defn

(defn name docstring? [params...] body...) (defn name docstring? ([params...] body...)+

Defines a function. The multi-arity spelling is one clause per arity plus a dispatch on the argument count; a single variadic clause (& rest) takes any count past its fixed parameters, and any other count signals (wrong number of arguments passed to: f). At most one variadic clause and one clause per arity. Parameters destructure, vector and map patterns alike. The name lowers to a direct call, so recursion is a call, not a value lookup; a head-position use of a value binding (a def, a parameter) is the funcall instead. A recur in the body jumps back to the enclosing clause with new argument values.

A ^:dynamic name holds its function in the var instead: the definition keeps its direct call shape (so recur still jumps straight to it), but calls go through the var's value, so binding rebinds them with dynamic extent (see binding). Any other defn without the marker stays refused by binding, like the oracle's non-dynamic error.

A definition may use a name defined below it: the file is pre-scanned for every top-level def/defn (and declare) name. Inside a body, defn works only in statement position, and a multi-arity one only at the top level.

Redefining a defn with def captures the function: (def p (memoize p)) evaluates its value against the OLD FUNCTION binding, so top-level calls hit the cache while the defun's own recursion stays direct.

Redefining a name with another defn gives each definition its own function: the call sites below each definition call the newest, while a (def g f) between two definitions keeps the older one ((g) answers the old value, (f) the new one). Each namespace versions its own names. A function defined above a redefinition still calls the older definition; only ^:dynamic names (whose calls go through the var) see the newest from every call site.