13.12.1 The value and proc slots

The value field has multiple purposes:

• When the symbol is the unique identifier of a data structure type: the value field holds a reference to the data struct–type descriptor.
• When the symbol is the unique identifier of a R6RS record type: the value field holds a reference to the record–type descriptor.
• Bindings defined at the top level of libraries, programs or standalone expressions to be evaluated by eval, have their current value stored in the value slot of symbols, or, more precisely, of gensyms; such symbols are called location gensyms or loc gensyms.
• Bindings defined at the top level of libraries, programs or standalone expressions to be evaluated by eval, have a label gensym associated to them: a symbol whose value slot holds a reference to the syntactic binding descriptor to be used by the expander.
• And more.

About the proc slot: when a top level binding is never assigned (in other words: it is immutable) and its value is a closure object, the associated loc gensym holds its value in both the value slot and the proc slot. Whenever binary code in a code object performs a call to a non–assigned top level closure object, it does the following:

1. From the relocation vector of the current code object: retrieve the loc gensym of the closure to call.
2. From the loc gensym: extract the value of the proc slot, which is meant to be a reference to the closure object. This is done by accessing the gensym object with a low–level assembly instruction, not necessarily by using the primitive operation $symbol-proc.
3. Actually call the closure object.

Values stored in the value slot of loc gensyms can be closure objects or not; they can be called by binary code, but they need to be validated as closure objects first. Values stored in the proc slot can only be references to closure objects and they can be called by binary code without being first validated.

Use in primitive function syntactic bindings

All the primitive functions implemented by code in Vicare’s boot image are accessible from libraries defined by the boot image; such primitive functions have a loc gensym associated to them, in which both the value and proc slots hold a reference to the actual closure object. The bindings of primitive functions are immutable.

Use in library top level syntactic bindings

All the top level bindings defined by a library form have a loc gensym associated to them:

• If a binding is never assigned in the source code of the library and its value is a closure object: both the value and proc slots of its loc gensym hold a reference to the actual closure object.
• If a binding is assigned in the source code of the library: its value is stored in the value slot of its loc gensym; the proc slot of the loc gensym is set to the unbound object.
• If a binding’s value is not a closure object: its value is stored in the value slot of its loc gensym; the proc slot of the loc gensym is set to the unbound object.