Question

Imagine you have a map like this:

(def person {
  :name {
    :first-name "John"
    :middle-name "Michael"
    :last-name "Smith" }})

What is the idiomatic way to change values associated with both :first-name and :last-name in one expression?

(Clarification: Let’s say you want to set :first-name to “Bob” and :last-name to “Doe”. Let’s also say that this map has some other values in it that we want to preserve, so constructing it from scratch is not an option)

Answer 1

Here are a couple of ways.

user> (update-in person [:name] assoc :first-name "Bob" :last-name "Doe")
{:name {:middle-name "Michael", :last-name "Doe", :first-name "Bob"}}

user> (update-in person [:name] merge {:first-name "Bob" :last-name "Doe"})
{:name {:middle-name "Michael", :last-name "Doe", :first-name "Bob"}}

user> (update-in person [:name] into {:first-name "Bob" :last-name "Doe"})
{:name {:middle-name "Michael", :last-name "Doe", :first-name "Bob"}}

user> (-> person 
          (assoc-in [:name :first-name] "Bob")
          (assoc-in [:name :last-name]  "Doe"))
{:name {:middle-name "Michael", :last-name "Doe", :first-name "Bob"}}

Edit

update-in does recursive assocs on your map. In this case it’s roughly equivalent to:

user> (assoc person :name 
             (assoc (:name person) 
                    :first-name "Bob" 
                    :last-name "Doe"))

The repetition of keys becomes more and more tedious as you go deeper into a series of nested maps. update-in‘s recursion lets you avoid repeating keys (e.g. :name) over and over; intermediary results are stored on the stack between recursive calls. Take a look at the source for update-in to see how it’s done.

user> (def foo {:bar {:baz {:quux 123}}})
#'user/foo

user> (assoc foo :bar 
             (assoc (:bar foo) :baz 
                    (assoc (:baz (:bar foo)) :quux 
                           (inc (:quux (:baz (:bar foo)))))))
{:bar {:baz {:quux 124}}}

user> (update-in foo [:bar :baz :quux] inc)
{:bar {:baz {:quux 124}}}

assoc is dynamic (as are update-in, assoc-in, and most other Clojure functions that operate on Clojure data structures). If assoc onto a map, it returns a map. If you assoc onto a vector, it returns a vector. Look at the source for assoc and take a look in in RT.java in the Clojure source for details.