Are you lazy?

are_you_lazy.clj

(ns are-you-lazy
  "Sometimes it's useful to check whether an object is lazy without 
  exploring the whole of it!"
  (:import [clojure.lang IPending LazilyPersistentVector LazySeq]))

(defn lazy?
  "Return `true` if this `coll` lazy, else `false`.
   
   **NOTE THAT** Probably all lazy sequences are instances of 
   `clojure.lang.LazySeq`, but `LazySeq` implements an interface `IPending`
   which as far as I can see nothing else implements, and I'm wondering 
   whether the existence of this interface implies that in some future
   release of Clojure, there may be lazy things which are not instances of
   `LazySeq`.
   
   **NOTE THAT** there's also a class `clojure.lang.LazilyPersistentVector`,
   which does *not* implement `IPending` and does not subclass `LazySeq`.
   I don't know what it's use case is; it is used internally by atoms and
   by regular expression matchers but I don't know whether it is expected to
   be present in clojure users (as opposed to implementors) code."
  [coll]
  (or
   (instance? LazySeq coll)
   (instance? IPending coll)))

;; Interestingly,
;;   (apply or (map #(instance? % coll)
;;                 (list LazySeq IPending LazilyPersistentVector)))
;; does not work because `or` is a macro.

(lazy? '(1 2 3))           ;; => false
(lazy? (range))            ;; => true
(lazy? (map inc '(1 2 3))) ;; => true

;; Interestingly, calling LazilyPersistentVector/create does not create a
;; LazilyPersistentVector:

(type (clojure.lang.LazilyPersistentVector/create (range 10))) ;; => clojure.lang.PersistentVector