102 lines
5.7 KiB
HTML
102 lines
5.7 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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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>2.2.2. Line Divisions</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Line Divisions">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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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=tex2html1796 HREF="node24.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html1794 HREF="node21.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html1788 HREF="node22.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html1798 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html1799 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html1797 HREF="node24.html"> Non-standard Characters</A>
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<B>Up:</B> <A NAME=tex2html1795 HREF="node21.html"> Characters</A>
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<B> Previous:</B> <A NAME=tex2html1789 HREF="node22.html"> Standard Characters</A>
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<HR> <P>
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<H2><A NAME=SECTION00622000000000000000>2.2.2. Line Divisions</A></H2>
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<P>
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The treatment of line divisions is one of the most difficult issues
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in designing portable software, simply because there is so little agreement
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among operating systems. Some use a single character to delimit lines;
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the recommended ASCII character for this purpose is the line feed character
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LF (also called the new line character, NL),
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but some systems use the carriage
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return character CR. Much more common is the two-character sequence
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CR followed by LF. Frequently line divisions have no representation
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as a character but are implicit in the structuring of a file into records,
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each record containing a line of text. A deck of punched cards has this
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structure, for example.
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<P>
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Common Lisp provides an abstract interface by requiring that there be a single
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character, <tt>#\Newline</tt>, that within the language serves as a line
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delimiter. (The language C has a similar requirement.)
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An implementation of Common Lisp must translate between this internal
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single-character representation and whatever external representation(s)
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may be used.
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<P>
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<hr>
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<b>Implementation note:</b> How the character called <tt>#\Newline</tt> is represented
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internally is not specified here, but it is strongly suggested that
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the ASCII LF character be used in Common Lisp implementations that use the
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ASCII character encoding. The ASCII CR character is a workable,
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but in most cases inferior, alternative.
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<hr>
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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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When the first edition was written it was not yet clear that UNIX would
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become so widely accepted. The decision to represent
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the line delimiter as a single character has proved to be a good one.
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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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The requirement that a line division be represented as a single character
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has certain consequences. A character string
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written in the middle of a program in such a way as to span more than
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one line must contain exactly one character to represent each line division.
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Consider this code fragment:
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<P><pre>
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(setq a-string "This string
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contains
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forty-two characters.")
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</pre><P>
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Between <tt>g</tt> and <tt>c</tt> there must be exactly one character,
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<tt>#\Newline</tt>; a two-character sequence, such as <tt>#\Return</tt> and then
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<tt>#\Newline</tt>, is not acceptable, nor is the absence of a character.
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The same is true between <tt>s</tt> and <tt>f</tt>.
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<P>
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When the character <tt>#\Newline</tt> is written to an output file,
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the Common Lisp implementation must take the appropriate action
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to produce a line division. This might involve writing out a
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record or translating <tt>#\Newline</tt> to a CR/LF sequence.
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<P>
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<hr>
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<b>Implementation note:</b> If an implementation uses the ASCII character encoding,
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uses the CR/LF sequence externally to delimit lines,
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uses LF to represent <tt>#\Newline</tt> internally, and supports <tt>#\Return</tt>
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as a data object corresponding to the ASCII character CR, the
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question arises as to what action to take when the program
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writes out <tt>#\Return</tt> followed by <tt>#\Newline</tt>.
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It should first be noted that <tt>#\Return</tt> is not a standard Common Lisp
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character, and the action to be taken when <tt>#\Return</tt> is written out
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is therefore not defined by the Common Lisp language. A plausible approach
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is to buffer the <tt>#\Return</tt> character and suppress it if and only if the
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next character is <tt>#\Newline</tt> (the net effect is to generate a CR/LF
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sequence).
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Another plausible
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approach is simply to ignore
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the difficulty and declare that writing <tt>#\Return</tt> and then
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<tt>#\Newline</tt> results in the sequence CR/CR/LF in the output.
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<hr>
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<P>
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<BR> <HR><A NAME=tex2html1796 HREF="node24.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html1794 HREF="node21.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html1788 HREF="node22.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html1798 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html1799 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html1797 HREF="node24.html"> Non-standard Characters</A>
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<B>Up:</B> <A NAME=tex2html1795 HREF="node21.html"> Characters</A>
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<B> Previous:</B> <A NAME=tex2html1789 HREF="node22.html"> Standard 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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</ADDRESS>
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</BODY>
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