One-off instances created with reify cause NPE

Question

I'm trying to write a Clojure layer around a Java API that looks like the following:

public class Executor {

  public interface ExecutorJob<Result> {
    public Result execute () throws Exception;
  }

  public static <R> R executeAsUser(RunAsWork<R> executorJob, String uid) {
    try {
      //...
      R result = executorJob.execute();
      return result;
    }
    finally {
      //...
    }
  }

}

My goal is to create a Clojure API that allows to execute a fn as the body of the execute method of of an ExecutorJob. This is what I came up with:

(defmacro execute-as
  "Runs the form f while impersonating the given user"
  [user f]
  `(let [work# (reify Executor$ExecutorJob
                     (~'execute [~'this]
                               (~f)))]
     (Executor/executeAsUser work# ~user)))

Unfortunately, given this call:

user> (macroexpand '(run-as "admin" (.println System/out "test")))
(let* [work__2928__auto__ (clojure.core/reify package.to.Executor$ExecutorJob (execute [this] ((.println System/out "test"))))] (package.to.Executor/executeAsUser work__2928__auto__ "admin"))

it causes an NPE:

user> (execute-as "admin" (.println System/out "test"))

No message.
  [Thrown class java.lang.NullPointerException]

Restarts:
 0: [QUIT] Quit to the SLIME top level

Backtrace:
  0: user$eval2936$reify__2937.doWork(NO_SOURCE_FILE:1)
  1: package.to.Executor.executeAsUser(Executor.java:508)
  2: user$eval2936.invoke(NO_SOURCE_FILE:1)
  3: clojure.lang.Compiler.eval(Compiler.java:5424)
  4: clojure.lang.Compiler.eval(Compiler.java:5391)
  5: clojure.core$eval.invoke(core.clj:2382)
 --more--

I tried to put some meaningful Java calls in the execute-as second parameters, which I can see being executed just fine with a debugger.

What's wrong with that macro?

Answer 1

Nevermind, I got it: I was misusing the macro parameter and trying to actually call the result of the execution of form f. And it was yielding nil, hence the NPE.

Corrected version:

(defmacro execute-as
  "Runs the form f while impersonating the given user"
  [user f]
  `(let [work# (reify Executor$ExecutorJob
                     (~'execute [~'this]
                               ~f))]
     (Executor/executeAsUser work# ~user)))