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02-substitutions.md

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Substitutions

What substitutions are for

In the tests of your application, or when developing it, you may want to mock the lifecycle expressions of a defstate. This way, the global state will result in another value when started.

Setting substitutions for (start)

To substitute a state, the call to the start function must be wrapped by the with-substitutes macro. A substitute is not defined with defstate, but with the state macro. For example:

(mount/with-substitutes [#'db (state :start (do (println "Starting fake DB") (atom {}))
                                     :stop  (println "Stopping fake DB"))]
  (start))
;>> Starting fake DB
;=> (#'your.app/db)

@db
;=> object[clojure.lang.Atom 0x2010a30b {:status :ready, :val {}}]

(mount/stop)
;>> Stopping fake DB
;=> (#'your.app/db #'your.app.config/config)

A substitution is only active for the current session. Thus, starting the state again (after is has been stopped), without a substitute configured for it, will start it with the original definition.

Note that substitution states don't need to be inline. And the with-substitutes wrappers can be nested. For example, the following is also possible:

(def sub (state :start {}))

(mount/with-substitutes [#'db sub]
  ...
  (mount/with-substitutes [#'config sub]
    ...
    (mount/start)
    ...))