def

(def name) (def name expr)

Defines a top-level variable, lowering to a setq of the mangled name. Without a value the var is unbound until a definition binds it, like the oracle's (see declare for the unbound root), and a bound root is left alone. Inside a body, def still sets the GLOBAL when the body runs -- it is not a local binding, which is what a let is for. The name is a VARIABLE, so a head-position call to it is a funcall of the value cell.

The value evaluates against the OLD binding, so (def p (memoize p)) after a (defn p ...) captures the function cell. Top-level calls then hit the cache; the defun's own recursion stays a direct call -- use ^:dynamic for a fully memoized recursion (see defn).