This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2021 Paul Brown <pbrown@optimusprime.ai> SPDX-FileCopyrightText: 1998-2021 Paulo Moura <pmoura@logtalk.org> SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

`html`

This library provides predicates for generating HTML content using either HTML 5 or XHTML 1.1 formats from a term representation. The library performs minimal validation, checking only that all elements are valid. No attempt is made to generate nicely indented output.

Normal elements are represented using a compound term with one argument (the element content) or two arguments (the element attributes represented by a list of Key=Value or Key-Value pairs and the element content). The element content can be another element or a list of elements. For example:

ol([type-a], [li(foo), li(bar), li(baz)])

The two exceptions are the pre or code elements whose content is never interpreted as an element or a list of elements. For example, the fragment:

pre([foo,bar,baz])

is translated to:

<pre>
[foo,bar,baz]
</pre>

Void elements are represented using a compound term with one argument, the (possibly empty) list of attributes represented by a list of Key=Value or Key-Value pairs. For example:

hr([class-separator])

Atomic arguments of the compound terms are interpreted as element content. Non-atomic element content can be represented as a quoted atom or by using the pre or code elements as explained above.

This library is a work in progress.

API documentation

Open the [../../docs/library_index.html#html](../../docs/library_index.html#html) link in a web browser.

Loading

To load all entities in this library, load the loader.lgt file:

| ?- logtalk_load(html(loader)).

Testing

To test this library predicates, load the tester.lgt file:

| ?- logtalk_load(html(tester)).

Generating a HTML document

HTML documents can be generated from a compound term representation and written to a file or a stream. For example, assuming we want to generate a HTML 5 file:

| ?- html5::

generate(
file('hello.html'),
html([lang=en], [head(title('Hello world!')), body(p('Bye!'))])
)

When the second argument is a html/1 or html/2 compound term, a doctype is automatically written. If we prefer instead e.g. a XHTML 1.1 document, we use the xhtml11 object:

| ?- xhtml11::

generate(
file('hello.html'),
html([lang=en], [head(title('Hello world!')), body(p('Bye!'))])
)

Generating a HTML fragment

It's also possible to generate just a fragment of a (X)HTML document by using a list of compound terms or a compound term for an element other than html. For example:

| ?- current_output(Stream), html5::generate(stream(Stream), ul([li(foo), li(bar), li(baz)])).

Stream = ...

Working with callbacks to generate content

Often we need to programmatically generate HTML content from queries. In other cases, we may have fixed content that we don't want to keep repeating (e.g., a navigation bar). The library supports a (::)/2 pseudo-element that sends a message to an object to retrieve content. As an example, assume the following predicate definition in user:

content(strong('Hello world!')).

This predicate can then be called from the HTML term representation. For example:

| ?- current_output(Stream), html5::generate(stream(Stream), span(user::content)).

<span><strong>Hello world!</strong></span>

Stream = ...

Note that the callback always takes the form `Object::Closure where Closure` is extended with a single argument (to be bound to the generated content). More complex callbacks are possible by using lambda expressions.

Working with custom elements

The html5 and xhtml11 objects recognize the same set of standard HTML 5 normal and void elements and generate an error for non-standard elements. If you need to generate HTML content containing custom elements, define a new object that extends one of the library objects. For example:

:- object(html5custom,
        extends(html5)).

        normal_element(foo, inline).
        normal_element(bar, block).
        normal_element(Name, Display) :-
                ^^normal_element(Name, Display).

:- end_object.