easily embedding html into Lisp

From: Andreas Thiele (nospam6329_at_nospam.com)
Date: 09/15/04 

Date: Wed, 15 Sep 2004 12:32:48 +0200

I'd like to introduce a simple approach of embedding html into Lisp.

Let's consider the following call to format

(format nil "~{ ~a ~}" '(1 2 3))

which produces " 1 2 3 " as output. This formatting capability can be used
to achive
a very simple embedding of html into Lisp.

Let's assume we already have three functions 'table 'tr and 'td. Our html
code to
make a table then might look like

(table :cellpadding 0 :cellspacing 5
  (tr (td "hello") (td "world")))

producing

<table cellpadding="0"
cellspacing="5"><tr><td>hello</td><td>world</td></tr></table>

I assume html tags can be described liked

<tag-name attribute1=value1 ... > content </tag-name>

So function 'table exspects a cons of a property list and the content as
parameters.
With the following helper function 'split-attribs

(defun split-attribs (lst)
  (let (attribs rest key value)
    (loop
      (if (zerop (length lst)) (return))
      (unless (keywordp (car lst)) (return))
      (setf key (string-downcase (pop lst)))
      (setf value (pop lst))
      (if value
        (push (format nil " ~a=\"~a\"" key value) attribs)
        (push (format nil " ~a" key) attribs)))
    (values (reverse attribs) lst)))

the function 'table could be

(defun table (&rest text)
  (multiple-value-bind (attribs rest)
      (split-attribs text)
    (format nil "<table~{~a~}>~{~a~}</table>~%" attribs rest)))

Now let's go one step ahead. For each html tag there has to be such a
function
definition. With the following macro we might generate these functions

(defun tag-format-string (tag)
  (concatenate 'string "<" tag "~{~a~}>~{~a~}</" tag ">~%"))

(defmacro deftag (name)
  (let ((f (tag-format-string (string-downcase (symbol-name name)))))
    `(defun ,name (&rest text)
       (multiple-value-bind (attribs rest)
           (split-attribs text)
         (format nil ,f attribs rest)))))

Thus defining our three functions 'table 'tr and 'td would reduce to

(deftag table)
(deftag tr)
(deftag td)

So the whole html thing can be reduced to approx. 60 lines of code, if we
consider
each 'deftag a line :-)

When talking about html I must mention Kevin Rosenberg's LML package at
http://lml.b9.com

I am interested in your criticism to my approach. With some slight
extensions (short tags
i.e. <br> ((no closing tag </br>)) and LF suppression) I use it to generate
html pages from Lisp.

Andreas

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