clojure - :refer vs :as

Question

I'm learning clojure. As part of the :require process, there is an option to use :refer for a specific method or all the methods.

Or to use the :as and then to choose the method you need.

I think I understand the difference between the 2 options, and also seen the documentation here that says:

:as takes a symbol as its argument and makes that symbol an alias to the lib's namespace in the current namespace.

:refer takes a list of symbols to refer from the namespace or the :all keyword to bring in all public vars.

But I'm still not sure about:

1. When should I use one option and not the other?

2. Is there a performance difference between the two options? (My thoughts say that there is non since the compilar will optimize both options

(I also did 2 small programs that uses core.asyc, one uses :as and one uses :refer. The time it took to run both was pretty much the same.

Answer 1

I pretty much always use :as like this:

(ns demo.core
  (:require
    [clojure.string :as str] ))

(println (str/join ["hello" "there"]))

This allows the reader to see that join belongs to str (a namespace alias), and they can easily see that join resolves to clojure.string/join.

Consider the alternate:

(ns demo.core
  (:require
    [clojure.string :refer [join] ))

<snip>
...397 lines of other code...
</snip>

(println (join ["hello" "there"]))

Here join looks like a local function defined in demo.core, and it can take a while for the reader to figure out where it comes from. They can still trace down the origin by looking in the ns declaration, but unless it is a very common function, most people agree that the namespace alias technique is easier/faster to grok when reading code you didn't write.

---

In execution, the compiler will convert both forms to the same machine code, so there is no difference there.