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206 lines
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>27.4. Format Directive Interface</TITLE>
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<meta name="description" value=" Format Directive Interface">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html4776 HREF="node258.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4774 HREF="node253.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4768 HREF="node256.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4778 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4779 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4777 HREF="node258.html"> Compiling Format Control </A>
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<B>Up:</B> <A NAME=tex2html4775 HREF="node253.html"> Pretty Printing</A>
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<B> Previous:</B> <A NAME=tex2html4769 HREF="node256.html"> Dynamic Control of </A>
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<HR> <P>
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<H1><A NAME=SECTION003140000000000000000>27.4. Format Directive Interface</A></H1>
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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<A NAME=PPRINTFORMATDIRECTIVESSECTION>The</A>
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primary interface to operations for dynamically determining the
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arrangement of output is provided through the functions above. However, an
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additional interface is provided via a set of format directives
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because, as shown by the examples in this section and the
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next, <tt>format</tt> strings are typically a much more compact way to specify
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pretty printing. In addition, without such an interface, one would have to
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abandon the use of <tt>format</tt> when interacting with the pretty printer.
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<P>
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<DL COMPACT><DT><tt>~W</tt>
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<DD>
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<i>Write.</i> An <i>arg</i>, any Lisp object, is printed obeying <i>every</i>
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printer control variable (as by <tt>write</tt>). In addition, <tt>~W</tt>
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interacts correctly with depth abbreviation by not resetting the depth
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counter to zero. <tt>~W</tt> does not accept parameters. If given the colon
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modifier, <tt>~W</tt> binds <tt>*print-pretty*</tt> to <tt>t</tt>. If given the atsign
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modifier, <tt>~W</tt> binds <tt>*print-level*</tt> and <tt>*print-length*</tt> to
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<tt>nil</tt>.
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<P>
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<tt>~W</tt> provides automatic support for circularity detection. If
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<tt>*print-circle*</tt> (and possibly also <tt>*print-shared*</tt>) is not <tt>nil</tt> and
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<tt>~W</tt> is applied to an argument that is a circular (or shared) reference,
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an appropriate ``<tt>#<i>n</i>#</tt>'' marker is inserted in the output
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instead of printing the argument.
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<P>
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<DT><tt>~_</tt>
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<DD>
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<i>Conditional newline.</i> Without any modifiers,
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<tt>~_</tt> is equivalent to
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<tt>(pprint-newline :linear)</tt>.
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The directive <tt>~@_</tt> is
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equivalent to <tt>(pprint-newline :miser)</tt>.
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The directive <tt>~:_</tt>
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is equivalent to <tt>(pprint-newline :fill)</tt>.
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The directive <tt>~:@_</tt> is
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equivalent to <tt>(pprint-newline :mandatory)</tt>.
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<P>
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<DT><tt>~<<i>str</i>~:></tt>
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<DD>
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<i>Logical block.</i> If <tt>~:></tt> is used to terminate a
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<tt>~<...</tt> directive, the directive is equivalent to a call on
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<tt>pprint-logical-block</tt>. The <tt>format</tt> argument corresponding to the
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<tt>~<...~:></tt> directive is treated in the same way as the <i>list</i>
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argument to <tt>pprint-logical-block</tt>, thereby providing automatic support for
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non-list arguments and the detection of circularity, sharing, and depth abbreviation.
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The portion of the <tt>format</tt> control string nested within the
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<tt>~<...~:></tt> specifies the <tt>:prefix</tt> (or <tt>:per-line-prefix</tt>),
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<tt>:suffix</tt>, and body of the <tt>pprint-logical-block</tt>.
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<P>
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The <tt>format</tt> string portion enclosed by <tt>~<...~:></tt> can be
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divided into segments <tt>~<<i>prefix</i>~;<i>body</i>~;<i>suffix</i>~:></tt> by <tt>~;</tt> directives. If the first section is
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terminated by <tt>~@;</tt>, it specifies a per-line prefix rather than a
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simple prefix. The prefix and suffix cannot contain <tt>format</tt> directives.
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An error is signaled if either the prefix or suffix fails to be a constant string
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or if the enclosed portion is divided into more than three segments.
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<P>
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If the enclosed portion is divided into only two segments, the suffix defaults
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to the null string. If the enclosed portion consists of only a single
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segment, both the prefix and the suffix default to the null string. If the
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colon modifier is used (that is, <tt>~:<...~:></tt>), the prefix and
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suffix default to <tt>"("</tt> and <tt>")"</tt>, respectively, instead of the null
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string.
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<P>
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The body segment can be any arbitrary <tt>format</tt> control string. This <tt>format</tt>
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control string is applied to the elements of the list corresponding to the
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<tt>~<...~:></tt> directive as a whole. Elements are extracted from this
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list using <tt>pprint-pop</tt>, thereby providing automatic support for malformed lists
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and the detection of circularity, sharing, and length abbreviation.
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Within the body segment, <tt>~^</tt> acts like
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<tt>pprint-exit-if-list-exhausted</tt>.
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<P>
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<tt>~<...~:></tt> supports a feature not supported by
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<tt>pprint-logical-block</tt>. If <tt>~:@></tt> is used to terminate the
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directive (that is, <tt>~<...~:@></tt>), then a fill-style
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conditional newline is automatically inserted after each group of blanks
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immediately contained in the body (except for blanks after a
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<tt>~<newline></tt> directive). This makes it easy to achieve the equivalent
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of paragraph filling.
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<P>
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If the atsign modifier is used with <tt>~<...~:></tt>, the
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entire remaining argument list is passed to the directive as its argument.
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All of the remaining arguments are always consumed by
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<tt>~@<...~:></tt>, even if they are not all used by the
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<tt>format</tt> string nested in the directive. Other than the difference in its
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argument, <tt>~@<...~:></tt> is exactly the same as
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<tt>~<...~:></tt>, except that circularity (and sharing) detection
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is not applied if the <tt>~@<...~:></tt> is at top level
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in a <tt>format</tt> string. This ensures that circularity detection is applied
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only to data lists and not to <tt>format</tt> argument lists.
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<P>
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To a considerable extent, the basic form of the directive
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<tt>~<...~></tt> is incompatible with the dynamic control of
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the arrangement of output by <tt>~W</tt>, <tt>~_</tt>,
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<tt>~<...~:></tt>, <tt>~I</tt>, and <tt>~:T</tt>. As
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a result, an error is signaled if any of these directives is nested within
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<tt>~<...~></tt>. Beyond this, an error is also signaled if
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the <tt>~<...~:;...~></tt> form of
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<tt>~<...~></tt> is used in the same <tt>format</tt> string with
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<tt>~W</tt>, <tt>~_</tt>,
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<tt>~<...~:></tt>, <tt>~I</tt>, or <tt>~:T</tt>.
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<P>
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<DT><tt>~I</tt>
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<DD>
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<i>Indent.</i> <tt>~<i>n</i>I</tt> is equivalent to
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<tt>(pprint-indent :block <i>n</i>)</tt>. <tt>~:<i>n</i>I</tt> is equivalent to
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<tt>(pprint-indent :current <i>n</i>)</tt>. In both cases, <i>n</i> defaults to zero
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if it is omitted.
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<P>
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<DT><tt>~:T</tt>
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<DD>
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<i>Tabulate.</i> If the colon modifier is used with the <tt>~T</tt>
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directive, the tabbing computation is done relative to the column where the
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section immediately containing the directive begins, rather than with
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respect to column zero. <tt>~<i>n</i>,<i>m</i>:T</tt> is equivalent to
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<tt>(pprint-tab :section <i>n</i> <i>m</i>)</tt>. <tt>~<i>n</i>,<i>m</i>:@T</tt>
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is equivalent to <tt>(pprint-tab :section-relative <i>n</i> <i>m</i>)</tt>. The numerical
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parameters are both interpreted as being in units of ems and both default
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to 1.
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<P>
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<DT><tt>~/<i>name</i>/</tt>
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<DD>
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<i>Call function.</i> User-defined functions can be called from within a
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<tt>format</tt> string by using the directive <tt>~/<i>name</i>/</tt>. The
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colon modifier, the atsign modifier, and arbitrarily many parameters can be
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specified with the <tt>~/<i>name</i>/</tt> directive. The <i>name</i>
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can be any string that does not contain ``<tt>/</tt>''. All of the characters
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in <i>name</i> are treated as if they were upper case. If <i>name</i>
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contains a ``<tt>:</tt>'' or ``<tt>::</tt>'', then everything up to but not
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including the first ``<tt>:</tt>'' or ``<tt>::</tt>'' is taken to be a string that
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names a package. Everything after the first ``<tt>:</tt>'' or ``<tt>::</tt>'' (if
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any) is taken to be a string that names a symbol. The function
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corresponding to a <tt>~/<i>name</i>/</tt> directive is obtained by
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looking up the symbol that has the indicated name in the indicated package.
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If <i>name</i> does not contain a ``<tt>:</tt>'' or ``<tt>::</tt>'', then the whole
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name string is looked up in the <tt>user</tt> package.
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<P>
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When a <tt>~/<i>name</i>/</tt> directive is encountered, the indicated
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function is called with four or more arguments. The first four arguments
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are the output stream, the <tt>format</tt> argument corresponding to the
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directive, the value <tt>t</tt> if the colon modifier was used (<tt>nil</tt>
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otherwise), and the value <tt>t</tt> if the atsign modifier was used (<tt>nil</tt>
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otherwise). The remaining arguments consist of any parameters specified
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with the directive. The function should print the argument appropriately.
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Any values returned by the function are ignored.
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<P>
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The three functions <tt>pprint-linear</tt>, <tt>pprint-fill</tt>, and
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<tt>pprint-tabular</tt> are designed so that they can be called by
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<tt>~/.../</tt> (that is, <tt>~/pprint-linear/</tt>,
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<tt>~/pprint-fill/</tt>, and <tt>~/pprint-tabular/</tt>. In
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particular they take colon and atsign arguments.
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<P>
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</DL>
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<P>
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As examples of the convenience of specifying pretty printing with
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<tt>format</tt> strings, consider the functions <tt>pprint-defun</tt>
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and <tt>pprint-let</tt> used as
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examples in the last section. They can be more compactly defined as follows. The
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function <tt>pprint-vector</tt> cannot be defined using <tt>format</tt>, because the data
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structure it traverses is not a list. The function <tt>pprint-tabular</tt> is
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inconvenient to define using <tt>format</tt>, because of the need to pass its
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<tt>tabsize</tt> argument through to a <tt>~:T</tt> directive nested within
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an iteration over a list.
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<P><pre>
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(defun pprint-defun (list)
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(format t
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"~:<~W ~@_~:I~W ~:_~W~1I ~_~W~:>"
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list))
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(defun pprint-let (list)
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(format t "~:<~W~^ ~:<~@{~:<~@{~W~^ ~_~}~:>~^ ~:_~}~:>~1I~
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~@{~^ ~_~W~}~:>"
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list))
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</pre><P>
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR> <HR><A NAME=tex2html4776 HREF="node258.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4774 HREF="node253.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4768 HREF="node256.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4778 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4779 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4777 HREF="node258.html"> Compiling Format Control </A>
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<B>Up:</B> <A NAME=tex2html4775 HREF="node253.html"> Pretty Printing</A>
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<B> Previous:</B> <A NAME=tex2html4769 HREF="node256.html"> Dynamic Control of </A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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