dpans/concept-syntax.tex

%-*- Mode: TeX -*-
%% Character Syntax

%% 2.2.1 1
%% 2.2.1 2
%% 22.1.5 1

The \term{Lisp reader} takes \term{characters} from a \term{stream}, 
interprets them as a printed representation of an \term{object},
constructs that \term{object}, and returns it.

\DefineSection{TheStandardSyntax}
The syntax described by this chapter is called the \newterm{standard syntax}.
Operations are provided by \clisp\ so that
various aspects of the syntax information represented by a \term{readtable} 
can be modified under program control; \seechapter\Reader.
Except as explicitly stated otherwise, 
the syntax used throughout this document is \term{standard syntax}.

\beginSubsection{Readtables}
\DefineSection{Readtables}

Syntax information for use by the \term{Lisp reader} is embodied in an
\term{object} called a \newterm{readtable}.  Among other things, 
the \term{readtable} contains the association between \term{characters} 
and \term{syntax types}.

\Thenextfigure\ lists some \term{defined names} that are applicable to
\term{readtables}.

\displaythree{Readtable defined names}{
*readtable*&readtable-case\cr
copy-readtable&readtablep\cr
get-dispatch-macro-character&set-dispatch-macro-character\cr
get-macro-character&set-macro-character\cr
make-dispatch-macro-character&set-syntax-from-char\cr
}


\beginsubsubsection{The Current Readtable}
\DefineSection{CurrentReadtable}

Several \term{readtables} describing different syntaxes can exist,
but at any given time only one, called the \newterm{current readtable}, 
affects the way in which \term{expressions}\meaning{2} are parsed 
into \term{objects} by the \term{Lisp reader}.
The \term{current readtable} in a given \term{dynamic environment}
is \thevalueof{*readtable*} in that \term{environment}.
To make a different \term{readtable} become the \term{current readtable},
\varref{*readtable*} can be \term{assigned} or \term{bound}.

\endsubsubsection%{The Current Readtable}

\beginsubsubsection{The Standard Readtable}

The \newterm{standard readtable} conforms to \term{standard syntax}.
The consequences are undefined if an attempt is made
to modify the \term{standard readtable}.
%% 22.1.5 3
To achieve the effect of altering or extending \term{standard syntax},
a copy of the \term{standard readtable} can be created; \seefun{copy-readtable}.

The \term{readtable case} of the \term{standard readtable} is \kwd{upcase}.

\endsubsubsection%{The Standard Readtable}

\beginsubsubsection{The Initial Readtable}

The \newterm{initial readtable} is
the \term{readtable} that is the \term{current readtable}
at the time when the \term{Lisp image} starts.
At that time, it conforms to \term{standard syntax}.
The \term{initial readtable} is \term{distinct} 
from the \term{standard readtable}.
It is permissible for a \term{conforming program} 
to modify the \term{initial readtable}.

\endsubsubsection%{The Initial Readtable}

\endSubsection%{Readtables}

\beginsubsection{Variables that affect the Lisp Reader}
\DefineSection{ReaderVars}

The \term{Lisp reader} is influenced not only by the \term{current readtable},
but also by various \term{dynamic variables}.  \Thenextfigure\ lists
the \term{variables} that influence the behavior of the \term{Lisp reader}.

\displaythree{Variables that influence the Lisp reader.}{
*package*&*read-default-float-format*&*readtable*\cr
*read-base*&*read-suppress*&\cr
}

\endsubsection%{Variables that affect the Lisp Reader}

\beginSubsection{Standard Characters}
\DefineSection{StandardChars}

%% Redundant.
% \clisp\ \term{characters} are used for writing programs 
% that can be read by a \clisp\ implementation, and for data.
\issue{CHARACTER-PROPOSAL:2-2-1}
All \term{implementations} must support a \term{character} \term{repertoire}
called \typeref{standard-char}; \term{characters} that are members of that
\term{repertoire} are called \newtermidx{standard characters}{standard character}.

The \typeref{standard-char} \term{repertoire} consists of
the \term{non-graphic} \term{character} \term{newline},
the \term{graphic} \term{character} \term{space},
and the following additional
ninety-four \term{graphic} \term{characters} or their equivalents:
 
\tablefigsix{Standard Character Subrepertoire (Part 1 of 3: Latin Characters)}%
{Graphic ID}{Glyph}{Description}%
{Graphic ID}{Glyph}{Description}{
  LA01  &  \f{a}  &  small a    &
  LN01  &  \f{n}  &  small n    \cr
  LA02  &  \f{A}  &  capital A  &
  LN02  &  \f{N}  &  capital N  \cr
  LB01  &  \f{b}  &  small b    &
  LO01  &  \f{o}  &  small o    \cr
  LB02  &  \f{B}  &  capital B  &
  LO02  &  \f{O}  &  capital O  \cr
  LC01  &  \f{c}  &  small c    &
  LP01  &  \f{p}  &  small p    \cr
  LC02  &  \f{C}  &  capital C  &
  LP02  &  \f{P}  &  capital P  \cr
  LD01  &  \f{d}  &  small d    &
  LQ01  &  \f{q}  &  small q    \cr
  LD02  &  \f{D}  &  capital D  &
  LQ02  &  \f{Q}  &  capital Q  \cr
  LE01  &  \f{e}  &  small e    &
  LR01  &  \f{r}  &  small r    \cr
  LE02  &  \f{E}  &  capital E  &
  LR02  &  \f{R}  &  capital R  \cr
  LF01  &  \f{f}  &  small f    &
  LS01  &  \f{s}  &  small s    \cr
  LF02  &  \f{F}  &  capital F  &
  LS02  &  \f{S}  &  capital S  \cr
  LG01  &  \f{g}  &  small g    &
  LT01  &  \f{t}  &  small t    \cr
  LG02  &  \f{G}  &  capital G  &
  LT02  &  \f{T}  &  capital T  \cr
  LH01  &  \f{h}  &  small h    &
  LU01  &  \f{u}  &  small u    \cr
  LH02  &  \f{H}  &  capital H  &
  LU02  &  \f{U}  &  capital U  \cr
  LI01  &  \f{i}  &  small i    &
  LV01  &  \f{v}  &  small v    \cr
  LI02  &  \f{I}  &  capital I  &
  LV02  &  \f{V}  &  capital V  \cr
  LJ01  &  \f{j}  &  small j    &
  LW01  &  \f{w}  &  small w    \cr
  LJ02  &  \f{J}  &  capital J  &
  LW02  &  \f{W}  &  capital W  \cr
  LK01  &  \f{k}  &  small k    &
  LX01  &  \f{x}  &  small x    \cr
  LK02  &  \f{K}  &  capital K  &
  LX02  &  \f{X}  &  capital X  \cr
  LL01  &  \f{l}  &  small l    &
  LY01  &  \f{y}  &  small y    \cr
  LL02  &  \f{L}  &  capital L  &
  LY02  &  \f{Y}  &  capital Y  \cr
  LM01  &  \f{m}  &  small m    &
  LZ01  &  \f{z}  &  small z    \cr
  LM02  &  \f{M}  &  capital M  &
  LZ02  &  \f{Z}  &  capital Z  \cr
}

\tablefigsix{Standard Character Subrepertoire (Part 2 of 3: Numeric Characters)}%
{Graphic ID}{Glyph}{Description}%
{Graphic ID}{Glyph}{Description}{
  ND01  &  \f{1}  &  digit 1 &
  ND06  &  \f{6}  &  digit 6 \cr
  ND02  &  \f{2}  &  digit 2 &
  ND07  &  \f{7}  &  digit 7 \cr
  ND03  &  \f{3}  &  digit 3 &
  ND08  &  \f{8}  &  digit 8 \cr
  ND04  &  \f{4}  &  digit 4 &
  ND09  &  \f{9}  &  digit 9 \cr
  ND05  &  \f{5}  &  digit 5 &
  ND10  &  \f{0}  &  digit 0 \cr
}

\DefineFigure{StdCharsThree}
\tablefigthree{Standard Character Subrepertoire (Part 3 of 3: Special Characters)}%
{Graphic ID}{Glyph}{Description}{
  SP02  &  \f{!}        &  exclamation mark                        \cr
  SC03  &  \f{\$}       &  dollar sign                             \cr
  SP04  &  \f{"}        &  quotation mark, or double quote         \cr
  SP05  &  \f{'}        &  apostrophe, or \brac{single} quote      \cr
  SP06  &  \f{(}        &  left parenthesis, or open parenthesis   \cr
  SP07  &  \f{)}        &  right parenthesis, or close parenthesis \cr
  SP08  &  \f{,}        &  comma                                   \cr
  SP09  &  \f{_}        &  low line, or underscore                 \cr
  SP10  &  \f{-}        &  hyphen, or minus \brac{sign}            \cr
  SP11  &  \f{.}        &  full stop, period, or dot               \cr
  SP12  &  \f{/}        &  solidus, or slash                       \cr
  SP13  &  \f{:}        &  colon                                   \cr
  SP14  &  \f{;}        &  semicolon                               \cr
  SP15  &  \f{?}        &  question mark                           \cr
  SA01  &  \f{+}        &  plus \brac{sign}                        \cr
  SA03  &  \f{<}        &  less-than \brac{sign}                   \cr
  SA04  &  \f{=}        &  equals \brac{sign}                      \cr
  SA05  &  \f{>}        &  greater-than \brac{sign}                \cr
  SM01  &  \f{\#}       &  number sign, or sharp\brac{sign}        \cr
  SM02  &  \f{\%}       &  percent \brac{sign}                     \cr
  SM03  &  \f{\&}       &  ampersand			           \cr
  SM04  &  \f{*}        &  asterisk, or star                       \cr
  SM05  &  \f{@}        &  commercial at, or at-sign               \cr
  SM06  &  \f{[}        &  left \brac{square} bracket              \cr
  SM07  &  \f{\\}       &  reverse solidus, or backslash           \cr
  SM08  &  \f{]}        &  right \brac{square} bracket             \cr
  SM11  &  \f{\{}       &  left curly bracket, or left brace       \cr
  SM13  &  \f{|}        &  vertical bar                            \cr
  SM14  &  \f{\}}       &  right curly bracket, or right brace     \cr
  SD13  &  \f{`}        &  grave accent, or backquote              \cr
  SD15  &  \f{\hat}     &  circumflex accent                       \cr
  SD19  &  \f{~}        &  tilde                                   \cr
}

The graphic IDs are not used within \clisp,
but are provided for cross reference purposes with {\ISOChars}.
Note that the first letter of the graphic ID 
categorizes the character as follows:
L---Latin, N---Numeric, S---Special.

\endSubsection%{Standard Characters}

\endissue{CHARACTER-PROPOSAL:2-2-1}

\beginSubsection{Character Syntax Types}
\DefineSection{CharacterSyntaxTypes}

%% 22.1.1 1

The \term{Lisp reader} constructs an \term{object} 
from the input text by interpreting each \term{character} 
according to its \term{syntax type}.
The \term{Lisp reader} cannot accept as input 
everything that the \term{Lisp printer} produces,
and the \term{Lisp reader} has features that are not used by the \term{Lisp printer}.
The \term{Lisp reader} can be used as a lexical analyzer 
for a more general user-written parser.

%% 22.1.1 2
%% 22.1.1 3
When the \term{Lisp reader} is invoked, it reads a single character from 
the \term{input} \term{stream} and dispatches according to the
\newterm{syntax type} of that \term{character}.
Every \term{character} that can appear in the \term{input} \term{stream}
is of one of the \term{syntax types} shown in \figref\PossibleSyntaxTypes.

\DefineFigure{PossibleSyntaxTypes}
\showthree{Possible Character Syntax Types}{
\term{constituent}&\term{macro character}&\term{single escape}\cr
\term{invalid}&\term{multiple escape}&\term{whitespace}\meaning{2}\cr
}

The \term{syntax type} of a \term{character} in a \term{readtable}
determines how that character is interpreted by the \term{Lisp reader}
while that \term{readtable} is the \term{current readtable}.
At any given time, every character has exactly one \term{syntax type}.

%% 22.1.1 4
%% 2.3.0 5
\Figref\CharSyntaxTypesInStdSyntax\ 
lists the \term{syntax type} of each \term{character} in \term{standard syntax}.

\DefineFigure{CharSyntaxTypesInStdSyntax}
%% 22.1.1 36
{\def\w{\term{whitespace}\meaning{2}}
\def\n{\term{non-terminating} \term{macro char}} %cheat on "char" => "character" to make fit
\def\t{\term{terminating} \term{macro char}}     %ditto
\def\c{\term{constituent}}
\def\C{\term{constituent}*}
\def\SE{\term{single escape}}
\def\ME{\term{multiple escape}}
\tablefigfour{Character Syntax Types in Standard Syntax}{character}{syntax type}{character}{syntax type}{
Backspace&\c&0--9&\c\cr
Tab&\w&:&\c\cr
Newline&\w&;&\t\cr
Linefeed&\w&{\tt<}&\c\cr
Page&\w&=&\c\cr
Return&\w&{\tt>}&\c\cr
Space&\w&?&\C\cr
!&\C&{\tt @}&\c\cr
{\tt"}&\t&A--Z&\c\cr
\#&\n&\f{[}&\C\cr
\$&\c&\f{\\}&\SE\cr
\%&\c&\f{]}&\C\cr
\&&\c&\hat&\c\cr
'&\t&\f{\_}&\c\cr
(&\t&`&\t\cr
)&\t&a--z&\c\cr
{\tt*}&\c&\f{\{}&\C\cr
+&\c&\f{|}&\ME\cr
,&\t&\f{\}}&\C\cr
-&\c&\f{~}&\c\cr
.&\c&Rubout&\c\cr
/&\c\cr
}}

%% The next two paragraphs were mutually redundant, so I collapsed them into one. -kmp 17-Jan-92
The characters marked with an asterisk (*) are initially \term{constituents},
% but are reserved to the user for use as \term{macro characters} or for any other
% desired purpose.
%                     
% Brackets (\f{[} and \f{]}),
% braces   (\f{\{}, \f{\}}),
% question mark (\f{?}),
% and exclamation point (\f{!})
% are defined to be constituents, 
but they are not used in any standard \clisp\ notations.
%!!! Maybe make a glossary term out of "reserved to the programmer"
%    and discuss the issue of who the "programmer" is in the face of
%    cascaded/layered/embedded implementations, some of which are conforming
%    and some of which are not.
These characters are explicitly reserved to the \term{programmer}.
\f{~} is not used in \clisp, and reserved to implementors.
\f{\$} and \f{\%} are \term{alphabetic}\meaning{2} \term{characters},
but are not used in the names of any standard \clisp\ \term{defined names}.

\term{Whitespace}\meaning{2} characters serve as separators but are otherwise
ignored.  \term{Constituent} and \term{escape} \term{characters} are accumulated
to make a \term{token}, which is then interpreted as a \term{number} or \term{symbol}.
\term{Macro characters} trigger the invocation of \term{functions} (possibly
user-supplied) that can perform arbitrary parsing actions.
\term{Macro characters} are divided into two kinds,
\term{terminating} and \term{non-terminating},
depending on whether or not they terminate a \term{token}.
The following are descriptions of each kind of \term{syntax type}.

\beginsubsubsection{Constituent Characters}
\DefineSection{ConstituentChars}

\term{Constituent} \term{characters} are used in \term{tokens}.
%% Sandra didn't like this wording, and wanted "representation of" added. -kmp 19-Nov-91
%A \term{token} is a \term{number} or a \term{symbol} name.
A \newterm{token} is a representation of a \term{number} or a \term{symbol}.  
Examples of \term{constituent} \term{characters} are letters and digits.

%% 2.3.0 6
Letters in symbol names are sometimes converted to 
letters in the opposite \term{case} when the name is read;
\seesection\ReadtableCaseReadEffect.
\term{Case} conversion can be suppressed by the use 
of \term{single escape} or \term{multiple escape} characters.
%% Moved to the section "Case in Symbols" -kmp 25-Jan-92
% The \term{symbols} that correspond to \clisp\ \term{defined names}
% have uppercase names even though their names generally appear
% in lowercase in this document.

\beginsubsubsection{Constituent Traits}
\DefineSection{ConstituentTraits}

Every \term{character} has one or more \term{constituent traits}
that define how the \term{character} is to be interpreted by the \term{Lisp reader}
when the \term{character} is a \term{constituent} \term{character}.
These \term{constituent traits} are 
     \term{alphabetic}\meaning{2},                  
     digit,
     \term{package marker},
     plus sign,
     minus sign, 
     dot,
     decimal point,
     \term{ratio marker},
     \term{exponent marker},
 and \term{invalid}.
\Figref\ConstituentTraitsOfStdChars\ shows the \term{constituent traits}
of the \term{standard characters}
and of certain \term{semi-standard} \term{characters};
% I added this next to clarify.  It effectively says the same thing in SET-SYNTAX-FROM-CHAR.
% -kmp 28-Jan-92
no mechanism is provided for changing the \term{constituent trait} of a \term{character}.
Any \term{character} with the alphadigit \term{constituent trait}
in that figure is a digit if the \term{current input base} is greater
than that character's digit value,
otherwise the \term{character} is \term{alphabetic}\meaning{2}.  
%\term{Alphabetic}\meaning{2} \term{constituents} are valid
%\term{characters} for \term{symbol} \term{names}.  
Any \term{character} quoted by a \term{single escape} 
is treated as an \term{alphabetic}\meaning{2} constituent, regardless of its normal syntax.

\DefineFigure{ConstituentTraitsOfStdChars}
\boxfig
{\dimen0=.75pc
\def\a{\term{alphabetic}\meaning{2}}
\def\ad{alphadigit}
\def\i{\term{invalid}}
\def\pm{\term{package marker}}
\tabskip \dimen0
\halign to \hsize {#\hfil\tabskip \dimen0&#\hfil\tabskip 0pt plus 1fil
&#\hfil\tabskip \dimen0&#\hfil\cr
\noalign{\vskip -9pt}
\bf constituent&\bf traits&\bf constituent&\bf traits\cr
\bf character&&\bf character\cr
\noalign{\vskip 2pt\hrule\vskip 2pt}
Backspace&\i&\f{\{}&\a\cr
Tab&\i*&\f{\}}&\a\cr
Newline&\i*&+&\a, plus sign\cr
Linefeed&\i*&-&\a, minus sign\cr
Page&\i*&.&\a, dot, decimal point\cr
Return&\i*&/&\a, \term{ratio marker}\cr
Space&\i*&A, a&\ad\cr
! &\a&B, b&\ad\cr
{\tt "}&\a*&C, c&\ad\cr
\#&\a*&D, d&\ad, double-float \term{exponent marker}\cr
\$&\a&E, e&\ad, float \term{exponent marker}\cr
\%&\a&F, f&\ad, single-float \term{exponent marker}\cr
\&&\a&G, g&\ad\cr
'&\a*&H, h&\ad\cr
(&\a*&I, i&\ad\cr
)&\a*&J, j&\ad\cr
{\tt *}&\a&K, k&\ad\cr
,&\a*&L, l&\ad, long-float \term{exponent marker}\cr
0-9&\ad&M, m&\ad\cr
:&\pm&N, n&\ad\cr
;&\a*&O, o&\ad\cr
{\tt<}&\a&P, p&\ad\cr
=&\a&Q, q&\ad\cr
{\tt>}&\a&R, r&\ad\cr
?&\a&S, s&\ad, short-float \term{exponent marker}\cr
\f{@}&\a&T, t&\ad\cr
\f{[}&\a&U, u&\ad\cr
\f{\\}&\a*&V, v&\ad\cr
\f{]}&\a&W, w&\ad\cr
\hat&\a&X, x&\ad\cr
\f{\_}&\a&Y, y&\ad\cr
`&\a*&Z, z&\ad\cr
\f{|}&\a*&Rubout&\i\cr
\f{~}&\a\cr
\noalign{\vskip -9pt}
}}
\caption{Constituent Traits of Standard Characters and Semi-Standard Characters}
\endfig
                   
The interpretations in this table apply only to \term{characters}
whose \term{syntax type} is \term{constituent}.
Entries marked with an asterisk (*) are normally \term{shadowed}\meaning{2} 
because the indicated \term{characters} are of \term{syntax type}
\term{whitespace}\meaning{2},
\term{macro character},
\term{single escape},
or \term{multiple escape};
these \term{constituent traits} apply to them only if their \term{syntax types} 
are changed to \term{constituent}.

\endsubsubsection%{Constituent Traits}

\endsubsubsection%{Constituent Characters}
 
\beginsubsubsection{Invalid Characters}
\DefineSection{InvalidChars}

\term{Characters} with the \term{constituent trait} \term{invalid} 
cannot ever appear in a \term{token} 
except under the control of a \term{single escape} \term{character}.
If an \term{invalid} \term{character} is encountered while an \term{object} is
being read, an error \oftype{reader-error} is signaled.
If an \term{invalid} \term{character} is preceded by a \term{single escape} \term{character},
it is treated as an \term{alphabetic}\meaning{2} \term{constituent} instead.

\endsubsubsection%{Invalid Characters}

\beginsubsubsection{Macro Characters}
\DefineSection{MacroChars}

When the \term{Lisp reader} encounters a \term{macro character} 
on an \term{input} \term{stream},
special parsing of subsequent \term{characters} 
on the \term{input} \term{stream} 
is performed.

A \term{macro character} has an associated \term{function}
called a \newterm{reader macro function} that implements its specialized parsing behavior.
An association of this kind can be established or modified under control of
a \term{conforming program} by using 
\thefunctions{set-macro-character} and \funref{set-dispatch-macro-character}.

Upon encountering a \term{macro character}, the \term{Lisp reader} calls its
\term{reader macro function}, which parses one specially formatted object 
from the \term{input} \term{stream}.
The \term{function} either returns the parsed \term{object},
or else it returns no \term{values} 
    to indicate that the characters scanned by the \term{function}
    are being ignored (\eg in the case of a comment).
Examples of \term{macro characters}
are \term{backquote}, \term{single-quote}, \term{left-parenthesis}, and 
\term{right-parenthesis}.

A \term{macro character} is either \term{terminating} or \term{non-terminating}.
The difference between \term{terminating} and \term{non-terminating} \term{macro characters} 
lies in what happens when such characters occur in the middle of a \term{token}.  
If a \newterm{non-terminating} \term{macro character} occurs in the middle of a \term{token},
the \term{function} associated 
with the \term{non-terminating} \term{macro character} is not called,
and the
\term{non-terminating} \term{macro character} does not terminate the \term{token}'s name; it
becomes part of the name as if the \term{macro character} were really a constituent
character.  A \newterm{terminating} \term{macro character} terminates any \term{token},
and its associated \term{reader macro function}
is called no matter where the \term{character} appears.
The only \term{non-terminating} \term{macro character} in \term{standard syntax} 
is \term{sharpsign}.

If a \term{character} is a \term{dispatching macro character} $C\sub 1$,
its \term{reader macro function} is a \term{function} supplied by the \term{implementation}.
This \term{function} reads decimal \term{digit} \term{characters} until a non-\term{digit}
$C\sub 2$ is read.
If any \term{digits} were read,
they are converted into a corresponding \term{integer} infix parameter $P$;
otherwise, the infix parameter $P$ is \nil.  
The terminating non-\term{digit} $C\sub 2$ is a \term{character} 
(sometimes called a ``sub-character'' to emphasize its subordinate role in the dispatching)
that is looked up in the dispatch table associated with
the \term{dispatching macro character} $C\sub 1$.
The \term{reader macro function} associated with the sub-character $C\sub 2$ 
is invoked with three arguments:
     the \term{stream},
     the sub-character $C\sub 2$,
 and the infix parameter $P$.
For more information about dispatch characters,
\seefun{set-dispatch-macro-character}.

For information about the \term{macro characters} 
that are available in \term{standard syntax},
\seesection\StandardMacroChars.

\endsubsubsection%{Macro Characters}

\beginsubsubsection{Multiple Escape Characters}
\DefineSection{MultipleEscapeChar}

A pair of \newterm{multiple escape} \term{characters}
is used to indicate that an enclosed sequence of characters,
including possible \term{macro characters} and \term{whitespace}\meaning{2} \term{characters},
are to be treated as \term{alphabetic}\meaning{2} \term{characters} 
with \term{case} preserved.
Any \term{single escape} and \term{multiple escape} \term{characters} 
that are to appear in the sequence must be preceded by a \term{single escape} 
\term{character}.  

\term{Vertical-bar} is a \term{multiple escape} \term{character}
in \term{standard syntax}.

\beginsubsubsubsection{Examples of Multiple Escape Characters}

\code
 ;; The following examples assume the readtable case of *readtable* 
 ;; and *print-case* are both :upcase.
 (eq 'abc 'ABC) \EV \term{true}
 (eq 'abc '|ABC|) \EV \term{true}
 (eq 'abc 'a|B|c) \EV \term{true}
 (eq 'abc '|abc|) \EV \term{false}
\endcode

\endsubsubsubsection%{Examples of Multiple Escape Characters}

\endsubsubsection%{Multiple Escape Characters}

\beginsubsubsection{Single Escape Character}
\DefineSection{SingleEscapeChar}

A \newterm{single escape} is used to indicate that 
the next \term{character} is to be treated as 
an \term{alphabetic}\meaning{2} \term{character}
with its \term{case} preserved,
no matter what the \term{character} is 
or which \term{constituent traits} it has.  

%% "slash" => "backslash" as an editorial change per Boyer/Kaufmann/Moore #4.
%% This was right everywhere else in the manual--shame for it to be wrong here.
%% -kmp 9-May-94
\term{Backslash} is a \term{single escape} \term{character} in \term{standard syntax}.

\beginsubsubsubsection{Examples of Single Escape Characters}

\code
 ;; The following examples assume the readtable case of *readtable* 
 ;; and *print-case* are both :upcase.
 (eq 'abc '\\A\\B\\C) \EV \term{true}
 (eq 'abc 'a\\Bc) \EV \term{true}
 (eq 'abc '\\ABC) \EV \term{true}
 (eq 'abc '\\abc) \EV \term{false}
\endcode

\endsubsubsubsection%{Examples of Single Escape Characters}

\endsubsubsection%{Single Escape Character}

\beginsubsubsection{Whitespace Characters}
\DefineSection{WhitespaceChars}

\term{Whitespace}\meaning{2} \term{characters} are used to separate \term{tokens}.

\term{Space} and \term{newline} are \term{whitespace}\meaning{2} \term{characters}
in \term{standard syntax}.

\beginsubsubsubsection{Examples of Whitespace Characters}

\code
 (length '(this-that)) \EV 1
 (length '(this - that)) \EV 3
 (length '(a
           b)) \EV 2
 (+ 34) \EV 34
 (+ 3 4) \EV 7
\endcode

\endsubsubsubsection%{Examples of Whitespace Characters}

\endsubsubsection%{Whitespace Characters}

\endSubsection%{Character Syntax Types}