299 lines
10 KiB
HTML
299 lines
10 KiB
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<head>
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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 4.</title>
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</head><body>
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<h2><a name="section4">4.</a> <tt> </tt>Defining Event Handlers
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>In the following list of Scheme primitives, the argument
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<i>name
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</i>denotes the name of a troff request, macro, escape sequence
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etc. (without any initial period or escape character) and can be
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supplied in form of a Scheme string, a Scheme symbol, or
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a Scheme character:
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<dl><dt><dd>
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<pre>
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(defrequest "ti" ...)
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(defrequest 'sp ...)
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(defescape #\h ...)
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</pre>
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</dl>
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(the primitives
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<i>defrequest
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</i>and
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<i>defescape
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</i>will be introduced in a moment).<tt> </tt>
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An argument named
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<i>handler
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</i>is either a procedure (usually a lambda expression) which returns
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a string, a symbol, or a character; or
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<i>handler
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</i>can itself be specified as a string, symbol, or character.<tt> </tt>
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In addition, the literal ``#f'' (false) can be supplied as a
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<i>handler
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</i>argument to remove any event handler that is currently associated with
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that event.<tt> </tt>
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Each of the ``def'' primitives listed below returns the handler
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that was previously associated with the corresponding event,
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or ``#f'' if the event was not handled.<tt> </tt>
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<h2>
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(defrequest <i>name</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 given handler with the given troff request.<tt> </tt>
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If
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<i>handler
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</i>is a procedure, it is passed the request's name and arguments
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as strings when called later.<tt> </tt>
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Passing the name of the request as the first argument aids in
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associating the same procedure with several different requests.<tt> </tt>
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<i>unroff
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</i>does not limit the number of arguments to requests, thus,
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an event handling procedure for a requests that takes a variable
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number of arguments could be defined like this:
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<dl><dt><dd>
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<pre>
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(defrequest 'rm
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(lambda (rm . args) ...))
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</pre>
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</dl>
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<p>
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If the request is invoked with fewer arguments than the procedure
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has formal arguments, the remaining arguments are bound to
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the empty string.<tt> </tt>
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If the request is invoked with
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<i>more
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</i>arguments than the procedure has formal arguments, the last lambda
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variable is assigned a string consisting of the (space-delimited)
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arguments left over after the other formal arguments have been bound to
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the other actual arguments.<tt> </tt>
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However, if
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<i>handler
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</i>has only one formal argument, an error message is displayed when the
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request is called with any arguments at all and the event is skipped.<tt> </tt>
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For example, consider the following handler for the (non-existing)
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request ``xx'':
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<dl><dt><dd>
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<pre>
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(defrequest 'xx
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(lambda (name a b) ...))
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</pre>
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</dl>
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The procedure's arguments
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<i>a
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</i>and
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<i>b
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</i>will be bound as follows when the request is invoked:
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<dl><dt><dd>
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<pre>
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.xx foo name="xx" a="foo" b=""
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.xx foo bar baz name="xx" a="foo" b="bar baz"
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</pre>
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</dl>
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<h2>
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(defmacro <i>name</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
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<i>handler
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</i>with the given troff macro, superseding
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any definition for this macro established by the ordinary ``.de''
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request.<tt> </tt>
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The only difference between
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<i>defrequest
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</i>and
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<i>defmacro
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</i>is the way arguments are bound in case
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<i>handler
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</i>is a procedure
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(troff employs slightly different rules when parsing the call
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to a request and a macro invocation).<tt> </tt>
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The quote character can be used in the latter case to surround
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arguments containing spaces, while quote characters are treated as
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normal characters in requests, which allows for the following
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remarkable troff idiom:
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<dl><dt><dd>
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<pre>
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.ds xy "hello
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</pre>
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</dl>
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In contrast to event handlers defined for requests, the formal
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arguments of a handler procedure associated with a macro must
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match the actual arguments in the normal way, that is, as if
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the procedure were invoked from within Scheme.<tt> </tt>
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A warning message is displayed if the number of macro arguments
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does not match the number of formal procedure arguments, and
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the event is skipped.<tt> </tt>
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<h2>
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(defspecial <i>name</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
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<i>handler
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</i>with the special character whose name is
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<i>name</i>.<tt> </tt>
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The name must have a length of 2.<tt> </tt>
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In addition, an empty name can be specified to define a
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``fallback'' handler that is called for special characters
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for which no handler exists.<tt> </tt>
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Like all event handler procedures,
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<i>handler
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</i>can have arbitrary side-effects in addition to returning a
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result; for example, the procedure may display a warning message
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if the special character cannot be represented in the target
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language and an approximation must be rendered instead.<tt> </tt>
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<h2>
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(defstring <i>name</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 a handler with the specified troff string.<tt> </tt>
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As
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<i>unroff
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</i>provides a default handler for the request ``.ds'' to implement
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used-defined strings,
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<i>defstring
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</i>is primarily used to give definitions for strings exported by
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troff macro packages.<tt> </tt>
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<h2>
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(defnumreg <i>name</i> <i>handler</i>)
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>This request behaves like
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<i>defstring</i>,
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except that it works on number registers.<tt> </tt>
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Note that the Scheme primitive
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<i>number->string
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</i>may have to be used by
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<i>handler
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</i>(if it is a procedure) to convert a numeric result into a string
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that can be returned from the handler.<tt> </tt>
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<p>
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In troff input, number registers as well as strings, special
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characters, and escape sequences can be denoted using the groff
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``long name'' syntax, unless troff compatibility has been enabled:
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<dl><dt><dd>
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<pre>
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\n[numreg] \n[string] \f[font] \[em] ...
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</pre>
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</dl>
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<h2>
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(defescape <i>name</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 an event handler with an escape sequence.<tt> </tt>
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<i>name
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</i>must have a length of 1, unless the empty string is
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given to define a ``fallback'' event handler (as with
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<i>defspecial</i>).<tt> </tt>
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Handlers defined for certain escape sequences are passed
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a second argument in addition to the name of the escape sequence.<tt> </tt>
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This is true for all escape sequences that have an argument
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according to the troff specification:
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<dl><dt><dd>
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<pre>
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\b \c \f \h \k \l \n \o \s \v \w \x \z
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\* \$ \"
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</pre>
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</dl>
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In addition, handlers for these groff escape sequences are passed an
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additional argument unless troff compatibility is enabled:
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<dl><dt><dd>
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<pre>
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\A \C \L \N \R \V \Y \Z
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</pre>
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</dl>
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The form of an escape sequence argument is determined by the
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troff specification and cannot be programmed; for example, the
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handler for `\z' is passed a character or a special character,
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and the handler for `\"' is invoked with the rest of the current
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input line sans the terminating newline.<tt> </tt>
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(The latter can be used to translate troff comments.)
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<p>
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Handlers registered for the escape sequences `\n' and '\s' are
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passed an optional third argument, one of the Scheme characters
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#\+ and #\-, if the escape sequence argument begins with a sign.<tt> </tt>
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The sign is then stripped from the actual argument.<tt> </tt>
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<p>
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As `\n' and `\*' are treated as ordinary escape sequences,
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handlers can be defined for them to achieve some form of fallback
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for number register and strings.<tt> </tt>
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<i>unroff
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</i>provides suitable default handlers for `\n', `\*', and '\$' as part
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of the implementation of user-defined number registers, strings,
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and macros.<tt> </tt>
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These handlers can be overridden if desired.<tt> </tt>
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<h2>
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(defchar <i>name</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
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<i>handler
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</i>with a character.<tt> </tt>
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<i>name
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</i>must have a length of 1.<tt> </tt>
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Each time the specified character is encountered in the troff
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input, the result (or value) of
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<i>handler
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</i>is output in place of the character.<tt> </tt>
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Character translations are not applied to the result of event
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handlers; event procedures can use the Scheme primitive
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<a href="m-8.html#.translate"><i>translate</i></a>
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(as described below) to execute the character translations
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established by calls to
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<i>defchar
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</i>if desired.<tt> </tt>
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<p>
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<i>defchar
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</i>currently has a number of weaknesses.<tt> </tt>
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The argument cannot be a special character
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(that is,
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<i>name
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</i>must be a plain character), and the mechanism cannot be used
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to achieve true
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<i>output
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</i>translations as with the troff request ``.tr'' or the groff
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request ``.char''.<tt> </tt>
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<h2>
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(defsentence <i>handler</i>)
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>Defines a handler to be consulted on end of sentence.<tt> </tt>
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If
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<i>handler
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</i>is a procedure, it is passed the punctuation mark ending the
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sentence as its argument (in form of a Scheme character).<tt> </tt>
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In any case, if an event handler has been specified, its result
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(or value) is output in place of the end-of-sentence mark and
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the newline character following it.<tt> </tt>
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<h2>
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(defequation <i>handler</i>)
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>Defines a handler for
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<i>eqn
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</i>inline equations.<tt> </tt>
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If
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<i>handler
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</i>is a procedure, it is passed the contents of the inline equation
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(with the delimiters stripped) as an argument.<tt> </tt>
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When an inline equation is encountered in the troff input and a handler
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has been defined for inline equations, the handler's result (or value)
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is output in place of the equation.<tt> </tt>
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<p>
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For inline equations to be recognized, delimiters must be defined first
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by passing
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<i>eqn
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</i>input that includes a ``delim'' directive to the Scheme primitive
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<a href="m-13.html#.filter-eqn-line"><i>filter-eqn-line</i></a>
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(explained below), as is usually done
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by the event handler associated with the request ``.EQ''.<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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