functions used in defroutes; an arity probem/mystery

[Ryan Waters]

Complete code here https://github.com/http-kit/chat-websocket/blob/master/src/main.clj

- - -

If defroutes specifies a response function with no args, how can the definition of that function be arity 1 (the request)?

(defn chat-handler [req]

  (with-channel req channel

    (info channel "connected")

    ...))

(defroutes chartrootm

  (GET "/ws" []  chat-handler)

  (files "" {:root "static"})

  (not-found "<p>Page not found.</p>" ))

When I try that style in my own code I get:

clojure.lang.ArityException: Wrong number of args (0) passed to: views$ws-test

        at clojure.lang.AFn.throwArity(AFn.java:437)

        at clojure.lang.AFn.invoke(AFn.java:35)

Thank you,

Ryan

[James Reeves]

I'm not quite sure what you're asking. What do you mean by "response function with no args"?

- James

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[Ryan Waters]

In the link below the function 'chat-handler' is apparently called with no args when someone does a GET on /ws, according to this:

(defroutes chartrootm

  (GET "/ws" []  chat-handler)

  (files "" {:root "static"})

  (not-found "<p>Page not found.</p>" ))

However, the actual function definition requires a single argument, according to this:

(defn chat-handler [req]

  (with-channel req channel

    (info channel "connected")

    ...))

I don't understand how that works.

- Ryan

[James Reeves]

Oh! That's because it's not called with no arguments. If it were called with no arguments, the code would look like:

    (GET "/ws" [] (chat-handler))

Instead, the code looks like:

    (GET "/ws" [] chat-handler)

We're not calling the function; we're returning it, in the same way as we'd return a value.

The reason it works is because if a function is returned by a route, it's treated as a handler and called with the current request. In other words, the above code is equivalent to:

    (GET "/ws" req (chat-handler req))

- James

[Ryan Waters]

Ah ha!  Now I understand - thank you for your time!