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doc: typed mid-rule actions

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* doc/bison.texi (Mid-Rule Actions): Restructure to insert...
(Typed Mid-Rule Actions): this new section.
Move the manual translation of mid-rule actions into regular actions
to...
(Mid-Rule Action Translation): here.
This commit is contained in:
Akim Demaille
2018-08-12 10:49:29 +02:00
parent adf0425d11
commit 005ea24cbb
1 changed files with 112 additions and 40 deletions
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doc/bison.texi
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@@ -225,6 +225,7 @@ Defining Language Semantics
Actions in Mid-Rule
* Using Mid-Rule Actions:: Putting an action in the middle of a rule.
* Typed Mid-Rule Actions:: Specifying the semantic type of their values.
* Mid-Rule Action Translation:: How mid-rule actions are actually processed.
* Mid-Rule Conflicts:: Mid-rule actions can cause conflicts.
@@ -4071,6 +4072,7 @@ are executed before the parser even recognizes the following components.
@menu
* Using Mid-Rule Actions:: Putting an action in the middle of a rule.
* Typed Mid-Rule Actions:: Specifying the semantic type of their values.
* Mid-Rule Action Translation:: How mid-rule actions are actually processed.
* Mid-Rule Conflicts:: Mid-rule actions can cause conflicts.
@end menu
@@ -4158,64 +4160,86 @@ earlier action is used to restore the prior list of variables. This
removes the temporary @code{let}-variable from the list so that it won't
appear to exist while the rest of the program is parsed.
Because the types of the semantic values of mid-rule actions are unknown to
Bison, type-based features (e.g., @samp{%printer}, @samp{%destructor}) do
not work, which could result in memory leaks. They also forbid the use of
the @code{variant} implementation of the @code{api.value.type} in C++
(@pxref{C++ Variants}).
@xref{Typed Mid-Rule Actions}, for one way to address this issue, and
@ref{Mid-Rule Action Translation}, for another: turning mid-action actions
into regular actions.
@node Typed Mid-Rule Actions
@subsubsection Typed Mid-Rule Actions
@findex %destructor
@cindex discarded symbols, mid-rule actions
@cindex error recovery, mid-rule actions
In the above example, if the parser initiates error recovery (@pxref{Error
Recovery}) while parsing the tokens in the embedded statement @code{stmt},
it might discard the previous semantic context @code{$<context>5} without
restoring it.
Thus, @code{$<context>5} needs a destructor (@pxref{Destructor Decl, , Freeing
Discarded Symbols}).
However, Bison currently provides no means to declare a destructor specific to
a particular mid-rule action's semantic value.
restoring it. Thus, @code{$<context>5} needs a destructor
(@pxref{Destructor Decl, , Freeing Discarded Symbols}), and Bison needs the
type of the semantic value (@code{context}) to select the right destructor.
One solution is to bury the mid-rule action inside a nonterminal symbol and to
declare a destructor for that symbol:
As an extension to Yacc's mid-rule actions, Bison offers a means to type
their semantic value: specify its type tag (@samp{<...>} before the mid-rule
action.
Consider the previous example, with an untyped mid-rule action:
@example
@group
%type <context> let
%destructor @{ pop_context ($$); @} let
@end group
%%
@group
stmt:
let stmt
@{
$$ = $2;
pop_context ($let);
@};
@end group
@group
let:
"let" '(' var ')'
@{
$let = push_context ();
$<context>$ = push_context (); // ***
declare_variable ($3);
@};
@}
stmt
@{
$$ = $6;
pop_context ($<context>5); // ***
@}
@end group
@end example
@noindent
Note that the action is now at the end of its rule.
Any mid-rule action can be converted to an end-of-rule action in this way, and
this is what Bison actually does to implement mid-rule actions.
If instead you write:
@example
@group
stmt:
"let" '(' var ')'
<context>@{ // ***
$$ = push_context (); // ***
declare_variable ($3);
@}
stmt
@{
$$ = $6;
pop_context ($5); // ***
@}
@end group
@end example
@noindent
then @code{%printer} and @code{%destructor} work properly (no more leaks!),
C++ @code{variant}s can be used, and redundancy is reduced (@code{<context>}
is specified once).
@node Mid-Rule Action Translation
@subsubsection Mid-Rule Action Translation
@vindex $@@@var{n}
@vindex @@@var{n}
As hinted earlier, mid-rule actions are actually transformed into regular
rules and actions. The various reports generated by Bison (textual,
graphical, etc., see @ref{Understanding, , Understanding Your Parser})
reveal this translation, best explained by means of an example. The
following rule:
Mid-rule actions are actually transformed into regular rules and actions.
The various reports generated by Bison (textual, graphical, etc., see
@ref{Understanding, , Understanding Your Parser}) reveal this translation,
best explained by means of an example. The following rule:
@example
exp: @{ a(); @} "b" @{ c(); @} @{ d(); @} "e" @{ f(); @};
@@ -4273,6 +4297,45 @@ mid.y:2.19-31: warning: unused value: $3
@end group
@end example
@sp 1
It is sometimes useful to turn mid-rule actions into regular actions, e.g.,
to factor them, or to escape from their limitations. For instance, as an
alternative to @emph{typed} mid-rule action, you may bury the mid-rule
action inside a nonterminal symbol and to declare a printer and a destructor
for that symbol:
@example
@group
%type <context> let
%destructor @{ pop_context ($$); @} let
%printer @{ print_context (yyo, $$); @} let
@end group
%%
@group
stmt:
let stmt
@{
$$ = $2;
pop_context ($let);
@};
@end group
@group
let:
"let" '(' var ')'
@{
$let = push_context ();
declare_variable ($var);
@};
@end group
@end example
@node Mid-Rule Conflicts
@subsubsection Conflicts due to Mid-Rule Actions
@@ -10523,7 +10586,7 @@ To enable variant-based semantic values, set @code{%define} variable
@code{%union} is ignored, and instead of using the name of the fields of the
@code{%union} to ``type'' the symbols, use genuine types.
For instance, instead of
For instance, instead of:
@example
%union
@@ -10536,7 +10599,7 @@ For instance, instead of
@end example
@noindent
write
write:
@example
%token <int> NUMBER;
@@ -10555,7 +10618,10 @@ Variants are stricter than unions. When based on unions, you may play any
dirty game with @code{yylval}, say storing an @code{int}, reading a
@code{char*}, and then storing a @code{double} in it. This is no longer
possible with variants: they must be initialized, then assigned to, and
eventually, destroyed.
eventually, destroyed. As a matter of fact, Bison variants forbid the use
of alternative types such as @samp{$<int>2} or @samp{$<std::string>$}, even
in mid-rule actions. It is mandatory to use typed mid-rule actions
(@pxref{Typed Mid-Rule Actions}).
@deftypemethod {semantic_type} {T&} build<T> ()
Initialize, but leave empty. Returns the address where the actual value may
@@ -10575,10 +10641,13 @@ Boost.Variant not only stores the value, but also a tag specifying its
type. But the parser already ``knows'' the type of the semantic value, so
that would be duplicating the information.
We do not use C++17's @code{std::variant} either: we want to support all the
C++ standards, and of course @code{std::variant} also stores a tag to record
the current type.
Therefore we developed light-weight variants whose type tag is external (so
they are really like @code{unions} for C++ actually). But our code is much
less mature that Boost.Variant. So there is a number of limitations in
(the current implementation of) variants:
they are really like @code{unions} for C++ actually). There is a number of
limitations in (the current implementation of) variants:
@itemize
@item
Alignment must be enforced: values should be aligned in memory according to
@@ -10588,6 +10657,9 @@ therefore, since, as far as we know, @code{double} is the most demanding
type on all platforms, alignments are enforced for @code{double} whatever
types are actually used. This may waste space in some cases.
@item
Move semantics is not yet supported, but will soon be added.
@item
There might be portability issues we are not aware of.
@end itemize