A simpler way to program the web.

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Clojure, everywhere.

Write everything in Clojure and ClojureScript, clientside and serverside. Even the page markup is ClojureScript that is evaluated to produce the DOM. Pages can still be written using the familiar HTML syntax if desired.

Simplicity on the client.

Use a spreadsheet-like dataflow programming environment to manage client state. Develop basic content-oriented pages and complex thick-client applications without descending into “callback hell”.

Simplicity on the server.

Decouple business logic from HTTP-related concerns while satisfying complex, fine-grained authorization requirements with transparent, performant transfer of Clojure data between server and client.

Overview Hoplon in a nutshell.

Hoplon provides a number of libraries promoting a simpler model for web applications. The project can be divided into three main components: the HTML evaluator, the clientside state machine, and serverside RPC middleware. These components can be used together as a full stack, or individually as a part of another stack.

More concretely, Hoplon ships with a compiler for compiling Hoplon pages into HTML and JavaScript (implemented as a build task for the boot build tool), ClojureScript libraries for creating spreadsheet-like dataflow programs and wiring them to the DOM, and ClojureScript and Clojure libraries for RPC interaction between client and server.

The source code for Hoplon is hosted on GitHub and Maven artifacts are deployed to Clojars. Contributions of code, feedback, feature requests, etc. are welcome. Please read the Community page for more information.

Web Apps Single-page applications, not documents.

Web applications are not documents. Users need to be able to interact with them. They contain state. Through them the user communicates with backend systems to have some effect on the real world.

Hoplon web apps are organized like applications. The Hoplon compiler produces artifacts, not documents. These artifacts are equivalent to JAR files in Maven–their coordinates being the URL on which they're deployed. The compilation unit is the “page”, which is compiled to produce an HTML file–a single-page app.

Hoplon does not perform any serverside HTML rendering; it is completely unnecessary, even for SEO purposes. (Reload this page with JavaScript disabled and see how the content was “prerendered” at compile time.)

HTML Evaluator Page markup is a program.

Web application user interfaces are specified as trees of nested elements in HTML markup, but this document-like appearance is misleading. Without JavaScript this markup cannot provide the dynamic behavior that web applications require.

It is at the boundary between the DOM and the JavaScript environment that the incidental complexities of web application UI development are found. There is a fundamental disconnect between the literal representation of the DOM (the HTML markup) and the dynamic environment (the JavaScript VM). The page markup is evaluated by the browser and the resulting DOM is then accessible from JavaScript, but it's a one-way linkage–there is no way to refer to objects created dynamically in the JavaScript environment from the markup. The browser's HTML evaluation model lacks a means of abstraction, preventing the development of new elements by composition.

Hoplon provides an HTML evaluator that closes the loop, unifying the page markup and the JavaScript environment. Page markup, expressed in either the standard HTML5 syntax or Clojure sexps, is evaluated as ClojureScript and the result becomes the DOM for the page. HTML tags are simply ClojureScript vars. Of course, Hoplon ships with all of the standard HTML5 primitives but the evaluation model now facilitates the use of custom compound elements defined in the environment, too.

Additionally, the unification of HTML and ClojureScript syntax, semantics, and namespace in the HTML evaluator facilitates the use of macros in markup. This makes it possible to write powerful looping and control structures for use in the page's HTML markup itself, bringing the power and expressiveness of Lisp to HTML.

Components First-class custom DOM elements.

While there is no shortage of templating systems to create blobs of DOM elements, there is no mechanism to create custom elements that are composable in any meaningful way. Consider the situation where a template engine binds a DOM template to some data and returns the filled-in DOM subtree. What happens when you append a child to, or set an attribute on that DOM element? The child gets appended to, or attribute set on the outermost element of the filled template, generally not what is required for the composition semantic.

Hoplon provides facilities for creating real, first-class components. That is to say, Hoplon components are first-class citizens in the DOM world–they are, essentially, user-defined custom DOM elements. They have a literal representation in the page markup and they implement the semantics of HTML: setting attributes and appending children. When a child is appended to a Hoplon component or an attribute is set, either in HTML markup or programmatically, the component's implementation is used instead of the built-in DOM one. In this way components can be built by composing simpler components and those may be used as a basis for even more complex components, without running into the limitations of template composition.

Spreadsheets Client state made simple.

Remember how awesome spreadsheets are? You never think about state when you’re programming a spreadsheet. Instead, you focus on values and the formulas that define the relationships between them. When a new value is entered into a cell the rest of the spreadsheet updates itself as necessary to maintain the correctness of the formulas.

After a spreadsheet is set up you can link its cells to charts and forms to create a user interface that manages its own state, automatically. User input flows from forms to input cells. Formulas then recompute their values as necessary. These values then flow to charts and graphs for the user to view.

Hoplon applies this crazy, futuristic technology to frontend web development. Forgoing the clunky grid metaphor and names like A1 and B27, Hoplon provides a cell reference type. These cells, like cells in a spreadsheet, are either updated directly (input cells), or automatically in accordance with a formula.

Charts and Forms Stateless, automatic UI.

The purpose of a program's user interface is to transmit input from the user to the underlying state model and to present output derived from the underlying state model to the user for viewing.

In a web application the user interface is derived from the page markup. User input consists entirely of DOM events: click, change, etc. Output is presented to the user by manipulating the properties of DOM elements: adding or removing CSS classes, changing the content of text nodes, etc.

Hoplon provides bindings that can be used to link DOM elements to the underlying state model in the same way that charts and forms are linked to cells in a spreadsheet. Once the structure of the DOM and the linkages between the DOM and cells are defined, the user interface manages itself.


Typical Page

Typical Page

The page can be coded in either ClojureScript or HTML syntax. The compiler can parse either one. It's easy to add a preprocessing step to parse HAML, etc. if desired. HTML is usually more verbose and noisy than ClojureScript, so most examples in this document will be using the latter syntax.

Using ClojureScript syntax:

(page "foo/bar.html"
  (:require [my.lib :as lib]))

(defc clicks 0)

    (title "Hello World")
    (link :rel "stylesheet" :href "css/main.css"))
    (lib/heading-large "Hello, world!")
    (p (text "You've clicked ~{clicks} times."))
    (button :on-click #(swap! clicks inc) "Click me!")))

Using HTML syntax:

<script type="text/hoplon">
(page "foo/bar.html"
  (:require [my.lib :as lib]))

(defc clicks 0)

<title>Hello World</title>
<link rel="stylesheet" href="css/main.css">
<lib.heading-large>Hello, world!</lib.heading-large>
<p><text>You've clicked ~{clicks} times.</text></p>
<button on-click="{{ #(swap! clicks inc) }}">
Click me!

BMI Calculator

BMI Calculator

This is an example of a spreadsheet-like application. The application's state is contained in cells which are wired up to the DOM. DOM events cause changes to the underlying cells and formula cells in turn cause changes to the DOM.

The demo:

The cells:

(def height (cell 180)) ; height in cm
(def weight (cell  80)) ; weight in kg

(def bmi ; formula: [bmi, description, color]
    (let [htm (/ height 100) ; height in meters
          bmi (/ weight (* htm htm))]
        (< bmi 18.5) [bmi "underweight"  "orange"]
        (< bmi 25.0) [bmi "normal"      "inherit"]
        (< bmi 30.0) [bmi "overweight"   "orange"]
        :else        [bmi "obese"           "red"]))))

The user interface:

;; Some details related to <input type="range">
;; have been omitted for clarity.
  (label (text "Height (~{height} cm)"))
    :id "bmi-h"
    :value @height
    :on-input #(reset! height (val-id "bmi-h")))

  (label (text "Weight (~{weight} kg)"))
    :id "bmi-w"
    :value @weight
    :on-input #(reset! weight (val-id "bmi-w")))

  (label (text "BMI ~(first bmi) (~(second bmi))"))
    :do-value (cell= (first bmi))
    :do-css (cell= {:background-color (nth bmi 2)})))