6.3 Veneers🔗ℹ

When a value has a veneer’s static information—typically because the veneer name is used as an annotation—then static lookup of methods, indexing operations, etc., use the veneer instead of the underlying representation. Within the veneer body, however, this has the static information of annot.

A veneer affects only static resolution of operations. To help avoid confusing behavior when dynamic resolution is chosen in a dynamic-mode context, a veneer can be used as an expression or annotation only in a static-mode context (see use_static). Using a veneer annotation in a dynamic-mode context is a syntax error.

The veneer’s id_name is bound in several spaces:

• in the expr space, a constructor function or form, which by default is a function that takes one argument and returns it, but with the static information of id_name;

• in the annot space, an annotation that is satisfied by any value that satisfies the annot specified for this;

• in the namespace space, a namespace to access exported bindings as well as a function id_name.method, a function id_name.property, and a syntactic form id_name.dot for each non-private/protected method, property, and dot syntax in the veneer (including inherited methods, properties, and dot syntax), respectively; and

• in the class space, a representation of the veneer for reference as a superveneer.

When the annot declared for this is checked, the check occurs whenever the veneer is used as an annotation, by the default veneer constructor, when a method or property of the veneer is called. In the case of a method or property, the check applies when using . after an expression that has the veneer’s static information or when using a function like id_name.method. An annot can be a converter annotation only if the converter veneer clause is specified, in which case the veneer is also a converter annotation.

Analogous to class, the body of a veneer form consists of definitions, exports, and veneer clauses.

When a veneer_clause is an extends form, the new veneer is created as a subveneer of the extended veneer. If both the superveneer and subveneer have checked annots, then both checks apply for any use of the subveneer. If a superveneer’s annot is a converter annotation, then the converter veneer clause is implicit. In the case of a converter subveneer, the subveneer’s conversion applies before the superveneer’s conversion (or predicate). In the case of a predicate annotation subveneer, the superveneer’s predicate is tried first.

A veneer can implement only specific interfaces that serve as bridges to static dispatch: Indexable, MutableIndexable, Appendable, and Comparable. Note that is_a is a dynamic operation, not a static operation. So, for example, a value with a veneer that implements Indexable is not an instance in the sense of is_a (unless the underlying representation is an instance). Instead, having a veneer implement Indexable only makes static #%index references (usually written with […]) use the veneer’s get method. A veneer cannot implement Sequenceable, but it can implement static sequence conversion with sequence.