Question

I need to define mutually-dependent vars. By this I mean that one var contains e.g. a vector with another var and vice versa. This is illustrated by the following code:

(declare a b)
(def a [1 b])
(def b [a 2])

But after loading this code I get this:

test=> (first a)
1
test=> (second a)
#<Unbound Unbound: #'test/b>
test=> (first b)
[1 #<Unbound Unbound: #'test/b>]
test=> (second b)
2

clearly, thats not how it should work.
I understand that printing such structure will give stack overflow, but I don’t need to print it. How should I do it?

Answer 1

You can do the following:

(declare a b)
(def a [1 #'b])
(def b [#'a 2])

@(a 1)
=> [#'user/a 2]

Note that #' is a reader macro for referring to a var.

I’m still not quite sure why you want to do this though….. trying to make vars mutually dependent like this seems like a pretty bad code smell to me. It’s likely that whatever you are trying to do would actually be best solved by a different approach.

EDIT

With the extra comment stating that the problem is related to having different types of entities referring to each other, I think a better approach is a map with keywords, e.g.

(def my-model
  {:a 
      {:name "Entity A" 
       :references [:b]}
   :b 
      {:name "Entity B"
       :references [:a]}}