Understanding Clojure: cond and condp
This is the first part of a multi-part series I want to start titled “Understanding Clojure”. Often times I come across Clojure code that can be refactored a bit better or even there’s an existing function to do X. Hopefully I can shine some light on some aspects of Clojure you didn’t know existed.
cond
(cond
(> 0 (:amount account)) "Your balance is negative"
(< 0 (:amount account)) "Your balance is positive"
(= 0 (:amount account)) "Your balance is zero")
There is also the option of a default case using :else.
(cond
(> 0 (:amount account)) "Your balance is negative"
(< 0 (:amount account)) "Your balance is positive"
:else "Your balance is zero")
condp
condp can be used if your predicate remains the same for each condition and you wish to switch on the result.
(condp < (:amount account)
10000 "top tier dividend"
5000 "second tier dividend"
1000 "first tier dividend"
0 "no dividend")
To supply an else condition to condp, you leave out the :else keyword and supply the result expression.
(condp < (:amount account)
10000 "top tier dividend"
5000 "second tier dividend"
1000 "first tier dividend"
"no dividend")
You can also supply condp with your own predicate function.
(defn describe [amount]
(cond
(> 0 amount) :negative
(< 0 amount) :positive
:else :zero))
(defn compare [descriptor amount]
(= descriptor (describe amount)))
(condp compare (:amount account)
:negative "Your balance is negative"
:positive "Your balance is positive"
:zero "Your balance is zero")
The predicate function (compare) must take exactly two arguments and must return a binary result. The predicate function would be called like:
(compare :negative (:amount account))
(compare :positive (:amount account))
(compare :zero (:amount account))