Question

This question is a based on a limitation of this answer.

If I have a macro that uses splicing unquote like this:

(defmacro instantiate [klass values] 
        `(new ~klass ~@values))

It will only work if values is a literal sequence or seq-able.

If it is passed a var holding a sequence like:

(def v [1 2 3]) 
(macroexpand '(instantiate Person v))

Then the output would be an error indicating that v is not a sequence.

Even a function call would be interpreted as a list:

(defn vf [] [1 2 3])
(macroexpand '(instantiate Person (vf)))
user=>(new Person vf)

My question is: Is there any way to use the splicing unquote in Clojure macros in those two cases where the sequence to be spliced isn’t a literal?

Answer 1

Macros receive their arguments unevaluated, so the behavior you are seeing is as intended.

Macros are expanded at compile time, not run time. The values of any variables passed into a macro may not be available at compile time, so dirty hacks like using eval will not work in the general case. Don’t create macros that require such tricks.