CODEWITHSUNDEEP
clojuremacrosclojurescript
estolua
estolua

Reputation: 676

Pass data to macro via variable in Clojure(Script)

Is there any trick to make this work without passing the data directly or using eval?

(defmacro pair-defs [data]
  (cons 'do
    (for [[k v] data]
      `(def ~k ~v))))

(def data '((a 1) (b 2) (c 3)))
(pair-defs data)

Upvotes: 1

Views: 421

Answers (1)

leetwinski
leetwinski

Reputation: 17859

If your data var is defined in namespace before the macro call,you can use some namespace manipulating functions to resolve it's value by name:

(defmacro pair-defs [data]
  `(do ~@(for [[k v] @(ns-resolve *ns* data)]
           `(def ~k ~v))))

user> (def data [['a 10] ['b 20]])
#'user/data

user> (pair-defs data)
#'user/b

or to handle both literal data and data by var name:

(defmacro pair-defs [data]
  (let [dt (if (symbol? data) @(ns-resolve *ns* data) data)]
    `(do ~@(for [[k v] dt]
             `(def ~k ~v)))))

the call is expanded to the desired form: (do (def a 10) (def b 20))

So resolving the namespace val without dirty tricks like eval is totally possible, but i find it to be quite an unneeded usage of macros. For example your task is easily replaceable by plain function call.

(defn pair-defs1 [data]
  (doseq [[k v] data]
    (intern *ns* k v)))

it would work on any data sequence, be it local or global, literal or generated

Upvotes: 3

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