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doc: introduce %empty and -Wempty-rule

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* doc/bison.texi (Grammar Rules): Make it a @section which
contains...
(Rules Syntax): this new subsection (with the previous contents of
"Grammar Rules".
(Empty Rules): New subsection, extracted from the former
"Grammar Rules".
Document %empty.
(Recursion): New a subsection of "Grammar Rules".
Complete a few index entries.
(Bison Options): Document -Wempty-rule.
This commit is contained in:
Akim Demaille
2013-02-16 13:23:23 +01:00
parent 8b807f1177
commit 09add9c24f
2 changed files with 96 additions and 20 deletions
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22
NEWS
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@@ -362,6 +362,28 @@ GNU Bison NEWS
%nonassoc '='
^^^
** Empty rules
Empty rules (i.e., with an empty right-hand side) can now be explicitly
marked by the new %empty directive. Using %empty on a non-empty rule is
an error. The new -Wempty-rule warning reports empty rules without
%empty. On the following grammar:
%%
s: a b c;
a: ;
b: %empty;
c: 'a' %empty;
bison reports:
3.4-5: warning: empty rule without %empty [-Wempty-rule]
a: {}
^^
5.8-13: error: %empty on non-empty rule
c: 'a' %empty {};
^^^^^^
** Java skeleton improvements
Contributed by Paolo Bonzini.

94
doc/bison.texi
View File

@@ -186,7 +186,6 @@ Bison Grammar Files
* Grammar Outline:: Overall layout of the grammar file.
* Symbols:: Terminal and nonterminal symbols.
* Rules:: How to write grammar rules.
* Recursion:: Writing recursive rules.
* Semantics:: Semantic values and actions.
* Tracking Locations:: Locations and actions.
* Named References:: Using named references in actions.
@@ -201,6 +200,13 @@ Outline of a Bison Grammar
* Grammar Rules:: Syntax and usage of the grammar rules section.
* Epilogue:: Syntax and usage of the epilogue.
Grammar Rules
* Rules Syntax:: Syntax of the rules.
* Empty Rules:: Symbols that can match the empty string.
* Recursion:: Writing recursive rules.
Defining Language Semantics
* Value Type:: Specifying one data type for all semantic values.
@@ -2789,7 +2795,6 @@ The Bison grammar file conventionally has a name ending in @samp{.y}.
* Grammar Outline:: Overall layout of the grammar file.
* Symbols:: Terminal and nonterminal symbols.
* Rules:: How to write grammar rules.
* Recursion:: Writing recursive rules.
* Semantics:: Semantic values and actions.
* Tracking Locations:: Locations and actions.
* Named References:: Using named references in actions.
@@ -3407,7 +3412,18 @@ value of the error token is 256, unless you explicitly assigned 256 to
one of your tokens with a @code{%token} declaration.
@node Rules
@section Syntax of Grammar Rules
@section Grammar Rules
A Bison grammar is a list of rules.
@menu
* Rules Syntax:: Syntax of the rules.
* Empty Rules:: Symbols that can match the empty string.
* Recursion:: Writing recursive rules.
@end menu
@node Rules Syntax
@subsection Syntax of Grammar Rules
@cindex rule syntax
@cindex grammar rule syntax
@cindex syntax of grammar rules
@@ -3481,33 +3497,57 @@ be joined with the vertical-bar character @samp{|} as follows:
@noindent
They are still considered distinct rules even when joined in this way.
If @var{components} in a rule is empty, it means that @var{result} can
match the empty string. For example, here is how to define a
comma-separated sequence of zero or more @code{exp} groupings:
@node Empty Rules
@subsection Empty Rules
@cindex empty rule
@cindex rule, empty
@findex %empty
A rule is said to be @dfn{empty} if its right-hand side (@var{components})
is empty. It means that @var{result} can match the empty string. For
example, here is how to define an optional semicolon:
@example
semicolon.opt: | ";";
@end example
@noindent
It is easy not to see an empty rule, especially when @code{|} is used. The
@code{%empty} directive allows to make explicit that a rule is empty on
purpose:
@example
@group
expseq:
/* empty */
| expseq1
;
@end group
@group
expseq1:
exp
| expseq1 ',' exp
semicolon.opt:
%empty
| ";"
;
@end group
@end example
Flagging a non-empty rule with @code{%empty} is an error. If run with
@option{-Wempty-rule}, @command{bison} will report empty rules without
@code{%empty}. Using @code{%empty} enables this warning, unless
@option{-Wno-empty-rule} was specified.
The @code{%empty} directive is a Bison extension, it does not work with
Yacc. To remain compatible with POSIX Yacc, it is customary to write a
comment @samp{/* empty */} in each rule with no components:
@example
@group
semicolon.opt:
/* empty */
| ";"
;
@end group
@end example
@noindent
It is customary to write a comment @samp{/* empty */} in each rule
with no components.
@node Recursion
@section Recursive Rules
@subsection Recursive Rules
@cindex recursive rule
@cindex rule, recursive
A rule is called @dfn{recursive} when its @var{result} nonterminal
appears also on its right hand side. Nearly all Bison grammars need to
@@ -3934,16 +3974,20 @@ declare a destructor for that symbol:
@group
%type <context> let
%destructor @{ pop_context ($$); @} let
@end group
%%
@group
stmt:
let stmt
@{
$$ = $2;
pop_context ($let);
@};
@end group
@group
let:
"let" '(' var ')'
@{
@@ -9734,6 +9778,11 @@ no effect on the conflict report.
Deprecated constructs whose support will be removed in future versions of
Bison.
@item empty-rule
Empty rules without @code{%empty}. @xref{Empty Rules}. Disabled by
default, but enabled by uses of @code{%empty}, unless
@option{-Wno-empty-rule} was specified.
@item precedence
Useless precedence and associativity directives. Disabled by default.
@@ -12428,6 +12477,11 @@ time to resolve reduce/reduce conflicts. @xref{GLR Parsers, ,Writing
GLR Parsers}.
@end deffn
@deffn {Directive} %empty
Bison declaration to declare make explicit that a rule has an empty
right-hand side. @xref{Empty Rules}.
@end deffn
@deffn {Symbol} $end
The predefined token marking the end of the token stream. It cannot be
used in the grammar.