avoid circular dependency when I access reitit route info from handler - clojure

Question

Assuming I have some kind of router set up that maps some routes to handlers something like this...
(ns myapp.user.api
(:require [reitit.core :as r]))
; define handlers here...
(def router
(r/router
[["/user" {:get {:name ::user-get-all
:handler get-all-users}}]
["/user/:id"
{:post {:name ::user-post
:handler user-post}}
{:get {:name ::user-get
:handler user-get}}]]))
And those handlers then call services that want access to the routing information...
(ns myapp.user-service
(:require [myapp.user.api :as api]))
; how can I get access to the route properties inside here..?
(defn get-all-users [])
(println (r/route-names api/router)))
When I try to import the router from the api file, into the service, I get a problem with circular dependencies, because the api requires handler, which requires service, so service can not then require api.
What's the best way to avoid this circular dependency? Can I look up values and properties of the router from within services?

Answer 1

I use six general approaches to avoid circular dependencies in clojure. They all have different tradeoffs and some situations one will fit better than another. I list them in order from what I prefer most to what I prefer least.
I show one example for each below. There may be more ways I haven't thought of, but hopefully this gives you some ways of thinking about the issue.
Refactor the code to remove the commonly referenced vars into a new namespace and require that namespace from both original namespaces. Often this is the best and simplest way. But can't be done here because the root handler var is a literal containing a var from the other namespace.
Pass in the dependent value into the function at runtime so as to avoid having to require the namespace literally.
(ns circular.a)
(defn make-handler [routes]
(fn []
(println routes)))
(ns circular.b
(:require [circular.a :as a]))
(def routes
{:handler (a/make-handler routes)})
;; 'run' route to test
((:handler routes))
Use multimethods to provide the dispatch mechanism, and then defmethod your binding from the other namespace.
(ns circular.a
(:require [circular.b :as b]))
(defmethod b/handler :my-handler [_]
(println b/routes))
(ns circular.b)
(defmulti handler identity)
(def routes
{:handler #(handler :my-handler)})
(ns circular.core
(:require [circular.b :as b]
;; now we bring in our handlers so as to define our method implementations
[circular.a :as a]))
;; 'run' route to test
((:handler b/routes))
Use a var literal that is resolved at runtime
(ns circular.a)
(defn handler []
(println (var-get #'circular.b/routes)))
(ns circular.b
(:require [circular.a :as a]))
(def routes
{:handler a/handler})
;; 'run' route to test
((:handler routes))
Move the code into the same namespace.
(ns circular.a)
(declare routes)
(defn handler []
(println routes))
(def routes
{:handler handler})
;; 'run' route to test
((:handler routes))
Use state. Store one of the values in an atom at runtime.
(ns circular.a
(:require [circular.c :as c]))
(defn handler []
(println #c/routes))
(ns circular.b
(:require [circular.a :as a]
[circular.c :as c]))
(def routes
{:handler a/handler})
(reset! c/routes routes)
((:handler routes))
(ns circular.c)
(defonce routes (atom nil))

Answer 2

You are making a simple mistake somewhere. My example:
(ns demo.core
(:use tupelo.core)
(:require
[reitit.core :as r]
[schema.core :as s]
))
(defn get-all-users [& args] (println :get-all-users))
(defn user-post [& args] (println :user-post))
(defn user-get [& args] (println :user-get))
; define handlers here...
(def router
(r/router
[
["/dummy" :dummy]
["/user" {:get {:name ::user-get-all
:handler get-all-users}}]
["/user/:id"
{:post {:name ::user-post
:handler user-post}}
{:get {:name ::user-get
:handler user-get}}]
]))
and use here:
(ns tst.demo.core
(:use demo.core tupelo.core tupelo.test)
(:require
[clojure.string :as str]
[reitit.core :as r]
))
(dotest
(spyx-pretty (r/router-name router))
(spyx-pretty (r/route-names router))
(spyx-pretty (r/routes router))
)
with result:
*************** Running tests ***************
:reloading (demo.core tst.demo.core)
Testing _bootstrap
-----------------------------------
Clojure 1.10.3 Java 15.0.2
-----------------------------------
Testing tst.demo.core
(r/router-name router) =>
:lookup-router
(r/route-names router) =>
[:dummy]
(r/routes router) =>
[["/dummy" {:name :dummy}]
["/user"
{:get
{:name :demo.core/user-get-all,
:handler
#object[demo.core$get_all_users 0x235a3fc "demo.core$get_all_users#235a3fc"]}}]]
Ran 2 tests containing 0 assertions.
0 failures, 0 errors.
based on my favorite template project