Question

I have a datatype defined with defrecord, which contains two vectors:

(defrecord MyType [a b])

(def mytype (->MyType [1 2 3] [4 5 6]))

I want to have a function update both vectors and return a new MyType. The only way I can think of to do that is via nested assoc calls:

(defn mutate-mytype [mytype x y]
  (assoc mytype :a (assoc (:a mytype) x y)
                :b (assoc (:b mytype) x y)))

Example output:

user=> (mutate-mytype mytype 1 7)
#user.MyType{:a [1 7 3], :b [4 7 6]}

Question: Is there a better way to write that mutate-mytype method?

Answer 1

Your implementation is perfectly fine.

There are a few alternatives, e.g. you might consider using assoc-in and the -> threading operator:

(defn mutate-mytype [mytype x y]
  (-> mytype 
      (assoc-in [:a x] y)
      (assoc-in [:b x] y)))

This doesn’t really have any advantages over your approach in this case, but it might make the code more readable if you had more deep nesting going on.