adventures in making stuff with Daniel Higginbotham

Look, Ma! No batteries! A Clojure Web App Without Noir

10 November 2012

UPDATE now uses a completely different codebase. You can find it on github. You can read about the new codebase at Building a Forum with Clojure, Datomic, Angular, and Ansible.

Awhile back I released OMG! SMACKDOWN!!!, which still holds the title for dumbest Clojure app in existence. To make up for that ludicrostrosity, I've created Grateful Place. This time I around I didn't use Noir, instead going closer to the metal with Compojure and Enlive. The experience was a lot easier than I anticipated, perhaps easier than using Noir. This post will cover some of the more interesting bits of code:

  • Templating heaven with Enlive and Middleman
  • Using macros to enforce full stack consistency
  • Roll-your-own validations
  • Pagination is more fun with macros
  • Using fierce mustaches to run tasks
  • Fun utilities

You can find all source code on github. Also, I'm going to be at the Clojure Conj and would love do meet folks. If you'd like to chat, please do tweet me (@nonrecursive) or find me at the conj!

App Overview

Grateful Place is a social app with only a few models: Users, Comments, Posts, and Likes. Nothing too fancy. If you're interested in why I made it, here's a post I wrote about gratitude.

Templating heaven with Enlive and Middleman

Enlive is the bee's knees

I can say without reservation that [Enlive]:( is the best templating library I've ever used. I absolutely love it! If you try out one thing from this post, try out the workflow I describe here. Enlive is completely different from Hiccup, ERB, HAML and the like and it's worth giving it a shot.

If you're not familiar with Enlive, it's a selector-based templating library. In the same way that CSS allows you to separate structure and presentation, Enlive lets you separate structure and view logic. This is huge! Unless you're the kind of person who still likes to use @font@ and @bold@ tags in your HTML, you should give Enlive a try.

Middleman/Enlive Workflow

Using Enlive allowed me to do all my design work using the Ruby library Middleman. This allowed me to build stand-alone web pages so that I could see how they looked without worrying about any view logic. For example, here's the example home page I put together:

title: Grateful Place
      %p Here is some content.
      .date Jan 03, 2012
        %a{:href => ""} Joe Schmoe
        %a{:href => "#"} ★
        .status Like this
      %a.comments{:href => ""} 35 Comments

      %p Here is some content, too.
      .date Jan 03, 2012
        %a{:href => ""} Joe Schmoe
        %a{:href => "#"} ★
        .status Like this
      %a.comments{:href => ""} 35 Comments

    %a.previous{:href => "#"}
      1{:href => "#"}
      2{:href => "#"}
      3{:href => "#"}

You might have noticed a few weird things. First, this is written using HAML. Yep, HAML. I prefer HAML over HTML because it's more concise and the consistent structure allows me to visually scan it more easily. And since my HTML templates and view logic are completely separate, I can write my designs in HAML, preview them with Middleman, and then generate an HTML file for Enlive to work with. Here's my process in full:

  • Build on every change by cd'ing to assets/generator/ and running fswatch . ./g. This runs a bash script which generates the HTML, CSS, and Javascript files and moves them to the proper directories.
  • Start up Middleman by cd'ing to assets/generator/ and running middleman
  • View site at http://localhost:4567 and make changes to my heart's content
  • When I'm happy with the changes, create or recompile a Clojure file which makes use of the HTML file with Enlive. It's necessary to recompile the files so that Enlive uses the updated HTML. That doesn't happen automatically.

Another thing you might have noticed is that in the HAML file I'm showing two example posts. With Enlive, you actually have to add some logic to remove one of the post divs:

(defpage all "index.html"
  [posts current-auth record-count page max-pages]
  ;; don't show the second post as it's just an example
  [[ (h/nth-of-type 2)]] nil

Some code to reduce boilerplate

Finally, here's some code I put together to reduce boilerplate:

(defonce template-dir "gratefulplace/templates/")

(h/defsnippet nav (str template-dir "index.html") [:nav]
  [:.auth] (if logged-in
             (h/do-> (h/content "Log Out")
                     (h/set-attr :href "/logout"))
             (h/do-> (h/content "Log In")
                     (h/set-attr :href "/login"))))

(h/deftemplate layout (str template-dir "index.html")
  [:html] (h/substitute html)
  [:nav] (h/substitute (nav (current-authentication)))

  [:nav :ul.secondary :#logged-in :a]
  (if-let [username (:username (current-authentication))]
     (h/content username)
     (h/set-attr :href (str "/users/" username))))

  [:nav :ul.secondary :#logged-in :span]
  (when (current-authentication)
    (h/content "Logged in as")))

;; Need to come up with better name
;; Bundles together some defsnippet commonalities for user with the
;; layout template
;; TODO destructuring doesn't work in argnames
(defmacro defpage
  [name file [& argnames] & body]
     (h/defsnippet ~(symbol (str name "*")) (str template-dir ~file) [:html]
     (defn ~name
       [{:keys [~@argnames]}]
       (layout (~(symbol (str name "*")) ~@argnames)))))

The ultimate purpose of this code is the defpage macro. This helps me in a few ways. It lets me easily write view code for each page without having to worry about the surrounding layout. For example, before writing the above code I had to include the navigation transformations in every page, which quickly got annoying.

The defpage macro also bundles up the template-dir variable, saving me some keystrokes and some duplication. Finally, it establishes a consistent way of interacting with the controller, a topic I'll explore more in the next section. Before that, here's an example of defpage in action:

(defpage show "posts/show.html"
  [post comments current-auth]
  [ :a]   (linked-username post)
  []        (h/content (created-on post))
  [ :.content]     (md-content post)

  [ :.edit]        (keep-when (can-modify-record? post))
  [ :.edit :a]     (set-path post post-edit-path)

  [ :.moderate]    (keep-when (moderator? (:username current-auth)))
  [ :.moderate :a] (h/do->
                          (set-path post post-path)
                          (h/content (if (:hidden post) "unhide" "hide")))

  [:#post_id]            (h/set-attr :value (:id post))
  [:.favorite]           (favorite current-auth post)

Thanks to defpage, my view code contains only logic specific to that view. I love it!

Using macros to enforce full-stack consistency

Creating a route->controller interface

When I was putting the app together, each of my compojure routes was a unique and beautiful snowflake:

  (GET  "/users/new" []

  (POST "/users" {params :params}
        (users/create! params))

  (GET  "/users/:username" [username]
        (users/show username))

As you can see, each route captures variables in a completely different way. At first I thought this made sense; I was passing exactly the data that was needed to each controller.

However, this quickly became a pain in the ass. For example, if I wanted to make a request param available to a view, I had to modify functions all up and down the stack: in the routes, in the controller function, and in the view function. I ended up making all routes send the full request object to their functions:

(defmacro route
  [method path & handlers]
  `(~method ~path req#
            (->> req#

(defroutes routes
  (compojure.route/files "/" {:root "public"})

  ;; posts
  (route GET "/" posts/all)
  (route GET "/posts" posts/all)
  (route GET "/posts/new" posts/show-new)
  (route POST "/posts" posts/create! (friend/authorize #{:user}))
  (route GET "/posts/:id" posts/show)
  (route GET "/posts/:id/edit" posts/edit)
  (route POST "/posts/:id" posts/update)

The @route@ macro is very handy. It saves me from typing and (for the most part) from burning precious brain cells on thinking, and it enforces the "send the full request" constraint by not allowing me to deviate. Of course, I can still deviate, and this is useful if you're using a library:

   (ANY "/logout" []
        (ring.util.response/redirect "/")))

If I want to get to the params, I just use a let; no big deal:

(defn show
  (let [id (get-in req [:params :id])

Creating a controller->view interface

In the same way that it was useful to create a route->controller interface, it became useful to create a controller->view interface. That was accomplished with this macro:

(defmacro view
  "provides defaults for the map provided to view functions and allows
  you to provide additional key value pairs. Assumes that a variable
  named req exists"

  [view-fn & keys]
  `(let [x# {:current-auth (friend/current-authentication)
             :errors {}
             :params (:params ~'req)
             :req ~'req}]
     (~view-fn (into x# (map vec (partition 2 ~(vec keys)))))))

This also helped with one weird problem I ran into with cemerick's friend library where (friend/current-authentication) returns nil from within an anonymous function (and there are many anonymous functions in my view code). I could circumvent this problem by assigning doing something like (let [current-auth (friend/current-authentication)]). But it seemed to make more sense to just always pass that data along when calling view functions, as you can see in the macro above.

The defpage macro also helped in establishing this interface in that it allowed me to destructure the arguments sent by the controller. For example, if a controller had

(view posts/show {:post post})

I could create the corresponding view with

(defpage show [post current-authentication])

And I wouldn't even have to worry about the order of the arguments. It really reduced some "incidental complexity" (and I'm almost certainly using that term incorrectly).

Roll-your-own validations

It wasn't too difficult to write my own way of doing validations, and I'm satisfied with it for now. Here's a description of the data structure, with an example following:

  • Validation: a combination of a key and "validation check groups"
  • Validation check group: first element is an error message and the rest are "validation checks". If any validation check returns false then the error message is added to a list of error messages for the given key.
  • Validation check: a boolean function to apply to one of a record's values. The value corresponds to the validation key

Here's an example:

 {:username "joebob", :password "hey"}
   ["Your password must be at least 4 characters long"
    #(>= (count %) 4)]})

This validation would fail because the value "hey" for the :password is not at least four characters long. Here's the main validation code:

(defmacro if-valid
  [to-validate validations errors-name & then-else]
  `(let [to-validate# ~to-validate
         validations# ~validations
         ~errors-name (validate to-validate# validations#)]
     (if (empty? ~errors-name)
       ~(first then-else)
       ~(second then-else))))

(defn error-messages-for
  "return a vector of error messages or nil if no errors
validation-check-groups is a seq of alternating messages and
validation checks"
  [value validation-check-groups]
    #(when (not ((last %) value)) (first %))
    (partition 2 validation-check-groups))))

(defn validate
  "returns a map of errors"
  [to-validate validations]
  (let [validations (vec validations)]
    (loop [errors {} v validations]
      (if-let [validation (first v)]
        (let [[fieldname validation-check-groups] validation
              value (get to-validate fieldname)
              error-messages (error-messages-for value validation-check-groups)]
          (if (empty? error-messages)
            (recur errors (rest v))
            (recur (assoc errors fieldname error-messages) (rest v))))

if-valid wasn't strictly necessary but I made it and ended up liking it. It's a small improvement, but a handy one.

Pagination is more fun with macros

It was a bit difficult for me to pull the pagination code together. I tried many approaches, and I'm not 100% satisfied with what I have, but it's better than nothing! The basic approach was to add a couple Korma clauses to a base set of clauses. Here are the base clauses, along with a macro that allows me to add more clauses willy-nilly:

(def base
   (select* e/post)
   (with e/user
         (fields :username))
   (with e/comment
         (aggregate (count :*) :count)
         (where {:hidden false}))
   (with e/favorite
         (aggregate (count :*) :count))
   (order :created_on :DESC)))

(defmacro all
  [& clauses]
  `(-> base

Here's the pagination macro:

(defmacro paginate
  ([page num-per-page query]
       (limit ~num-per-page)
       (offset (dec ~page)))))

And here's the whole thing in action:

(defn all
  (let [current-auth (friend/current-authentication)
        per-page 20
        page (str->int (or (get-in req [:params :page] 1)))
        conditions (with-visibility
                     {:moderator true
                      :logged-in (or {:hidden false}
                                     {:user_id [= (:id current-auth)]})
                      :not-logged-in {:hidden false}})
        record-count (post/record-count (where conditions))]
     :posts (paginate page per-page (post/all (where conditions)))
     :record-count record-count
     :page page
     :max-pages (ceil (/ record-count per-page)))))

I definitely think there's room for improvement here and would love suggestions.

Using fierce mustaches to run tasks

So it turns out that you can use leiningen to run tasks, kind of like Rake. As far as I know, it's not as powerful in that you can't specify dependencies, but it's still better than nothing! Here's a file I wrote to allow me to rebuild and re-seed by db:

(ns tasks.db
  (:refer-clojure :exclude [alter drop complement
                            bigint boolean char double float time])
  (:require [gratefulplace.models.user :as user])
  (:use (lobos core connectivity)))

(defn rebuild
  (rollback :all)

(defn seed
  (println (user/create! {:username     "higginbotham"
                          :email        ""
                          :display_name "higginbotham"
                          :password     "password"})))
(defn -main
  (condp = task-name
    "rebuild" (rebuild)
    "seed" (seed)))

I ran lein run -m tasks.db rebuild and lein run -m tasks.db seed.

Fun utilities

Here's some stuff that was fun to write:

(ns gratefulplace.utils)

(defn str->int
     (Integer.  str))

  ([m & keys]
      (fn [m k]
        (if-let [val (k m)]
          (assoc m k (str->int val))
      m keys)))

(defn deserialize
  [m & ks]
  (reduce #(assoc % %2 (read-string (%2 %))) m ks))

(defmacro self-unless-fn
  [self fn otherwise]
  `(let [self# ~self]
     (if (~fn self#)

The End

And that's it! I hope you've found this useful. I'd love any feedback, so please do leave a comment or tweet me or email me. And like I mentioned above - it'd be great to meet some folks at the Conj next week!