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<hr>
<h3 class="section" id="Punning-Names-1">5.30 Punning Names</h3>
<a class="index-entry-id" id="index-diversions-1"></a>
<p>Macros, strings, and diversions share a name space; recall
<a class="ref" href="Identifiers.html">Identifiers</a>. Internally, the same mechanism is used to store
them. You can thus call a macro with string interpolation syntax and
vice versa.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.de subject
Typesetting
..
.de predicate
rewards attention to detail
..
\*[subject] \*[predicate].
Truly.
⇒ Typesetting
⇒ rewards attention to detail Truly.
</pre></div></div>
<p>What went wrong? Strings don’t contain newlines, but macros do. String
interpolation placed a newline at the end of ‘<samp class="samp">\*[subject]</samp>’, and the
next thing on the input was a space. Then when ‘<samp class="samp">\*[predicate]</samp>’ was
interpolated, it was followed by the empty request ‘<samp class="samp">.</samp>’ on a line by
itself. If we want to use macros as strings, we must take interpolation
behavior into account.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.de subject
Typesetting\\
..
.de predicate
rewards attention to detail\\
..
\*[subject] \*[predicate].
Truly.
⇒ Typesetting rewards attention to detail. Truly.
</pre></div></div>
<p>By ending each text line of the macros with an escaped
<code class="code">\<kbd class="key">RET</kbd></code>, we get the desired effect (see <a class="pxref" href="Line-Continuation.html">Line Continuation</a>).<a class="footnote" id="DOCF114" href="groff.html_fot.html#FOOT114"><sup>114</sup></a>
What would have happened if we had used only one backslash at a time
instead?
</p>
<p>Interpolating a string does not hide existing macro arguments. We can
also place the escaped newline outside the string interpolation instead
of within the string definition. Thus, in a macro, a more efficient way
of doing
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.xx \\$@
</pre></div></div>
<p>is
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">\\*[xx]\\
</pre></div></div>
<p>The latter calling syntax doesn’t change the value of <code class="code">\$0</code>, which
is then inherited from the calling macro (see <a class="pxref" href="Parameters.html">Parameters</a>).
</p>
<p>Diversions can be also called with string syntax. It is sometimes
convenient to copy one-line diversions to a string.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.di xx
the
.ft I
interpolation system
.ft
.br
.di
.ds yy This is a test of \*(xx\c
\*(yy.
⇒ This is a test of the <i class="i">interpolation system</i>.
</pre></div></div>
<p>As the previous example shows, it is possible to store formatted output
in strings. The <code class="code">\c</code> escape sequence prevents the subsequent
newline from being interpreted as a break (again,
see <a class="pxref" href="Line-Continuation.html">Line Continuation</a>).
</p>
<p>Copying multi-output line diversions produces unexpected results.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.di xxx
a funny
.br
test
.br
.di
.ds yyy This is \*[xxx]\c
\*[yyy].
⇒ test This is a funny.
</pre></div></div>
<p>Usually, it is not predictable whether a diversion contains one or more
output lines, so this mechanism should be avoided. With <abbr class="acronym">AT&T</abbr>
<code class="code">troff</code>, this was the only solution to strip off a final newline
from a diversion. Another disadvantage is that the spaces in the copied
string are already formatted, preventing their adjustment. This can
cause ugly results.
</p>
<a class="index-entry-id" id="index-stripping-final-newline-in-diversions"></a>
<a class="index-entry-id" id="index-diversion_002c-stripping-final-newline"></a>
<a class="index-entry-id" id="index-final-newline_002c-stripping-in-diversions"></a>
<a class="index-entry-id" id="index-newline_002c-final_002c-stripping-in-diversions"></a>
<a class="index-entry-id" id="index-horizontal-space_002c-unformatting"></a>
<a class="index-entry-id" id="index-space_002c-horizontal_002c-unformatting"></a>
<a class="index-entry-id" id="index-unformatting-horizontal-space"></a>
<p>A clean solution to this problem is available in GNU <code class="code">troff</code>, using
the requests <code class="code">chop</code> to remove the final newline of a diversion, and
<code class="code">unformat</code> to make the horizontal spaces adjustable again.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.box xxx
a funny
.br
test
.br
.box
.chop xxx
.unformat xxx
This is \*[xxx].
⇒ This is a funny test.
</pre></div></div>
<p>See <a class="xref" href="Gtroff-Internals.html"><code class="code">gtroff</code> Internals</a>.
</p>
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