What is the idiomatic way to assoc several keys/values in a nested map in Clojure?

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.