Future/bison
1
0
Fork 0
mirror of https://git.savannah.gnu.org/git/bison.git synced 2026-03-09 12:23:04 +00:00
Code Issues Packages Projects Releases Wiki Activity

generate the default semantic action

Browse Source 
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
Download Patch File Download Diff File Expand all files Collapse all files

21
NEWS
View File

@@ -68,6 +68,27 @@ GNU Bison NEWS
The new examples/variant-11.yy shows these features in action.
*** C++: The implicit default semantic action is always run
When variants are enabled, the default action was not run, so
exp: "number"
was equivalent to
exp: "number" {}
It now behaves like in all the other cases, as
exp: "number" { $$ = $1; }
possibly using std::move if automove is enabled.
We do not expect backward compatibility issues. However, beware of
forward compatibility issues: if you rely on default actions with
variants, be sure to '%require "3.2"' to avoid older versions of Bison to
generate incorrect parsers.
*** C++: Renaming location.hh
When both %defines and %locations are enabled, Bison generates a

9
doc/bison.texi
View File

@@ -6348,8 +6348,7 @@ Obsoleted by @code{api.namespace}
@item Purpose: Issue runtime assertions to catch invalid uses.
In C++, when variants are used (@pxref{C++ Variants}), symbols must be
constructed and
destroyed properly. This option checks these constraints.
constructed and destroyed properly. This option checks these constraints.
@item Accepted Values: Boolean
@@ -11594,13 +11593,13 @@ assignment:
%left "+" "-";
%left "*" "/";
exp:
exp "+" exp @{ $$ = $1 + $3; @}
"number"
| "identifier" @{ $$ = drv.variables[$1]; @}
| exp "+" exp @{ $$ = $1 + $3; @}
| exp "-" exp @{ $$ = $1 - $3; @}
| exp "*" exp @{ $$ = $1 * $3; @}
| exp "/" exp @{ $$ = $1 / $3; @}
| "(" exp ")" @{ std::swap ($$, $2); @}
| "identifier" @{ $$ = drv.variables[$1]; @}
| "number" @{ std::swap ($$, $1); @};
%%
@end example

2
examples/variant-11.yy
View File

@@ -101,7 +101,7 @@ list:
;
item:
TEXT { $$ = $1; }
TEXT
| NUMBER { $$ = make_string_uptr (to_string ($1)); }
;
%%

2
examples/variant.yy
View File

@@ -89,7 +89,7 @@ list:
;
item:
TEXT { std::swap ($$, $1); }
TEXT
| NUMBER { $$ = to_string ($1); }
;
%%

50
src/reader.c
View File

@@ -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. |
`----------------------------------------------------------------*/
| 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);

2
tests/calc.at
View File

@@ -303,7 +303,7 @@ line:
;
exp:
NUM { $$ = $1; }
NUM
| exp '=' exp
{
if ($1 != $3)