CODEWITHSUNDEEP
clojure
Zaz
Zaz

Reputation: 48769

What is the difference between true? and boolean

In Clojure, what is the difference between the true? function and the boolean function?

I see from the source code that a difference does exist (meta-information removed):

(defn boolean [x] (clojure.lang.RT/booleanCast x))
(defn true? [x] (clojure.lang.Util/identical x true))

Upvotes: 2

Views: 116

Answers (2)

Thumbnail
Thumbnail

Reputation: 13473

Function boolean is a type cast to Clojure's boolean values true or false. It works according to the rules of truthiness as exercised by if and all its progeny: nil and false act false; everything else acts true.

You could define it as 

(defn boolean [x]
  (case x
    (nil false) false
    true))

Function true? determines whether the argument is the Clojure value true. You could define it as

(defn true? [x] (identical? true x))

Thus (boolean :whatever) is true, whereas (true? :whatever) is false.

There are some nasties lurking under the surface, due to Java allowing new Boolean objects to be created. More later.

Upvotes: 1

Zaz
Zaz

Reputation: 48769

As you can see from the source code, true? returns true if the value is identical to true. boolean returns true if the value is merely truthy (all values except false and nil).

=> (map true? [true 0 1 :a])
(true false false false)
=> (map boolean [true 0 1 :a])
(true true true true)

Upvotes: 3

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