3.6 Named References

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.

This document was generated by Rick Perry on December 29, 2013 using texi2html 1.82.