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How do I check if a value is a promise?

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(def x (promise))

How can I check if type x is a promise?

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2 answers


I would say that you cannot at all. Since both parties are created as reify

several interfaces: IDeref

, IBlockingDeref

, IPending

, IFn

. But you can check whether the value of one of them implements: clojure.lang.IPending

.

(defn promise? [v]
  (instance? clojure.lang.IPending p))


Or maybe even everything:

(defn promise? [v]
  (every? #(instance? % v) 
         [clojure.lang.IPending 
          clojure.lang.IFn 
          clojure.lang.IBlockingDeref 
          clojure.lang.IDeref]))
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I would go with



(defn promise? [p]
  (isa? (class p) clojure.lang.IPending))
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