Question

In clojure, apply cannot be applied to a macro. For instance (apply and [true false]) raises an exception. I was thinking about following workaround:

(defmacro apply-macro[func args] `(~func ~@args))

At first glance, it seemed to work pretty well:

(apply-macro and [true 5]); 5
(apply-macro and [true 5 0]); 0
(let [a 0] (apply-macro and [true a])); 0

But, when I passed to it a variable that points to a vector, it collapsed.

(let [a [true]] (apply-macro and a));  java.lang.IllegalArgumentException:
   ;Don't know how to create ISeq from: clojure.lang.Symbol

What a disappointment!!!!

Any idea how to fix apply-macro?

Answer 1

The problem is that a is just a symbol at compile time. So there’s no way for a compile-time macro to see what it contains and do the necessary expansion. As a result, you need to expand the macro at run-time using eval.

One way to do this is just to wrap the macro in a function that calls eval, which can be done with this handy “functionize” macro:

(defmacro functionize [macro]
  `(fn [& args#] (eval (cons '~macro args#))))

(let [a [true]] (apply (functionize and) a))
=> true

If you like, you could also define apply-macro in terms of functionize:

(defmacro apply-macro [macro args]
   `(apply (functionize ~macro) ~args))

(let [a [true false]] (apply-macro and a))
=> false

Having said all this, I still think the best thing to do is to avoid macros entirely when they are not really needed: they add extra complexity and are best reserved for cases when you really need compile time code generation. In this case you don’t: Alex Taggart’s answer gives a good example of how to achieve a similar objective without any macros, which is probably more appropriate in most situations.