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\subsubsection{Repositioning HTML for CSS and javascript links}

\label{sec:html-post}

Modern HTML commonly uses CSS and Javascript. This requires $<$link$>$
elements in the HTML $<$head$>$ element or $<$script$>$ elements in the $<$body$>$.
Unfortunately this seriously harms re-using HTML DCG rules as components
as each of these components may rely on their own style sheets or
JavaScript code. We added a `mailing' system to reposition and collect
fragments of HTML. This is implemented by \dcgref{html_post}{2}, \dcgref{html_receive}{1}
and \dcgref{html_receive}{2}.

\begin{description}
    \dcg[det]{html_post}{2}{+Id, :HTML}
Reposition \arg{HTML} to the receiving \arg{Id}. The \dcgref{html_post}{2} call
processes \arg{HTML} using \dcgref{html}{1}. Embedded \bsl{}-commands are executed
by \predref{mailman}{1} from \predref{print_html}{1} or \predref{html_print_length}{2}. These
commands are called in the calling context of the \dcgref{html_post}{2}
call.

A typical usage scenario is to get required CSS links in the
document head in a reusable fashion. First, we define \dcgref{css}{1} as:

\begin{code}
css(URL) -->
        html_post(css,
                  link([ type('text/css'),
                         rel('stylesheet'),
                         href(URL)
                       ])).
\end{code}

Next we insert the \textit{unique} CSS links, in the pagehead using the
following call to \predref{reply_html_page}{2}:

\begin{code}
        reply_html_page([ title(...),
                          \html_receive(css)
                        ],
                        ...)
\end{code}

    \dcg[det]{html_receive}{1}{+Id}
Receive posted HTML tokens. Unique sequences of tokens posted
with \dcgref{html_post}{2} are inserted at the location where
\dcgref{html_receive}{1} appears.

\begin{tags}
\mtag{See also}- The local predicate \dcgref{sorted_html}{1} handles the output of
\dcgref{html_receive}{1}. \\- \dcgref{html_receive}{2} allows for post-processing the posted
material.
\end{tags}

    \dcg[det]{html_receive}{2}{+Id, :Handler}
This extended version of \dcgref{html_receive}{1} causes \arg{Handler} to be
called to process all messages posted to the channal at the time
output is generated. \arg{Handler} is called as below, where
\arg{PostedTerms} is a list of Module:Term created from calls to
\dcgref{html_post}{2}. Module is the context module of html_post and Term
is the unmodified term. Members in \arg{PostedTerms} are in the
order posted and may contain duplicates.

\begin{code}
  phrase(Handler, PostedTerms, HtmlTerms, Rest)
\end{code}

Typically, \arg{Handler} collects the posted terms, creating a term
suitable for \dcgref{html}{1} and finally calls \dcgref{html}{1}.
\end{description}

The library predefines the receiver channel \const{head} at the end of the
\const{head} element for all pages that write the html \const{head} through this
library. The following code can be used anywhere inside an HTML
generating rule to demand a javascript in the header:

\begin{code}
js_script(URL) -->
        html_post(head, script([ src(URL),
                                 type('text/javascript')
                               ], [])).
\end{code}

This mechanism is also exploited to add XML namespace (\const{xmlns})
declarations to the (outer) \const{html} element using \dcgref{xhml_ns}{2}:

\begin{description}
    \dcg{xhtml_ns}{2}{+Id, +Value}
Demand an xmlns:id=\arg{Value} in the outer html tag. This uses the
\predref{html_post}{2} mechanism to post to the \const{xmlns} channel. Rdfa
(\url{http://www.w3.org/2006/07/SWD/RDFa/syntax/)}, embedding RDF in
(x)html provides a typical usage scenario where we want to
publish the required namespaces in the header. We can define:

\begin{code}
rdf_ns(Id) -->
        { rdf_global_id(Id:'', Value) },
        xhtml_ns(Id, Value).
\end{code}

After which we can use \dcgref{rdf_ns}{1} as a normal rule in \dcgref{html}{1} to
publish namespaces from \file{library(semweb/rdf_db)}. Note that this
macro only has effect if the dialect is set to \const{xhtml}. In
\const{html} mode it is silently ignored.

The required \const{xmlns} receiver is installed by \dcgref{html_begin}{1}
using the \const{html} tag and thus is present in any document that
opens the outer \const{html} environment through this library.
\end{description}