implement program execution.

src/execute.mjs

@@ -0,0 +1,24 @@
+const id = x => x;
+
+export function execute (program, arg = id) {
+	if (!program.env.has("main")) {
+		return arg;
+	}
+
+	const main = program.env.get("main");
+	return call_fn(main, [arg])();
+}
+
+function call_fn (fn, env = []) {
+	if (typeof fn === "number") {
+		while (fn > 0) {
+			fn--;
+			env = env[1];
+		}
+		return env[0];
+	} else if (fn[0] === 0) {
+		return arg => call_fn(fn[1], [arg, env]);
+	} else {
+		return call_fn(fn[0], env)(call_fn(fn[1], env));
+	}
+}