clojure / if|when let-multi

if-let-multi-and-when-let-multi.clj

(defmacro assert-all
  [& pairs]
  `(do (when-not ~(first pairs)
         (throw (IllegalArgumentException.
                  (str (first ~'&form) " requires " ~(second pairs) " in " ~'*ns* ":" (:line (meta ~'&form))))))
       ~(let [more (nnext pairs)]
          (when more
            (list* `assert-all more)))))

(defmacro when-let*
  "Multiple binding version of when-let"
  [bindings & body]
  (when (seq bindings)
    (assert-all
      (vector? bindings) "a vector for its binding"
      (even? (count bindings)) "exactly even forms in binding vector"))
  (if (seq bindings)
    `(when-let [~(first bindings) ~(second bindings)]
       (when-let* ~(vec (drop 2 bindings)) ~@body))
    `(do ~@body)))
  
  ;;Example when-let*
  
  (when-let* [a 1 
             b 2 
             c (+ a b)]
             (println "yeah!")
             c)
  ;;=>yeah!
  ;;=>3
  
  (when-let* [a 1 
             b nil 
             c 3]
             (println "damn! b is nil")
             a)
  ;;=>nil

(defmacro if-let*
  "Multiple binding version of if-let"
  ([bindings then]
   `(if-let* ~bindings ~then nil))
  ([bindings then else]
   (when (seq bindings)
     (assert-all
       (vector? bindings) "a vector for its binding"
       (even? (count bindings)) "exactly even forms in binding vector"))
   (if (seq bindings)
     `(if-let [~(first bindings) ~(second bindings)]
        (if-let* ~(vec (drop 2 bindings)) ~then ~else)
        ~(if-not (second bindings) else))
     then)))
   
;;Example if-let*

(if-let* [a 1
          b (+ a 1) ]
          b)
;;=> 2

(if-let* [a 1
          b (+ a 1)
          c false] ;;false or nil - does not matter
          b
          a)

;;=> 1