Clojurescript decoupled file & namespace?

Question

I'm using reagent to build several alternate root components, only one of which will be mounted on any given page; definitely either/or. These have a degree of commonality in their makeup, hence it will be convenient to move what is common among them to a common namespace.

What would be ideal is if in the file for each of these components I had the option to switch namespace into common, and add defs particular to the component, then switch back, thus avoiding circular dependencies nor needing any kind of inheritance.

I recalled this being possible in common lisp, how wonderful it was, and it also seems possible in clojure.

From Clojurescript docs: ns must be the first form and can only be used once, and in-ns is only usable from the repl.

I'm wondering if there's a way to achieve this kind of thing in clojurescript which is still eluding me.

If not I may need to reconsider my assumptions behind multiple alternate root components; the "many builds within one build" kind of idea, if that makes sense.

Update after some futher experimentation and confusion: another option might be to split a single namespace across multiple files (is this possible?). Not sure what direction to turn in here.

The fact that in reagent I am using atoms in the global namespace is what's creating the need for circular dependencies if I use a separate namespace for common. Hence, wonder about one global namespace, in which case multiple files might help. Or is the way forward one giant file and one namespace??

Update: I've realised there is a great tension between keeping all app state globally (in my current case, multiple atoms), and passing app state around. My pattern currently is everything global, don't pass any of it around. Passing the necessary state as parameters to fns in the common namespace would solve the problem here (duh!), but then there's the question of what principles are being followed here regarding app state. If I just added a param whenever I needed one, but started with the idea that everything was global, there'd be no real principle to it...

Answer 1

In ClojureScript, everything is pre-compiled into a single static JavaScript "executable", so there is nothing like the repl you are used to in Clojure. Indeed, in CLJS the "Var" concept doesn't really after the compiler, they are just static (constant) variables and cannot be rebound.

Having said that, CLJS does emulate the behavior of Clojure dynamic variables via the binding form, so that may help you to reach your goal. As in CLJ, it creates what amounts to a (thread-local) global variable. This is a degenerate case in CLJS since there is only one thread. However, the source code looks identical to the CLJ case.

Another way to accomplish this is to just use a plain atom as a global variable so you don't have to pass a parameter around.

As always, when using a global variable, it reduces the number of parameters in function call trees, but it creates invisible dependencies between different parts of the code. Somethimes convenient, but usually a bad tradeoff.