Yet another template system for Common Lisp.
TEN is a fork of ECO template system by Fernando Borretti.
Like ECO, TEN compiles templates to Lisp code, but has some differences:
- Two types of tags only. Control and output.
- Support for templates inheritance.
- Dot syntax for accessing template data.
- Convenient syntax for applying filters.
- Configurable syntax delimiters (planned, not done yet).
My reasons for writing yet another template system for Common Lisp is to combine the simplicity and usability of ECO (the whole Lisp language at your disposal for writing the templates), with features available in more complex template systems like Djula that makes things easier for web development (inheritance, dot syntax, etc.).
A TEN template looks like this:
{% template ex1 () (user enabled &key items) %}
<html>
<head>
</head>
<body>
{{ user.name | string-capitalize }}
{% if enabled %}
Enabled
{% else %}
Disabled
{% end %}
{% when items %}
<ul>
{% loop for item in items do %}
<li>{{ item }}</li>
{% end %}
</ul>
{% end %}
{% when (not items) %}
There are no items
{% end %}
</body>
</html>
{% end %}
These are the types of tags:
- Output tags:
{{ <var> }}, becomes<var>, and{{ <fn> &rest args }}, that becomes(fn arg1 arg2 .. argn). - Control tags:
{% <expr> %} body {% end %}, becomes(<expr> body). - Comments tags: Use
{#and#}to comment out a piece of template.
Control tags control which parts of the tamplate are rendered; their return value is ignored.
The value returned by output tags are interpolated into the template. The function called can be any Lisp function, or another template (because templates are compiled to functions).
For example:
{{ user }}=>user{{ name user }}=>(name user){% when (name user) %} ... {% end %}=>(when (name user) ...)
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 %}Also, more advanced control expressions are possible, like let, case, cond, etc.
Templates are defined with the following syntax:
{% template name (&rest options) (&rest args) %}
... body ...
{% end %}
Template options are:
:extends: The template to extend from.:dot-syntax: If T, templates are compiled with dot syntax enabled. Dot syntax is implemented via the Lisp libraryaccess. Default is T.:package: The package in which to compile and export the template. By default, templates are compiled and exported inTEN-TEMPLATESpackage.:export: When T, export the generated template function. Otherwise, the template is not exported. Default is T.:escape-html: Whether to escape html in output tags. Default is T.:output-whitespace. Default is T. When NIL, expressions that just spit whitespace are discarded.
For manually compiling templates, use ten:compile-template function.
But more useful is to include them in the ASDF system definition of your project.
First, add :ten as ASDF system definition dependency:
:defsystem-depends-on (:ten)
Then, use :ten-template in to include the template files:
(:ten-template "filename")The default file extension is "ten", but another can be specified via the :file-extension option; and the template package can be specified with the :package option. Look at ten.examples ASDF system for an example.
You can also compile all the templates in some directory using this ASDF recipe:
:perform (asdf:compile-op :after (o c)
(dolist (template (uiop:directory-files (asdf:system-relative-pathname :my-app "templates/*.ten")))
(uiop:symbol-call :ten 'compile-template template)))Templates are compiled into functions and exported in the indicated package. The default package is ten-templates, but that can be changed from either the ASDF system definition, the ten:compile-template parameters, or the {% template %} options.
If CREATE-STREAM-WRITING-FUNCTION option is enabled, then two functions are compiled, the default one, that compiles to a function that takes template arguments and renders the template to a string. And a function that renders the template to the *TEMPLATE-OUTPUT* stream. So, for a template named my-template, a my-template function that renders to a string is created, and a my-template* function that renders to *TEMPLATE-OUTPUT* stream is created.
When developing your project it is useful to be able to compile templates in an interactive way.
If you are using Emacs + SLIME, load ten.el file.
Then use M-X ten-compile-template when on the template buffer to compile templates. Note that you may want to have :package option specified in the template so that it gets compiled into the correct package.
For debugging, you can inspect the expanded template using ten:expand-template function. In Emacs, go to template buffer an do M-x ten-expand-template.
If you enable ten minor mode, template compilation gets conveniently bound to C-c C-c, and template expansion to C-c RET. Best is to automatically enable the minor mode for template files adding something like (add-hook 'web-mode-hook 'ten-mode) to your .emacs initialization file.
To make a template inherit from anohter, use the :extends option in template definition.
Templates are organized in sections. sections are the parts of the templates that are inherited.
Use {{super}} inside a section to render the parent section.
TEN leverages CLOS for implementing template inheritance. Templates are compiled to classes and generic functions render-template and render-section.
Have a look at some examples of template inheritance.
To include other templates, just use the output tag with the name of the included template. Remember that templates are compiled to functions; just call those functions from the template to include them.
Have a look at an example.
When dot syntax is enabled (it is, by default), it is possible to conveniently access objects with dot syntax in templates:
{{ object.key1.key2 }}
that gets translated by access library to:
(access:accesses obj 'key1 'key2)
Have a look at an example.
TEN implements some convenient syntax for filters.
{{ value | func1 arg1 .. argN| func2 arg1 .. argN| .. | funcN arg1 .. argN}}
Filters are just normal functions that get applied to the value.
Filters are translated to functions application like this:
(funcN (.. (func2 (func1 value arg1 .. argN) arg1 .. argN))) arg1 .. argN)
In general, filter functions are expected to receive the value to be filtered as first parameter.
But, for several Lisp functions that's not the case. In those cases, it is possible to use _ to indicate where the filter function should receive the value.
For example, string-trim receives the string to trim as second value, so, to apply it as filter we do:
{{str | string-trim '(#\%) _}}
Filters syntax is completly optional, you can disregard it and just apply functions instead:
{{ string-trim '(#\%) (string-capitalize str) }}
Have a look at some examples of filters.
Load and have a look at the examples.
(require :ten.examples)Example templates get compiled and exported to ten/examples package.
- When specifying a template package other than
ten-templates, if the package specified doesn't:usetenorten-templatepackages, then you may run into problems trying to compile your templates. That may be because thetemplateandsectionmacros are not found in the specified package. In that case, make sure to prefix yourtemplateandsectiondeclarations withten:, like:
{% ten:template my-template (:package my-package) %}
{% ten:section my-section %}
{% end %}
{% end %}- You can use different implementation and export packages. Qualify the template name with the package name, like:
{% ten:template my-templates:my-template (:package my-package) %}
{{ foo "bar" }}
{% ten:section my-section %}
{% end %}
{% end %}That will define MY-TEMPLATE on the MY-TEMPLATES package, but use MY-PACKAGE in its source (the {{ foo "bar" }} expression expands to to (MY-PACKAGE::FOO "bar")). Note that you need to previously define a package MY-TEMPLATE that exports MY-TEMPLATE. Or otherwise use MY-TEMPLATE::MY-TEMPLATE as template name.
- Some "complex" expressions, like
condandcase, require that you turn:output-whitespacetoNIL. Otherwise, template compilation putswrite-stringexpressions right in the middle of thecaseandcondbodies. Have a look at this template.
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