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(ns user
(:use clojure.walk))
(defn unzip [n s]
(if (seq s)
(lazy-seq
(map cons (first s) (unzip n (rest s))))
(replicate n ())))
(defn- deref-vars [vars form]
(let [replacement-map (into {} (map (fn [x] [x `(deref ~x)]) vars))]
(postwalk-replace replacement-map (macroexpand-all form))))
(defmacro letrec [bindings & body]
(let [[vars forms] (unzip 2 (partition 2 bindings))
gensyms (repeatedly gensym)]
`(let [~@(mapcat (fn [var] `(~var (promise))) vars)
~@(mapcat (fn [var form] `(~var ~(deref-vars vars form))) gensyms forms)]
~@(map (fn [var val] `(deliver ~var ~val)) vars gensyms)
(let [~@(mapcat (fn [var] `(~var (deref ~var))) vars)]
~@body))))
;; Examples
(letrec [xs (lazy-seq (cons 1 ys))
ys (lazy-seq (cons 2 xs))]
(let [zs (take 10 xs)]
(println "test1:" (identical? xs (rest xs))) ;; false
(println "test2:" (identical? xs (rest (rest xs)))) ;; true
(println "test3:" (identical? (rest xs) (rest (rest (rest xs))))) ;; true
(println zs)))
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