121 lines
3.8 KiB
HTML
121 lines
3.8 KiB
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<!-- This file has been generated by unroff 1.0, 03/21/96 19:29:43. -->
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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 6.</title>
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</head><body>
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<h2><a name="section6">6.</a> <tt> </tt>Event Procedures with Side-Effects
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>Besides the basic events described in the
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<a href="m-3.html#.events">preceding sections</a>,
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another group of--slightly different--events exist and can
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be handled by user-defined Scheme procedures.<tt> </tt>
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These events are not related to troff functions, but to a number of
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other conditions that are encountered when processing documents:
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<ul>
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<li>
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the end of an input line
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<li>
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the beginning of a troff input file processed by
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<i>unroff
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</i><li>
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the end of a troff input file
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<li>
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startup of the program
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<li>
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termination of the program
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<li>
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a keyword/value option encountered in the command line.<tt> </tt>
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</ul>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>Among other tasks, these events can be used to generate a prologue and
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epilogue for each input file.<tt> </tt>
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In contrast to the events described in the previous section, handlers for
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these events are called solely for their side-effects.<tt> </tt>
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Each event handler must be a Scheme procedure.<tt> </tt>
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Their results are ignored, thus the procedures must have side-effects
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to be useful.<tt> </tt>
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Another difference is that more than one event handler can be associated
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with each request.<tt> </tt>
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A numeric
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<i>level
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</i>(a small integer number) is specified together with each event handler,
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and when the corresponding event is triggered, all procedures
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defined for this event are executed in increasing order as indicated by
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their levels.<tt> </tt>
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<h2>
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(defevent <i>event</i> <i>level</i> <i>handler</i>)
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>Associates the procedure
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<i>handler
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</i>with an event and returns the previous event handler registered
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for this combination of event and level.<tt> </tt>
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<i>level
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</i>is an integer between 0 and 99;
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<i>handler
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</i>is a procedure, or the literal #f to remove a previously defined handler.<tt> </tt>
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<i>event
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</i>indicates the type of event and is one of the following Scheme symbols:
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<i>line
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</i>(end of input line),
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<i>prolog
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</i>(beginning of input file),
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<i>epilog
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</i>(end of input file),
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<i>start
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</i>(program start),
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<i>exit
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</i>(program termination),
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<i>option
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</i>(keyword/value command line option).<tt> </tt>
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<p>
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Procedures defined for the events
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<i>prolog
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</i>and
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<i>epilog
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</i>are called with two string arguments:
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the path name (as specified by the user) and the file name component of
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the troff input file whose processing has just begun or finished,
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or the string ``stdin'' if
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<i>unroff
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</i>is taking its input from standard input.<tt> </tt>
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Procedures defined for the event
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<i>option
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</i>are passed the option's name and value as strings.<tt> </tt>
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All other event procedures are invoked without arguments.<tt> </tt>
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<i>unroff
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</i>provides a default handler for
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<i>option
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</i>(see the
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<a href="m-15.html#.options">primitives for options</a>
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below).<tt> </tt>
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<p>
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Example:
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<dl><dt><dd>
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<pre>
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(defevent 'exit 50 ; cleanup on exit
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(lambda ()
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...))
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</pre>
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</dl>
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The handler defined in this way will be executed on termination,
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after any handlers with levels 0-49.<tt> </tt>
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<h2>
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(eventdef <i>event</i> <i>level</i>)
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>Returns the procedure defined as a handler for
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<i>event
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</i>and
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<i>level</i>,
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or #f if no such handler exists.<tt> </tt>
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See
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<i>defevent
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</i>above for a description of the arguments.<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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