setf

Setf is a macro in Common Lisp that generalizes assignment to a destination, or place. It is not a function; its purpose is to store a value in a place that can be a simple variable or a more complex location such as an array element, a slot in a structure, or a user-defined location expanded by a setf mechanic. The setf macro takes a place designator and a value expression, evaluating the value and storing it in the specified place.

A place, in this context, is any location capable of holding a value. Simple examples include a variable (setq is used in conjunction with setf for variables) and an element of an array or list, such as (setf (aref arr i) 42) or (setf (nth 2 list) 'x). More complex places include structure slots (setf (slot-name obj) value) and results of functions that are defined to act as setf places, via setf expanders. The setf form thus enables a uniform syntax for a wide range of assignment targets.

In Common Lisp, (setf place new-value) evaluates new-value and stores it in place, returning the stored value. The mechanism can be customized for user types using defsetf and define-modify-macro, allowing new kinds of places to participate in setf semantics without changing their call sites.

Related concepts include setq for variable assignment and the general notion of places in the language. Setf is a core tool for writing generic, flexible assignment code in Lisp programs.

See also: defsetf, define-modify-macro, setq.