15.3.3 Syntactic binding’s descriptors

A syntactic binding’s descriptor is a pair with the format:

(?binding-type . ?binding-value)

where ?binding-type is a symbol and the format of ?binding-value depends on the binding type.

Every syntactic binding has a unique label gensym associated to it; during the expansion process, no matter the current lexical context, given a label gensym we can retrieve the associated binding descriptor.

Under Vicare, symbol objects are data structures with a slot named value, accessor symbol-value and mutator set-symbol-value!. This slot is usually initialised to the built–in object returned by (unbound-object); we can inspect the state of the slot with symbol-bound?. The expander makes use of this slot in label gensyms:

• Syntactic bindings imported from libraries, including those established by the boot image, have the binding descriptor referenced by the value slot of their label gensym.
• Syntactic bindings established by the library being expanded, have the association between label gensyms and binding descriptors stored in the LEXENV component of the lexical environment. The value slot of these gensyms is set to the return value of (unbound-object).

Given a label gensym we can determine if it is associated to an imported syntactic binding by inspecting its value slot: if symbol-bound? applied to the label returns #t, the binding is imported; otherwise it was established by the code being expanded.