what happens if a variable is declared lexical but defined dynamically

Fri, 23 Jul 2021 17:59:04 +0000

The following two code snippets behave very differently, depending only on the order you define the functions. I find this behavior surprising, and probably not intended.

(ns jimka-test
  (:require [clojure.pprint :refer [cl-format]]))

(declare f)

(defn g []
  (map f '(1 2 3)))

(def ^:dynamic f (fn [x] (* x x)))

(defn h []
  (assert (= (g) '(1 4 9)))
  (binding [f (fn [x] (+ x x))]
    (assert (= (g) '(2 4 6))
            (cl-format false "(g) returned ~A" (g)))))

(h)

Now swap the order of definitions of f and g.

(ns jimka-test
  (:require [clojure.pprint :refer [cl-format]]))

(declare f)

(def ^:dynamic f (fn [x] (* x x)))

(defn g []
  (map f '(1 2 3)))

(defn h []
  (assert (= (g) '(1 4 9)))
  (binding [f (fn [x] (+ x x))]
    (assert (= (g) '(2 4 6))
            (cl-format false "(g) returned ~A" (g)))))

(h)

My suggestion is that if a variable is declared non-dynamic, then a later definition as dynamic should issue a warning.

This issue was also discussed on clojureverse

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what happens if a variable is declared lexical but defined dynamically