As hinted earlier, mid-rule 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 mid-rule action is expected to generate a value if it uses `$$`

, or
the (final) action uses `$`

`n` where `n` denote the mid-rule
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 mid-rule
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 -fcaret -Wmidrule-value mid.y mid.y:2.6-13: warning: unset value: $$ exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; }; ^^^^^^^^ mid.y:2.19-31: warning: unused value: $3 exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; }; ^^^^^^^^^^^^^