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clojure

What is the use of “true” when writing a function, which given a key and map, returns “true”

Write a function which, given a key and map, returns true if the map contains an entry with that key and its value is nil.

Solution I came across: 

#(nil? (get %2 % true))

Can someone please explain the use of true in 

(get %2 % true) ?

Thank you!

Solution

  • that's a default value that will be returned in the case of absence of the key

    ;; key exists
(get {:a 1} :a 2)
#=> 1
;; key doesn't exist (default value is returned)
(get {:a 1} :b 2)
#=> 2
;; key exists and it's value is nil
(get {:a nil} :a 2)
#=> nil
;; key doesn't exist, nil is returned
(get {:a 1} :b)
#=> nil

    some docs could be found here

    https://clojuredocs.org/clojure.core/get

    so the idea is that (get {:a 1} :b) will always return nil because key doesn't exist. In this case (nil? (get {:a 1} :b)) will return true, which is not what we want. That's why adding this default value is necessary. So nil will be returned only in the case when the actual value is nil.

    The value true is not special here. 85 would work just as well: any value other than nil fixes the problem.