Conditionals in Hiccup, can I make this more idiomatic?

Question

Clojure beginner here! I added flash message support to my Hiccup code in a Noir project.

What I'm trying to do is check if the message string for each specific was set or not. If there's no message, then I don't want to display the specific flash element containing that message.

(defpartial success-flash [msg]
            [:div.alert.notice.alert-success
             [:a.close {:data-dismiss "alert"} "x"]
             [:div#flash_notice msg]])

(defpartial error-flash [msg]
            [:div.alert.notice.alert-error
             [:a.close {:data-dismiss "alert"} "x"]
             [:div#flash_notice msg]])

[..]

(defpartial layout [& content]
            (html5
              [:head
                [...]
              [:body
               (list
                [...]
                [:div.container
                 (let [error-msg (session/flash-get :error-message)
                       error-div (if (nil? error-msg) () (error-flash error-msg))
                       success-msg (session/flash-get :success-message)
                       success-div (if (nil? success-msg) () (success-flash success-msg))]
                       warning-msg (session/flash-get :warning-message)
                       warning-div (if (nil? warning-msg) () (warning-flash warning-msg))]

                   (list error-div success-div warning-div content))])]))

Disclaimer: I completely agree that you won't likely ever be in a situation where you'll need more than one of those specific flashes on at once, but indulge me in my attempt at figuring out a better and more functional way of implementing this.

I'm confident that there's a pattern out there for handling similar situations. Basically I check the value of several expressions, do a bunch of stuff with those values, and then act based on the results. You could pull this off with a progressively more and more monstrous (cond), but my solution is at least somewhat cleaner.

Tips?

Answer 1

You could also use when-let.

(defpartial layout
  [& contents]
  (html5
    [:body
     (when-let [msg (session/flash-get :error-message)]
       (error-flash msg))
     (when-let [msg (session/flash-get :warning-message)]
       (warning-flash msg))
     (when-let [msg (session/flash-get :success-message)]
       (success-flash msg))
     contents))

I'm not a hiccup expert, but I think this should work. I find it a little clearer on what's going on, although it's slightly more verbose.

Answer 2

The pattern is called mapping value. Below is an example that uses keep function to apply the pattern of mapping values and then filtering them

(use 'clojure.contrib.core)


(def flash-message
[[:error-message error-flash]
 [:success-message success-flash]
 [:warning-message warning-flash]])

(keep (fn [m f] (-?>> m (session/flash-get) (f))) flash-message)