105 lines
4.6 KiB
HTML
105 lines
4.6 KiB
HTML
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<!-- This file has been generated by unroff 1.0, 03/21/96 19:29:41. -->
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<!-- Do not edit! -->
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<link rev="made" href="mailto:net@informatik.uni-bremen.de">
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<title>unroff Programmer's Manual, section 3.</title>
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</head><body>
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<h2><a name="section3">3.</a> <tt> </tt><a name=".events">Events and Event Handling</a>
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt><i>unroff
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</i>interprets a troff document as a sequence of chunks of normal
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text and interspersed ``events''.<tt> </tt>
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Plain text is usually just copied to the current output (a file or
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standard output).<tt> </tt>
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The output produced for an event is determined by an ``event
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handler'' (usually a Scheme procedure) that can be associated
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with each event.<tt> </tt>
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If no event handler can be found for an event encountered in the
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currently processed document (with a few exceptions), a warning message
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is displayed and the input that triggered the event is skipped
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(in case of requests and macros) or treated like normal text.<tt> </tt>
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For events such as troff requests, a separate Scheme procedure
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can be defined for each request, and the name of the request that
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triggered the event is then passed to the procedure as an argument.<tt> </tt>
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An event handling procedure can be defined for
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<ul>
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<li>
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each troff request, including requests that perform intrinsic troff
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functions, such as ``.de'' and ``.if''
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<li>
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each troff macro, whether user-defined or part of a macro
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package
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<li>
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each troff string
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<li>
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each number register
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<li>
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each special character
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<li>
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each escape sequence
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<li>
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each character (to provide character translations)
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<li>
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each inline equation enclosed by the current
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<i>eqn</i>(1)
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delimiter characters
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<li>
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each end of sentence (defined as a period, exclamation mark, or
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question mark, followed by a newline).<tt> </tt>
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</ul>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>When invoked, every Scheme procedure associated with one of
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the above events receives one or more arguments.<tt> </tt>
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For example, a procedure registered for the escape sequence `\h'
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(horizontal space) is passed the name of the escape sequence
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(the letter `h') as well as the argument to `\h' (i.e. the amount
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of space).<tt> </tt>
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Likewise, event handling procedures for requests and macros are
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called with the name of the request or macro as well as any
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arguments specified in the troff input.<tt> </tt>
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The exact arguments passed to each type of event handler will be
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explained below.<tt> </tt>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>A Scheme procedure associated with an event must return a string
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which is then output in place of whatever input triggered the
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event.<tt> </tt>
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Here, and in a number of other places, a Scheme symbol or a Scheme
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is accepted as an alternative to a string return value.<tt> </tt>
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Event handling procedures are free to directly produce output
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in addition to returning it as a result.<tt> </tt>
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As procedures associated with events frequently just return a
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fixed text, the text itself may be defined as the event handler
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in place of the procedure to save the overhead of the procedure
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call.<tt> </tt>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>Predefined Scheme procedures are supplied for events such as the
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requests ``.de'', ``.nr'', ``.ds'', and the corresponding escape
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sequences `\n' and `\*' to support user-defined macros, strings,
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and number registers.<tt> </tt>
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In any case, specific event handlers registered for macros,
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strings, and number registers supersede any user-supplied
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definitions.<tt> </tt>
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Thus, the author of a document can attach a
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special translation rule to a macro, string, or number register
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defined in the document to take effect when the document is processed by
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<i>unroff</i>.<tt> </tt>
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This is particularly important for high-level, structure-oriented
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target languages like SGML, as the the micro-formatting
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used by typical, more complex troff macros and by many low-level requests
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may not be expressible in such languages.<tt> </tt>
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As a case in point, it would obviously be impossible to translate, for
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example, the ``.IP'' macro defined by the ``ms'' package to a
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language such as HTML just by looking at the definition of the macro.<tt> </tt>
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For this reason,
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<i>unroff
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</i>does not really load the actual macro definitions for a troff macro
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package selected via the ``-m'' option; instead, an event handler
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is defined for each macro exported by the package to generate
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whatever represents the corresponding macro's function in the
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target language.<tt> </tt>
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<p><hr>
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Markup created by <em>unroff</em> 1.0, <tt> </tt> <tt> </tt>March 21, 1996, <tt> </tt> <tt> </tt>net@informatik.uni-bremen.de</body>
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</html>
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