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Common Lisp the Language, 2nd Edition
Next: Data Types
Up: Notational Conventions
Previous: The Lisp Reader
Certain characters are used in special ways in the syntax of Common Lisp. The complete syntax is explained in detail in chapter 22, but a quick summary here may be useful:
(
A left parenthesis begins a list of items. The list may contain any
number of items, including zero. Lists may be nested. For example,
(cons (car x) (cdr y)) is a list of three things, of which
the last two are themselves lists.
)
A right parenthesis ends a list of items.
'
An acute accent (also called single quote or apostrophe) followed by an
expression form is an abbreviation for
(quoteform). Thus
'foo means (quote foo) and
'(cons 'a 'b) means
(quote (cons (quote a) (quote b))).
;
Semicolon is the comment character. It and all characters up to the end
of the line are discarded.
"
Double quotes surround character strings:
"This is a thirty-nine-character string."
\
Backslash is an escape character. It causes the next character to be
treated as a letter rather than for its usual syntactic purpose. For
example, A\(B denotes a symbol whose name consists of the
three characters A, (, and B.
Similarly, "\"" denotes a character string containing one
character, a double quote, because the first and third double quotes
serve to delimit the string, and the second double quote serves as the
contents of the string. The backslash causes the second double quote to
be taken literally and prevents it from being interpreted as the
terminating delimiter of the string.
|
Vertical bars are used in pairs to surround the name (or part of the
name) of a symbol that has many special characters in it. It is roughly
equivalent to putting a backslash in front of every character so
surrounded. For example, |A(B)|, A|(|B|)|, and
A\(B\) all mean the symbol whose name consists of the four
characters A, (, B, and
).
#
The number sign signals the beginning of a complicated syntactic
structure. The next character designates the precise syntax to follow.
For example, #o105 means 
(105 in octal notation);
#x105 means 
(105 in hexadecimal notation); #b1011 means 
(1011 in binary notation);
#\L denotes a character object for the character
L; and #(a b c) denotes a vector of three
elements a, b, and c. A
particularly important case is that
#'fn means
(functionfn), in a
manner analogous to 'form meaning
(quoteform).
`
Grave accent (``backquote’’) signals that the next expression is a
template that may contain commas. The backquote syntax represents a
program that will construct a data structure according to the
template.
,
Commas are used within the backquote syntax.
:
Colon is used to indicate which package a symbol belongs to. For
example, network:reset denotes the symbol named
reset in the package named network. A leading
colon indicates a keyword, a symbol that always evaluates to
itself. The colon character is not actually part of the print name of
the symbol. This is all explained in chapter 11; until you read that, just keep in mind
that a symbol notated with a leading colon is in effect a constant that
evaluates to itself.
Notice of correction. In the first edition, the characters
``,’’ and ``:’’ at the left margin above were
inadvertently omitted.
Brackets, braces, question mark, and exclamation point (that is,
[, ], {, },
?, and !) are not used for any purpose in
standard Common Lisp syntax. These characters are explicitly reserved to
the user, primarily for use as macro characters for
user-defined lexical syntax extensions (see section 22.1.3).
All code in this book is written using lowercase letters. Common Lisp is
generally insensitive to the case in which code is written. Internally,
names of symbols are ordinarily converted to and stored in uppercase
form. There are ways to force case conversion on output if desired; see
*print-case*. In this book, wherever an interactive
exchange between a user and the Lisp system is shown, the input is
exhibited with lowercase letters and the output with uppercase
letters.
X3J13 voted in June 1989 (READ-CASE-SENSITIVITY) to introduce
readtable-case. Certain settings allow the names of symbols
to be case-sensitive. The default behavior, however, is as described in
the previous paragraph. In any event, only uppercase letters appear in
the internal print names of symbols naming the standard Common Lisp
facilities described in this book.
Next: Data Types
Up: Notational Conventions
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