Bison - News
Bison 3.6 released
Item posted by Akim Demaille <akim> on Sat 09 May 2020 08:48:38 AM UTC.
We are extremely happy to announce the release of Bison 3.6:
- the developer can forge syntax error messages the way she wants.
- token string aliases can be internationalized, and UTF-8 sequences
are properly preserved.
- push parsers can ask at any moment for the list of "expected tokens",
which can be used to provide syntax-driven autocompletion.
- yylex may now tell the parser to enter error-recovery without issuing an
error message (when the error was already reported by the scanner).
- several new examples were added, in particular "bistromathic" demonstrates
almost all the existing bells and whistles, including interactive
autocompletion on top of GNU readline.
Please see the much more detailed release notes below.
Many thanks to testers, bug reporters, contributors and feature requesters:
Adrian Vogelsgesang, Ahcheong Lee, Alexandre Duret-Lutz, Andy Fiddaman,
Angelo Borsotti, Arthur Schwarz, Christian Schoenebeck, Dagobert Michelsen,
Denis Excoffier, Dennis Clarke, Don Macpherson, Evan Lavelle, Frank
Heckenbach, Horst von Brand, Jannick, Nikki Valen, Paolo Bonzini, Paul
Eggert, Pramod Kumbhar and Victor Morales Cayuela. The author also thanks
an anonymous reviewer for his precious comments.
Special thanks to Bruno Haible for his investment into making Bison
portable.
Happy parsing!
Akim
PS/ The experimental back-end for the D programming language is still
looking for active support from the D community.
==================================================================
GNU Bison is a general-purpose parser generator that converts an annotated
context-free grammar into a deterministic LR or generalized LR (GLR) parser
employing LALR(1) parser tables. Bison can also generate IELR(1) or
canonical LR(1) parser tables. Once you are proficient with Bison, you can
use it to develop a wide range of language parsers, from those used in
simple desk calculators to complex programming languages.
Bison is upward compatible with Yacc: all properly-written Yacc grammars
work with Bison with no change. Anyone familiar with Yacc should be able to
use Bison with little trouble. You need to be fluent in C, C++ or Java
programming in order to use Bison.
Bison and the parsers it generates are portable, they do not require any
specific compilers.
GNU Bison's home page is https://gnu.org/software/bison/.
==================================================================
Here are the compressed sources:
https://ftp.gnu.org/gnu/bison/bison-3.6.tar.gz (5.1MB)
https://ftp.gnu.org/gnu/bison/bison-3.6.tar.xz (3.1MB)
Here are the GPG detached signatures[*]:
https://ftp.gnu.org/gnu/bison/bison-3.6.tar.gz.sig
https://ftp.gnu.org/gnu/bison/bison-3.6.tar.xz.sig
Use a mirror for higher download bandwidth:
https://www.gnu.org/order/ftp.html
[*] Use a .sig file to verify that the corresponding file (without the
.sig suffix) is intact. First, be sure to download both the .sig file
and the corresponding tarball. Then, run a command like this:
gpg --verify bison-3.6.tar.gz.sig
If that command fails because you don't have the required public key,
then run this command to import it:
gpg --keyserver keys.gnupg.net --recv-keys 0DDCAA3278D5264E
and rerun the 'gpg --verify' command.
This release was bootstrapped with the following tools:
Autoconf 2.69
Automake 1.16.2
Flex 2.6.4
Gettext 0.19.8.1
Gnulib v0.1-3382-g2ac33b29f
==================================================================
* Noteworthy changes in release 3.6 (2020-05-08) [stable]
** Backward incompatible changes
TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
The YYERROR_VERBOSE macro is no longer supported; the parsers that still
depend on it will now produce Yacc-like error messages (just "syntax
error"). It was superseded by the "%error-verbose" directive in Bison
1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
(2013-07-25), "%error-verbose" is deprecated in favor of "%define
parse.error verbose".
** Deprecated features
The YYPRINT macro, which works only with yacc.c and only for tokens, was
obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
It is deprecated and its support will be removed eventually.
** New features
*** Improved syntax error messages
Two new values for the %define parse.error variable offer more control to
the user. Available in all the skeletons (C, C++, Java).
**** %define parse.error detailed
The behavior of "%define parse.error detailed" is closely resembling that
of "%define parse.error verbose" with a few exceptions. First, it is safe
to use non-ASCII characters in token aliases (with 'verbose', the result
depends on the locale with which bison was run). Second, a yysymbol_name
function is exposed to the user, instead of the yytnamerr function and the
yytname table. Third, token internationalization is supported (see
below).
**** %define parse.error custom
With this directive, the user forges and emits the syntax error message
herself by defining the yyreport_syntax_error function. A new type,
yypcontext_t, captures the circumstances of the error, and provides the
user with functions to get details, such as yypcontext_expected_tokens to
get the list of expected token kinds.
A possible implementation of yyreport_syntax_error is:
int
yyreport_syntax_error (const yypcontext_t *ctx)
{
int res = 0;
YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
fprintf (stderr, ": syntax error");
// Report the tokens expected at this point.
{
enum { TOKENMAX = 10 };
yysymbol_kind_t expected[TOKENMAX];
int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
if (n < 0)
// Forward errors to yyparse.
res = n;
else
for (int i = 0; i < n; ++i)
fprintf (stderr, "%s %s",
i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
}
// Report the unexpected token.
{
yysymbol_kind_t lookahead = yypcontext_token (ctx);
if (lookahead != YYSYMBOL_YYEMPTY)
fprintf (stderr, " before %s", yysymbol_name (lookahead));
}
fprintf (stderr, "\n");
return res;
}
**** Token aliases internationalization
When the %define variable parse.error is set to `custom` or `detailed`,
one may specify which token aliases are to be translated using _(). For
instance
%token
PLUS "+"
MINUS "-"
<double>
NUM _("number")
<symrec*>
FUN _("function")
VAR _("variable")
In that case the user must define _() and N_(), and yysymbol_name returns
the translated symbol (i.e., it returns '_("variable")' rather that
'"variable"'). In Java, the user must provide an i18n() function.
*** List of expected tokens (yacc.c)
Push parsers may invoke yypstate_expected_tokens at any point during
parsing (including even before submitting the first token) to get the list
of possible tokens. This feature can be used to propose autocompletion
(see below the "bistromathic" example).
It makes little sense to use this feature without enabling LAC (lookahead
correction).
*** Returning the error token
When the scanner returns an invalid token or the undefined token
(YYUNDEF), the parser generates an error message and enters error
recovery. Because of that error message, most scanners that find lexical
errors generate an error message, and then ignore the invalid input
without entering the error-recovery.
The scanners may now return YYerror, the error token, to enter the
error-recovery mode without triggering an additional error message. See
the bistromathic for an example.
*** Deep overhaul of the symbol and token kinds
To avoid the confusion with types in programming languages, we now refer
to token and symbol "kinds" instead of token and symbol "types". The
documentation and error messages have been revised.
All the skeletons have been updated to use dedicated enum types rather
than integral types. Special symbols are now regular citizens, instead of
being declared in ad hoc ways.
**** Token kinds
The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
LPAREN, etc. While backward compatibility is of course ensured, users are
nonetheless invited to replace their uses of "enum yytokentype" by
"yytoken_kind_t".
This type now also includes tokens that were previously hidden: YYEOF (end
of input), YYUNDEF (undefined token), and YYerror (error token). They
now have string aliases, internationalized when internationalization is
enabled. Therefore, by default, error messages now refer to "end of file"
(internationalized) rather than the cryptic "$end", or to "invalid token"
rather than "$undefined".
Therefore in most cases it is now useless to define the end-of-line token
as follows:
%token T_EOF 0 "end of file"
Rather simply use "YYEOF" in your scanner.
**** Symbol kinds
The "symbol kinds" is what the parser actually uses. (Unless the
api.token.raw %define variable is used, the symbol kind of a terminal
differs from the corresponding token kind.)
They are now exposed as a enum, "yysymbol_kind_t".
This allows users to tailor the error messages the way they want, or to
process some symbols in a specific way in autocompletion (see the
bistromathic example below).
*** Modernize display of explanatory statements in diagnostics
Since Bison 2.7, output was indented four spaces for explanatory
statements. For example:
input.y:2.7-13: error: %type redeclaration for exp
input.y:1.7-11: previous declaration
Since the introduction of caret-diagnostics, it became less clear. This
indentation has been removed and submessages are displayed similarly as in
GCC:
input.y:2.7-13: error: %type redeclaration for exp
2 | %type <float> exp
| ^~~~~~~
input.y:1.7-11: note: previous declaration
1 | %type <int> exp
| ^~~~~
Contributed by Victor Morales Cayuela.
*** C++
The token and symbol kinds are yy::parser::token_kind_type and
yy::parser::symbol_kind_type.
The symbol_type::kind() member function allows to get the kind of a
symbol. This can be used to write unit tests for scanners, e.g.,
yy::parser::symbol_type t = make_NUMBER ("123");
assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
assert (t.value.as<int> () == 123);
** Documentation
*** User Manual
In order to avoid ambiguities with "type" as in "typing", we now refer to
the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
etc.).
*** Examples
There are now examples/java: a very simple calculator, and a more complete
one (push-parser, location tracking, and debug traces).
The lexcalc example (a simple example in C based on Flex and Bison) now
also demonstrates location tracking.
A new C example, bistromathic, is a fully featured interactive calculator
using many Bison features: pure interface, push parser, autocompletion
based on the current parser state (using yypstate_expected_tokens),
location tracking, internationalized custom error messages, lookahead
correction, rich debug traces, etc.
It shows how to depend on the symbol kinds to tailor autocompletion. For
instance it recognizes the symbol kind "VARIABLE" to propose
autocompletion on the existing variables, rather than of the word
"variable".
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