3.4.8.3 Midrule Action Translation

Midrule actions are actually transformed into regular rules and actions. The various reports generated by Bison (textual, graphical, etc., see Understanding Your Parser) reveal this translation, best explained by means of an example. The following rule:

exp: { a(); } "b" { c(); } { d(); } "e" { f(); };

is translated into:

$@1: %empty { a(); };
$@2: %empty { c(); };
$@3: %empty { d(); };
exp: $@1 "b" $@2 $@3 "e" { f(); };

with new nonterminal symbols $@n, where n is a number.

A midrule action is expected to generate a value if it uses $$, or the (final) action uses $n where n denote the midrule action. In that case its nonterminal is rather named @n:

exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };

is translated into

@1: %empty { a(); };
@2: %empty { $$ = c(); };
$@3: %empty { d(); };
exp: @1 "b" @2 $@3 "e" { f = $1; }

There are probably two errors in the above example: the first midrule action does not generate a value (it does not use $$ although the final action uses it), and the value of the second one is not used (the final action does not use $3). Bison reports these errors when the midrule-value warnings are enabled (see Invoking Bison):

$ bison -Wmidrule-value mid.y

mid.y:2.6-13: warning: unset value: $$
    2 | exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };
      |      ^~~~~~~~

mid.y:2.19-31: warning: unused value: $3
    2 | exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };
      |                   ^~~~~~~~~~~~~

It is sometimes useful to turn midrule actions into regular actions, e.g., to factor them, or to escape from their limitations. For instance, as an alternative to typed midrule action, you may bury the midrule action inside a nonterminal symbol and to declare a printer and a destructor for that symbol:

%nterm <context> let
%destructor { pop_context ($$); } let
%printer { print_context (yyo, $$); } let

%%

stmt:
  let stmt
    {
      $$ = $2;
      pop_context ($let);
    };

let:
  "let" '(' var ')'
    {
      $let = push_context ();
      declare_variable ($var);
    };