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139 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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<TITLE>13.1. Character Attributes</TITLE>
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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=tex2html3226 HREF="node137.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3224 HREF="node135.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3218 HREF="node135.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3228 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3229 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3227 HREF="node137.html"> Predicates on Characters</A>
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<B>Up:</B> <A NAME=tex2html3225 HREF="node135.html"> Characters</A>
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<B> Previous:</B> <A NAME=tex2html3219 HREF="node135.html"> Characters</A>
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<HR> <P>
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<H1><A NAME=SECTION001710000000000000000>13.1 Character Attributes</A></H1>
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<P>
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Every character has three attributes: code, bits, and font.
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The code attribute is intended to distinguish among the printed glyphs
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and formatting functions for characters. The bits attribute allows extra
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flags to be associated with a character. The font attribute permits
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a specification of the style of the glyphs (such as italics).
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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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The treatment of character attributes in Common Lisp has not been
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entirely successful. The font attribute has not been widely used,
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for two reasons. First, a single integer, limited in most
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implementations to 255 at most, is not an adequate, convenient, or portable
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representation for a font. Second, in many applications where font
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information matters it is more convenient or more efficient to represent
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font information as shift codes that apply to many characters, rather than
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attaching font information separately to each character.
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<P>
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As for the bits attribute, it was intended to support
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character input from extended keyboards having extra ``shift'' keys.
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This, in turn, was imagined to support the programming of a portable
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EMACS-like editor in Common Lisp. (The EMACS command set
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is most convenient when the keyboard has separate ``control'' and
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``meta'' keys.) The bits attribute has been used in the implementation
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of such editors and other interactive interfaces. However, software
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that relies crucially on these extended characters will not be portable
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to Common Lisp implementations that do not support them.
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<P>
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X3J13 voted in March 1989 (CHARACTER-PROPOSAL) <A NAME=14447> </A>
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and in June 1989 (MORE-CHARACTER-PROPOSAL) <A NAME=14448> </A>
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to revise considerably the treatment
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of characters in the language. The bits and font attributes are eliminated;
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instead a character may have <i>implementation-defined attributes</i>.
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The treatment of such attributes by existing character-handling functions
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is carefully constrained by certain rules.
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<P>
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Implementations are free to
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continue to support bits and font attributes, but they are
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formally regarded as implementation-defined attributes.
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The rules are generally consistent with the previous
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treatment of the bits and font attributes.
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My guess is that
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the font attribute as currently defined will wither away,
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but the bits attribute as defined by the first edition will
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continue to be supported as a <i>de facto</i> standard extension,
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because it fills a useful small purpose.
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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><b>[Constant]</b><BR>
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<tt>char-code-limit</tt><P>The value of <tt>char-code-limit</tt> is a non-negative
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integer that is the upper exclusive bound on values produced
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by the function <tt>char-code</tt>, which returns the <i>code</i> component
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of a given character; that is, the values returned by <tt>char-code</tt>
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are non-negative and strictly less than the value of
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<tt>char-code-limit</tt>.
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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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Common Lisp does not at present explicitly guarantee that all integers between
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zero and the value of <tt>char-code-limit</tt> are valid character codes, and so
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it is wise in any case for the programmer to assume that the space of
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assigned character codes may be sparse.
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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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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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<BR><b>[Constant]</b><BR>
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<tt>char-font-limit</tt><P>The value of <tt>char-font-limit</tt> is a non-negative
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integer that is the upper exclusive bound on values produced
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by the function <tt>char-font</tt>, which returns the <i>font</i> component
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of a given character; that is, the values returned by <tt>char-font</tt>
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are non-negative and strictly less than the value of
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<tt>char-font-limit</tt>.
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<P>
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<hr>
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<b>Implementation note:</b> No Common Lisp implementation is required to support
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non-zero font attributes; if it does not, then <tt>char-font-limit</tt>
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should be <tt>1</tt>.
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<hr>
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<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
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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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X3J13 voted in March 1989 (CHARACTER-PROPOSAL) <A NAME=14476> </A>
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to eliminate <tt>char-font-limit</tt>.
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<P>
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Experience has shown that numeric codes are not an especially
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convenient, let alone portable, representation for font information.
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A system based on typeface names, type styles, and point sizes would be much better.
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(Macintosh software developers made the same discovery and have recently
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converted to a new font identification scheme.)
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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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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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<BR><b>[Constant]</b><BR>
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<tt>char-bits-limit</tt><P>The value of <tt>char-bits-limit</tt> is a non-negative
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integer that is the upper exclusive bound on values produced
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by the function <tt>char-bits</tt>, which returns the <i>bits</i> component
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of a given character; that is, the values returned by <tt>char-bits</tt>
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are non-negative and strictly less than the value of
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<tt>char-bits-limit</tt>. Note that the value of <tt>char-bits-limit</tt>
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will be a power of 2.
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<P>
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<hr>
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<b>Implementation note:</b> No Common Lisp implementation is required to support
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non-zero bits attributes; if it does not, then <tt>char-bits-limit</tt>
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should be <tt>1</tt>.
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<hr>
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<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
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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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X3J13 voted in March 1989 (CHARACTER-PROPOSAL) <A NAME=14494> </A>
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to eliminate <tt>char-bits-limit</tt>.
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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=tex2html3226 HREF="node137.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3224 HREF="node135.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3218 HREF="node135.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3228 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3229 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3227 HREF="node137.html"> Predicates on Characters</A>
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<B>Up:</B> <A NAME=tex2html3225 HREF="node135.html"> Characters</A>
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<B> Previous:</B> <A NAME=tex2html3219 HREF="node135.html"> Characters</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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