sucks-rocks.clj

(defmacro when-let->
  "Takes an expression and a set of binding+test/form pairs. Threads expr (via ->)
  through each form for which the corresponding test
  expression is true, binding the result of the test."
  [expr & clauses]
  (assert (even? (count clauses)))
  (let [g (gensym)
        pstep (fn [[[res test] step]] `(if-let [~res ~test] (-> ~g ~step) ~g))]
    `(let [~g ~expr
           ~@(interleave (repeat g) (map pstep (partition 2 clauses)))]
       ~g)))

(comment
  ;; Lame example: real world use is in a prismatic/schema schema I am trying to make
  (let [[a b c] (repeatedly 3 (partial rand 10))
        check (fn [n] (when (< 5 n) (str "ERROR: value to big:" n)))]
    (when-let-> {}
                [msg (check a)] (assoc :a (str msg ", regarding a"))
                [msg (check b)] (assoc :b (str msg ", regarding b"))
                [msg (check c)] (assoc :c (str msg ", regarding c"))))

  RESULT: {:c "ERROR: value to big:6.131820288094671, regarding c"}
                
  )