3.21.1 Emitting HTML documents

The non-terminal html//1 translates a specification into a list of atoms and layout instructions. Currently the layout instructions are terms of the format nl(N), requesting at least N newlines. Multiple consecutive nl(1) terms are combined to an atom containing the maximum of the requested number of newline characters.

To simplify handing the data to a client or storing it into a file, the following predicates are available from this library:

reply_html_page(:Head, :Body)

Same as reply_html_page(default, Head, Body).

reply_html_page(+Style, :Head, :Body)

Writes an HTML page preceded by an HTTP header as required by library(http_wrapper) (CGI-style). Here is a simple typical example:

reply(Request) :-
        reply_html_page(title('Welcome'),
                        [ h1('Welcome'),
                          p('Welcome to our ...')
                        ]).

The header and footer of the page can be hooked using the grammar-rules user:head//2 and user:body//2. The first argument passed to these hooks is the Style argument of reply_html_page/3 and the second is the 2nd (for head//2) or 3rd (for body//2) argument of reply_html_page/3. These hooks can be used to restyle the page, typically by embedding the real body content in a div. E.g., the following code provides a menu on top of each page of that is identified using the style myapp.

:- multifile
        user:body//2.

user:body(myapp, Body) -->
        html(body([ div(id(top), \application_menu),
                    div(id(content), Body)
                  ])).

Redefining the head can be used to pull in scripts, but typically html_requires//1 provides a more modular approach for pulling scripts and CSS-files.

reply_html_partial(+HTML)

Reply with partial HTML document. The reply only contains the element from HTML, i.e., this predicate does not add a DOCTYPE header, html, head or body. It is intended for JavaScript handlers that request a partial document and insert that somewhere into the existing page DOM. See reply_html_page/3 to reply with a complete (valid) HTML page.

print_html(+List)

Print the token list to the Prolog current output stream.

print_html(+Stream, +List)

Print the token list to the specified output stream

html_print_length(+List, -Length)

When calling html_print/[1,2] on List, Length characters will be produced. Knowing the length is needed to provide the Content-length field of an HTTP reply-header.