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generate the default semantic action

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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 commit is contained in:
Akim Demaille
2018-10-14 11:27:05 +02:00
parent 20a6c3175f
commit 01898726e2
6 changed files with 60 additions and 28 deletions
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52
src/reader.c
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@@ -275,13 +275,14 @@ symbol_should_be_used (symbol_list const *s, bool *midrule_warning)
return false;
}
/*----------------------------------------------------------------.
| Check that the rule R is properly defined. For instance, there |
| should be no type clash on the default action. |
`----------------------------------------------------------------*/
/*-----------------------------------------------------------------.
| Check that the rule R is properly defined. For instance, there |
| should be no type clash on the default action. Possibly install |
| the default action. |
`-----------------------------------------------------------------*/
static void
grammar_rule_check (const symbol_list *r)
grammar_rule_check_and_complete (symbol_list *r)
{
/* Type check.
@@ -303,6 +304,16 @@ grammar_rule_check (const symbol_list *r)
complain (&r->location, Wother,
_("type clash on default action: <%s> != <%s>"),
lhs_type, rhs_type);
else
{
/* Install the default action. */
code_props_rule_action_init (&r->action_props, "{ $$ = $1; }",
r->location, r,
/* name */ NULL,
/* type */ NULL,
/* is_predicate */ false);
code_props_translate_code (&r->action_props);
}
}
/* Warn if there is no default for $$ but we need one. */
else
@@ -439,14 +450,14 @@ grammar_current_rule_prec_set (symbol *precsym, location loc)
{
/* POSIX says that any identifier is a nonterminal if it does not
appear on the LHS of a grammar rule and is not defined by %token
or by one of the directives that assigns precedence to a token. We
ignore this here because the only kind of identifier that POSIX
allows to follow a %prec is a token and because assuming it's a
token now can produce more logical error messages. Nevertheless,
grammar_rule_check does obey what we believe is the real intent of
POSIX here: that an error be reported for any identifier that
appears after %prec but that is not defined separately as a
token. */
or by one of the directives that assigns precedence to a token.
We ignore this here because the only kind of identifier that
POSIX allows to follow a %prec is a token and because assuming
it's a token now can produce more logical error messages.
Nevertheless, grammar_rule_check_and_complete does obey what we
believe is the real intent of POSIX here: that an error be
reported for any identifier that appears after %prec but that is
not defined separately as a token. */
symbol_class_set (precsym, token_sym, loc, false);
if (current_rule->ruleprec)
duplicate_directive ("%prec",
@@ -593,7 +604,7 @@ packgram (void)
symbols may appear unused, but the parsing algorithm ensures that
%destructor's are invoked appropriately. */
if (p != grammar)
grammar_rule_check (p);
grammar_rule_check_and_complete (p);
rules[ruleno].user_number = ruleno;
rules[ruleno].number = ruleno;
@@ -799,12 +810,13 @@ check_and_convert_grammar (void)
symbols_pack above) so that token types are set correctly before the rule
action type checking.
Before invoking grammar_rule_check (in packgram below) on any rule, make
sure all actions have already been scanned in order to set 'used' flags.
Otherwise, checking that a midrule's $$ should be set will not always work
properly because the check must forward-reference the midrule's parent
rule. For the same reason, all the 'used' flags must be set before
checking whether to remove '$' from any midrule symbol name (also in
Before invoking grammar_rule_check_and_complete (in packgram
below) on any rule, make sure all actions have already been
scanned in order to set 'used' flags. Otherwise, checking that a
midrule's $$ should be set will not always work properly because
the check must forward-reference the midrule's parent rule. For
the same reason, all the 'used' flags must be set before checking
whether to remove '$' from any midrule symbol name (also in
packgram). */
for (symbol_list *sym = grammar; sym; sym = sym->next)
code_props_translate_code (&sym->action_props);