How does ClojureScript compile closures?
-
14-11-2019 - |
Question
When using ClojureScript I tried to define a function that is a closure over a variable like this:
(let [x 42]
(defn foo [n] (+ x n)))
That prints the following source at the Rhino REPL:
function foo(n){
return cljs.core._PLUS_.call(null,x__43,n);
}
The function works as I expect but when trying to get at the variable named x__43
I can't get it. Where did it go?
Solution
(let [x 42]
(defn foo [n] (+ x n)))
is currently compiled to
var x__1311 = 42;
cljs.user.foo = (function foo(n){
return (x__1311 + n);
});
The exact number attached to the x
may of course vary from compilation to compilation and cljs.user
would be replaced by the appropriate namespace name.
There is no attempt to conceal the generated variable from unrelated code in a JavaScript closure, so in principle it could still be modified if one went out of one's way to do so. Accidental collisions are extremely unlikely and simply will not happen with regular ClojureScript.
To discover things like the above, you can either call the compiler with {:optimizations :simple :pretty-print true}
among the options or ask it to emit some JavaScript at the REPL (as provided by script/repl
in the ClojureScript source tree or lein repl
in a Leiningen project with ClojureScript declared as a dependency):
(require '[cljs.compiler :as comp])
(binding [comp/*cljs-ns* 'cljs.user]
(comp/emit
(comp/analyze {:ns {:name 'cljs.user} :context :statement :locals {}}
'(let [x 42] (defn foo [n] (+ x n))))))
OTHER TIPS
the x variable is defined outside the foo function, in the let binding. you can't "get it" because you're not in the scope of the let binding. that's more or less the whole point of using closures.
conceptually, let bindings are implemented as function calls:
(let [x 2] ...)
is equivalent to
((fn [x] ...) 2)
which is probably similar to let
is implemented in ClojureScript - either as a macro transformation to fn
or directly to (function(x){...})(2)
.