token types and value types is checked via 'assert'. For instance, 'symbol_type (ID, 42)' would abort (while 'make_ID (42)' would not even compile).

• C++: Variadic emplace

If your application requires C++11 and you don't use symbol constructors, you may now use a variadic emplace for semantic values:

    %define api.value.type variant
    %token <std::pair<int, int>> PAIR

in your scanner:

    int yylex (parser::semantic_type *lvalp)
    {
      lvalp->emplace <std::pair<int, int>> (1, 2);
      return parser::token::PAIR;
    }

• C++: Syntax error exceptions in GLR

The glr.cc skeleton now supports syntax_error exceptions thrown from user actions, or from the scanner.

• More POSIX Yacc compatibility warnings

More Bison specific directives are now reported with -y or -Wyacc. This change was ready since the release of Bison 3.0 in September 2015. It was delayed because Autoconf used to define YACC as bison -y, which resulted in numerous warnings for Bison users that use the GNU Build System.

If you still experience that problem, either redefine YACC as `bison -o y.tab.c`, or pass -Wno-yacc to Bison.

• The tables yyrhs and yyphrs are back

Because no Bison skeleton uses them, these tables were removed (no longer passed to the skeletons, not even computed) in 2008. However, some users have expressed interest in being able to use them in their own

Bug fixes

• Incorrect number of reduce-reduce conflicts

On a grammar such as

exp: "num" | "num" | "num"

bison used to report a single RR conflict, instead of two. This is now fixed. This was the oldest (known) bug in Bison: it was there when Bison was entered in the RCS version control system, in December 1987.

Some grammar files might have to adjust their %expect-rr.

• Parser directives that were not careful enough

Passing invalid arguments to %nterm, for instance character literals, used to result in unclear error messages.

Documentation

The examples/ directory (installed in .../share/doc/bison/examples) has been restructured per language for clarity. The examples come with a README and a Makefile. Not only can they be used to toy with Bison, they can also be starting points for your own grammars.

There is now a Java example, and a simple example in C based on Flex and Bison (examples/c/lexcalc/).

Changes

• Parsers in C++

They now use noexcept and constexpr. Please, report missing annotations.

• Symbol Declarations

The syntax of the variation directives to declare symbols was overhauled for more consistency, and also better POSIX Yacc compliance (which, for instance, allows "%type" without actually providing a type). The %nterm directive, supported by Bison since its inception, is now documented and officially supported.

The syntax is now as follows:

    %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
    %left  TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
    %type  TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
    %nterm TAG? ID+ ( TAG ID+ )*

where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or one), ‘*’ (zero or more) and ‘+’ (one or more).

[Bruce Dubbs]

glibc-2.29 is now available. It looks like pythin3 will be required and we man need to add it to Chapter 5.

[Bruce Dubbs]

Fixed at revision 11506.