Clojure: add element to a vector inside a map

Question

I have this map

(def m {:a "aaa" :b {:c ["ss" "gg"]}})

I want to update it to this (add "uu" to the vector :c):

{:a "aaa" :b {:c ["ss" "gg" "uu"]}}

This is what I came up with and I hate it:

(assoc-in m [:b :c] (conj (get-in m [:b :c]) "uu"))

How should I be doing it?

Answer 1

(update-in m [:b :c] conj "uu")

The way I think of it is that update-in gets you there and calls a function that receives not only the state that is there, but also the remaining parameters. So here conj will be called with ["ss" "gg"] and "uu", and the value of key :c in the data structure will become ["ss" "gg" "uu"].

assoc-in does not get any initial state, hence in your example you are having to labour to create what is there all over again.