如何在Clojure中映射PersistentHashMaps的PersistentVector？

Question

I have a PersistentVector of PersistentHashMaps:

[
  {:url http://www.url.com, id: some_id ...},
  {:url http://www.url.com, id: some_id ...},
  {:url http://www.url.com, id: some_id ...},
]

What I need here to have a collection of all id s, so I need to map over a vector and from each map select id value.

I do:

(map #(get % :id) (all-users))

Is there a better way to do this?

Answer 1

Keywords can be used as functions.

(map :id (all-users))

If you are on Clojure 1.7 and don't need laziness, eg want to realize they entire collection as a vector you can use the map transducer:

(into [] (map :id) (all-users))

Answer 2

Unless you know that you need the lazy behavior of map, I would always suggest using mapv :

(def all-users [ {:id 111 ...} 
                 {:id 222 ...}
                 ... ] )
(mapv :id all-users)

The difference is that mapv is always evaluated immediately, and it stuffs the results in to a vector for easy access. Lazy sequences are great for things that are either very large or very slow, but they introduces an added layer of timing uncertainty that is unneeded by 99% of use cases.