Next: Declarations, Previous: Tracking Locations, Up: Grammar File [Contents][Index]
As described in the preceding sections, the traditional way to refer to any
semantic value or location is a positional reference, which takes the
form $n
, $$
, @n
, and @$
. However,
such a reference is not very descriptive. Moreover, if you later decide to
insert or remove symbols in the right-hand side of a grammar rule, the need
to renumber such references can be tedious and error-prone.
To avoid these issues, you can also refer to a semantic value or location using a named reference. First of all, original symbol names may be used as named references. For example:
invocation: op '(' args ')' { $invocation = new_invocation ($op, $args, @invocation); }
Positional and named references can be mixed arbitrarily. For example:
invocation: op '(' args ')' { $$ = new_invocation ($op, $args, @$); }
However, sometimes regular symbol names are not sufficient due to ambiguities:
exp: exp '/' exp { $exp = $exp / $exp; } // $exp is ambiguous. exp: exp '/' exp { $$ = $1 / $exp; } // One usage is ambiguous. exp: exp '/' exp { $$ = $1 / $3; } // No error.
When ambiguity occurs, explicitly declared names may be used for values and locations. Explicit names are declared as a bracketed name after a symbol appearance in rule definitions. For example:
exp[result]: exp[left] '/' exp[right] { $result = $left / $right; }
In order to access a semantic value generated by a mid-rule action, an explicit name may also be declared by putting a bracketed name after the closing brace of the mid-rule action code:
exp[res]: exp[x] '+' {$left = $x;}[left] exp[right] { $res = $left + $right; }
In references, in order to specify names containing dots and dashes, an explicit
bracketed syntax $[name]
and @[name]
must be used:
if-stmt: "if" '(' expr ')' "then" then.stmt ';' { $[if-stmt] = new_if_stmt ($expr, $[then.stmt]); }
It often happens that named references are followed by a dot, dash or other
C punctuation marks and operators. By default, Bison will read
‘$name.suffix’ as a reference to symbol value $name
followed by
‘.suffix’, i.e., an access to the suffix
field of the semantic
value. In order to force Bison to recognize ‘name.suffix’ in its
entirety as the name of a semantic value, the bracketed syntax
‘$[name.suffix]’ must be used.
The named references feature is experimental. More user feedback will help to stabilize it.
Next: Declarations, Previous: Tracking Locations, Up: Grammar File [Contents][Index]