Fast, flexible, designer-friendly templates.

Upstream URL


Fernando Borretti



Eco - Fast, flexible, designer-friendly templates.

Build Status


  • Simple: Eco is essentially just a string concatenator. It introduces no constructs of its own: Every tag is pure Common Lisp code, plain and simple.
  • Easy to Use: Proper use of Eco should not require more than a cursory read of this README from time to time.
  • Designer-friendly: Lispers have written template engine after template engine that turns S-expressions into HTML. Which is great, if you're a lisper. Eco is meant to be used for more than HTML and also to be usable to the many designers and programmers who don't know the language and should not be expected to learn an obscure template syntax to be able to contribute.
  • Performance: Eco uses the many performance advantages of Common Lisp. Templates are not interpreted or run in a VM, but compiled to Common Lisp, which is then compiled down to efficient machine code. By making each template a function that takes an (Optionally typed) argument list rather than passing an environment hash table like most other template engines, one can leverage the type inference features of modern Common Lisp implementations to create performant templates.


A basic template (.eco extension) looks like this:

<% deftemplate index (title &optional posts) () %>
  <!DOCTYPE html>
      <title><%= title %></title>
      <% if posts %>
        <h1>Recent Posts</h1>
        <ul id="post-list">
          <% loop for (title . snippet) in posts do %>
            <li><%= title %> - <%= snippet %></li>
          <% end %>
      <% else %>
        <span>No recent posts.</span>
      <% end %>
<% end %>

To load this template, put this in your system definition file:

(:eco-template "filename")

For interactive or quick tests on the REPL, templates can be loaded using ECO:COMPILE-STRING,

(eco:compile-string "<% deftemplate inline-test () () %>This is a test.<% end %>")

All templates are eventually compiled into Lisp functions. To get their outputs, call the templates like any Lisp function:

(eco-template:index "My Blog" nil)

Eco is designed to be output-agnostic, however, by default it will autoescape HTML for convenience. Specify :ESCAPE-HTML NIL when defining each template to disable this behaviour individually for each template.

<% deftemplate output-js-logger (module) (:escape-html nil) %>
   function logDebug (message) {
       console.log("<%= module %>: " + message);
<% end %>


There are three types of tags in Eco:

  • Expressions: <%= <expr> %> becomes <expr>.
  • Blocks: <% <code> <arg1> <arg2> %><body><% end %> becomes (<code> <arg1> <arg2> <body>).
  • Calls: <%- <fun> <arg1> <arg2> %><body><% end %> becomes (<fun> <arg1> <arg2> <body>).

Blocks are used to specify Lisp code "inline" in the template, and tend to contain imperative code, their return values are ignored. Expressions and calls are more functional as they return values to be interpolated into their templates. The function called by the CALL construct may be another templates, or any arbitrary Lisp function.

The if tag is a special case: it supports using an else tag to separate the true and false branches. For example:

<% if posts %>
  <h1>Recent Posts</h1>
  ... loop over posts ...
<% else %>
  No recent posts.
<% end %>


  • *template-package*: The package the templates will be defined it. Defaults to :eco-template.




<% deftemplate name (&rest args) (&rest options) %>
<% end %>

Defines a template. The only option that is currently defined is :ESCAPE-HTML (default T)




<% if cond %>
  true branch
<% else %>
  false branch
<% end %>


Eco uses esrap to parse templates, which it then compiles down to Common Lisp code.


Copyright (c) 2014-2019 Fernando Borretti (

Licensed under the MIT License.

Dependencies (5)

  • alexandria
  • cl-who
  • esrap
  • fiveam
  • split-sequence

Dependents (0)

    • GitHub
    • Quicklisp