:static-fns

Controls how multi-arity function calls are generated. A "static" call means the dispatch to the appropriate arity is handled at compile time (better performance), whereas a "dynamic" call means the dispatch is handled at run time (allows safer redefinition in REPL). Static is enabled by default in advanced :optimizations.

:static-fns true

See Static-Free ClojureScript REPL for details.

With the following multi-arity function foo:

(defn foo
  ([] "zero")
  ([x] "one")
  ([x & y] "multi"))

When static is disabled,:

          ;; compiles to...
(foo)     ;;  foo.call(null)
(foo 1)   ;;  foo.call(null,(1))
(foo 1 2) ;;  foo.call(null,(1),(2))

When static is enabled:

          ;; compiles to...
(foo)     ;;  foo..._arity$0()
(foo 1)   ;;  foo..._arity$1((1))
(foo 1 2) ;;  foo..._arity$variadic((1),cljs.core.array_seq([(2)], 0))

If foo was redefined to have an arity removed, any previous static calls to the removed arity would be broken, whereas previous dynamic calls would remain working as expected.