Come fa le chiusure della compilazione di clojecrit?

Question

Quando si utilizza Clojursept ho provato a definire una funzione che è una chiusura su una variabile come questa:

(let [x 42] 
  (defn foo [n] (+ x n)))

.

che stampa la sorgente seguente al Rhino Repl:

function foo(n){
  return cljs.core._PLUS_.call(null,x__43,n);
}

.

La funzione funziona come mi aspetto, ma quando si tenta di ottenere alla variabile denominata x__43 non riesco a ottenerlo.Dove è andato?

Answer 1

(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))))))

Answer 2

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).