while->

while.clj

(defmacro while->
  "As long as the predicate holds true, threads
  the expr through each form (via `->`). Once pred returns
  false, the current value of all forms is returned."
  [pred expr & forms]
  (let [g (gensym)
        pstep (fn [step] `(if (~pred ~g) (-> ~g ~step) ~g))]
    `(let [~g ~expr
           ~@(interleave (repeat g) (map pstep forms))]
       ~g)))

(defn valid? [v]
  (not (contains? v :errors)))

(while-> valid?
         {:hello "world"}
         (assoc :errors "my errors")
         (assoc :and "goodbye"))

;; => {:hello "world", :errors "my errors"}

(while-> valid?
         {:hello "world"}
         (assoc :and "goodbye"))

;; => {:hello "world" :and "goodbye"}

Interesting. This seems like a generalization of some->, but that fact doesn't seem useful.

You'll want to gensym and let-bind pred because it could be a computed value.

Ah, good point. I'll make that change...