Question

Novice question, but I don’t really understand why there are so many operations for constructing maps in clojure.

You have conj, assoc and merge, but they seem to more or less do the same thing?

(assoc {:a 1 :b 2} :c 3)
(conj {:a 1 :b 2} {:c 3})
(merge {:a 1 :b 2} {:c 3})

What’s really the difference and why are all these methods required when they do more or less the same thing?

Answer 1

assoc and conj behave very differently for other data structures:

user=> (assoc [1 2 3 4] 1 5)
[1 5 3 4]
user=> (conj [1 2 3 4] 1 5)
[1 2 3 4 1 5]

If you are writing a function that can handle multiple kinds of collections, then your choice will make a big difference.

Treat merge as a maps-only function (its similar to conj for other collections).

My opinion:

• assoc – use when you are ‘changing’ existing key/value pairs
• conj – use when you are ‘adding’ new key/value pairs
• merge – use when you are combining two or more maps