dotfiles/gcl-info/chap-2.texi

@node Syntax, Evaluation and Compilation, Introduction (Introduction), Top
@chapter Syntax

@menu
* Character Syntax::		
* Reader Algorithm::		
* Interpretation of Tokens::	
* Standard Macro Characters::	
@end menu

@node Character Syntax, Reader Algorithm, Syntax, Syntax
@section Character Syntax

@c including concept-syntax

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

The syntax described by this chapter is called the @i{standard syntax}
@IGindex{standard syntax}
.
Operations are provided by @r{Common Lisp} so that
various aspects of the syntax information represented by a @i{readtable} 
can be modified under program control; see @ref{Reader}.
Except as explicitly stated otherwise, 
the syntax used throughout this document is @i{standard syntax}.

@menu
* Readtables::			
* Variables that affect the Lisp Reader::  
* Standard Characters::		
* Character Syntax Types::	
@end menu

@node Readtables, Variables that affect the Lisp Reader, Character Syntax, Character Syntax
@subsection Readtables

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

Figure 2--1 lists some @i{defined names} that are applicable to
@i{readtables}.

@group
@noindent
@w{  *readtable*                    readtable-case                }
@w{  copy-readtable                 readtablep                    }
@w{  get-dispatch-macro-character   set-dispatch-macro-character  }
@w{  get-macro-character            set-macro-character           }
@w{  make-dispatch-macro-character  set-syntax-from-char          }

@noindent
@w{              Figure 2--1: Readtable defined names             }

@end group

@menu
* The Current Readtable::	
* The Standard Readtable::	
* The Initial Readtable::	
@end menu

@node The Current Readtable, The Standard Readtable, Readtables, Readtables
@subsubsection The Current Readtable

Several @i{readtables} describing different syntaxes can exist,
but at any given time only one, called the @i{current readtable}
@IGindex{current readtable}
, 
affects the way in which @i{expressions}_2 are parsed 
into @i{objects} by the @i{Lisp reader}.
The @i{current readtable} in a given @i{dynamic environment}
is the @i{value} of @b{*readtable*} in that @i{environment}.
To make a different @i{readtable} become the @i{current readtable},
@b{*readtable*} can be @i{assigned} or @i{bound}.

@node The Standard Readtable, The Initial Readtable, The Current Readtable, Readtables
@subsubsection The Standard Readtable

The @i{standard readtable}
@IGindex{standard readtable}
 conforms to @i{standard syntax}.
The consequences are undefined if an attempt is made
to modify the @i{standard readtable}.
To achieve the effect of altering or extending @i{standard syntax},
a copy of the @i{standard readtable} can be created; see the @i{function} @b{copy-readtable}.

The @i{readtable case} of the @i{standard readtable} is @t{:upcase}.

@node The Initial Readtable,  , The Standard Readtable, Readtables
@subsubsection The Initial Readtable

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

@node Variables that affect the Lisp Reader, Standard Characters, Readtables, Character Syntax
@subsection Variables that affect the Lisp Reader

The @i{Lisp reader} is influenced not only by the @i{current readtable},
but also by various @i{dynamic variables}.  Figure 2--2 lists
the @i{variables} that influence the behavior of the @i{Lisp reader}.

@group
@noindent
@w{  *package*    *read-default-float-format*  *readtable*  }
@w{  *read-base*  *read-suppress*                           }

@noindent
@w{  Figure 2--2: Variables that influence the Lisp reader. }

@end group

@node Standard Characters, Character Syntax Types, Variables that affect the Lisp Reader, Character Syntax
@subsection Standard Characters

All @i{implementations} must support a @i{character} @i{repertoire}
called @b{standard-char}; @i{characters} that are members of that
@i{repertoire} are called @i{standard characters}
@IGindex{standard character}
.

The @b{standard-char} @i{repertoire} consists of
the @i{non-graphic} @i{character} @i{newline},
the @i{graphic} @i{character} @i{space},
and the following additional
ninety-four @i{graphic} @i{characters} or their equivalents:

@group
@noindent
@w{  Graphic ID  Glyph  Description  Graphic ID  Glyph  Description  }
@w{  LA01        @t{a}      small a      LN01        @t{n}      small n      }
@w{  LA02        @t{A}      capital A    LN02        @t{N}      capital N    }
@w{  LB01        @t{b}      small b      LO01        @t{o}      small o      }
@w{  LB02        @t{B}      capital B    LO02        @t{O}      capital O    }
@w{  LC01        @t{c}      small c      LP01        @t{p}      small p      }
@w{  LC02        @t{C}      capital C    LP02        @t{P}      capital P    }
@w{  LD01        @t{d}      small d      LQ01        @t{q}      small q      }
@w{  LD02        @t{D}      capital D    LQ02        @t{Q}      capital Q    }
@w{  LE01        @t{e}      small e      LR01        @t{r}      small r      }
@w{  LE02        @t{E}      capital E    LR02        @t{R}      capital R    }
@w{  LF01        @t{f}      small f      LS01        @t{s}      small s      }
@w{  LF02        @t{F}      capital F    LS02        @t{S}      capital S    }
@w{  LG01        @t{g}      small g      LT01        @t{t}      small t      }
@w{  LG02        @t{G}      capital G    LT02        @t{T}      capital T    }
@w{  LH01        @t{h}      small h      LU01        @t{u}      small u      }
@w{  LH02        @t{H}      capital H    LU02        @t{U}      capital U    }
@w{  LI01        @t{i}      small i      LV01        @t{v}      small v      }
@w{  LI02        @t{I}      capital I    LV02        @t{V}      capital V    }
@w{  LJ01        @t{j}      small j      LW01        @t{w}      small w      }
@w{  LJ02        @t{J}      capital J    LW02        @t{W}      capital W    }
@w{  LK01        @t{k}      small k      LX01        @t{x}      small x      }
@w{  LK02        @t{K}      capital K    LX02        @t{X}      capital X    }
@w{  LL01        @t{l}      small l      LY01        @t{y}      small y      }
@w{  LL02        @t{L}      capital L    LY02        @t{Y}      capital Y    }
@w{  LM01        @t{m}      small m      LZ01        @t{z}      small z      }
@w{  LM02        @t{M}      capital M    LZ02        @t{Z}      capital Z    }

@noindent
@w{  Figure 2--3: Standard Character Subrepertoire (Part 1 of 3: Latin Characters)}

@end group

@group
@noindent
@w{  Graphic ID  Glyph  Description  Graphic ID  Glyph  Description  }
@w{  ND01        @t{1}      digit 1      ND06        @t{6}      digit 6      }
@w{  ND02        @t{2}      digit 2      ND07        @t{7}      digit 7      }
@w{  ND03        @t{3}      digit 3      ND08        @t{8}      digit 8      }
@w{  ND04        @t{4}      digit 4      ND09        @t{9}      digit 9      }
@w{  ND05        @t{5}      digit 5      ND10        @t{0}      digit 0      }

@noindent
@w{  Figure 2--4: Standard Character Subrepertoire (Part 2 of 3: Numeric Characters)}

@end group

@group
@noindent
@w{  Graphic ID  Glyph  Description                              }
@w{  SP02        @t{!}      exclamation mark                         }
@w{  SC03        @t{$}     dollar sign                              }
@w{  SP04        @t{"}      quotation mark, or double quote          }
@w{  SP05        @t{'}      apostrophe, or @r{[}single@r{]} quote            }
@w{  SP06        @t{(}      left parenthesis, or open parenthesis    }
@w{  SP07        @t{)}      right parenthesis, or close parenthesis  }
@w{  SP08        @t{,}      comma                                    }
@w{  SP09        @t{_}      low line, or underscore                  }
@w{  SP10        @t{-}      hyphen, or minus @r{[}sign@r{]}                  }
@w{  SP11        @t{.}      full stop, period, or dot                }
@w{  SP12        @t{/}      solidus, or slash                        }
@w{  SP13        @t{:}      colon                                    }
@w{  SP14        @t{;}      semicolon                                }
@w{  SP15        @t{?}      question mark                            }
@w{  SA01        @t{+}      plus @r{[}sign@r{]}                              }
@w{  SA03        @t{<}      less-than @r{[}sign@r{]}                         }
@w{  SA04        @t{=}      equals @r{[}sign@r{]}                            }
@w{  SA05        @t{>}      greater-than @r{[}sign@r{]}                      }
@w{  SM01        @t{#}      number sign, or sharp@r{[}sign@r{]}              }
@w{  SM02        @t{%}      percent @r{[}sign@r{]}                           }
@w{  SM03        @t{&}      ampersand                                }
@w{  SM04        @t{*}      asterisk, or star                        }
@w{  SM05        @t{@@}      commercial at, or at-sign                }
@w{  SM06        @t{[}      left @r{[}square@r{]} bracket                    }
@w{  SM07        @t{\}      reverse solidus, or backslash            }
@w{  SM08        @t{]}      right @r{[}square@r{]} bracket                   }
@w{  SM11        @t{@{}      left curly bracket, or left brace        }
@w{  SM13        @t{|}      vertical bar                             }
@w{  SM14        @t{@}}      right curly bracket, or right brace      }
@w{  SD13        @t{`}      grave accent, or backquote               }
@w{  SD15        @t{@t{^}}      circumflex accent                        }
@w{  SD19        @t{~}      tilde                                    }

@noindent
@w{  Figure 2--5: Standard Character Subrepertoire (Part 3 of 3: Special Characters)}

@end group

The graphic IDs are not used within @r{Common Lisp},
but are provided for cross reference purposes with {@r{ISO 6937/2}}.
Note that the first letter of the graphic ID 
categorizes the character as follows:
L---Latin, N---Numeric, S---Special.

@node Character Syntax Types,  , Standard Characters, Character Syntax
@subsection Character Syntax Types

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

When the @i{Lisp reader} is invoked, it reads a single character from 
the @i{input} @i{stream} and dispatches according to the
@i{syntax type}
@IGindex{syntax type}
 of that @i{character}.
Every @i{character} that can appear in the @i{input} @i{stream}
is of one of the @i{syntax types} shown in @i{Figure~2--6}.

@group
@noindent
@w{  @i{constituent}  @i{macro character}  @i{single escape}  }
@w{  @i{invalid}      @i{multiple escape}  @i{whitespace}_2   }

@noindent
@w{  Figure 2--6: Possible Character Syntax Types }

@end group

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

@i{Figure~2--7} 
lists the @i{syntax type} of each @i{character} in @i{standard syntax}.

{

@group
@noindent
@w{  character  syntax type                 character  syntax type             }
@w{  Backspace  @i{constituent}                 0--9       @i{constituent}             }
@w{  Tab        @i{whitespace}_2                :          @i{constituent}             }
@w{  Newline    @i{whitespace}_2                ;          @i{terminating} @i{macro char}  }
@w{  Linefeed   @i{whitespace}_2                @t{<}          @i{constituent}             }
@w{  Page       @i{whitespace}_2                =          @i{constituent}             }
@w{  Return     @i{whitespace}_2                @t{>}          @i{constituent}             }
@w{  Space      @i{whitespace}_2                ?          @i{constituent}*            }
@w{  !          @i{constituent}*                @t{@@}          @i{constituent}             }
@w{  @t{"}          @i{terminating} @i{macro char}      A--Z       @i{constituent}             }
@w{  #          @i{non-terminating} @i{macro char}  @t{[}          @i{constituent}*            }
@w{  $         @i{constituent}                 @t{\}          @i{single escape}           }
@w{  %          @i{constituent}                 @t{]}          @i{constituent}*            }
@w{  &          @i{constituent}                 @t{^}          @i{constituent}             }
@w{  '          @i{terminating} @i{macro char}      @t{_}          @i{constituent}             }
@w{  (          @i{terminating} @i{macro char}      `          @i{terminating} @i{macro char}  }
@w{  )          @i{terminating} @i{macro char}      a--z       @i{constituent}             }
@w{  @t{*}          @i{constituent}                 @t{@{}          @i{constituent}*            }
@w{  +          @i{constituent}                 @t{|}          @i{multiple escape}         }
@w{  ,          @i{terminating} @i{macro char}      @t{@}}          @i{constituent}*            }
@w{  -          @i{constituent}                 @t{~}          @i{constituent}             }
@w{  .          @i{constituent}                 Rubout     @i{constituent}             }
@w{  /          @i{constituent}                 }

@noindent
@w{            Figure 2--7: Character Syntax Types in Standard Syntax          }

@end group
}

The characters marked with an asterisk (*) are initially @i{constituents},
but they are not used in any standard @r{Common Lisp} notations.
These characters are explicitly reserved to the @i{programmer}.
@t{~} is not used in @r{Common Lisp}, and reserved to implementors.
@t{$} and @t{%} are @i{alphabetic}_2 @i{characters},
but are not used in the names of any standard @r{Common Lisp} @i{defined names}.

@i{Whitespace}_2 characters serve as separators but are otherwise
ignored.  @i{Constituent} and @i{escape} @i{characters} are accumulated
to make a @i{token}, which is then interpreted as a @i{number} or @i{symbol}.
@i{Macro characters} trigger the invocation of @i{functions} (possibly
user-supplied) that can perform arbitrary parsing actions.
@i{Macro characters} are divided into two kinds,
@i{terminating} and @i{non-terminating},
depending on whether or not they terminate a @i{token}.
The following are descriptions of each kind of @i{syntax type}.

@menu
* Constituent Characters::	
* Constituent Traits::		
* Invalid Characters::		
* Macro Characters::		
* Multiple Escape Characters::	
* Examples of Multiple Escape Characters::  
* Single Escape Character::	
* Examples of Single Escape Characters::  
* Whitespace Characters::	
* Examples of Whitespace Characters::  
@end menu

@node Constituent Characters, Constituent Traits, Character Syntax Types, Character Syntax Types
@subsubsection Constituent Characters

@i{Constituent} @i{characters} are used in @i{tokens}.
A @i{token}
@IGindex{token}
 is a representation of a @i{number} or a @i{symbol}.  
Examples of @i{constituent} @i{characters} are letters and digits.

Letters in symbol names are sometimes converted to 
letters in the opposite @i{case} when the name is read;
see @ref{Effect of Readtable Case on the Lisp Reader}.
@i{Case} conversion can be suppressed by the use 
of @i{single escape} or @i{multiple escape} characters.

@node Constituent Traits, Invalid Characters, Constituent Characters, Character Syntax Types
@subsubsection Constituent Traits

Every @i{character} has one or more @i{constituent traits}
that define how the @i{character} is to be interpreted by the @i{Lisp reader}
when the @i{character} is a @i{constituent} @i{character}.
These @i{constituent traits} are 
     @i{alphabetic}_2,                  
     digit,
     @i{package marker},
     plus sign,
     minus sign, 
     dot,
     decimal point,
     @i{ratio marker},
     @i{exponent marker},
 and @i{invalid}.
@i{Figure~2--8} shows the @i{constituent traits}
of the @i{standard characters}
and of certain @i{semi-standard} @i{characters};
no mechanism is provided for changing the @i{constituent trait} of a @i{character}.
Any @i{character} with the alphadigit @i{constituent trait}
in that figure is a digit if the @i{current input base} is greater
than that character's digit value,
otherwise the @i{character} is @i{alphabetic}_2.  
Any @i{character} quoted by a @i{single escape} 
is treated as an @i{alphabetic}_2 constituent, regardless of its normal syntax.

@group
@noindent
@w{ constituent traits         constituent traits                                   }
@w{ character                  character   }
@w{ ________________________________________________________________________________}
@w{ Backspace   @i{invalid}        @t{@{}           @i{alphabetic}_2                             }
@w{ Tab         @i{invalid}*       @t{@}}           @i{alphabetic}_2                             }
@w{ Newline     @i{invalid}*       +           @i{alphabetic}_2, plus sign                  }
@w{ Linefeed    @i{invalid}*       -           @i{alphabetic}_2, minus sign                 }
@w{ Page        @i{invalid}*       .           @i{alphabetic}_2, dot, decimal point         }
@w{ Return      @i{invalid}*       /           @i{alphabetic}_2, @i{ratio marker}               }
@w{ Space       @i{invalid}*       A, a        alphadigit                               }
@w{ !           @i{alphabetic}_2   B, b        alphadigit                               }
@w{ @t{"}           @i{alphabetic}_2*  C, c        alphadigit                               }
@w{ #           @i{alphabetic}_2*  D, d        alphadigit, double-float @i{exponent marker} }
@w{ $          @i{alphabetic}_2   E, e        alphadigit, float @i{exponent marker}        }
@w{ %           @i{alphabetic}_2   F, f        alphadigit, single-float @i{exponent marker} }
@w{ &           @i{alphabetic}_2   G, g        alphadigit                               }
@w{ '           @i{alphabetic}_2*  H, h        alphadigit                               }
@w{ (           @i{alphabetic}_2*  I, i        alphadigit                               }
@w{ )           @i{alphabetic}_2*  J, j        alphadigit                               }
@w{ @t{*}           @i{alphabetic}_2   K, k        alphadigit                               }
@w{ ,           @i{alphabetic}_2*  L, l        alphadigit, long-float @i{exponent marker}   }
@w{ 0-9         alphadigit     M, m        alphadigit                               }
@w{ :           @i{package marker} N, n        alphadigit                               }
@w{ ;           @i{alphabetic}_2*  O, o        alphadigit                               }
@w{ @t{<}           @i{alphabetic}_2   P, p        alphadigit                               }
@w{ =           @i{alphabetic}_2   Q, q        alphadigit                               }
@w{ @t{>}           @i{alphabetic}_2   R, r        alphadigit                               }
@w{ ?           @i{alphabetic}_2   S, s        alphadigit, short-float @i{exponent marker}  }
@w{ @t{@@}           @i{alphabetic}_2   T, t        alphadigit                               }
@w{ @t{[}           @i{alphabetic}_2   U, u        alphadigit                               }
@w{ @t{\}           @i{alphabetic}_2*  V, v        alphadigit                               }
@w{ @t{]}           @i{alphabetic}_2   W, w        alphadigit                               }
@w{ @t{^}           @i{alphabetic}_2   X, x        alphadigit                               }
@w{ @t{_}           @i{alphabetic}_2   Y, y        alphadigit                               }
@w{ `           @i{alphabetic}_2*  Z, z        alphadigit                               }
@w{ @t{|}           @i{alphabetic}_2*  Rubout      @i{invalid}                                  }
@w{ @t{~}           @i{alphabetic}_2   }
@end group

@w{  Figure 2--8: Constituent Traits of Standard Characters and Semi-Standard Characters}

The interpretations in this table apply only to @i{characters}
whose @i{syntax type} is @i{constituent}.
Entries marked with an asterisk (*) are normally @i{shadowed}_2 
because the indicated @i{characters} are of @i{syntax type}
@i{whitespace}_2,
@i{macro character},
@i{single escape},
or @i{multiple escape};
these @i{constituent traits} apply to them only if their @i{syntax types} 
are changed to @i{constituent}.

@node Invalid Characters, Macro Characters, Constituent Traits, Character Syntax Types
@subsubsection Invalid Characters

@i{Characters} with the @i{constituent trait} @i{invalid} 
cannot ever appear in a @i{token} 
except under the control of a @i{single escape} @i{character}.
If an @i{invalid} @i{character} is encountered while an @i{object} is
being read, an error of @i{type} @b{reader-error} is signaled.
If an @i{invalid} @i{character} is preceded by a @i{single escape} @i{character},
it is treated as an @i{alphabetic}_2 @i{constituent} instead.

@node Macro Characters, Multiple Escape Characters, Invalid Characters, Character Syntax Types
@subsubsection Macro Characters

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

A @i{macro character} has an associated @i{function}
called a @i{reader macro function}
@IGindex{reader macro function}
 that implements its specialized parsing behavior.
An association of this kind can be established or modified under control of
a @i{conforming program} by using 
the @i{functions} @b{set-macro-character} and @b{set-dispatch-macro-character}.

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

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

If a @i{character} is a @i{dispatching macro character} C_1,
its @i{reader macro function} is a @i{function} supplied by the @i{implementation}.
This @i{function} reads decimal @i{digit} @i{characters} until a non-@i{digit}
C_2 is read.
If any @i{digits} were read,
they are converted into a corresponding @i{integer} infix parameter P;
otherwise, the infix parameter P is @b{nil}.  
The terminating non-@i{digit} C_2 is a @i{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 @i{dispatching macro character} C_1.
The @i{reader macro function} associated with the sub-character C_2 
is invoked with three arguments:
     the @i{stream},
     the sub-character C_2,
 and the infix parameter P.
For more information about dispatch characters,
see the @i{function} @b{set-dispatch-macro-character}.

For information about the @i{macro characters} 
that are available in @i{standard syntax},
see @ref{Standard Macro Characters}.

@node Multiple Escape Characters, Examples of Multiple Escape Characters, Macro Characters, Character Syntax Types
@subsubsection Multiple Escape Characters

A pair of @i{multiple escape}
@IGindex{multiple escape}
 @i{characters}
is used to indicate that an enclosed sequence of characters,
including possible @i{macro characters} and @i{whitespace}_2 @i{characters},
are to be treated as @i{alphabetic}_2 @i{characters} 
with @i{case} preserved.
Any @i{single escape} and @i{multiple escape} @i{characters} 
that are to appear in the sequence must be preceded by a @i{single escape} 
@i{character}.  

@i{Vertical-bar} is a @i{multiple escape} @i{character}
in @i{standard syntax}.

@node Examples of Multiple Escape Characters, Single Escape Character, Multiple Escape Characters, Character Syntax Types
@subsubsection Examples of Multiple Escape Characters

@example
 ;; The following examples assume the readtable case of *readtable* 
 ;; and *print-case* are both :upcase.
 (eq 'abc 'ABC) @result{}  @i{true}
 (eq 'abc '|ABC|) @result{}  @i{true}
 (eq 'abc 'a|B|c) @result{}  @i{true}
 (eq 'abc '|abc|) @result{}  @i{false}
@end example

@node Single Escape Character, Examples of Single Escape Characters, Examples of Multiple Escape Characters, Character Syntax Types
@subsubsection Single Escape Character

A @i{single escape}
@IGindex{single escape}
 is used to indicate that 
the next @i{character} is to be treated as 
an @i{alphabetic}_2 @i{character}
with its @i{case} preserved,
no matter what the @i{character} is 
or which @i{constituent traits} it has.  

@i{Slash} is a @i{single escape} @i{character}
in @i{standard syntax}.

@node Examples of Single Escape Characters, Whitespace Characters, Single Escape Character, Character Syntax Types
@subsubsection Examples of Single Escape Characters

@example
 ;; The following examples assume the readtable case of *readtable* 
 ;; and *print-case* are both :upcase.
 (eq 'abc '\A\B\C) @result{}  @i{true}
 (eq 'abc 'a\Bc) @result{}  @i{true}
 (eq 'abc '\ABC) @result{}  @i{true}
 (eq 'abc '\abc) @result{}  @i{false}
@end example

@node Whitespace Characters, Examples of Whitespace Characters, Examples of Single Escape Characters, Character Syntax Types
@subsubsection Whitespace Characters

@i{Whitespace}_2 @i{characters} are used to separate @i{tokens}.

@i{Space} and @i{newline} are @i{whitespace}_2 @i{characters}
in @i{standard syntax}.

@node Examples of Whitespace Characters,  , Whitespace Characters, Character Syntax Types
@subsubsection Examples of Whitespace Characters

@example
 (length '(this-that)) @result{}  1
 (length '(this - that)) @result{}  3
 (length '(a
           b)) @result{}  2
 (+ 34) @result{}  34
 (+ 3 4) @result{}  7
@end example

@c end of including concept-syntax

@node Reader Algorithm, Interpretation of Tokens, Character Syntax, Syntax
@section Reader Algorithm

@c including concept-reader-algorithm

This section describes the algorithm used by the @i{Lisp reader}
to parse @i{objects} from an @i{input} @i{character} @i{stream},
including how the @i{Lisp reader} processes @i{macro characters}.

When dealing with @i{tokens}, the reader's basic function is to distinguish
representations of @i{symbols} from those of @i{numbers}.
When a @i{token} is accumulated, it is assumed to represent a @i{number} if it
satisfies the syntax for numbers listed in @i{Figure~2--9}.
If it does not represent a @i{number},
it is then assumed to be a @i{potential number} 
if it satisfies the rules governing the syntax for a @i{potential number}.
If a valid @i{token} is neither a representation of a @i{number} 
			       nor a @i{potential number},
it represents a @i{symbol}.

The algorithm performed by the @i{Lisp reader} is as follows:

@table @asis

@item 1.  
If at end of file, end-of-file processing is performed as specified
in @b{read}.
Otherwise,
one @i{character}, @i{x},  is read from the @i{input} @i{stream}, and
dispatched according to the @i{syntax type} of @i{x} to one
of steps 2 to 7.

@item 2.  
If @i{x} is an @i{invalid} @i{character},
an error of @i{type} @b{reader-error} is signaled.

@item 3.  
If @i{x} is a @i{whitespace}_2 @i{character},
then it is discarded and step 1 is re-entered.

@item 4.  
If @i{x} is a @i{terminating} or @i{non-terminating} @i{macro character}
then its associated @i{reader macro function} is called with two @i{arguments},
the @i{input} @i{stream} and @i{x}.

The @i{reader macro function} may read @i{characters} 
from the @i{input} @i{stream}; 
if it does, it will see those @i{characters} following the @i{macro character}.
The @i{Lisp reader} may be invoked recursively from the @i{reader macro function}.

The @i{reader macro function} must not have any side effects other than on the
@i{input} @i{stream};
because of backtracking and restarting of the @b{read} operation,
front ends to the @i{Lisp reader} (@i{e.g.}, ``editors'' and ``rubout handlers'') 
may cause the @i{reader macro function} to be called repeatedly during the
reading of a single @i{expression} in which @i{x} only appears once.

The @i{reader macro function} may return zero values or one value.
If one value is returned,
then that value is returned as the result of the read operation;
the algorithm is done.
If zero values are returned, then step 1 is re-entered.

@item 5.  
If @i{x} is a @i{single escape} @i{character}
then the next @i{character}, @i{y}, is read, or an error of @i{type} @b{end-of-file} 
is signaled if at the end of file.
@i{y} is treated as if it is a @i{constituent} 
whose only @i{constituent trait} is @i{alphabetic}_2.
@i{y} is used to begin a @i{token}, and step 8 is entered.

@item 6.  
If @i{x} is a @i{multiple escape} @i{character}
then a @i{token} (initially
containing no @i{characters}) is  begun and step 9 is entered.

@item 7.  
If @i{x} is a @i{constituent} @i{character}, then it begins a @i{token}.
After the @i{token} is read in, it will be interpreted
either as a @r{Lisp} @i{object} or as being of invalid syntax.
If the @i{token} represents an @i{object},
that @i{object} is returned as the result of the read operation.
If the @i{token} is of invalid syntax, an error is signaled.
If @i{x} is a @i{character} with @i{case},
it might be replaced with the corresponding @i{character} of the opposite @i{case}, 
depending on the @i{readtable case} of the @i{current readtable},
as outlined in @ref{Effect of Readtable Case on the Lisp Reader}.
@i{X} is used to begin a @i{token}, and step 8 is entered.

@item 8.  
At this point a @i{token} is being accumulated, and an even number
of @i{multiple escape} @i{characters} have been encountered.
If at end of file, step 10 is entered.
Otherwise, a @i{character}, @i{y}, is read, and
one of the following actions is performed according to its @i{syntax type}:

@table @asis

@item @t{*}  
If @i{y} is a @i{constituent} or @i{non-terminating} @i{macro character}:
@table @asis

@item --  
If @i{y} is a @i{character} with @i{case},
it might be replaced with the corresponding @i{character} of the opposite @i{case}, 
depending on the @i{readtable case} of the @i{current readtable},
as outlined in @ref{Effect of Readtable Case on the Lisp Reader}.
@item --  
@i{Y} is appended to the @i{token} being built.
@item --  
Step 8 is repeated.
@end table

@item @t{*}  
If @i{y} is a @i{single escape} @i{character}, then the next @i{character},
@i{z}, is read, or an error of @i{type} @b{end-of-file} is signaled if at end of file.
@i{Z} is treated as if it is a @i{constituent} 
whose only @i{constituent trait} is @i{alphabetic}_2.
@i{Z} is appended to the @i{token} being built,
and step 8 is repeated.

@item @t{*}  
If @i{y} is a @i{multiple escape} @i{character},
then step 9 is entered.

@item @t{*}  
If @i{y} is an @i{invalid} @i{character},
an error of @i{type} @b{reader-error} is signaled.

@item @t{*}  
If @i{y} is a @i{terminating} @i{macro character},
then it terminates the @i{token}.
First the @i{character} @i{y} is unread (see @b{unread-char}),
and then step 10 is entered.

@item @t{*}  
If @i{y} is a @i{whitespace}_2 @i{character}, then it terminates
the @i{token}.  First the @i{character} @i{y} is unread
if appropriate (see @b{read-preserving-whitespace}),
and then step 10 is entered.
@end table

@item 9.  
At this point a @i{token} is being accumulated, and an odd number
of @i{multiple escape} @i{characters} have been encountered.
If at end of file, an error of @i{type} @b{end-of-file} is signaled.
Otherwise, a @i{character}, @i{y}, is read, and
one of the following actions is performed according to its @i{syntax type}:

@table @asis

@item @t{*}  
If @i{y} is a @i{constituent}, macro, or @i{whitespace}_2 @i{character},
@i{y} is treated as a @i{constituent} 
whose only @i{constituent trait} is @i{alphabetic}_2.             
@i{Y} is appended to the @i{token} being built, and step 9 is repeated.

@item @t{*}  
If @i{y} is a @i{single escape} @i{character}, then the next @i{character},
@i{z}, is read, or an error of @i{type} @b{end-of-file} is signaled if at end of file.
@i{Z} is treated as a @i{constituent}
whose only @i{constituent trait} is @i{alphabetic}_2.
@i{Z} is appended to the @i{token} being built,
and step 9 is repeated.

@item @t{*}  
If @i{y} is a @i{multiple escape} @i{character},
then step 8 is entered.

@item @t{*}  
If @i{y} is an @i{invalid} @i{character},
an error of @i{type} @b{reader-error} is signaled.
@end table

@item 10.  
An entire @i{token} has been accumulated.
The @i{object} represented by the @i{token} is returned 
as the result of the read operation,
or an error of @i{type} @b{reader-error} is signaled if the @i{token} is not of valid syntax.
@end table

@c end of including concept-reader-algorithm

@node Interpretation of Tokens, Standard Macro Characters, Reader Algorithm, Syntax
@section Interpretation of Tokens

@c including concept-tokens

@menu
* Numbers as Tokens::		
* Constructing Numbers from Tokens::  
* The Consing Dot::		
* Symbols as Tokens::		
* Valid Patterns for Tokens::	
* Package System Consistency Rules::  
@end menu

@node Numbers as Tokens, Constructing Numbers from Tokens, Interpretation of Tokens, Interpretation of Tokens
@subsection Numbers as Tokens

When a @i{token} is read,
it is interpreted as a @i{number} or @i{symbol}.
The @i{token} is interpreted as a @i{number} if it satisfies
the syntax for numbers specified in Figure 2--9.

@group
@noindent
@w{ @i{numeric-token} ::= !@i{integer} | !@i{ratio} | !@i{float}                                              }
@w{ @i{integer}       ::= @t{[}@i{sign}@t{]} @{@i{decimal-digit}@}^+ @i{decimal-point} | @t{[}@i{sign}@t{]} @{@i{digit}@}^+               }
@w{ @i{ratio}         ::= @t{[}@i{sign}@t{]} @{@i{digit}@}^+ @i{slash} @{@i{digit}@}^+                                        }
@w{ @i{float}         ::= @t{[}@i{sign}@t{]} @{@i{decimal-digit}@}{*} @i{decimal-point} @{@i{decimal-digit}@}^+ @t{[}!@i{exponent}@t{]}   }
@w{                   | @t{[}@i{sign}@t{]} @{@i{decimal-digit}@}^+ @t{[}@i{decimal-point} @{@i{decimal-digit}@}{*}@t{]} !@i{exponent} }
@w{ @i{exponent}      ::= @i{exponent-marker} @t{[}@i{sign}@t{]} @{@i{digit}@}^+                                        }
@w{ @i{sign}---a @i{sign}.}
@w{ @i{slash}---a @i{slash}}
@w{ @i{decimal-point}---a @i{dot}.}
@w{ @i{exponent-marker}---an @i{exponent marker}.}
@w{ @i{decimal-digit}---a @i{digit} in @i{radix} @t{10}.}
@w{ @i{digit}---a @i{digit} in the @i{current input radix}.}
@end group

@w{  Figure 2--9: Syntax for Numeric Tokens}

@menu
* Potential Numbers as Tokens::	 
* Escape Characters and Potential Numbers::  
* Examples of Potential Numbers::  
@end menu

@node Potential Numbers as Tokens, Escape Characters and Potential Numbers, Numbers as Tokens, Numbers as Tokens
@subsubsection Potential Numbers as Tokens

To allow implementors and future @r{Common Lisp} standards
to extend the syntax of numbers, a
syntax for @i{potential numbers} is defined that is
more general than the syntax for numbers.
A @i{token} is a @i{potential number} if it satisfies all of the following
requirements:

@table @asis

@item 1.  
The @i{token} consists entirely of 
  @i{digits},
  @i{signs},
  @i{ratio markers},
  decimal points (@t{.}),
  extension characters (@t{^} or @t{_}),
  and number markers.
A number marker is a letter. 
Whether a letter may be treated as a number marker depends on context,
but no letter that is adjacent to another letter may ever be treated as a number marker.
@i{Exponent markers} are number markers.

@item 2.  
The @i{token} contains at least one digit.  Letters may be considered to be
digits, depending on the @i{current input base}, but only
in @i{tokens} containing no decimal points.

@item 3.  
The @i{token} begins with a @i{digit}, @i{sign}, decimal point, or extension character,

[Reviewer Note by Barmar: This section is unnecessary because the first bullet already 
		  omits discussion of a colon (@i{package marker}).]
but not a 
@i{package marker}.
The syntax involving a leading 
@i{package marker} followed by a @i{potential number} is
not well-defined. The consequences of the use 
of notation such as @t{:1}, @t{:1/2}, and @t{:2{@t{^}}3} in a
position where an expression appropriate for @b{read} 
is expected are unspecified.

@item 4.  
The @i{token} does not end with a sign.
@end table

If a @i{potential number} has number syntax, 
a @i{number} of the appropriate type is constructed and returned, 
if the @i{number} is representable in an implementation.
A @i{number} will not be representable in an implementation 
if it is outside the boundaries set by the @i{implementation-dependent} 
constants for @i{numbers}.
For example, specifying too large or too small an exponent for a @i{float}
may make the @i{number} impossible to represent in the implementation.
A @i{ratio} with denominator zero (such as @t{-35/000})
is not represented in any implementation.
When a @i{token} with the syntax of a number cannot be converted to an internal
@i{number}, an error of @i{type} @b{reader-error} is signaled.  An error
must not be signaled for specifying too many significant digits
for a @i{float}; a truncated or rounded value should be produced.

If there is an ambiguity as to whether
a letter should be treated as a digit or as a number marker,
the letter is treated as a digit.

@node Escape Characters and Potential Numbers, Examples of Potential Numbers, Potential Numbers as Tokens, Numbers as Tokens
@subsubsection Escape Characters and Potential Numbers

A @i{potential number} cannot contain any @i{escape}
@i{characters}.  An @i{escape} @i{character} robs the following
@i{character} of all syntactic qualities, forcing it to be strictly
@i{alphabetic}_2 and therefore unsuitable for use in a
@i{potential number}.  For example, all of the following
representations are interpreted as @i{symbols}, not @i{numbers}:

@example
 \256   25\64   1.0\E6   |100|   3\.14159   |3/4|   3\/4   5||
@end example

In each case, removing the @i{escape} @i{character} (or @i{characters}) 
would 
cause the token to be a @i{potential number}.

@node Examples of Potential Numbers,  , Escape Characters and Potential Numbers, Numbers as Tokens
@subsubsection Examples of Potential Numbers

As examples, the @i{tokens} in Figure 2--10 are @i{potential numbers},
but they are not actually numbers, and so are reserved @i{tokens};
a @i{conforming implementation} is permitted, but not required,
to define their meaning.

@group
@noindent
@w{  @t{1b5000}                       @t{777777q}                @t{1.7J}  @t{-3/4+6.7J}  @t{12/25/83}  }
@w{  @t{27{@t{^}}19}                      @t{3{@t{^}}4/5}                @t{6//7}  @t{3.1.2.6}    @t{{@t{^}}-43@t{^}}   }
@w{  @t{3.141_592_653_589_793_238_4}  @t{-3.7+2.6i-6.17j+19.6k}  }

@noindent
@w{                     Figure 2--10: Examples of reserved tokens                   }

@end group

The @i{tokens} in Figure 2--11 are not @i{potential numbers}; 
they are always treated as @i{symbols}:

@group
@noindent
@w{  @t{/}     @t{/5}     @t{+}  @t{1+}  @t{1-}     }
@w{  @t{foo+}  @t{ab.cd}  @t{_}  @t{@t{^}}   @t{{@t{^}}/-}  }

@noindent
@w{  Figure 2--11: Examples of symbols}

@end group

The @i{tokens} in Figure 2--12 are @i{potential numbers}
if the @i{current input base} is @t{16}, 
but they are always treated as @i{symbols} if the @i{current input base} is @t{10}. 

@group
@noindent
@w{  @t{bad-face}  @t{25-dec-83}  @t{a/b}  @t{fad_cafe}  @t{f{@t{^}}}  }

@noindent
@w{  Figure 2--12: Examples of symbols or potential numbers}

@end group

@node Constructing Numbers from Tokens, The Consing Dot, Numbers as Tokens, Interpretation of Tokens
@subsection Constructing Numbers from Tokens

A @i{real} is constructed directly from a corresponding numeric @i{token};
see @i{Figure~2--9}.

A @i{complex} is notated as a @t{#C} (or @t{#c}) followed by a @i{list}
of two @i{reals}; see @ref{Sharpsign C}.

The @i{reader macros} @t{#B}, @t{#O}, @t{#X}, and @t{#R} may also be useful
in controlling the input @i{radix} in which @i{rationals} are parsed;
 see @ref{Sharpsign B},
     @ref{Sharpsign O},
     @ref{Sharpsign X},
 and @ref{Sharpsign R}.

This section summarizes the full syntax for @i{numbers}.

@menu
* Syntax of a Rational::	
* Syntax of an Integer::	
* Syntax of a Ratio::		
* Syntax of a Float::		
* Syntax of a Complex::		
@end menu

@node Syntax of a Rational, Syntax of an Integer, Constructing Numbers from Tokens, Constructing Numbers from Tokens
@subsubsection Syntax of a Rational

@node Syntax of an Integer, Syntax of a Ratio, Syntax of a Rational, Constructing Numbers from Tokens
@subsubsection Syntax of an Integer

@i{Integers} can be written as a sequence of @i{digits}, 
optionally preceded by a @i{sign} and optionally followed by a decimal point;
see @i{Figure~2--9}.
When a decimal point is used,
the @i{digits} are taken to be in @i{radix} @t{10};
when no decimal point is used,
the @i{digits} are taken to be in radix given by the @i{current input base}.

For information on how @i{integers} are printed, see @ref{Printing Integers}.

@node Syntax of a Ratio, Syntax of a Float, Syntax of an Integer, Constructing Numbers from Tokens
@subsubsection Syntax of a Ratio

@i{Ratios} can be written as an optional @i{sign} followed by two
non-empty sequences of @i{digits} separated by a @i{slash};
see @i{Figure~2--9}.
The second sequence may not consist
entirely of zeros.
Examples of @i{ratios} are in Figure 2--13.

@group
@noindent
@w{  @t{2/3}                 ;This is in canonical form                  }
@w{  @t{4/6}                 ;A non-canonical form for 2/3               }
@w{  @t{-17/23}              ;A ratio preceded by a sign                 }
@w{  @t{-30517578125/32768}  ;This is (-5/2)^{15}                        }
@w{  @t{10/5}                ;The canonical form for this is @t{2}           }
@w{  @t{#o-101/75}           ;Octal notation for -65/61                  }
@w{  @t{#3r120/21}           ;Ternary notation for 15/7                  }
@w{  @t{#Xbc/ad}             ;Hexadecimal notation for 188/173           }
@w{  @t{#xFADED/FACADE}      ;Hexadecimal notation for 1027565/16435934  }

@noindent
@w{                  Figure 2--13: Examples of Ratios                }

@end group

[Reviewer Note by Barmar: #o, #3r, #X, and #x mentioned above 
		  are not in the syntax rules defined just above that.]

For information on how @i{ratios} are printed,
see @ref{Printing Ratios}.

@node Syntax of a Float, Syntax of a Complex, Syntax of a Ratio, Constructing Numbers from Tokens
@subsubsection Syntax of a Float

@i{Floats} can be written in either decimal fraction or computerized
scientific notation: an optional sign, then a non-empty sequence of digits
with an embedded decimal point,
then an optional decimal exponent specification.
If there is no exponent specifier, then
the decimal point is required, and there must be digits
after it.
The exponent specifier consists of an @i{exponent marker},
an optional sign, and a non-empty sequence of digits.
If no exponent specifier is present, or if the @i{exponent marker} @t{e}
(or @t{E}) is used, then
the format specified
by @b{*read-default-float-format*} is used.
See @i{Figure~2--9}.

An implementation may provide one or more kinds of @i{float}
that collectively make up the @i{type} @b{float}.
The letters @t{s}, @t{f}, @t{d}, and @t{l} (or their
respective uppercase equivalents) explicitly specify the
use of the @i{types} @b{short-float}, @b{single-float}, 
@b{double-float}, and @b{long-float}, respectively.

The internal format used for an external representation depends only
on the @i{exponent marker}, and not on the number of decimal digits
in the external representation.

Figure 2--14 contains examples of notations for @i{floats}: 

@group
@noindent
@w{  @t{0.0}       ;Floating-point zero in default format                          }
@w{  @t{0E0}       ;As input, this is also floating-point zero in default format.  }
@w{            ;As output, this would appear as @t{0.0}.                           }
@w{  @t{0e0}       ;As input, this is also floating-point zero in default format.  }
@w{            ;As output, this would appear as @t{0.0}.                           }
@w{  @t{-.0}       ;As input, this might be a zero or a minus zero,                }
@w{            ; depending on whether the implementation supports              }
@w{            ; a distinct minus zero.                                        }
@w{            ;As output, @t{0.0} is zero and @t{-0.0} is minus zero.                 }
@w{  @t{0.}        ;On input, the integer zero---@i{not} a floating-point number!      }
@w{            ;Whether this appears as @t{0} or @t{0.} on output depends              }
@w{            ;on the @i{value} of @b{*print-radix*}.                                 }
@w{  @t{0.0s0}     ;A floating-point zero in short format                          }
@w{  @t{0s0}       ;As input, this is a floating-point zero in short format.       }
@w{            ;As output, such a zero would appear as @t{0.0s0}                   }
@w{            ; (or as @t{0.0} if @b{short-float} was the default format).            }
@w{  @t{6.02E+23}  ;Avogadro's number, in default format                           }
@w{  @t{602E+21}   ;Also Avogadro's number, in default format                      }

@noindent
@w{               Figure 2--14: Examples of Floating-point numbers             }

@end group

For information on how @i{floats} are printed,
see @ref{Printing Floats}.

@node Syntax of a Complex,  , Syntax of a Float, Constructing Numbers from Tokens
@subsubsection Syntax of a Complex

A @i{complex} has a Cartesian structure, 
with a real part and an imaginary part each of which is a 

@i{real}.

The parts of a @i{complex} are not necessarily @i{floats} 
but both parts must be of the same @i{type}: 

[Editorial Note by KMP: This is not the same as saying they must be the same type.
                 Maybe we mean they are of the same `precision' or `format'?
		 GLS had suggestions which are not yet merged.]
either both are @i{rationals}, or both are of the same @i{float} @i{subtype}.
When constructing a @i{complex}, if the specified parts are not the
same @i{type}, the parts are converted to be the same @i{type}
internally (@i{i.e.}, the @i{rational} part is converted to a @i{float}). 
An @i{object} of type @t{(complex rational)} is converted internally
and represented thereafter as a @i{rational} if its imaginary part is an 
@i{integer} whose value is 0.

For further information, see @ref{Sharpsign C} and @ref{Printing Complexes}.

@node The Consing Dot, Symbols as Tokens, Constructing Numbers from Tokens, Interpretation of Tokens
@subsection The Consing Dot

If a @i{token} consists solely of dots (with no escape characters),
then an error of @i{type} @b{reader-error} is signaled,
except in one circumstance:
if the @i{token} is a single @i{dot}
and appears in a situation where @i{dotted pair} notation permits a @i{dot},
then it is accepted as part of such syntax and no error is signaled. 
See @ref{Left-Parenthesis}.

@node Symbols as Tokens, Valid Patterns for Tokens, The Consing Dot, Interpretation of Tokens
@subsection Symbols as Tokens

Any @i{token} that is not a @i{potential number},
does not contain a @i{package marker},
and does not consist entirely of dots
will always be interpreted as a @i{symbol}.
Any @i{token} that is a @i{potential number} but does not fit the       
number syntax is a reserved @i{token} and
has an @i{implementation-dependent} interpretation.
In all other cases, the @i{token} is construed to be the name of a @i{symbol}.

Examples of the printed representation of @i{symbols} are in Figure 2--15. 
For presentational simplicity,
these examples assume that
the @i{readtable case} of the @i{current readtable} is @t{:upcase}.

@group
@noindent
@w{  @t{FROBBOZ}         The @i{symbol} whose @i{name} is @t{FROBBOZ}.                }
@w{  @t{frobboz}         Another way to notate the same @i{symbol}.           }
@w{  @t{fRObBoz}         Yet another way to notate it.                    }
@w{  @t{unwind-protect}  A @i{symbol} with a hyphen in its @i{name}.              }
@w{  @t{+$}             The @i{symbol} named @t{+$}.                            }
@w{  @t{1+}              The @i{symbol} named @t{1+}.                             }
@w{  @t{+1}              This is the @i{integer} @t{1}, not a @i{symbol}.             }
@w{  @t{pascal_style}    This @i{symbol} has an underscore in its @i{name}.       }
@w{  @t{file.rel.43}     This @i{symbol} has periods in its @i{name}.             }
@w{  @t{\(}              The @i{symbol} whose @i{name} is @t{(}.                      }
@w{  @t{\+1}             The @i{symbol} whose @i{name} is @t{+1}.                     }
@w{  @t{+\1}             Also the @i{symbol} whose @i{name} is @t{+1}.                }
@w{  @t{\frobboz}        The @i{symbol} whose @i{name} is @t{fROBBOZ}.                }
@w{  @t{3.14159265\s0}   The @i{symbol} whose @i{name} is @t{3.14159265s0}.           }
@w{  @t{3.14159265\S0}   A different @i{symbol}, whose @i{name} is @t{3.14159265S0}.  }
@w{  @t{3.14159265s0}    A possible @i{short float} approximation to \pi.     }

@noindent
@w{  Figure 2--15: Examples of the printed representation of symbols (Part 1 of 2)}

@end group

@group
@noindent
@w{  @t{APL\\360}               The @i{symbol} whose @i{name} is @t{APL\360}.       }
@w{  @t{apl\\360}               Also the @i{symbol} whose @i{name} is @t{APL\360}.  }
@w{  @t{\(b{@t{^}}2\)\ -\ 4*a@t{*c}}    The @i{name} is @t{(B{@t{^}}2) - 4*A*C}.            }
@w{                         Parentheses and two spaces in it.       }
@w{  @t{\(\b{@t{^}}2\)\ -\4*\a*\c}  The @i{name} is @t{(b{@t{^}}2) - 4*a*c}.            }
@w{                         Letters explicitly lowercase.           }
@w{  @t{|"|}                    The same as writing @t{\"}.                 }
@w{  @t{|(b{@t{^}}2) - 4*a*c|}      The @i{name} is @t{(b{@t{^}}2) - 4*a*c}.            }
@w{  @t{|frobboz|}              The @i{name} is @t{frobboz}, not @t{FROBBOZ}.       }
@w{  @t{|APL\360|}              The @i{name} is @t{APL360}.                     }
@w{  @t{|APL\\360|}             The @i{name} is @t{APL\360}.                    }
@w{  @t{|apl\\360|}             The @i{name} is @t{apl\360}.                    }
@w{  @t{|\|\||}                 Same as @t{\|\|} ---the @i{name} is @t{||}.          }
@w{  @t{|(B{@t{^}}2) - 4*A*C|}      The @i{name} is @t{(B{@t{^}}2) - 4*A*C}.            }
@w{                         Parentheses and two spaces in it.       }
@w{  @t{|(b{@t{^}}2) - 4*a*c|}      The @i{name} is @t{(b{@t{^}}2) - 4*a*c}.            }

@noindent
@w{  Figure 2--16: Examples of the printed representation of symbols (Part 2 of 2)}

@end group

In the process of parsing a @i{symbol},
it is @i{implementation-dependent} which
@i{implementation-defined} @i{attributes} are removed
from the @i{characters} forming a @i{token} that represents a @i{symbol}.

When parsing the syntax for a @i{symbol},
the @i{Lisp reader} looks up the @i{name} of that @i{symbol} 
in the @i{current package}.
This lookup may involve looking in other 
@i{packages} whose @i{external symbols}
are inherited by the @i{current package}.  If the name is found,
the corresponding @i{symbol} is returned.  If the name is not found
(that is, there is no @i{symbol} 
of that name @i{accessible} in the @i{current package}),
a new @i{symbol} is created and is placed in the @i{current package}
as an @i{internal symbol}.  The @i{current package} becomes the owner
(@i{home package}) of the @i{symbol}, 
and the @i{symbol} becomes interned in the @i{current package}.
If the name is later read again while this same @i{package} is
current, the same @i{symbol} will be found and returned.

@node Valid Patterns for Tokens, Package System Consistency Rules, Symbols as Tokens, Interpretation of Tokens
@subsection Valid Patterns for Tokens

The valid patterns for @i{tokens} are summarized in Figure 2--17. 

@group
@noindent
@w{  @t{@i{nnnnn}}              a @i{number}                                           }
@w{  @t{@i{xxxxx}}              a @i{symbol} in the @i{current package}                    }
@w{  @t{:@i{xxxxx}}             a @i{symbol} in the the @t{KEYWORD} @i{package}                }
@w{  @t{@i{ppppp}:@i{xxxxx}}        an @i{external symbol} in the @i{ppppp} @i{package}            }
@w{  @t{@i{ppppp}::@i{xxxxx}}       a (possibly internal) @i{symbol} in the @i{ppppp} @i{package}  }
@w{  @t{:@i{nnnnn}}             undefined                                          }
@w{  @t{@i{ppppp}:@i{nnnnn}}        undefined                                          }
@w{  @t{@i{ppppp}::@i{nnnnn}}       undefined                                          }
@w{  @t{::@i{aaaaa}}            undefined                                          }
@w{  @t{@i{aaaaa}:}             undefined                                          }
@w{  @t{@i{aaaaa}:@i{aaaaa}:@i{aaaaa}}  undefined                                          }

@noindent
@w{                 Figure 2--17: Valid patterns for tokens                }

@end group

Note that @i{nnnnn} has number syntax,
          neither @i{xxxxx} nor @i{ppppp} has number syntax,
      and @i{aaaaa} has any syntax.

A summary of rules concerning @i{package markers} follows.
In each case, examples are offered to illustrate the case;
for presentational simplicity, the examples assume that
the @i{readtable case} of the @i{current readtable} is @t{:upcase}.

@table @asis

@item 1.  
If there is a single @i{package marker}, and it occurs at the beginning of the
@i{token}, then the @i{token} is interpreted as a @i{symbol} in the @t{KEYWORD} @i{package}.
It also sets the @b{symbol-value} of the newly-created @i{symbol} to that
same @i{symbol} so that the @i{symbol} will self-evaluate.

For example, 
@t{:bar}, when read, interns @t{BAR} as an @i{external symbol} in the @t{KEYWORD} @i{package}.

@item 2.  
If there is a single @i{package marker} not at the beginning or end of the
@i{token}, then it divides the @i{token} into two parts.  The first part
specifies a @i{package}; 
the second part is the name of an @i{external symbol}
available in that package.  

For example, 
@t{foo:bar}, when read, looks up @t{BAR} among the @i{external symbols} of
the @i{package} named @t{FOO}.

@item 3.  
If there are two adjacent @i{package markers} not at the beginning or end of the
@i{token}, then they divide the @i{token} into two parts.  The first part
specifies a @i{package};
the second part is the name of a @i{symbol} within
that @i{package} (possibly an @i{internal symbol}).

For example, 
@t{foo::bar}, when read, interns @t{BAR} in the @i{package} named @t{FOO}.

@item 4.  
If the @i{token} contains no @i{package markers}, 
and does not have @i{potential number} syntax,
then the entire @i{token} is the name of the @i{symbol}.
The @i{symbol} is looked up in the @i{current package}.

For example, 
@t{bar}, when read, interns @t{BAR} in the @i{current package}.

@item 5.  
The consequences are unspecified if any other pattern of @i{package markers}
in a @i{token} is used.
All other uses of @i{package markers} within names of @i{symbols} 
are not defined by this standard 
but are reserved for @i{implementation-dependent} use.
@end table

For example,
assuming the @i{readtable case} of the @i{current readtable} is @t{:upcase},
@t{editor:buffer} refers to the @i{external symbol} 
named @t{BUFFER} present in the @i{package} named @t{editor},
regardless of whether there is a @i{symbol} named @t{BUFFER} in
the @i{current package}.  If there is no @i{package} named
@t{editor}, or if no @i{symbol} named @t{BUFFER}
is present in @t{editor}, or if @t{BUFFER} is not exported by
@t{editor}, the reader signals
a correctable error.
If @t{editor::buffer} is seen, the effect is exactly the same as
reading @t{buffer} with the @t{EDITOR} @i{package} being the @i{current package}.

@node Package System Consistency Rules,  , Valid Patterns for Tokens, Interpretation of Tokens
@subsection Package System Consistency Rules

The following rules apply to the package system as long as 
the @i{value} of @b{*package*} is not changed:

@table @asis

@item @b{Read-read consistency}  
Reading the same @i{symbol} @i{name}
always results in the @i{same} @i{symbol}.

@item @b{Print-read consistency}  
An @i{interned symbol} always prints as a sequence of characters that, 
when read back in, yields the @i{same} @i{symbol}.

For information about how the @i{Lisp printer} treats @i{symbols},
see @ref{Printing Symbols}.

@item @b{Print-print consistency}  
If two interned @i{symbols} are not the @i{same},
then their printed representations will be different sequences of characters.
@end table

These rules are true regardless of any implicit interning.
As long as the @i{current package} is not changed,
results are reproducible regardless of the order of @i{loading} files 
or the exact history of what @i{symbols} were typed in when.  
If the @i{value} of @b{*package*} is changed and then changed back to the previous value,
consistency is maintained.
The rules can be violated by
    changing the @i{value} of @b{*package*},
    forcing a change to @i{symbols} 
                  or to @i{packages} 
    	          or to both
    by continuing from an error,
or calling one of the following @i{functions}:
    @b{unintern},
    @b{unexport},
    @b{shadow},
    @b{shadowing-import},
 or @b{unuse-package}.

An inconsistency only applies if one of the restrictions is violated
between two of the named @i{symbols}.
@b{shadow}, @b{unexport}, @b{unintern},
and @b{shadowing-import} can only affect the consistency of
@i{symbols} with the same @i{names} (under @b{string=})
as the ones supplied as arguments.

@c end of including concept-tokens

@node Standard Macro Characters,  , Interpretation of Tokens, Syntax
@section Standard Macro Characters

@c including concept-macro-chars

If the reader encounters a @i{macro character},
then its associated @i{reader macro function} 
is invoked and may produce an @i{object} to be returned.
This @i{function} may read the @i{characters} 
following the @i{macro character} in the @i{stream} 
in any syntax and return the @i{object} represented by that syntax.

Any @i{character} can be made to be a @i{macro character}.
The @i{macro characters} defined initially in a @i{conforming implementation}
include
the following:

@menu
* Left-Parenthesis::		
* Right-Parenthesis::		
* Single-Quote::		
* Semicolon::			
* Double-Quote::		
* Backquote::			
* Comma::			
* Sharpsign::			
* Re-Reading Abbreviated Expressions::	
@end menu

@node Left-Parenthesis, Right-Parenthesis, Standard Macro Characters, Standard Macro Characters
@subsection Left-Parenthesis

The @i{left-parenthesis} initiates reading of a @i{list}.
@b{read} is called recursively to read successive @i{objects}
until a right parenthesis is found in the input @i{stream}.
A @i{list} of the @i{objects} read is returned.  Thus

@example
 (a b c)
@end example

is read as a @i{list} of three @i{objects}
(the @i{symbols} @t{a}, @t{b}, and @t{c}).
The right parenthesis need not immediately follow the printed representation of
the last @i{object}; @i{whitespace}_2                              
characters and comments may precede it.

If no @i{objects} precede the right parenthesis, 
it reads as a @i{list} of zero @i{objects} 
(the @i{empty list}).

If a @i{token} that is just a dot
not immediately preceded by an escape character
is read after some @i{object}
then exactly one more @i{object} must follow the dot,
possibly preceded or followed by @i{whitespace}_2 or a comment,
followed by the right parenthesis:

@example
 (a b c . d)
@end example

This means that the @i{cdr} of the last @i{cons} in the 
@i{list} is not @b{nil},
but rather the @i{object} whose representation followed the dot.
The above example might have been the result of evaluating

@example
 (cons 'a (cons 'b (cons 'c 'd)))
@end example

Similarly,

@example
 (cons 'this-one 'that-one) @result{}  (this-one . that-one)
@end example

It is permissible for the @i{object} 
following the dot to be a @i{list}:

@example
 (a b c d . (e f . (g))) @equiv{} (a b c d e f g)
@end example

For information on how the @i{Lisp printer} prints @i{lists} and @i{conses},
see @ref{Printing Lists and Conses}.

@node Right-Parenthesis, Single-Quote, Left-Parenthesis, Standard Macro Characters
@subsection Right-Parenthesis

The @i{right-parenthesis} is invalid 
except when used in conjunction with the left parenthesis character.
For more information, see @ref{Reader Algorithm}.

@node Single-Quote, Semicolon, Right-Parenthesis, Standard Macro Characters
@subsection Single-Quote

@b{Syntax:}  @t{'<<@i{exp}>>}

A @i{single-quote} introduces an @i{expression} to be ``quoted.''
@i{Single-quote} followed by an @i{expression} @i{exp} 
is treated by the @i{Lisp reader} as an abbreviation for
and is parsed identically to the @i{expression} @t{(quote @i{exp})}.
See the @i{special operator} @b{quote}.

@menu
* Examples of Single-Quote::	
@end menu

@node Examples of Single-Quote,  , Single-Quote, Single-Quote
@subsubsection Examples of Single-Quote

@example
 'foo @result{}  FOO
 ''foo @result{}  (QUOTE FOO)
 (car ''foo) @result{}  QUOTE
@end example

@node Semicolon, Double-Quote, Single-Quote, Standard Macro Characters
@subsection Semicolon

@b{Syntax:} @t{;<<@i{text}>>}

A @i{semicolon} introduces @i{characters} that are to be ignored,
such as comments.  The @i{semicolon} and all @i{characters} up to
and including the next @i{newline} or end of file are ignored.

@menu
* Examples of Semicolon::	
* Notes about Style for Semicolon::  
* Use of Single Semicolon::	
* Use of Double Semicolon::	
* Use of Triple Semicolon::	
* Use of Quadruple Semicolon::	
* Examples of Style for Semicolon::  
@end menu

@node Examples of Semicolon, Notes about Style for Semicolon, Semicolon, Semicolon
@subsubsection Examples of Semicolon

@example
 (+ 3 ; three
    4)
@result{}  7    
@end example

@node Notes about Style for Semicolon, Use of Single Semicolon, Examples of Semicolon, Semicolon
@subsubsection Notes about Style for Semicolon

Some text editors make assumptions about desired indentation based on
the number of @i{semicolons} that begin a comment.  The following style 
conventions are common, although not by any means universal.  

@node Use of Single Semicolon, Use of Double Semicolon, Notes about Style for Semicolon, Semicolon
@subsubsection Use of Single Semicolon

Comments that begin with a single @i{semicolon} are all aligned to 
the same column at the right (sometimes called the ``comment column'').
The text of such a comment generally applies only to the line on which it appears.
Occasionally two or three contain a single sentence together;
this is sometimes indicated by indenting all but the first with an additional
space (after the @i{semicolon}).

@node Use of Double Semicolon, Use of Triple Semicolon, Use of Single Semicolon, Semicolon
@subsubsection Use of Double Semicolon

Comments that begin with a double @i{semicolon} are all aligned to
the same level of indentation as a @i{form} would be at that same
position in the @i{code}.
The text of such a comment usually describes
    the state of the @i{program} at the point where the comment occurs,
    the @i{code} which follows the comment,
 or both.

@node Use of Triple Semicolon, Use of Quadruple Semicolon, Use of Double Semicolon, Semicolon
@subsubsection Use of Triple Semicolon

Comments that begin with a triple @i{semicolon} are all aligned to
the left margin.  Usually they are used prior to a definition or set
of definitions, rather than within a definition.

@node Use of Quadruple Semicolon, Examples of Style for Semicolon, Use of Triple Semicolon, Semicolon
@subsubsection Use of Quadruple Semicolon

Comments that begin with a quadruple @i{semicolon} are all aligned to
the left margin, and generally contain only a short piece of text that
serve as a title for the code which follows, and might be used in the
header or footer of a program that prepares code for presentation as 
a hardcopy document.

@node Examples of Style for Semicolon,  , Use of Quadruple Semicolon, Semicolon
@subsubsection Examples of Style for Semicolon

@example
;;;; Math Utilities

;;; FIB computes the the Fibonacci function in the traditional
;;; recursive way.

(defun fib (n)
  (check-type n integer)
  ;; At this point we're sure we have an integer argument.
  ;; Now we can get down to some serious computation.
  (cond ((< n 0)
         ;; Hey, this is just supposed to be a simple example.
         ;; Did you really expect me to handle the general case?
         (error "FIB got ~D as an argument." n))
        ((< n 2) n)             ;fib[0]=0 and fib[1]=1
        ;; The cheap cases didn't work.
        ;; Nothing more to do but recurse.
        (t (+ (fib (- n 1))     ;The traditional formula
              (fib (- n 2)))))) ; is fib[n-1]+fib[n-2].
@end example

@node Double-Quote, Backquote, Semicolon, Standard Macro Characters
@subsection Double-Quote

@b{Syntax:} @t{"<<@i{text}>>"}

The @i{double-quote} is used to begin and end a @i{string}.
When a @i{double-quote} is encountered,
@i{characters} are read from the @i{input} @i{stream} 
and accumulated until another @i{double-quote} is encountered.
If a @i{single escape} @i{character} is seen,
the @i{single escape} @i{character} is discarded,
the next @i{character} is accumulated, and accumulation continues.
The accumulated @i{characters} 
up to but not including the matching @i{double-quote} 
are made into a @i{simple string} and returned.

It is @i{implementation-dependent}
which @i{attributes} of the accumulated characters are removed in this process.

Examples of the use of the @i{double-quote} character are in Figure 2--18. 

@group
@noindent
@w{  @t{"Foo"}                      ;A string with three characters in it  }
@w{  @t{""}                         ;An empty string                       }
@w{  @t{"\"APL\\360?\" he cried."}  ;A string with twenty characters       }
@w{  @t{"|x| = |-x|"}               ;A ten-character string                }

@noindent
@w{          Figure 2--18: Examples of the use of double-quote         }

@end group

Note that to place a single escape character or a @i{double-quote} into a string,
such a character must be preceded by a single escape character.
Note, too, that a multiple escape character need not be quoted by a 
single escape character within a string.

For information on how the @i{Lisp printer} prints @i{strings},
see @ref{Printing Strings}.

@node Backquote, Comma, Double-Quote, Standard Macro Characters
@subsection Backquote

The @i{backquote} introduces a template of a data structure to be built.  
For example, writing

@example
 `(cond ((numberp ,x) ,@@y) (t (print ,x) ,@@y))
@end example

is roughly equivalent to writing

@example
 (list 'cond 
       (cons (list 'numberp x) y) 
       (list* 't (list 'print x) y))
@end example

Where a comma
occurs in the template, 
the @i{expression}
following the comma is to be evaluated to produce an @i{object} to
be inserted at that point.  Assume @t{b} has the value 3, for example, then
evaluating the @i{form} denoted by @t{`(a b ,b ,(+ b 1) b)} produces
the result @t{(a b 3 4 b)}.

If a comma is immediately followed by an @i{at-sign}, 
then the @i{form} following the @i{at-sign}
is evaluated to produce a @i{list} of @i{objects}.
These @i{objects} are then ``spliced'' into place in the template.  For
example, if @t{x} has the value @t{(a b c)}, then

@example
 `(x ,x ,@@x foo ,(cadr x) bar ,(cdr x) baz ,@@(cdr x))
@result{}  (x (a b c) a b c foo b bar (b c) baz b c)
@end example

The backquote syntax can be summarized formally as follows.

@table @asis

@item @t{*}  
@t{`@i{basic}} is the same as @t{'@i{basic}},
that is, @t{(quote @i{basic})}, for any @i{expression} 
@i{basic} that is not a @i{list} or a general @i{vector}.

@item @t{*}  
@t{`,@i{form}} is the same as @i{form}, for any @i{form}, provided
that the representation of @i{form} does not begin with @i{at-sign}
or @i{dot}.  (A similar caveat holds for all occurrences of a form after a @i{comma}.)

@item @t{*}  
@t{`,@@@i{form}} has undefined consequences.

@item @t{*}  
@t{`(x1 x2 x3 ... xn . atom)}
may be interpreted to mean

@example
 (append {[} x1{]} {[} x2{]} {[} x3{]} ... {[} xn{]} (quote atom))
@end example

where the brackets are used to indicate
a transformation of an @i{xj} as follows:

@table @asis

@item --  
@t{[@i{form}]} is interpreted as @t{(list `@i{form})}, 
which contains a backquoted form that must then be further interpreted.

@item --  
@t{[,@i{form}]} is interpreted as @t{(list @i{form})}.

@item --  
@t{[,@@@i{form}]} is interpreted as @i{form}.
@end table

@item @t{*}  
@t{`(x1 x2 x3 ... xn)} may be interpreted to mean
the same as the backquoted form
@t{`(x1 x2 x3 ... xn . @b{nil})},
thereby reducing it to the previous case.

@item @t{*}  
@t{`(x1 x2 x3 ... xn . ,form)} may be interpreted to mean

@example
 (append {[} x1{]} {[} x2{]} {[} x3{]} ... {[} xn{]} form)
@end example

where the brackets indicate a transformation of an @t{xj} as described above.

@item @t{*}  
@t{`(x1 x2 x3 ... xn . ,@@form)} has undefined consequences.

@item @t{*}  
@t{`#(x1 x2 x3 ... xn)} may be interpreted to mean
@t{(apply #'vector `(x1 x2 x3 ... xn))}.
@end table

Anywhere ``@t{,@@}'' may be used, the syntax ``@t{,.}'' may be used instead
to indicate that it is permissible to operate @i{destructively} on 
the @i{list structure}  produced by the form following the ``@t{,.}'' 
(in effect, to use @b{nconc} instead of @b{append}).

If the backquote syntax is nested, the innermost backquoted form
should be expanded first.  This means that if several commas occur
in a row, the leftmost one belongs to the innermost @i{backquote}.

An @i{implementation} is free to interpret a backquoted @i{form} F_1
as any @i{form} F_2 that, when evaluated, will produce a result that is
the @i{same} under @b{equal} as the result implied by the above definition, 
provided that the side-effect behavior of the substitute @i{form} F_2 
is also consistent with the description given above.
The constructed
copy of the template might or might not share @i{list} structure with the
template itself.  As an example, the above definition implies that

@example
 `((,a b) ,c ,@@d)
@end example

will be interpreted as if it were

@example
 (append (list (append (list a) (list 'b) '@b{nil})) (list c) d '@b{nil})
@end example

but it could also be legitimately interpreted to mean any of the following:

@example
 (append (list (append (list a) (list 'b))) (list c) d)
 (append (list (append (list a) '(b))) (list c) d)
 (list* (cons a '(b)) c d)
 (list* (cons a (list 'b)) c d)
 (append (list (cons a '(b))) (list c) d)
 (list* (cons a '(b)) c (copy-list d))
@end example

@menu
* Notes about Backquote::	
@end menu

@node Notes about Backquote,  , Backquote, Backquote
@subsubsection Notes about Backquote

Since the exact manner in which the @i{Lisp reader} will parse
an @i{expression} involving the @i{backquote} @i{reader macro} 
is not specified, an @i{implementation} is free to choose any
representation that preserves the semantics described.

Often an @i{implementation} will choose a representation that facilitates
pretty printing of the expression, so that @t{(pprint `(a ,b))} will display
@t{`(a ,b)} and not, for example, @t{(list 'a b)}.  However, this is not a
requirement.

Implementors who have no particular reason to make one choice or another
might wish to refer to {IEEE Standard for the Scheme Programming Language}, which identifies a popular choice of
representation for such expressions that might provide useful to be useful
compatibility for some user communities.  There is no requirement, however,
that any @i{conforming implementation} use this particular representation.
This information is provided merely for cross-reference purposes.

@node Comma, Sharpsign, Backquote, Standard Macro Characters
@subsection Comma

The @i{comma} is part of the backquote syntax; see @ref{Backquote}.
@i{Comma} is invalid if used other than inside the body of a 
backquote @i{expression} as described above.

@node Sharpsign, Re-Reading Abbreviated Expressions, Comma, Standard Macro Characters
@subsection Sharpsign

@i{Sharpsign} is a @i{non-terminating} @i{dispatching macro character}.
It reads an optional 
sequence of digits and then one more character,
and uses that character to select a @i{function} to run as a
@i{reader macro function}.

The @i{standard syntax} includes constructs introduced by the @t{#} character.
The syntax of these constructs is as follows:
a character that identifies the type of construct is 
followed by arguments in some form.
If the character is a letter, its @i{case} is not important;
@t{#O} and @t{#o} are considered to be equivalent, for example.

Certain @t{#} constructs allow an unsigned decimal number to appear
between the @t{#} and the character.

The @i{reader macros} associated with the @i{dispatching macro character} @t{#}
are described later in this section and summarized in Figure 2--19.

{

@group
@noindent
@w{  dispatch char  purpose                  dispatch char  purpose                }
@w{  Backspace      signals error            @t{@{}              undefined*             }
@w{  Tab            signals error            @t{@}}              undefined*             }
@w{  Newline        signals error            +              read-time conditional  }
@w{  Linefeed       signals error            -              read-time conditional  }
@w{  Page           signals error            .              read-time evaluation   }
@w{  Return         signals error            /              undefined              }
@w{  Space          signals error            A, a           array                  }
@w{  !              undefined*               B, b           binary rational        }
@w{  @t{"}              undefined                C, c           complex number         }
@w{  #              reference to = label     D, d           undefined              }
@w{  $             undefined                E, e           undefined              }
@w{  %              undefined                F, f           undefined              }
@w{  &              undefined                G, g           undefined              }
@w{  '              function abbreviation    H, h           undefined              }
@w{  (              simple vector            I, i           undefined              }
@w{  )              signals error            J, j           undefined              }
@w{  @t{*}              bit vector               K, k           undefined              }
@w{  ,              undefined                L, l           undefined              }
@w{  :              uninterned symbol        M, m           undefined              }
@w{  ;              undefined                N, n           undefined              }
@w{  @t{<}              signals error            O, o           octal rational         }
@w{  @t{=}              labels following object  P, p           pathname               }
@w{  @t{>}              undefined                Q, q           undefined              }
@w{  ?              undefined*               R, r           radix-n rational       }
@w{  @@              undefined                S, s           structure              }
@w{  [              undefined*               T, t           undefined              }
@w{  @t{\}              character object         U, u           undefined              }
@w{  ]              undefined*               V, v           undefined              }
@w{  {@t{^}}            undefined                W, w           undefined              }
@w{  @t{_}              undefined                X, x           hexadecimal rational   }
@w{  `              undefined                Y, y           undefined              }
@w{  @t{|}              balanced comment         Z, z           undefined              }
@w{  @t{~}              undefined                Rubout         undefined              }

@noindent
@w{           Figure 2--19: Standard # Dispatching Macro Character Syntax         }

@end group
}

The combinations marked by an asterisk (*) are explicitly reserved to the
user.  No @i{conforming implementation} defines them.

Note also that @i{digits} do not appear in the preceding table.  This is
because the notations @t{#0}, @t{#1}, ..., @t{#9} are
reserved for another purpose which occupies the same syntactic space.
When a @i{digit} follows a @i{sharpsign},
it is not treated as a dispatch character.
Instead, an unsigned integer argument is accumulated 
and passed as an @i{argument} to the @i{reader macro} 
for the @i{character} that follows the digits.
For example,
@t{#2A((1 2) (3 4))} is a use of @t{#A} with an argument of @t{2}.

@menu
* Sharpsign Backslash::		
* Sharpsign Single-Quote::	
* Sharpsign Left-Parenthesis::	
* Sharpsign Asterisk::		
* Examples of Sharpsign Asterisk::  
* Sharpsign Colon::		
* Sharpsign Dot::		
* Sharpsign B::			
* Sharpsign O::			
* Sharpsign X::			
* Sharpsign R::			
* Sharpsign C::			
* Sharpsign A::			
* Sharpsign S::			
* Sharpsign P::			
* Sharpsign Equal-Sign::	
* Sharpsign Sharpsign::		
* Sharpsign Plus::		
* Sharpsign Minus::		
* Sharpsign Vertical-Bar::	
* Examples of Sharpsign Vertical-Bar::	
* Notes about Style for Sharpsign Vertical-Bar::  
* Sharpsign Less-Than-Sign::	
* Sharpsign Whitespace::	
* Sharpsign Right-Parenthesis::	 
@end menu

@node Sharpsign Backslash, Sharpsign Single-Quote, Sharpsign, Sharpsign
@subsubsection Sharpsign Backslash

@b{Syntax:} @t{#\<<@i{x}>>}

When the @i{token} @i{x} is a single @i{character} long, 
this parses as the literal @i{character} @i{char}.
@i{Uppercase} and @i{lowercase} letters are distinguished after @t{#\};
@t{#\A} and @t{#\a} denote different @i{character} @i{objects}.
Any single @i{character} works after @t{#\},
even those that are normally special to @b{read}, 
such as @i{left-parenthesis} and @i{right-parenthesis}.

In the single @i{character} case,
the @i{x} must be followed by a non-constituent @i{character}.
After @t{#\} is read,
the reader backs up over the @i{slash} and then reads a @i{token},
treating the initial @i{slash} as a @i{single escape} @i{character}
(whether it really is or not in the @i{current readtable}).

When the @i{token} @i{x} is more than one @i{character} long,
the @i{x} must have the syntax of a @i{symbol} 
with no embedded @i{package markers}.
In this case, the @i{sharpsign} @i{backslash} notation
parses as the @i{character} whose @i{name} is @t{(string-upcase @i{x})};
see @ref{Character Names}.

For information about how the @i{Lisp printer} prints @i{character} @i{objects},
see @ref{Printing Characters}.

@node Sharpsign Single-Quote, Sharpsign Left-Parenthesis, Sharpsign Backslash, Sharpsign
@subsubsection Sharpsign Single-Quote

Any @i{expression} preceded by @t{#'} 
(@i{sharpsign} followed by @i{single-quote}),
as in @t{#'@i{expression}},
is treated by the @i{Lisp reader} as an abbreviation for and parsed identically 
to the @i{expression} @t{(function @i{expression})}.
See @b{function}.  For example,

@example
(apply #'+ l) @equiv{} (apply (function +) l)
@end example

@node Sharpsign Left-Parenthesis, Sharpsign Asterisk, Sharpsign Single-Quote, Sharpsign
@subsubsection Sharpsign Left-Parenthesis

@t{#(} and @t{)} are used to notate a @i{simple vector}. 

If an unsigned decimal integer
appears between the @t{#} and @t{(},
it specifies explicitly the length of the @i{vector}.  
The consequences are undefined if the number of @i{objects} 
specified before the closing @t{)}
exceeds the unsigned decimal integer.
If the number of @i{objects} supplied before the closing @t{)}
is less than the unsigned decimal integer but greater than zero,
the last @i{object}
is used to fill all
remaining elements of the @i{vector}.

[Editorial Note by Barmar: This should say "signals...".]
The consequences are undefined if the unsigned decimal integer is non-zero and
number of @i{objects} supplied before the closing @t{)}
is zero.
For example,

@example
 #(a b c c c c)
 #6(a b c c c c)
 #6(a b c)
 #6(a b c c)
@end example

all mean the same thing: a @i{vector} of length @t{6}
with @i{elements} @t{a}, @t{b}, and four occurrences of @t{c}.  
Other examples follow:

@example
 #(a b c)               ;A vector of length 3
 #(2 3 5 7 11 13 17 19 23 29 31 37 41 43 47)
                        ;A vector containing the primes below 50
 #()                    ;An empty vector
@end example

The notation @t{#()} denotes an empty @i{vector}, as does @t{#0()}.

For information on how the @i{Lisp printer} prints @i{vectors},
see @ref{Printing Strings},
    @ref{Printing Bit Vectors},
 or @ref{Printing Other Vectors}.

@node Sharpsign Asterisk, Examples of Sharpsign Asterisk, Sharpsign Left-Parenthesis, Sharpsign
@subsubsection Sharpsign Asterisk

@b{Syntax:} @t{#*<<@i{bits}>>}

A @i{simple bit vector} is constructed containing the indicated @i{bits}
(@t{0}'s and @t{1}'s), where the leftmost @i{bit} has index zero 
and the subsequent @i{bits} have increasing indices.

@b{Syntax:} @t{#<<@i{n}>>*<<@i{bits}>>}

With an argument @i{n},
the @i{vector} to be created is of @i{length} @i{n}.
If the number of @i{bits} is less than @i{n} but greater than zero,
the last bit is used to fill all remaining bits of the @i{bit vector}.

The notations @t{#*} and @t{#0*} each denote an empty @i{bit vector}.

Regardless of whether the optional numeric argument @i{n} is provided,
the @i{token} that follows the @i{asterisk} is delimited by 
a normal @i{token} delimiter.
However, (unless the @i{value} of @b{*read-suppress*} is @i{true})
an error of @i{type} @b{reader-error} is signaled 
     if that @i{token} is not composed entirely of @t{0}'s and @t{1}'s,
  or if @i{n} was supplied 
        and the @i{token} is composed of more than @i{n} @i{bits},
  or if @i{n} is greater than one, but no @i{bits} were specified.
Neither a @i{single escape} nor a @i{multiple escape} is permitted in this @i{token}.

For information on how the @i{Lisp printer} prints @i{bit vectors},
see @ref{Printing Bit Vectors}.

@node Examples of Sharpsign Asterisk, Sharpsign Colon, Sharpsign Asterisk, Sharpsign
@subsubsection Examples of Sharpsign Asterisk

For example, 
@example
  #*101111
 #6*101111
 #6*101
 #6*1011
@end example

all mean the same thing: a @i{vector} of length @t{6}
with @i{elements} @t{1}, @t{0}, @t{1}, @t{1}, @t{1}, and @t{1}.

For example:

@example
 #*         ;An empty bit-vector
@end example

@node Sharpsign Colon, Sharpsign Dot, Examples of Sharpsign Asterisk, Sharpsign
@subsubsection Sharpsign Colon

@b{Syntax:}  @t{#:<<@i{symbol-name}>>}

@t{#:} introduces an @i{uninterned} @i{symbol} whose @i{name} 
is @i{symbol-name}.  Every time this syntax is encountered,
a @i{distinct} @i{uninterned} @i{symbol} is created.
The @i{symbol-name} must have the syntax of a @i{symbol} 
with no @i{package prefix}.

For information on how the @i{Lisp reader} 
prints @i{uninterned} @i{symbols},
see @ref{Printing Symbols}.

@node Sharpsign Dot, Sharpsign B, Sharpsign Colon, Sharpsign
@subsubsection Sharpsign Dot

@t{#.@i{foo}} is read as the @i{object} resulting from the evaluation
of the @i{object} represented by @i{foo}.
The evaluation is done during the @b{read} process,
when the @t{#.} notation is encountered.
The @t{#.} syntax therefore performs a read-time evaluation of @i{foo}.

The normal effect of @t{#.} is inhibited when the @i{value} of @b{*read-eval*} is @i{false}.

In that situation, an error of @i{type} @b{reader-error} is signaled.

For an @i{object}
that does not have a convenient printed
representation, a @i{form} that computes the @i{object} can be given using
the @t{#.} notation.

@node Sharpsign B, Sharpsign O, Sharpsign Dot, Sharpsign
@subsubsection Sharpsign B

@t{#B}@i{rational} reads @i{rational} in binary (radix 2).
For example, 

@example
 #B1101 @equiv{} 13 ;1101{{}_2}
 #b101/11 @equiv{} 5/3
@end example

The consequences are undefined if the token immediately following
the @t{#B} does not have the syntax of a binary (@i{i.e.}, radix 2) @i{rational}.

@node Sharpsign O, Sharpsign X, Sharpsign B, Sharpsign
@subsubsection Sharpsign O

@t{#O}@i{rational} reads @i{rational} in octal (radix 8).
For example, 

@example
 #o37/15 @equiv{} 31/13
 #o777 @equiv{} 511
 #o105 @equiv{} 69 ;105{{}_8}
@end example

The consequences are undefined if the token immediately following
the @t{#O} does not have the syntax of an octal (@i{i.e.}, radix 8) @i{rational}.

@node Sharpsign X, Sharpsign R, Sharpsign O, Sharpsign
@subsubsection Sharpsign X

@t{#X}@i{rational} reads @i{rational} in hexadecimal (radix 16).
The digits above @t{9} are the letters @t{A} through @t{F} (the lowercase
letters @t{a} through @t{f} are also acceptable).  For example,

@example
 #xF00 @equiv{} 3840             
 #x105 @equiv{} 261 ;105{{}_@{16@}}
@end example

The consequences are undefined if the token immediately following
the @t{#X} does not have the syntax of a hexadecimal (@i{i.e.}, radix 16) @i{rational}.

@node Sharpsign R, Sharpsign C, Sharpsign X, Sharpsign
@subsubsection Sharpsign R

@t{#@i{n}R}

@t{#@i{radix}R@i{rational}} reads @i{rational} in radix @i{radix}.
@i{radix} must consist of only digits
that are interpreted as an @i{integer}
in decimal radix; its value must be between 2 and 36 (inclusive).
Only valid digits
for the specified radix may be used.

For example, @t{#3r102} is another way of writing @t{11} (decimal), 
and @t{#11R32}
is another way of writing @t{35} (decimal).  
For radices larger than 10, letters of
the alphabet are used in order for the digits after @t{9}.
No alternate @t{#} notation exists for the decimal radix since a
decimal point suffices.

Figure 2--20 contains examples of the use of @t{#B},
@t{#O}, @t{#X}, and @t{#R}.

@group
@noindent
@w{  @t{#2r11010101}  ;Another way of writing @t{213} decimal  }
@w{  @t{#b11010101}   ;Ditto                               }
@w{  @t{#b+11010101}  ;Ditto                               }
@w{  @t{#o325}        ;Ditto, in octal radix               }
@w{  @t{#xD5}         ;Ditto, in hexadecimal radix         }
@w{  @t{#16r+D5}      ;Ditto                               }
@w{  @t{#o-300}       ;Decimal @t{-192}, written in base 8     }
@w{  @t{#3r-21010}    ;Same thing in base 3                }
@w{  @t{#25R-7H}      ;Same thing in base 25               }
@w{  @t{#xACCEDED}    ;@t{181202413}, in hexadecimal radix     }

@noindent
@w{        Figure 2--20: Radix Indicator Example       }

@end group

The consequences are undefined if the token immediately following
the @t{#@i{n}R} does not have the syntax of a @i{rational} in radix @i{n}.

@node Sharpsign C, Sharpsign A, Sharpsign R, Sharpsign
@subsubsection Sharpsign C

@t{#C} reads a following @i{object}, which must be a @i{list} of
length two whose @i{elements} are both @i{reals}.
These @i{reals} denote, respectively,
the real and imaginary parts of a @i{complex} number.

If the two parts as notated are not of the same data type,
then they are converted 
according to the rules of floating-point @i{contagion}
described in @ref{Contagion in Numeric Operations}.

@t{#C(@i{real} @i{imag})} is equivalent to 
@t{#.(complex (quote @i{real}) (quote @i{imag}))},
except that @t{#C} is not affected by @b{*read-eval*}.
See the @i{function} @b{complex}.

Figure 2--21 contains examples of the use of @t{#C}.

@group
@noindent
@w{  @t{#C(3.0s1 2.0s-1)}  ;A @i{complex} with @i{small float} parts.                }
@w{  @t{#C(5 -3) }         ;A ``Gaussian integer''                             }
@w{  @t{#C(5/3 7.0) }      ;Will be converted internally to @t{#C(1.66666 7.0)}  }
@w{  @t{#C(0 1)}           ;The imaginary unit; that is, i.                  }

@noindent
@w{                  Figure 2--21: Complex Number Example                }

@end group

For further information, 
see @ref{Printing Complexes} and @ref{Syntax of a Complex}.

@node Sharpsign A, Sharpsign S, Sharpsign C, Sharpsign
@subsubsection Sharpsign A

@t{#@i{n}A}

@t{#@i{n}@t{A}@i{object}} constructs an @i{n}-dimensional @i{array},
using @i{object} as the value of the @t{:initial-contents} argument
to @b{make-array}.

For example, @t{#2A((0 1 5) (foo 2 (hot dog)))} represents a 2-by-3 matrix:

@example
 0       1       5
 foo     2       (hot dog)
@end example

In contrast, @t{#1A((0 1 5) (foo 2 (hot dog)))} 
represents a @i{vector} of @i{length} @t{2} 
whose @i{elements} are @i{lists}:

@example
 (0 1 5) (foo 2 (hot dog))
@end example

@t{#0A((0 1 5) (foo 2 (hot dog)))} represents a zero-dimensional
@i{array} whose sole element is a @i{list}:

@example
 ((0 1 5) (foo 2 (hot dog)))
@end example

@t{#0A foo} represents 
a zero-dimensional @i{array} whose sole element is the 
@i{symbol} @t{foo}.
The notation @t{#1A foo} is not valid because @t{foo} is
not a @i{sequence}.

If some @i{dimension} of the @i{array}
whose representation is being parsed is found to be @t{0},
all @i{dimensions} to the right 
(@i{i.e.}, the higher numbered @i{dimensions})
are also considered to be @t{0}.

For information on how the @i{Lisp printer} prints @i{arrays},
see @ref{Printing Strings},
    @ref{Printing Bit Vectors},
    @ref{Printing Other Vectors},
 or @ref{Printing Other Arrays}.

@node Sharpsign S, Sharpsign P, Sharpsign A, Sharpsign
@subsubsection Sharpsign S

@t{#s(name slot1 value1 slot2 value2 ...)}
denotes a @i{structure}.  This is valid only if @i{name} is the name
of a @i{structure} @i{type} already defined by @b{defstruct} 
and if the @i{structure} @i{type} has a standard constructor function.
Let @i{cm} stand for the name of this constructor function;
then this syntax is equivalent to

@example
 #.(cm keyword1 'value1 keyword2 'value2 ...)
@end example

where each @i{keywordj} is the result of computing

@example
 (intern (string slotj) (find-package 'keyword))
@end example

The net effect is that the constructor function is called with the specified
slots having the specified values. 

(This coercion feature is deprecated; in the future, keyword names will 
 be taken in the package they are read in, so @i{symbols} that are 
 actually in the @t{KEYWORD} @i{package} should be used if that is what is desired.)

Whatever @i{object} the constructor function returns
is returned by the @t{#S} syntax.

For information on how the @i{Lisp printer} prints @i{structures},
see @ref{Printing Structures}.

@node Sharpsign P, Sharpsign Equal-Sign, Sharpsign S, Sharpsign
@subsubsection Sharpsign P

@t{#P} reads a following @i{object}, which must be a @i{string}.

@t{#P<<@i{expression}>>} is equivalent to 
@t{#.(parse-namestring '<<@i{expression}>>)},
except that @t{#P} is not affected by @b{*read-eval*}.

For information on how the @i{Lisp printer} prints @i{pathnames},
see @ref{Printing Pathnames}.

@node Sharpsign Equal-Sign, Sharpsign Sharpsign, Sharpsign P, Sharpsign
@subsubsection Sharpsign Equal-Sign

@t{#@i{n}=}

@t{#@i{n}=@i{object}} reads as whatever @i{object}
has @i{object} as its printed representation.  However, that @i{object}
is labeled by @i{n}, a required unsigned decimal integer, for
possible reference by the syntax @t{#@i{n}#}.
The scope of the label is the @i{expression} being read by the outermost
call to @b{read}; within this @i{expression},
the same label may not appear twice.

@node Sharpsign Sharpsign, Sharpsign Plus, Sharpsign Equal-Sign, Sharpsign
@subsubsection Sharpsign Sharpsign

@t{#@i{n}#}

@t{#@i{n}#}, where @i{n} is a required unsigned decimal
@i{integer},
provides a reference to some @i{object} labeled by @t{#@i{n}=};
that is, @t{#@i{n}#} represents a pointer to the same 
(@b{eq}) @i{object} labeled by @t{#@i{n}=}.
For example, a structure created in the variable @t{y} by this code:

@example
 (setq x (list 'p 'q))
 (setq y (list (list 'a 'b) x 'foo x))
 (rplacd (last y) (cdr y))
@end example

could be represented in this way:

@example
 ((a b) . #1=(#2=(p q) foo #2# . #1#))
@end example

Without this notation, but with @b{*print-length*} set to @t{10}
and @b{*print-circle*} set to @b{nil},
the structure would print in this way:

@example
 ((a b) (p q) foo (p q) (p q) foo (p q) (p q) foo (p q) ...)
@end example

A reference @t{#@i{n}#} may only occur after a label @t{#@i{n}=};
forward references are not permitted.  The reference
may not appear as the labeled object itself (that is,
@t{#@i{n}=#@i{n}#}) may not be written 
because the @i{object}
labeled by @t{#@i{n}=} is not well defined in this case.

@node Sharpsign Plus, Sharpsign Minus, Sharpsign Sharpsign, Sharpsign
@subsubsection Sharpsign Plus

@t{#+} provides a read-time conditionalization facility;
the syntax is @t{#+@i{test} @i{expression}}.
If the @i{feature expression} @i{test} succeeds,
then this textual notation represents an @i{object}
 whose printed representation is @i{expression}.
If the @i{feature expression} @i{test} fails,
then this textual notation is treated as @i{whitespace}_2; 
 that is, it is as if the ``@t{#+} @i{test} @i{expression}'' 
 did not appear and only a @i{space} appeared in its place.

For a detailed description of success and failure in @i{feature expressions},
see @ref{Feature Expressions}.

@t{#+} operates by first reading the @i{feature expression}
and then skipping over the @i{form} if the @i{feature expression} fails.

While reading the @i{test}, the @i{current package} is the @t{KEYWORD} @i{package}.

Skipping over the @i{form} is accomplished by @i{binding} 
@b{*read-suppress*} to @i{true} and then calling @b{read}.

For examples, see @ref{Examples of Feature Expressions}.

@node Sharpsign Minus, Sharpsign Vertical-Bar, Sharpsign Plus, Sharpsign
@subsubsection Sharpsign Minus

@t{#-} is like @t{#+} 
except that it skips the @i{expression} if the @i{test} succeeds; 
that is,

@example
#-@i{test} @i{expression} @equiv{} #+(not @i{test}) @i{expression}
@end example

For examples, see @ref{Examples of Feature Expressions}.

@node Sharpsign Vertical-Bar, Examples of Sharpsign Vertical-Bar, Sharpsign Minus, Sharpsign
@subsubsection Sharpsign Vertical-Bar

@t{#|...|#} is treated as a comment by the reader.
It must be balanced with respect to other occurrences of @t{#|} and @t{|#},
but otherwise may contain any characters whatsoever.

@node Examples of Sharpsign Vertical-Bar, Notes about Style for Sharpsign Vertical-Bar, Sharpsign Vertical-Bar, Sharpsign
@subsubsection Examples of Sharpsign Vertical-Bar

The following are some examples that exploit the @t{#|...|#} notation:

@example
;;; In this example, some debugging code is commented out with #|...|#
;;; Note that this kind of comment can occur in the middle of a line
;;; (because a delimiter marks where the end of the comment occurs)
;;; where a semicolon comment can only occur at the end of a line 
;;; (because it comments out the rest of the line).
 (defun add3 (n) #|(format t "~&Adding 3 to ~D." n)|# (+ n 3))

;;; The examples that follow show issues related to #| ... |# nesting.

;;; In this first example, #| and |# always occur properly paired,
;;; so nesting works naturally.
 (defun mention-fun-fact-1a ()
   (format t "CL uses ; and #|...|# in comments."))
@result{}  MENTION-FUN-FACT-1A
 (mention-fun-fact-1a)
@t{ |> } CL uses ; and #|...|# in comments.
@result{}  NIL
 #| (defun mention-fun-fact-1b ()
      (format t "CL uses ; and #|...|# in comments.")) |#
 (fboundp 'mention-fun-fact-1b) @result{}  NIL

;;; In this example, vertical-bar followed by sharpsign needed to appear
;;; in a string without any matching sharpsign followed by vertical-bar
;;; having preceded this.  To compensate, the programmer has included a
;;; slash separating the two characters.  In case 2a, the slash is 
;;; unnecessary but harmless, but in case 2b, the slash is critical to
;;; allowing the outer #| ... |# pair match.  If the slash were not present,
;;; the outer comment would terminate prematurely.
 (defun mention-fun-fact-2a ()
   (format t "Don't use |\# unmatched or you'll get in trouble!"))
@result{}  MENTION-FUN-FACT-2A
 (mention-fun-fact-2a)
@t{ |> } Don't use |# unmatched or you'll get in trouble!
@result{}  NIL
 #| (defun mention-fun-fact-2b ()
      (format t "Don't use |\# unmatched or you'll get in trouble!") |#
 (fboundp 'mention-fun-fact-2b) @result{}  NIL

;;; In this example, the programmer attacks the mismatch problem in a
;;; different way.  The sharpsign vertical bar in the comment is not needed
;;; for the correct parsing of the program normally (as in case 3a), but 
;;; becomes important to avoid premature termination of a comment when such 
;;; a program is commented out (as in case 3b).
 (defun mention-fun-fact-3a () ; #|
   (format t "Don't use |# unmatched or you'll get in trouble!"))
@result{}  MENTION-FUN-FACT-3A
 (mention-fun-fact-3a)
@t{ |> } Don't use |# unmatched or you'll get in trouble!
@result{}  NIL
 #|
 (defun mention-fun-fact-3b () ; #|
   (format t "Don't use |# unmatched or you'll get in trouble!"))
 |#
 (fboundp 'mention-fun-fact-3b) @result{}  NIL
@end example

@node Notes about Style for Sharpsign Vertical-Bar, Sharpsign Less-Than-Sign, Examples of Sharpsign Vertical-Bar, Sharpsign
@subsubsection Notes about Style for Sharpsign Vertical-Bar

Some text editors that purport to understand Lisp syntax treat any @t{|...|}
as balanced pairs that cannot nest (as if they were just balanced pairs of 
the multiple escapes used in notating certain symbols).  To compensate for 
this deficiency, some programmers use the notation @t{#||...#||...||#...||#}
instead of @t{#|...#|...|#...|#}.   Note that this alternate usage is not
a different @i{reader macro}; it merely exploits the fact that the additional
vertical-bars occur within the comment in a way that tricks certain text editor
into better supporting nested comments.  As such, one might sometimes see code
like:

@example
 #|| (+ #|| 3 ||# 4 5) ||# 
@end example

Such code is equivalent to:

@example
 #| (+ #| 3 |# 4 5) |#
@end example

@node Sharpsign Less-Than-Sign, Sharpsign Whitespace, Notes about Style for Sharpsign Vertical-Bar, Sharpsign
@subsubsection Sharpsign Less-Than-Sign

@t{#<} is not valid reader syntax.
The @i{Lisp reader} will signal an error 

of @i{type} @b{reader-error}

on encountering @t{#<}.
This syntax is typically used in the printed representation 
of @i{objects} that cannot be read back in.  

@node Sharpsign Whitespace, Sharpsign Right-Parenthesis, Sharpsign Less-Than-Sign, Sharpsign
@subsubsection Sharpsign Whitespace

@t{#} followed immediately by @i{whitespace}_1 is not valid reader syntax.
The @i{Lisp reader} will signal an error of @i{type} @b{reader-error} if it
encounters the reader macro notation @t{#<@i{Newline}>} or @t{#<@i{Space}>}.

@node Sharpsign Right-Parenthesis,  , Sharpsign Whitespace, Sharpsign
@subsubsection Sharpsign Right-Parenthesis

This is not valid reader syntax.

The @i{Lisp reader} will signal an error 

of @i{type} @b{reader-error}

upon encountering @t{#)}.

@node Re-Reading Abbreviated Expressions,  , Sharpsign, Standard Macro Characters
@subsection Re-Reading Abbreviated Expressions

Note that the @i{Lisp reader} will 
generally
signal an error of @i{type} @b{reader-error}
when reading an @i{expression}_2 that has been
abbreviated because of length or level limits 
(see @b{*print-level*},
     @b{*print-length*},
 and @b{*print-lines*})
due to restrictions on ``@t{..}'', ``@t{...}'', ``@t{#}'' followed by @i{whitespace}_1,
and ``@t{#)}''.

@c end of including concept-macro-chars

@c %**end of chapter