Occurs declarations

Quick examples:

attribute pp occurs on Expr;

attribute fst<a> occurs on Pair<a b>;

An occurs-on declaration indicates that a (separately declared) attribute occurs on the (separately declared) nonterminal specified. Most commonly, a nonterminal with clause will be used instead of this form of declaration directly.

For parameterized attributes, an occurs declaration also plays the crucial role of indicating how the attribute’s type parameters should be determined, given a nonterminal and its type parameters. If the attribute is not parameterized, the angle brackets are omitted.

attribute name < types... > occurs on nonterminal type;

Note that attributes can only occurs on nonterminal types. Also note that inside the angle brackets is a type list as opposed to a type variable list. Thus, the following is a valid occurs on declaration:

synthesized attribute ast<a> :: a;
attribute ast<AbstractExpr> occurs on ConcreteExpr;

Only those type variables that appear in the nonterminal type on the right may appear in the type list for the attribute on the left.

The most strongly preferred, whenever possible, means of declaring attribute occurrences is described on the nonterminal declaration page. There is also a means of merging occurs on declarations with the attribute declarations themselves. Occurs declarations by themselves are rare in Silver code.

Additionally it is possible to declare more than one attribute, more than one nonterminal, or both in one occurs on declaration:

attribute env, pp, errors occurs on Expr, Stmt;

However, this syntax also falls prey to the same limitation described on the attributes page.