Clojure operator & # 8594; (arrow) and additional operations

Question

I worked in several situations when I want to make a chain of operations on an object with additional functions. "->" works fine for sequences of commands on the same object (for example, (c (b (ax))) becomes (-> xabc)), unless some operations are optional. For example, suppose I wanted:

(c (if (> (ax) 2) (b (ax)) (ax) ) ) 

Is there a way to do this more clearly using an operation like "->"?

Answer 1

You can do this with cond-> recently introduced in Clojure 1.5:

 (cond-> x true a (> (ax) 2) b true c) 

Or better

 (-> x a (cond-> (> (ax) 2) b) c) 

Meaning: "take x , pass it through a , take the result and pass it through b if (> (ax) 2) or leave it unchanged, finally take what you have and pass it through c ."

In other words, cond-> is similar to -> , except that instead of separate forms for streaming your expression through it, pairs of test + forms are taken, where the form is skipped if the test is false and used for streaming if the test is plausible

 (cond-> x test-1 form-1 test-2 form-2) ;; if test-1 is truthy and test-2 is falsey, output is as if from (-> x test-1) ;; if it the other way around, output is as if from (-> x test-2) ;; if both tests are truthy: (-> x test-1 test-2) ;; if both tests are falsey: x