[joshlemer]

You can pretty efficiently do this in Clojure

    (update-in
     {:foo [{:bar ["a"], :baz ["b" "c" "d"]}]}
     [:foo 0 :baz 1]
     clojure.string/capitalize)
    
    
    ;; => {:foo [{:bar ["a"], :baz ["b" "C" "d"]}]}

[tomp]

This only works if you know what to edit up-front.

If you're processing a tree-like structure via recursion (e.g. type-checking an AST implemented with persistent immutable structures in a compiler) it's no longer easy.