Rendering HTML with SXML and GNU Guile

April 10, 2015

GNU Guile provides modules for working with XML documents called SXML. SXML provides an elegant way of writing XML documents as s-expressions that can be easily manipulated in Scheme. Here’s an example:

(sxml->xml '(foo (bar (@ (attr "something")))))
<foo><bar attr="something" /></foo>

I don’t know about you, but I work with HTML documents much more often than XML. Since HTML is very similar to XML, we should be able to represent it with SXML, too!

(sxml->xml
 '(html
   (head
    (title "Hello, world!")
    (script (@ (src "foo.js"))))
   (body
    (h1 "Hello!"))))
<html>
  <head>
    <title>Hello, world!</title>
    <script src="foo.js" /> <!-- what? -->
  </head>
  <body>
    <h1>Hello!</h1>
  </body>
</html>

That <script> tag doesn’t look right! Script tags don’t close themselves like that. Well, we could hack around it:

(sxml->xml
 '(html
   (head
    (title "Hello, world!")
    (script (@ (src "foo.js")) ""))
   (body
    (h1 "Hello!"))))
<html>
  <head>
    <title>Hello, world!</title>
    <script src="foo.js"></script>
  </head>
  <body>
    <h1>Hello!</h1>
  </body>
</html>

Note the use of the empty string in (script (@ (src "foo.js")) ""). The output looks correct now, great! But what about the other void elements? We’ll have to remember to use the empty string hack each time we use one. That doesn’t sound very elegant.

Furthermore, text isn’t even escaped properly!

(sxml->xml "Copyright © 2015  David Thompson <davet@gnu.org>")
Copyright © 2015  David Thompson &lt;davet@gnu.org&gt;

The < and > braces were escaped, but © should’ve been rendered as &copy;. Why does this fail, too? Is there a bug in SXML?

There’s no bug. The improper rendering happens because HTML, while similar to XML, has some different syntax rules. Instead of using sxml->xml, a new procedure that is tailored to the HTML syntax is needed. Introducing sxml->html:

(define* (sxml->html tree #:optional (port (current-output-port)))
  "Write the serialized HTML form of TREE to PORT."
  (match tree
    (() *unspecified*)
    (('doctype type)
     (doctype->html type port))
    ;; Unescaped, raw HTML output
    (('raw html)
     (display html port))
    (((? symbol? tag) ('@ attrs ...) body ...)
     (element->html tag attrs body port))
    (((? symbol? tag) body ...)
     (element->html tag '() body port))
    ((nodes ...)
     (for-each (cut sxml->html <> port) nodes))
    ((? string? text)
     (string->escaped-html text port))
    ;; Render arbitrary Scheme objects, too.
    (obj (object->escaped-html obj port))))

In addition to being aware of void elements and escape characters, it can also render '(doctype "html") as <!DOCTYPE html>, or render an unescaped HTML string using '(raw "frog &amp; toad"). If we replace sxml->xml with sxml->html in the failing example above we can see that it does the right thing.

(sxml->html
 '((script (@ (src "foo.js")))
   "Copyright © 2015  David Thompson <davet@gnu.org>"))
<script src="foo.js"></script>
Copyright &copy; 2015  David Thompson &lt;davet@gnu.org&gt;

Here’s the full version of my (sxml html) module. It’s quite brief, if you don’t count the ~250 lines of escape codes! This code requires Guile 2.0.11 or greater.

Happy hacking!

;; Copyright © 2015  David Thompson <davet@gnu.org>
;;
;; This library is free software; you can redistribute it and/or
;; modify it under the terms of the GNU Lesser General Public License
;; as published by the Free Software Foundation; either version 3 of
;; the License, or (at your option) any later version.
;;
;; This library is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
;; Lesser General Public License for more details.
;;
;; You should have received a copy of the GNU Lesser General Public
;; License along with this library.  If not, see
;; <http://www.gnu.org/licenses/>.

define-module (sxml html)
 #:use-module (sxml simple)
 #:use-module (srfi srfi-26)
 #:use-module (ice-9 match)
 #:use-module (ice-9 format)
 #:use-module (ice-9 hash-table)
 #:export (sxml->html))

define %void-elements
 '(area
   base
   br
   col
   command
   embed
   hr
   img
   input
   keygen
   link
   meta
   param
   source
   track
   wbr))

define (void-element? tag)
 "Return #t if TAG is a void element."
 (pair? (memq tag %void-elements)))

define %escape-chars
 (alist->hash-table
  '((#\" . "quot")
    (#\& . "amp")
    (#\' . "apos")
    (#\< . "lt")
    (#\> . "gt")
    (#\¡ . "iexcl")
    (#\¢ . "cent")
    (#\£ . "pound")
    (#\¤ . "curren")
    (#\¥ . "yen")
    (#\¦ . "brvbar")
    (#\§ . "sect")
    (#\¨ . "uml")
    (#\© . "copy")
    (#\ª . "ordf")
    (#\« . "laquo")
    (#\¬ . "not")
    (#\® . "reg")
    (#\¯ . "macr")
    (#\° . "deg")
    (#\± . "plusmn")
    (#\² . "sup2")
    (#\³ . "sup3")
    (#\´ . "acute")
    (#\µ . "micro")
    (#\¶ . "para")
    (#\· . "middot")
    (#\¸ . "cedil")
    (#\¹ . "sup1")
    (#\º . "ordm")
    (#\» . "raquo")
    (#\¼ . "frac14")
    (#\½ . "frac12")
    (#\¾ . "frac34")
    (#\¿ . "iquest")
    (#\À . "Agrave")
    (#\Á . "Aacute")
    (#\Â . "Acirc")
    (#\Ã . "Atilde")
    (#\Ä . "Auml")
    (#\Å . "Aring")
    (#\Æ . "AElig")
    (#\Ç . "Ccedil")
    (#\È . "Egrave")
    (#\É . "Eacute")
    (#\Ê . "Ecirc")
    (#\Ë . "Euml")
    (#\Ì . "Igrave")
    (#\Í . "Iacute")
    (#\Î . "Icirc")
    (#\Ï . "Iuml")
    (#\Ð . "ETH")
    (#\Ñ . "Ntilde")
    (#\Ò . "Ograve")
    (#\Ó . "Oacute")
    (#\Ô . "Ocirc")
    (#\Õ . "Otilde")
    (#\Ö . "Ouml")
    (#\× . "times")
    (#\Ø . "Oslash")
    (#\Ù . "Ugrave")
    (#\Ú . "Uacute")
    (#\Û . "Ucirc")
    (#\Ü . "Uuml")
    (#\Ý . "Yacute")
    (#\Þ . "THORN")
    (#\ß . "szlig")
    (#\à . "agrave")
    (#\á . "aacute")
    (#\â . "acirc")
    (#\ã . "atilde")
    (#\ä . "auml")
    (#\å . "aring")
    (#\æ . "aelig")
    (#\ç . "ccedil")
    (#\è . "egrave")
    (#\é . "eacute")
    (#\ê . "ecirc")
    (#\ë . "euml")
    (#\ì . "igrave")
    (#\í . "iacute")
    (#\î . "icirc")
    (#\ï . "iuml")
    (#\ð . "eth")
    (#\ñ . "ntilde")
    (#\ò . "ograve")
    (#\ó . "oacute")
    (#\ô . "ocirc")
    (#\õ . "otilde")
    (#\ö . "ouml")
    (#\÷ . "divide")
    (#\ø . "oslash")
    (#\ù . "ugrave")
    (#\ú . "uacute")
    (#\û . "ucirc")
    (#\ü . "uuml")
    (#\ý . "yacute")
    (#\þ . "thorn")
    (#\ÿ . "yuml")
    (#\Π. "OElig")
    (#\œ . "oelig")
    (#\Š . "Scaron")
    (#\š . "scaron")
    (#\Ÿ . "Yuml")
    (#\ƒ . "fnof")
    (#\ˆ . "circ")
    (#\˜ . "tilde")
    (#\Α . "Alpha")
    (#\Β . "Beta")
    (#\Γ . "Gamma")
    (#\Δ . "Delta")
    (#\Ε . "Epsilon")
    (#\Ζ . "Zeta")
    (#\Η . "Eta")
    (#\Θ . "Theta")
    (#\Ι . "Iota")
    (#\Κ . "Kappa")
    (#\Λ . "Lambda")
    (#\Μ . "Mu")
    (#\Ν . "Nu")
    (#\Ξ . "Xi")
    (#\Ο . "Omicron")
    (#\Π . "Pi")
    (#\Ρ . "Rho")
    (#\Σ . "Sigma")
    (#\Τ . "Tau")
    (#\Υ . "Upsilon")
    (#\Φ . "Phi")
    (#\Χ . "Chi")
    (#\Ψ . "Psi")
    (#\Ω . "Omega")
    (#\α . "alpha")
    (#\β . "beta")
    (#\γ . "gamma")
    (#\δ . "delta")
    (#\ε . "epsilon")
    (#\ζ . "zeta")
    (#\η . "eta")
    (#\θ . "theta")
    (#\ι . "iota")
    (#\κ . "kappa")
    (#\λ . "lambda")
    (#\μ . "mu")
    (#\ν . "nu")
    (#\ξ . "xi")
    (#\ο . "omicron")
    (#\π . "pi")
    (#\ρ . "rho")
    (#\ς . "sigmaf")
    (#\σ . "sigma")
    (#\τ . "tau")
    (#\υ . "upsilon")
    (#\φ . "phi")
    (#\χ . "chi")
    (#\ψ . "psi")
    (#\ω . "omega")
    (#\ϑ . "thetasym")
    (#\ϒ . "upsih")
    (#\ϖ . "piv")
    (#\. "ensp")
    (#\. "emsp")
    (#\. "thinsp")
    (#\– . "ndash")
    (#\— . "mdash")
    (#\‘ . "lsquo")
    (#\’ . "rsquo")
    (#\‚ . "sbquo")
    (#\“ . "ldquo")
    (#\” . "rdquo")
    (#\„ . "bdquo")
    (#\† . "dagger")
    (#\‡ . "Dagger")
    (#\• . "bull")
    (#\… . "hellip")
    (#\‰ . "permil")
    (#\′ . "prime")
    (#\″ . "Prime")
    (#\‹ . "lsaquo")
    (#\› . "rsaquo")
    (#\‾ . "oline")
    (#\⁄ . "frasl")
    (#\€ . "euro")
    (#\ℑ . "image")
    (#\℘ . "weierp")
    (#\ℜ . "real")
    (#\™ . "trade")
    (#\ℵ . "alefsym")
    (#\← . "larr")
    (#\↑ . "uarr")
    (#\→ . "rarr")
    (#\↓ . "darr")
    (#\↔ . "harr")
    (#\↵ . "crarr")
    (#\⇐ . "lArr")
    (#\⇑ . "uArr")
    (#\⇒ . "rArr")
    (#\⇓ . "dArr")
    (#\⇔ . "hArr")
    (#\∀ . "forall")
    (#\∂ . "part")
    (#\∃ . "exist")
    (#\∅ . "empty")
    (#\∇ . "nabla")
    (#\∈ . "isin")
    (#\∉ . "notin")
    (#\∋ . "ni")
    (#\∏ . "prod")
    (#\∑ . "sum")
    (#\− . "minus")
    (#\∗ . "lowast")
    (#\√ . "radic")
    (#\∝ . "prop")
    (#\∞ . "infin")
    (#\∠ . "ang")
    (#\∧ . "and")
    (#\∨ . "or")
    (#\∩ . "cap")
    (#\∪ . "cup")
    (#\∫ . "int")
    (#\∴ . "there4")
    (#\∼ . "sim")
    (#\≅ . "cong")
    (#\≈ . "asymp")
    (#\≠ . "ne")
    (#\≡ . "equiv")
    (#\≤ . "le")
    (#\≥ . "ge")
    (#\⊂ . "sub")
    (#\⊃ . "sup")
    (#\⊄ . "nsub")
    (#\⊆ . "sube")
    (#\⊇ . "supe")
    (#\⊕ . "oplus")
    (#\⊗ . "otimes")
    (#\⊥ . "perp")
    (#\⋅ . "sdot")
    (#\⋮ . "vellip")
    (#\⌈ . "lceil")
    (#\⌉ . "rceil")
    (#\⌊ . "lfloor")
    (#\⌋ . "rfloor")
    (#\〈 . "lang")
    (#\〉 . "rang")
    (#\◊ . "loz")
    (#\♠ . "spades")
    (#\♣ . "clubs")
    (#\♥ . "hearts")
    (#\♦ . "diams"))))

define (string->escaped-html s port)
 "Write the HTML escaped form of S to PORT."
 (define (escape c)
   (let ((escaped (hash-ref %escape-chars c)))
     (if escaped
         (format port "&~a;" escaped)
         (display c port))))
 (string-for-each escape s))

define (object->escaped-html obj port)
 "Write the HTML escaped form of OBJ to PORT."
 (string->escaped-html
  (call-with-output-string (cut display obj <>))
  port))

define (attribute-value->html value port)
 "Write the HTML escaped form of VALUE to PORT."
 (if (string? value)
     (string->escaped-html value port)
     (object->escaped-html value port)))

define (attribute->html attr value port)
 "Write ATTR and VALUE to PORT."
 (format port "~a=\"" attr)
 (attribute-value->html value port)
 (display #\" port))

define (element->html tag attrs body port)
 "Write the HTML TAG to PORT, where TAG has the attributes in the
ist ATTRS and the child nodes in BODY."
 (format port "<~a" tag)
 (for-each (match-lambda
            ((attr value)
             (display #\space port)
             (attribute->html attr value port)))
           attrs)
 (if (and (null? body) (void-element? tag))
     (display " />" port)
     (begin
       (display #\> port)
       (for-each (cut sxml->html <> port) body)
       (format port "</~a>" tag))))

define (doctype->html doctype port)
 (format port "<!DOCTYPE ~a>" doctype))

define* (sxml->html tree #:optional (port (current-output-port)))
 "Write the serialized HTML form of TREE to PORT."
 (match tree
   (() *unspecified*)
   (('doctype type)
    (doctype->html type port))
   ;; Unescaped, raw HTML output
   (('raw html)
    (display html port))
   (((? symbol? tag) ('@ attrs ...) body ...)
    (element->html tag attrs body port))
   (((? symbol? tag) body ...)
    (element->html tag '() body port))
   ((nodes ...)
    (for-each (cut sxml->html <> port) nodes))
   ((? string? text)
    (string->escaped-html text port))
   ;; Render arbitrary Scheme objects, too.
   (obj (object->escaped-html obj port))))