From be22823e78808e3d9b267919a769a3111fd0cb2a Mon Sep 17 00:00:00 2001 From: Akim Demaille Date: Sun, 9 Dec 2012 16:49:58 +0100 Subject: [PATCH] doc: explain how mid-rule actions are translated * doc/bison.texi (Actions in Mid-Rule): Mention and use named references. Split into three subsections, among which... (Mid-Rule Action Translation): this new section. --- NEWS | 2 + doc/bison.texi | 122 +++++++++++++++++++++++++++++++++++++++++++++++-- 2 files changed, 120 insertions(+), 4 deletions(-) diff --git a/NEWS b/NEWS index cecc6760..d937bea7 100644 --- a/NEWS +++ b/NEWS @@ -106,6 +106,8 @@ GNU Bison NEWS The sections about shift/reduce and reduce/reduce conflicts resolution have been fixed and extended. + The translation of mid-rule actions is now described. + * Noteworthy changes in release 2.6.5 (2012-11-07) [stable] We consider compiler warnings about Bison generated parsers to be bugs. diff --git a/doc/bison.texi b/doc/bison.texi index 5a6cb686..a508b9c1 100644 --- a/doc/bison.texi +++ b/doc/bison.texi @@ -208,6 +208,12 @@ Defining Language Semantics This says when, why and how to use the exceptional action in the middle of a rule. +Actions in Mid-Rule + +* Using Mid-Rule Actions:: Putting an action in the middle of a rule. +* Mid-Rule Action Translation:: How mid-rule actions are actually processed. +* Mid-Rule Conflicts:: Mid-rule actions can cause conflicts. + Tracking Locations * Location Type:: Specifying a data type for locations. @@ -3739,6 +3745,15 @@ Occasionally it is useful to put an action in the middle of a rule. These actions are written just like usual end-of-rule actions, but they are executed before the parser even recognizes the following components. +@menu +* Using Mid-Rule Actions:: Putting an action in the middle of a rule. +* Mid-Rule Action Translation:: How mid-rule actions are actually processed. +* Mid-Rule Conflicts:: Mid-rule actions can cause conflicts. +@end menu + +@node Using Mid-Rule Actions +@subsubsection Using Mid-Rule Actions + A mid-rule action may refer to the components preceding it using @code{$@var{n}}, but it may not refer to subsequent components because it is run before they are parsed. @@ -3791,8 +3806,27 @@ list of accessible variables) as its semantic value, using alternative @code{context} in the data-type union. Then it calls @code{declare_variable} to add the new variable to that list. Once the first action is finished, the embedded statement @code{stmt} can be -parsed. Note that the mid-rule action is component number 5, so the -@samp{stmt} is component number 6. +parsed. + +Note that the mid-rule action is component number 5, so the @samp{stmt} is +component number 6. Named references can be used to improve the readability +and maintainability (@pxref{Named References}): + +@example +@group +stmt: + "let" '(' var ')' + @{ + $let = push_context (); + declare_variable ($3); + @}[let] + stmt + @{ + $$ = $6; + pop_context ($let); + @} +@end group +@end example After the embedded statement is parsed, its semantic value becomes the value of the entire @code{let}-statement. Then the semantic value from the @@ -3826,13 +3860,13 @@ stmt: let stmt @{ $$ = $2; - pop_context ($1); + pop_context ($let); @}; let: "let" '(' var ')' @{ - $$ = push_context (); + $let = push_context (); declare_variable ($3); @}; @@ -3844,6 +3878,76 @@ Note that the action is now at the end of its rule. Any mid-rule action can be converted to an end-of-rule action in this way, and this is what Bison actually does to implement mid-rule actions. +@node Mid-Rule Action Translation +@subsubsection Mid-Rule Action Translation +@vindex $@@@var{n} +@vindex @@@var{n} + +As hinted earlier, mid-rule actions are actually transformed into regular +rules and actions. The various reports generated by Bison (textual, +graphical, etc., see @ref{Understanding, , Understanding Your Parser}) +reveal this translation, best explained by means of an example. The +following rule: + +@example +exp: @{ a(); @} "b" @{ c(); @} @{ d(); @} "e" @{ f(); @}; +@end example + +@noindent +is translated into: + +@example +$@@1: /* empty */ @{ a(); @}; +$@@2: /* empty */ @{ c(); @}; +$@@3: /* empty */ @{ d(); @}; +exp: $@@1 "b" $@@2 $@@3 "e" @{ f(); @}; +@end example + +@noindent +with new nonterminal symbols @code{$@@@var{n}}, where @var{n} is a number. + +A mid-rule action is expected to generate a value if it uses @code{$$}, or +the (final) action uses @code{$@var{n}} where @var{n} denote the mid-rule +action. In that case its nonterminal is rather named @code{@@@var{n}}: + +@example +exp: @{ a(); @} "b" @{ $$ = c(); @} @{ d(); @} "e" @{ f = $1; @}; +@end example + +@noindent +is translated into + +@example +@@1: /* empty */ @{ a(); @}; +@@2: /* empty */ @{ $$ = c(); @}; +$@@3: /* empty */ @{ d(); @}; +exp: @@1 "b" @@2 $@@3 "e" @{ f = $1; @} +@end example + +There are probably two errors in the above example: the first mid-rule +action does not generate a value (it does not use @code{$$} although the +final action uses it), and the value of the second one is not used (the +final action does not use @code{$3}). Bison reports these errors when the +@code{midrule-value} warnings are enabled (@pxref{Invocation, ,Invoking +Bison}): + +@example +$ bison -fcaret -Wmidrule-value mid.y +@group +mid.y:2.6-13: warning: unset value: $$ + exp: @{ a(); @} "b" @{ $$ = c(); @} @{ d(); @} "e" @{ f = $1; @}; + ^^^^^^^^ +@end group +@group +mid.y:2.19-31: warning: unused value: $3 + exp: @{ a(); @} "b" @{ $$ = c(); @} @{ d(); @} "e" @{ f = $1; @}; + ^^^^^^^^^^^^^ +@end group +@end example + + +@node Mid-Rule Conflicts +@subsubsection Conflicts due to Mid-Rule Actions Taking action before a rule is completely recognized often leads to conflicts since the parser must commit to a parse in order to execute the action. For example, the following two rules, without mid-rule actions, @@ -3941,6 +4045,7 @@ compound: Now Bison can execute the action in the rule for @code{subroutine} without deciding which rule for @code{compound} it will eventually use. + @node Tracking Locations @section Tracking Locations @cindex location @@ -11404,8 +11509,12 @@ In an action, the location of the left-hand side of the rule. @end deffn @deffn {Variable} @@@var{n} +@deffnx {Symbol} @@@var{n} In an action, the location of the @var{n}-th symbol of the right-hand side of the rule. @xref{Tracking Locations}. + +In a grammar, the Bison-generated nonterminal symbol for a mid-rule action +with a semantical value. @xref{Mid-Rule Action Translation}. @end deffn @deffn {Variable} @@@var{name} @@ -11414,6 +11523,11 @@ In an action, the location of a symbol addressed by @var{name}. @xref{Tracking Locations}. @end deffn +@deffn {Symbol} $@@@var{n} +In a grammar, the Bison-generated nonterminal symbol for a mid-rule action +with no semantical value. @xref{Mid-Rule Action Translation}. +@end deffn + @deffn {Variable} $$ In an action, the semantic value of the left-hand side of the rule. @xref{Actions}. -- 2.47.2