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Currently, in C, the default semantic action is implemented by being always run before running the actual user semantic action. As a consequence, when the user action is run, $$ is already set as $1. In C++ with variants, we don't do that, since we cannot manipulate the semantic value without knowing its exact type. When variants are enabled, the only guarantee is that $$ is default contructed and ready to the used. Some users still would like the default action to be run with variants. Frank Heckenbach's parser in C++17 (http://lists.gnu.org/archive/html/bug-bison/2018-04/msg00011.html) provides this feature, but relying on std::variant's dynamic typing, which we forbid in lalr1.cc. The simplest seems to be actually generating the default semantic action (in all languages/skeletons). This makes the pre-action (that sets $$ to $1) useless. But... maybe some users depend on this, in spite of the comments that clearly warn againt this. So let's not turn this off just yet. * src/reader.c (grammar_rule_check_and_complete): Rename as... (grammar_rule_check_and_complete): this. Install the default semantic action when applicable. * examples/variant-11.yy, examples/variant.yy, tests/calc.at: Exercise the default semantic action, even with variants.
This directory contains examples of Bison grammar files. * mfcalc A C example of a multi-function calculator. Extracted from the documentation. * calc++ A C++ version of the canonical example for parsers: a calculator. Also uses Flex for the scanner. Extracted from the documentation. * variant.yy A C++ example that uses variants (they allow to use any C++ type as semantic value type) and symbol constructors (they ensure consistency between declared token type and effective semantic value). * variant-11.yy Another C++ example, closely related to the previous one, but exhibiting support for C++11's move semantics. ----- Local Variables: mode: outline fill-column: 76 End: Copyright (C) 2018 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the "GNU Free Documentation License" file as part of this distribution. # LocalWords: mfcalc calc parsers yy