hey, mount newbie here. Do you use partially applied functions as states in mount? E.g. I have (defn find-user [db id] …) function and turn it into state that depends on db state like (defstate user-finder :start (partial find-user db)).

This works fine, but doesn’t obviously “react” properly when db state is stopped

I mean it still keeps the “running” version of db state. Is there a way to combine those two: partial application to avoid dragging all the “deps” as well as having “live-link” to db state? Gist here: https://gist.github.com/mostr/372f65ece0ad6ea218d1

@mostr: welcome!

looks like you are missing a :stop

(defstate db :start users)

that's why it is not stopped when (mount/stop) is called

try (defstate db :start users :stop #{})

or whatever stop value makes more sense

in reality it will be a real db disconnect

hm, so I did the following:

(defn stop []
  (prn "Stopping users db")
  #{})

(defstate db
          :start users
          :stop (stop))

but still get

Loading src/clj_sandbox/ms/users_db.clj... done
(mount/stop)
=> {:stopped []}
(mount/start)
=>
{:started ["#'clj-sandbox.ms.users-db/db"
           "#'clj-sandbox.ms.user-finder/user-finder"
           "#'clj-sandbox.ms.user-finder/user-finder2"]}
(user-finder 2)
=> {:id 2, :name "bar"}
(mount/stop #'clj-sandbox.ms.users-db/db)
"Stopping users db"
=> {:stopped ["#'clj-sandbox.ms.users-db/db"]}
(user-finder 2)
=> {:id 2, :name "bar”}

not sure if this is real-life case (stop db while running), but trying to understand how things work in mount

Actually this works as expected as there is (partial…) in user-finder state definition which just probably takes “current” value/reference to db and doesn’t care about state changes. But I wonder whether there is a way to combine these two things: partial application + being aware of state changes

I'll check it out in a couple hours.. will let you know

sure, no worries

looking at it with less distractions... yea, this is expected, here is just plain Clojure example:

boot.user=> (def db {:a 42})
#'boot.user/db
boot.user=> (def find-key (partial db))
#'boot.user/find-key
boot.user=> (find-key :a)
42
boot.user=> (def a {})
#'boot.user/a
boot.user=> (find-key :a)
42

in case you are ok with closing state over in function, you can simply (:require [users-db :refer [db]]) in user-finder and use it within a function.

you are of course have freedom of where to use defstate, but I found it is best used for resources, like db in this case

and API, such as user-finder could just use it as functions, rather than be defstates themselves

there you can choose whether to close over that db and pass it there directly

for example, say you have your db API that have access to db state live in app.db namespace. then you can do:

(:require [app.db :as db])

(GET "/find-user" [user]
  (db/find-user user))

(:require [app.db :refer [db]])

(GET "/find-user" [user]
  (find-user db user))