fboundp

(fboundp function-name)

Returns t when function-name names something callable or expandable: a function (built-in or defun), a macro (built-in or defmacro), a special form, or a car/cdr composition like cadr. This matches Common Lisp, where fboundp is true of macros and special operators too. A (setf name) list is a function name too, quoted or built at run time: t when a (setf name) function is defined.

On the compiled backends a literal quoted argument is decided at compile time with full knowledge (macros and special forms included); a computed argument is checked at runtime against the function registries, which only know real functions — so (fboundp (intern "cond")) is nil in compiled code but t in the interpreter, and defmacro macros are likewise compile-time-only there.

As a function value (#'fboundp, as in (mapcar #'fboundp names)) it is a computed probe on every backend, with the run-time answers above; the eval runtime is pulled in only for a program that names #'fboundp.

t, nil and keywords are symbols that name no function, so the answer is nil for them on every backend.

A name retired by fmakunbound answers nil again, at a literal call site too.

A function a defun below the top level defines is bound once that definition has run: a literal or computed argument answers nil before and t after on every backend.