clojure template variadic macro

Question

I tried the following code (learning about macros):

(require '[clojure.template :as temp])

(defn fna [x] (println "a" x) x)
(defn fnb [x] (println "b" x) x)

(temp/apply-template '[& fns]
                     '(fn [payload] (-> payload fns))
                     '[fna fnb])
;; returns (fn [payload] (-> payload fnb))

I am surprised by the output, as I was expecting:

(fn [payload] (-> payload fna fnb))

Why is it producing this output, and what would be a way to achieve the above? I specifically says in the doc:

argv is an argument list, as in defn.

Note: it could be written like so, but I am trying to understand how to use template:

(defmacro wrap-fn-> [& fns]
  `(fn [~'x#]
     (-> ~'x
         ~@fns)))

I would say, if you are learning to use template because you hope it will be useful, don't bother. It is not used in real life outside of clojure.test, and it is a fairly limited tool; knowing how to write the macro yourself will serve you far better. — amalloy
– amalloy, Commented Mar 3, 2017 at 18:28

Alan Thompson · Accepted Answer · 2017-03-03 15:10:39Z

1

The examples here

do not show the use of "rest args" like [& fns]. I think that is where you are going wrong. Try this:

(println
  (temp/apply-template
    '[x y]
    '(fn [payload] (-> payload x y))
    '[fna fnb]))

=> (fn [payload] (-> payload fna fnb))

BTW, I've never seen clojure.template in use before - it looks like a very specialized tool.

answered Mar 3, 2017 at 15:10

Alan Thompson

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2 Comments

nha Over a year ago

Yes it would work like that, but for a fixed number of fns, I am trying to make it take a variadic number of arguments. Note, this is for learning purposes (never saw it before looking in the clojure.test source). And in practice I would use the defmacro to take a variable number of arguments, and pass it to apply-template anyway. This is pure curiosity (so far). Moreover I don't see why it would take fnb rather than fna.

amalloy Over a year ago

It "takes" fnb because [& fns] is an argument list containing two symbols, & and fns. Your template definition ignores the first argument, named & and passed fna, and uses only the second argument, named fns and passed fnb.

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