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Templating System for Clojure


  1. Template is function of its arguments.
  2. HTML is better for HTML than some host language DSL (just cause HTML is DSL).
  3. DOM manipulation tools and XSLT are good for transforming, not for templating (yes, opinionated).
  4. Clojure is good :)
  5. HTML isn't the only language that needs templating.

Why Fleet?

— I wanted template engine for Clojure
— close to Clojure
— and I hate writing HTML not in HTML (clj-html, haml, etc).

So I've started by reviewing few Clojure and CL implementations of ERB/JSP-like templates. Their syntax looks like
<p><%= (post :body) %></p>
<p><?clj (post :body) ?></p>
Also, in order to prevent XSS attacks, one needs to append some escape-xml function call to all this code:
<p><%= (escape-html (post :body)) %></p> or (Rails-like) <p><%=(h(post :body))%></p>
10 chars <%=(h())%> just to safely insert expression value seems to be a little too much...

Then I realized that the following syntax is usable:
<p><(post :body)></p>
Not a big deal, but... that's all. Really, <%= (escape-html ... is here, and all other constructions are here too.

Rails-like partials like
<%= render :partial => 'post', :collection => posts %>
are here too:
<(map post-tpl posts)>
...just plain Clojure code.

Need to bypass escaping?
<(raw "<script>alert('Hello!')</script>")>

Not writing HTML at all? Use fleet with default (bypassing) escape function.
Use some escape-mylang to work with other languages.


Single anonymous template: fleet

(fleet [& args] template-str options)

Creates anonymous function from template-str using provided options map.

Main option is :escaping. It can be function of one String argument or keyword specifying one of predefined functions:
:bypass — default, no escaping;
:xml — XML (or HTML) rules;
:str — Java-compatible string escaping;
:clj-str — Clojure string escaping (\n is allowed);
:regex — Escaping of Regex special symbols.

Options :file-name and :file-path of type String are used for error reporting (e.g. file-name will be in stacktrace along with line number).

Template namespace: fleet-ns

(fleet-ns root-ns root-path filters)

Treats root-path as root of template namespace with prefix root-ns. and creates template functions for each file in it with name and samespace according to relative path.

Template function creation conventions:
— Several functions will be created for each file. E.g. file posts.html.fleet will produce 3 functions: posts, posts-html and posts-html-fleet.
— Template function will take one or two arguments: first named same as shortest function name for file (posts in previous example) and second named data.
— When it's called with one arguments both symbols (fn-name and data) are bound to same value of this argument.
— When it's called with no arguments both symbols (fn-name and data) are bound to nil.

Filters argument is vector of file-filter escaping-fn pairs used to filter which files to process and with which escaping function. File filters could be defined as function, string, regex, :fleet or :all.
— Function should have Boolean type and one File argument.
— String filter definition treated as *.string.fleet mask, e.g. "js" mask will match update.js.fleet.
— Regex filter matches whole filename, e.g. #".*.html" will match posts.html.
:fleet filter is treated as "others". If it is set all *.fleet files will be processed.
:all means, literally, all.

Template Language

Main Fleet construction is Spaceship <()>.

...just cause (star)fleet consists of many spaceships.

<()> is almost equivalent to (), so <h1><(body)></h1> in Fleet is nearly the same as (str "<h1>" (body) "</h1>") in Clojure.

The only difference is that (body) output gets escaped (e.g. html-encoded to prevent XSS).
Use raw function to prevent encoding: <(raw "<br/>")>.
Use str function to place value <(str posts-count)>.

This seems to be complete system, but writing something like

<(raw (for [p posts]
  (str "<li class=\"post\">" (p :title) "</li>")))>

is too ugly..
And defining <li class="post"><(p :title)></li> as separate template can be overkill in many cases. So there should be the way of embedding strings and anonymous templates.

Slipway construction "><" intended for embedding strings.

The previous example could be rewritten using Slipway as

<(for [p posts] ">
  <li class="post"><(p :title)></li>

This example has two points worth mentioning. Result of "><" construction processing is an expression of String type. Strings in Slipway concidered raw by default.

Next case is something like this:

<(raw (map (fn [post]
  (str "<li class=\"post\">" (post :title) "</li>")) posts))>

With Slipway it can be replaced with

<(map (fn [post] ">
  <li class="post"><(post :title)></li>
<") posts)>

Need to mention that all this supports lexical scoping and other Clojure fectures just like reference (previous) expression.

More on escaping

If you need to insert Fleet constructions into text you can escape them using backslash.
You only need escaping to remove ambiguity, so use \<( and \<" only outside embedded clojure code, \"> and \)> only inside embedded clojure code.


This is not intended to work out-of-box, only to show some bits of a language / system.


Template file (post_dedicated.fleet):

  <title><(post :title)></title>

  <(stylesheet :main)>
  <(raw "<script>alert('Hello!')</script>")>

<p><(str notice)></p>

<p>Spaceship \<()> is landing.</p>

; Begin of post
<(inside-frame (let [p post] ">
  Author: <(p :author)><br/>
  Date: <(p :date)><br/>

<p><(post :body)></p>
  <(for [tag (post :tags] ">
    <li><(str tag)></li>
; End of post



(deftemplate post-page [post] "post_dedicated")


(post-page p)



(def footer (fleet "<p>&copy; <(.get (Calendar/getInstance) Calendar/YEAR)> Flamefork</p>"))


Directory tree


will be treated and processed by (fleet-ns templates "path/to/root_dir" [:fleet :xml]) as functions


and (for example) first function will be like

(defn file-a
  ([file-a data] ...)
  ([file-a] (recur file-a file-a)))
  ([] (recur nil nil)))


Copyright (c) 2010 Ilia Ablamonov, released under the MIT license.

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