How to Clojure.Spec a reference type (like atom)?

Question

I wonder how I would spec a function, that has a parameter, that hold a map in an atom.

(defn do-something [a]
  (prn (vals @a)))

This those obviously not work:

(s/fdef do-something
  :args (s/cat :a map?))

How would I spec that a is a reference to map?

Answer 1

Don't. clojure.spec is about specifying the structure of data and atoms are state, not data. Not every function necessarily needs to have (or check) a spec.

My general advice for dealing with stateful data is to:

• Define your data
• Define pure functions to that take and return your data
• Create specs for those data functions
• Manipulate atoms using only those pure functions in as few places as possible

With some care, you can often reduce the number of functions that take or return atoms to 0 (by closing over the atom in the place where it's managed), which is imho a worthy goal.

Answer 2

You don't. For one reason, it wouldn't be thread-safe. If you did somehow spec that a atom contained a map, it could change to an integer in the time it takes you to check the atom, and continue with your function.

One option, however, is to provide a validator to the atom. You could easily use partial to do this: (set-validator! my-atom (partial s/valid? :my-spec)). Now the atom will fail to update unless the value conforms to :my-spec.

Another option is to add validation logic to all the functions that update the atom. Which of these two approaches works best depends on the application.