with-slots

(with-slots (slot-or-pair...) instance body...)

Binds the slot names of a CLOS-subset instance (defclass / define-condition) as symbol-macro-style places for the body: each entry is a slot name, or a (var slot) pair binding var to slot slot. The instance form is evaluated once. Reads see the slots, and setf/push/incf of a bound name writes back to the slot (the substitution is textual over the body; an inner binding shadowing a slot variable is still substituted).

Lite: code GENERATED at run time inside the body (e.g. a macrolet template mentioning a slot name) resolves the name through a fallback binding holding the slot's value at entry -- reads work, but a write from such generated code updates that local copy only.

with-slots only BINDS -- it never reads a slot on entry -- so a body that merely assigns a slot declared without an :initform works, and the fallback binding above holds nil for such a slot. A read the body really performs still signals unbound-slot.