SWI-Prolog -- html/3

Documentation
- Reference manual
- Packages
  - SWI-Prolog HTTP support
    - The HTTP server libraries
      - The library(http/html_write) library
        
        html//1
        
        page//2
        
        page//1
        
        html_begin//1
        
        html_end//1
        
        Emitting HTML documents
        
        Repositioning HTML for CSS and javascript links
        
        Adding rules for html//1
        
        Generating layout
        
        Examples for using the HTML write library
        
        Remarks on the library(http/html_write) library

Availability::- use_module(library(http/html_write)).

html(:Spec)//

The DCG non-terminal html//1 is the main predicate of this library. It translates the specification for an HTML page into a list of atoms that can be written to a stream using print_html/[1,2]. The expansion rules of this predicate may be extended by defining the multifile DCG html_write:expand//1. Spec is either a single specification or a list of single specifications. Using nested lists is not allowed to avoid ambiguity caused by the atom []

Atomic data
Atomic data is quoted using html_quoted//1.
Fmt - Args
Fmt and Args are used as format-specification and argument list to format/3. The result is quoted and added to the output list.
\List
Escape sequence to add atoms directly to the output list. This can be used to embed external HTML code or emit script output. List is a list of the following terms:
- Fmt - Args
  Fmt and Args are used as format-specification and argument list to format/3. The result is added to the output list.
- Atomic
  Atomic values are added directly to the output list.
\Term
Invoke the non-terminal Term in the calling module. This is the common mechanism to realise abstraction and modularisation in generating HTML.
Module:Term
Invoke the non-terminal <Module>:<Term>. This is similar to \Term but allows for invoking grammar rules in external packages.
&(Entity)
Emit &<Entity>; or &#<Entity>; if Entity is an integer. SWI-Prolog atoms and strings are represented as Unicode. Explicit use of this construct is rarely needed because code-points that are not supported by the output encoding are automatically converted into character-entities.
Tag(Content)
Emit HTML element Tag using Content and no attributes. Content is handed to html//1. See section 3.21.4 for details on the automatically generated layout.
Tag(Attributes, Content)
Emit HTML element Tag using Attributes and Content. Attributes is either a single attribute of a list of attributes. Each attributes is of the format Name(Value) or Name=Value. Value is the atomic attribute value but allows for a limited functional notation:
- A + B
  Concatenation of A and B
- Format-Arguments
  Use format/3 and emit the result as quoted value.
- encode(Atom)
  Use uri_encoded/3 to create a valid URL query component.
- location_by_id(ID)
  HTTP location of the HTTP handler with given ID. See http_location_by_id/2.
- #(ID)
  Abbreviated for for location_by_id(ID).
- A + List
  List is handled as a URL‘search’component. The list members are terms of the format Name = Value or Name(Value). Values are encoded as in the encode option described above.
- List
  Emit SGML multi-valued attributes (e.g., NAMES). Each value in list is separated by a space. This is particularly useful for setting multiple class attributes on an element. For example:
```
        ...
        span(class([c1,c2]), ...),
```
The example below generates a URL that references the predicate set_lang/1 in the application with given parameters. The http_handler/3 declaration binds /setlang to the predicate set_lang/1 for which we provide a very simple implementation. The code between ... is part of an HTML page showing the english flag which, when pressed, calls set_lang(Request) where Request contains the search parameter lang = en. Note that the HTTP location (path) /setlang can be moved without affecting this code.
```
:- http_handler('/setlang', set_lang, []).

set_lang(Request) :-
        http_parameters(Request,
                        [ lang(Lang, [])
                        ]),
        http_session_retractall(lang(_)),
        http_session_assert(lang(Lang)),
        reply_html_page(title('Switched language'),
                        p(['Switch language to ', Lang])).


        ...
        html(a(href(location_by_id(set_lang) + [lang(en)]),
               img(src('/www/images/flags/en.png')))),
        ...
```