d9a9b054ae
The user gives yyexpected_tokens a limit: the max number of tokens she
wants to hear about. That's because an error message that reports a
bazillion of possible tokens is useless.
In that case yyexpected_tokens returned 0, so the user would not know
if there are too many expected tokens or none (yes, that's possible).
There are several ways to tell the user in which situation she's in:
- return some E2MANY, a negative value. Then it makes the pattern
int argsize = yypcontext_expected_tokens (ctx, arg, ARGS_MAX);
if (argsize < 0)
return argsize;
no longer valid, as for E2MANY (i) the user must generate the error
message anyway, and (ii) she should not return E2MANY
- return ARGS_MAX + 1. Then it makes it dangerous for the user, as
she has to iterate update `min (ARGS_MAX, argsize)`.
Returning 0 is definitely simpler and safer for the user, as it tells
her "this is not an error, just generate your message without a list
of expecting tokens". So let's still return 0, but set arg[0] to the
empty token when the list is really empty.
* data/skeletons/glr.c, data/skeletons/lalr1.cc, data/skeletons/lalr1.java
* data/skeletons/yacc.c (yyexpected_tokens): Put the empty symbol
first if there are no possible tokens at all.
* examples/c/bistromathic/parse.y: Demonstrate how to use that.
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/.
Installation
Build from git
Here are basic installation instructions for a repository checkout:
$ git submodule update --init
$ ./bootstrap
then proceed with the usual configure && make steps.
The file README-hacking.md is about building, modifying and checking Bison.
Build from tarball
See the file INSTALL for generic compilation and installation instructions.
Bison requires GNU m4 1.4.6 or later. See https://ftp.gnu.org/gnu/m4/m4-1.4.6.tar.gz.
Running a non installed bison
Once you ran make, you might want to toy with this fresh bison before
installing it. In that case, do not use src/bison: it would use the
installed files (skeletons, etc.), not the local ones. Use tests/bison.
Colored diagnostics
As an experimental feature, diagnostics are now colored, controlled by the
--color and --style options.
To use them, install the libtextstyle library before configuring Bison. It is available from https://alpha.gnu.org/gnu/gettext/, for instance https://alpha.gnu.org/pub/gnu/gettext/libtextstyle-0.20.5.tar.gz.
The option --color supports the following arguments:
- always, yes: Enable colors.
- never, no: Disable colors.
- auto, tty (default): Enable colors if the output device is a tty.
To customize the styles, create a CSS file, say bison-bw.css, similar to
/* bison-bw.css */
.warning { }
.error { font-weight: 800; text-decoration: underline; }
.note { }
then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
environment variable to bison-bw.css.
Relocatability
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 symlinks to the installed and moved programs, and invoke them through the symlink.
See "Enabling Relocatability" in the documentation.
Internationalization
Bison supports two catalogs: one for Bison itself (i.e., for the maintainer-side parser generation), and one for the generated parsers (i.e., for the user-side parser execution). The requirements between both differ: bison needs ngettext, the generated parsers do not. To simplify the build system, neither are installed if ngettext is not supported, even if generated parsers could have been localized. See http://lists.gnu.org/archive/html/bug-bison/2009-08/msg00006.html for more details.
Questions
See the section FAQ in the documentation (doc/bison.info) for frequently
asked questions. The documentation is also available in PDF and HTML,
provided you have a recent version of Texinfo installed: run make pdf or
make html.
If you have questions about using Bison and the documentation does not answer them, please send mail to help-bison@gnu.org.
Bug reports
Please send bug reports to bug-bison@gnu.org. Be sure to include the
version number from bison --version, and a complete, self-contained test
case in each bug report.
Copyright statements
For any copyright year range specified as YYYY-ZZZZ in this package, note that the range specifies every single year in that closed interval.