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\section{Standard Module \sectcode{htmllib}}
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\stmodindex{htmllib}
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\index{HTML}
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\index{hypertext}
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\renewcommand{\indexsubitem}{(in module htmllib)}
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This module defines a number of classes which can serve as a basis for
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parsing text files formatted in HTML (HyperText Mark-up Language).
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The classes are not directly concerned with I/O --- the have to be fed
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their input in string form, and will make calls to methods of a
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``formatter'' object in order to produce output. The classes are
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designed to be used as base classes for other classes in order to add
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functionality, and allow most of their methods to be extended or
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overridden. In turn, the classes are derived from and extend the
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class \code{SGMLParser} defined in module \code{sgmllib}.
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\index{SGML}
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\stmodindex{sgmllib}
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\ttindex{SGMLParser}
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\index{formatter}
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The following is a summary of the interface defined by
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\code{sgmllib.SGMLParser}:
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\begin{itemize}
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\item
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The interface to feed data to an instance is through the \code{feed()}
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method, which takes a string argument. This can be called with as
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little or as much text at a time as desired;
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\code{p.feed(a); p.feed(b)} has the same effect as \code{p.feed(a+b)}.
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When the data contains complete
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HTML elements, these are processed immediately; incomplete elements
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are saved in a buffer. To force processing of all unprocessed data,
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call the \code{close()} method.
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Example: to parse the entire contents of a file, do\\
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\code{parser.feed(open(file).read()); parser.close()}.
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\item
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The interface to define semantics for HTML tags is very simple: derive
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a class and define methods called \code{start_\var{tag}()},
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\code{end_\var{tag}()}, or \code{do_\var{tag}()}. The parser will
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call these at appropriate moments: \code{start_\var{tag}} or
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\code{do_\var{tag}} is called when an opening tag of the form
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\code{<\var{tag} ...>} is encountered; \code{end_\var{tag}} is called
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when a closing tag of the form \code{<\var{tag}>} is encountered. If
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an opening tag requires a corresponding closing tag, like \code{<H1>}
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... \code{</H1>}, the class should define the \code{start_\var{tag}}
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method; if a tag requires no closing tag, like \code{<P>}, the class
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should define the \code{do_\var{tag}} method.
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\end{itemize}
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The module defines the following classes:
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\begin{funcdesc}{HTMLParser}{}
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This is the most basic HTML parser class. It defines one additional
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entity name over the names defined by the \code{SGMLParser} base
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class, \code{\•}. It also defines handlers for the following
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tags: \code{<LISTING>...</LISTING>}, \code{<XMP>...</XMP>}, and
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\code{<PLAINTEXT>} (the latter is terminated only by end of file).
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\end{funcdesc}
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\begin{funcdesc}{CollectingParser}{}
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This class, derived from \code{HTMLParser}, collects various useful
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bits of information from the HTML text. To this end it defines
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additional handlers for the following tags: \code{<A>...</A>},
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\code{<HEAD>...</HEAD>}, \code{<BODY>...</BODY>},
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\code{<TITLE>...</TITLE>}, \code{<NEXTID>}, and \code{<ISINDEX>}.
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\end{funcdesc}
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\begin{funcdesc}{FormattingParser}{formatter\, stylesheet}
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This class, derived from \code{CollectingParser}, interprets a wide
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selection of HTML tags so it can produce formatted output from the
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parsed data. It is initialized with two objects, a \var{formatter}
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which should define a number of methods to format text into
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paragraphs, and a \var{stylesheet} which defines a number of static
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parameters for the formatting process. Formatters and style sheets
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are documented later in this section.
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\index{formatter}
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\index{style sheet}
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\end{funcdesc}
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\begin{funcdesc}{AnchoringParser}{formatter\, stylesheet}
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This class, derived from \code{FormattingParser}, extends the handling
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of the \code{<A>...</A>} tag pair to call the formatter's
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\code{bgn_anchor()} and \code{end_anchor()} methods. This allows the
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formatter to display the anchor in a different font or color, etc.
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\end{funcdesc}
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Instances of \code{CollectingParser} (and thus also instances of
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\code{FormattingParser} and \code{AnchoringParser}) have the following
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instance variables:
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\begin{datadesc}{anchornames}
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A list of the values of the \code{NAME} attributes of the \code{<A>}
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tags encountered.
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\end{datadesc}
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\begin{datadesc}{anchors}
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A list of the values of \code{HREF} attributes of the \code{<A>} tags
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encountered.
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\end{datadesc}
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\begin{datadesc}{anchortypes}
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A list of the values of the \code{TYPE} attributes of the \code{<A>}
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tags encountered.
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\end{datadesc}
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\begin{datadesc}{inanchor}
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Outside an \code{<A>...</A>} tag pair, this is zero. Inside such a
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pair, it is a unique integer, which is positive if the anchor has a
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\code{HREF} attribute, negative if it hasn't. Its absolute value is
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one more than the index of the anchor in the \code{anchors},
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\code{anchornames} and \code{anchortypes} lists.
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\end{datadesc}
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\begin{datadesc}{isindex}
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True if the \code{<ISINDEX>} tag has been encountered.
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\end{datadesc}
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\begin{datadesc}{nextid}
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The attribute list of the last \code{<NEXTID>} tag encountered, or
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an empty list if none.
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\end{datadesc}
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\begin{datadesc}{title}
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The text inside the last \code{<TITLE>...</TITLE>} tag pair, or
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\code{''} if no title has been encountered yet.
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\end{datadesc}
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The \code{anchors}, \code{anchornames} and \code{anchortypes} lists
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are ``parallel arrays'': items in these lists with the same index
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pertain to the same anchor. Missing attributes default to the empty
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string. Anchors with neither a \code{HREF} nor a \code{NAME}
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attribute are not entered in these lists at all.
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The module also defines a number of style sheet classes. These should
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never be instantiated --- their class variables are the only behavior
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required. Note that style sheets are specifically designed for a
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particular formatter implementation. The currently defined style
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sheets are:
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\index{style sheet}
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\begin{datadesc}{NullStylesheet}
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A style sheet for use on a dumb output device such as an \ASCII{}
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terminal.
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\end{datadesc}
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\begin{datadesc}{X11Stylesheet}
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A style sheet for use with an X11 server.
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\end{datadesc}
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\begin{datadesc}{MacStylesheet}
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A style sheet for use on Apple Macintosh computers.
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\end{datadesc}
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\begin{datadesc}{StdwinStylesheet}
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A style sheet for use with the \code{stdwin} module; it is an alias
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for either \code{X11Stylesheet} or \code{MacStylesheet}.
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\bimodindex{stdwin}
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\end{datadesc}
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\begin{datadesc}{GLStylesheet}
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A style sheet for use with the SGI Graphics Library and its font
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manager (the SGI-specific built-in modules \code{gl} and \code{fm}).
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\bimodindex{gl}
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\bimodindex{fm}
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\end{datadesc}
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Style sheets have the following class variables:
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\begin{datadesc}{stdfontset}
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A list of up to four font definititions, respectively for the roman,
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italic, bold and constant-width variant of a font for normal text. If
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the list contains less than four font definitions, the last item is
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used as the default for missing items. The type of a font definition
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depends on the formatter in use; its only use is as a parameter to the
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formatter's \code{setfont()} method.
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\end{datadesc}
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\begin{datadesc}{h1fontset}
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\dataline{h2fontset}
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\dataline{h3fontset}
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The font set used for various headers (text inside \code{<H1>...</H1>}
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tag pairs etc.).
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\end{datadesc}
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\begin{datadesc}{stdindent}
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The indentation of normal text. This is measured in the ``native''
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units of the formatter in use; for some formatters these are
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characters, for others (especially those that actually support
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variable-spacing fonts) in pixels or printer points.
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\end{datadesc}
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\begin{datadesc}{ddindent}
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The indentation used for the first level of \code{<DD>} tags.
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\end{datadesc}
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\begin{datadesc}{ulindent}
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The indentation used for the first level of \code{<UL>} tags.
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\end{datadesc}
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\begin{datadesc}{h1indent}
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The indentation used for level 1 headers.
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\end{datadesc}
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\begin{datadesc}{h2indent}
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The indentation used for level 2 headers.
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\end{datadesc}
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\begin{datadesc}{literalindent}
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The indentation used for literal text (text inside
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\code{<PRE>...</PRE>} and similar tag pairs).
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\end{datadesc}
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Although no documented implementation of a formatter exists, the
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\code{FormattingParser} class assumes that formatters have a
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certain interface. This interface requires the following methods:
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\index{formatter}
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\begin{funcdesc}{setfont}{fontspec}
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Set the font to be used subsequently. The \var{fontspec} argument is
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an item in a style sheet's font set.
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\end{funcdesc}
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\begin{funcdesc}{flush}{}
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Finish the current line, if not empty, and begin a new one.
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\end{funcdesc}
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\begin{funcdesc}{setleftindent}{n}
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Set the left indentation of the following lines to \var{n} units.
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\end{funcdesc}
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\begin{funcdesc}{needvspace}{n}
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Require at least \var{n} blank lines before the next line. Implies
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\code{flush()}.
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\end{funcdesc}
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\begin{funcdesc}{addword}{word\, space}
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Add a \var{word} to the current paragraph, followed by \var{space}
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spaces.
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\end{funcdesc}
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\begin{datadesc}{nospace}
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If this instance variable is true, empty words should be ignored by
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\code{addword}. It should be set to false after a non-empty word has
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been added.
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\end{datadesc}
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\begin{funcdesc}{setjust}{justification}
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Set the justification of the current paragraph. The
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\var{justification} can be \code{'c'} (center), \code{'l'} (left
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justified), \code{'r'} (right justified) or \code{'lr'} (left and
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right justified).
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\end{funcdesc}
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\begin{funcdesc}{bgn_anchor}{id}
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Begin an anchor. The \var{id} parameter is the value of the parser's
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\code{inanchor} attribute.
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\end{funcdesc}
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\begin{funcdesc}{end_anchor}{id}
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End an anchor. The \var{id} parameter is the value of the parser's
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\code{inanchor} attribute.
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\end{funcdesc}
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A sample formatter implementation can be found in the module
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\code{fmt}, which in turn uses the module \code{Para}. These modules are
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not intended as standard library modules; they are available as an
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example of how to write a formatter.
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\ttindex{fmt}
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\ttindex{Para}
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