Question

The macro, transform!, as defined below seems to work for => (transform! ["foo" 1 2 3]). The purpose is to take in a list, with the first element being a string that represents a function in the namespace. Then wrapping everything into swap!.

The problem is that transform! doesn’t work for => (transform! coll), where (def coll ["foo" 1 2 3]). I am getting this mystery exception:

#<UnsupportedOperationException java.lang.UnsupportedOperationException: nth not supported on this type: Symbol>

The function:

(defmacro transform! 
" Takes string input and update data with corresponding command function.
"  
  [[f & args]]  ;; note double brackets
 `(swap! *image* ~(ns-resolve *ns* (symbol f)) ~@args))

I find it strange that it works for one case and not the other.

Answer 1

Macros work at compile-time and operate on code, not on runtime data. In the case of (transform! coll), the macro is being passed a single, unevaluated argument: the symbol coll.

You don’t actually need a macro; a regular function will suffice:

(defn transform! [[f & args]]
  (apply swap! *image* (resolve (symbol f)) args)))

Resolving vars at runtime could be considered a code smell, so think about whether you really need to do it.