Question

I’m in the process of learning Clojure macros, and I’m getting a NullPointerException when trying to use macroexpand-1 on this macro:

(def config {:ns 'bulbs.neo4jserver.client, 
             :root-uri "http://localhost:7474/db/data/"})
(def data {:name "James"})

(defmacro create
  [config data]
  `(~(ns-resolve (:ns config) 'create-vertex) config data))

(macroexpand-1 '(create config data))

Trying to compile this returns:

 Unknown location:
   error: java.lang.NullPointerException

 Compilation failed.

But evaluating the macro’s body…

`(~(ns-resolve (:ns config) 'create-vertex) config data)

…returns this…

(#'bulbs.neo4jserver.client/create-vertex bulbs.vertices/config bulbs.vertices/data)

…which is what I think I want.

UPDATE: If I manually replace (:ns config) with 'bulbs.neo4jserver.client then the error goes away — how do you make (:ns config) play nice?

Answer 1

You’re trying to mix macroexpand-time and runtime information. The local “config” does not contain the contents of the #’config var, but instead is the symbol ‘config.

If you look at the full stack trace, not just the error message, you’ll see that ns-resolve is being passed a nil:

user=> (pst)
NullPointerException 
        java.util.concurrent.ConcurrentHashMap.get (ConcurrentHashMap.java:796)
        clojure.lang.Namespace.find (Namespace.java:188)
        clojure.core/find-ns (core.clj:3657)
        clojure.core/the-ns (core.clj:3689)
        clojure.core/ns-resolve (core.clj:3879)
        clojure.core/ns-resolve (core.clj:3876)
        clj.core/create (NO_SOURCE_FILE:7)

Once you understand the following you will understand your original problem:

user=> (def bar [1 2 3])
user=> (defmacro foo [x] [(class x) (pr-str x)])
user=> (foo (get bar 2))
[clojure.lang.PersistentList "(get bar 2)"]