A macro for "deferring" a ClojureScript function call within a core.async `go` block. Could be used, for example, in conjunction with "subroutines" in a `go-loop` so that those function calls yield back a little time to the CPU before they are executed, thus allowing for a little breathing room between or within expensive subroutines.

defer.clj

;; use only inside a go block; body should be of the form (fn-name arg1 arg2 ...)
;; fn-name must be apply-able, e.g. it cannot be a macro; assumes `setImmediate`
;; is available in the browser's global scope

(defmacro defer [body]
  `(let [yield# (cljs.core.async/chan)]
     (js/setImmediate #(cljs.core.async/put! yield# (list ~@(rest body))))
     (apply ~(first body) (cljs.core.async/<! yield#))))


;; sample usage :: will log `6` to the console
;; -----------------------------------------------
;; (go (.log js/console (+ 1 (defer (+ 2 3)))))