481 lines
25 KiB
HTML
481 lines
25 KiB
HTML
|
<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
|
||
|
|
<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
|
||
|
|
<HEAD>
|
||
|
|
<TITLE>22.1.2. Parsing of Numbers and Symbols</TITLE>
|
||
|
|
</HEAD>
|
||
|
|
<BODY>
|
||
|
|
<meta name="description" value=" Parsing of Numbers and Symbols">
|
||
|
|
<meta name="keywords" value="clm">
|
||
|
|
<meta name="resource-type" value="document">
|
||
|
|
<meta name="distribution" value="global">
|
||
|
|
<P>
|
||
|
|
<b>Common Lisp the Language, 2nd Edition</b>
|
||
|
|
<BR> <HR><A NAME=tex2html3903 HREF="node190.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3901 HREF="node187.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3895 HREF="node188.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3905 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3906 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html3904 HREF="node190.html"> Macro Characters</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html3902 HREF="node187.html"> Printed Representation of </A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html3896 HREF="node188.html"> What the Read </A>
|
||
|
|
<HR> <P>
|
||
|
|
<H2><A NAME=SECTION002612000000000000000>22.1.2. Parsing of Numbers and Symbols</A></H2>
|
||
|
|
<P>
|
||
|
|
<A NAME=PARSETOKENSSECTION>When</A>
|
||
|
|
an extended token is read, it is interpreted as a number or symbol.
|
||
|
|
In general, the token is interpreted as a number if it satisfies
|
||
|
|
the syntax for numbers specified in table <A HREF="node189.html#NUMBERSYNTAXTABLE">22-2</A>;
|
||
|
|
this is discussed in more detail below.
|
||
|
|
<P>
|
||
|
|
The characters of the extended token may serve various syntactic
|
||
|
|
functions as shown
|
||
|
|
in table <A HREF="node189.html#StandardReadtableAttributesTable">22-3</A>, but it must be
|
||
|
|
remembered that any character included in a token under the control
|
||
|
|
of an escape character is treated as <i>alphabetic</i> rather than
|
||
|
|
according to the attributes shown in the table.
|
||
|
|
One consequence of this rule is that a whitespace, macro, or escape
|
||
|
|
character will always be treated as alphabetic within an extended token
|
||
|
|
because such a character cannot be included in an extended
|
||
|
|
token except under the control of an escape character.
|
||
|
|
<P>
|
||
|
|
To allow for extensions to the syntax of numbers, a
|
||
|
|
syntax for <i>potential numbers</i> is defined in Common Lisp that is
|
||
|
|
more general than the actual syntax for numbers.
|
||
|
|
Any token that is not a potential number and does not consist
|
||
|
|
entirely of dots will always be taken to be a symbol,
|
||
|
|
now and in the future; programs may rely on this fact.
|
||
|
|
Any token that is a potential number but does not fit the
|
||
|
|
actual number syntax defined below is a <i>reserved token</i> and
|
||
|
|
has an implementation-dependent interpretation;
|
||
|
|
an implementation may signal an error, quietly treat the token
|
||
|
|
as a symbol, or take some other action. Programmers should avoid
|
||
|
|
the use of such reserved tokens. (A symbol whose name looks like a reserved
|
||
|
|
token can always be written using one or more escape characters.)
|
||
|
|
<P>
|
||
|
|
<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
|
||
|
|
Just as <i>bignum</i> is the standard term used by Lisp implementors for
|
||
|
|
very large integers, and <i>flonum</i> (rhymes with ``low hum'') refers
|
||
|
|
to a floating-point number, the term <i>potnum</i> has been used widely
|
||
|
|
as an abbreviation for ``potential number.'' ``Potnum'' rhymes with ``hot rum.''
|
||
|
|
<br><img align=bottom alt="change_end" src="gif/change_end.gif">
|
||
|
|
|
||
|
|
<A NAME=NUMBERSYNTAXTABLE> </A>
|
||
|
|
<pre>
|
||
|
|
----------------------------------------------------------------
|
||
|
|
Table 22-2: Actual Syntax of Numbers
|
||
|
|
|
||
|
|
<i>number</i> ::= <i>integer</i> | <i>ratio</i> | <i>floating-point-number</i>
|
||
|
|
<i>integer</i> ::= [<i>sign</i>] {<i>digit</i>}+ [<i>decimal-point</i>]
|
||
|
|
<i>ratio</i> ::= [<i>sign</i>] {<i>digit</i>}+ / {<i>digit</i>}+
|
||
|
|
<i>floating-point-number</i> ::= [<i>sign</i>] {<i>digit</i>}* <i>decimal-point</i> {<i>digit</i>}+ [<i>exponent</i>]
|
||
|
|
| [<i>sign</i>] {<i>digit</i>}+ [<i>decimal-point</i> {<i>digit</i>}*] <i>exponent</i>
|
||
|
|
<i>sign</i> ::= + | -
|
||
|
|
<i>decimal-point</i> ::= .
|
||
|
|
<i>digit</i> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
|
||
|
|
<i>exponent</i> ::= <i>exponent-marker</i> [<i>sign</i>] {<i>digit</i>}+
|
||
|
|
<i>exponent-marker</i> ::= e | s | f | d | l | E | S | F | D | L
|
||
|
|
----------------------------------------------------------------
|
||
|
|
</pre>
|
||
|
|
<P>
|
||
|
|
<A NAME=StandardReadtableAttributesTable> </A>
|
||
|
|
<pre>
|
||
|
|
----------------------------------------------------------------
|
||
|
|
Table 22-3: Standard Constituent Character Attributes
|
||
|
|
|
||
|
|
! <i>alphabetic</i> <page> <i>illegal</i> <backspace> <i>illegal</i>
|
||
|
|
" <i>alphabetic</i> * <return> <i>illegal</i> * <tab> <i>illegal</i> *
|
||
|
|
# <i>alphabetic</i> * <space> <i>illegal</i> * <newline> <i>illegal</i> *
|
||
|
|
$ <i>alphabetic</i> <rubout> <i>illegal</i> <linefeed> <i>illegal</i> *
|
||
|
|
% <i>alphabetic</i> . <i>alphabetic</i>, <i>dot</i>, <i>decimal point</i>
|
||
|
|
& <i>alphabetic</i> + <i>alphabetic</i>, <i>plus sign</i>
|
||
|
|
' <i>alphabetic</i> * - <i>alphabetic</i>, <i>minus sign</i>
|
||
|
|
( <i>alphabetic</i> * * <i>alphabetic</i>
|
||
|
|
) <i>alphabetic</i> * / <i>alphabetic</i>, <i>ratio marker</i>
|
||
|
|
, <i>alphabetic</i> * @ <i>alphabetic</i>
|
||
|
|
0 <i>alphadigit</i> A, a <i>alphadigit</i>
|
||
|
|
1 <i>alphadigit</i> B, b <i>alphadigit</i>
|
||
|
|
2 <i>alphadigit</i> C, c <i>alphadigit</i>
|
||
|
|
3 <i>alphadigit</i> D, d <i>alphadigit</i>, <i>double-float exponent marker</i>
|
||
|
|
4 <i>alphadigit</i> E, e <i>alphadigit</i>, <i>float exponent marker</i>
|
||
|
|
5 <i>alphadigit</i> F, f <i>alphadigit</i>, <i>single-float exponent marker</i>
|
||
|
|
6 <i>alphadigit</i> G, g <i>alphadigit</i>
|
||
|
|
7 <i>alphadigit</i> H, h <i>alphadigit</i>
|
||
|
|
8 <i>alphadigit</i> I, i <i>alphadigit</i>
|
||
|
|
9 <i>alphadigit</i> J, j <i>alphadigit</i>
|
||
|
|
: <i>package marker</i> K, k <i>alphadigit</i>
|
||
|
|
; <i>alphabetic</i> * L, l <i>alphadigit</i>, <i>long-float exponent marker</i>
|
||
|
|
< <i>alphabetic</i> M, m <i>alphadigit</i>
|
||
|
|
= <i>alphabetic</i> N, n <i>alphadigit</i>
|
||
|
|
> <i>alphabetic</i> O, o <i>alphadigit</i>
|
||
|
|
? <i>alphabetic</i> P, p <i>alphadigit</i>
|
||
|
|
[ <i>alphabetic</i> Q, q <i>alphadigit</i>
|
||
|
|
\ <i>alphabetic</i> * R, r <i>alphadigit</i>
|
||
|
|
] <i>alphabetic</i> S, s <i>alphadigit</i>, <i>short-float exponent marker</i>
|
||
|
|
^ <i>alphabetic</i> T, t <i>alphadigit</i>
|
||
|
|
_ <i>alphabetic</i> U, u <i>alphadigit</i>
|
||
|
|
` <i>alphabetic</i> * V, v <i>alphadigit</i>
|
||
|
|
{ <i>alphabetic</i> W, w <i>alphadigit</i>
|
||
|
|
| <i>alphabetic</i> * X, x <i>alphadigit</i>
|
||
|
|
} <i>alphabetic</i> Y, y <i>alphadigit</i>
|
||
|
|
~ <i>alphabetic</i> Z, z <i>alphadigit</i>
|
||
|
|
|
||
|
|
----------------------------------------------------------------
|
||
|
|
|
||
|
|
</pre>
|
||
|
|
<P>
|
||
|
|
A token is a potential number if it satisfies the following
|
||
|
|
requirements:
|
||
|
|
<UL><LI>
|
||
|
|
It consists entirely of digits, signs (<tt>+</tt> or <tt>-</tt>),
|
||
|
|
ratio markers (<tt>/</tt>), decimal points (<tt>.</tt>), extension characters
|
||
|
|
(<tt>^</tt> or <tt>_</tt>), 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. Floating-point exponent markers are instances
|
||
|
|
of number markers.)
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
It contains at least one digit. (Letters may be considered to be
|
||
|
|
digits, depending on the value of <tt>*read-base*</tt>, but only
|
||
|
|
in tokens containing no decimal points.)
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
It begins with a digit, sign, decimal point, or extension character.
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
It does not end with a sign.
|
||
|
|
</UL>
|
||
|
|
As examples, the following tokens are potential numbers,
|
||
|
|
but they are <i>not</i> actually numbers as defined below, and so are
|
||
|
|
reserved tokens. (They do indicate some interesting possibilities
|
||
|
|
for future extensions.)
|
||
|
|
<pre>
|
||
|
|
1b5000 777777q 1.7J -3/4+6.7J 12/25/83
|
||
|
|
27^19 3^4/5 6//7 3.1.2.6 ^-43^
|
||
|
|
3.141_592_653_589_793_238_4 -3.7+2.6i-6.17j+19.6k
|
||
|
|
</pre>
|
||
|
|
The following tokens are <i>not</i> potential numbers but are always
|
||
|
|
treated as symbols:
|
||
|
|
<P><pre>
|
||
|
|
/ /5 + 1+ 1-
|
||
|
|
foo+ ab.cd _ ^ ^/-
|
||
|
|
</pre><P>
|
||
|
|
The following tokens are potential numbers if the value of
|
||
|
|
<tt>*read-base*</tt> is <tt>16</tt> (an abnormal situation), but they are
|
||
|
|
always treated as symbols if the value of <tt>*read-base*</tt>
|
||
|
|
is <tt>10</tt> (the usual value):
|
||
|
|
<P><pre>
|
||
|
|
bad-face 25-dec-83 a/b fad_cafe f^
|
||
|
|
</pre><P>
|
||
|
|
It is possible for there to be an ambiguity as to whether
|
||
|
|
a letter should be treated as a digit or as a number marker.
|
||
|
|
In such a case, the letter is always treated as a digit
|
||
|
|
rather than as a number marker.
|
||
|
|
<P>
|
||
|
|
Note that the printed representation for a potential
|
||
|
|
number may not contain any escape characters.
|
||
|
|
An escape character robs the following character of all syntactic
|
||
|
|
qualities, forcing it to be strictly alphabetic and therefore unsuitable
|
||
|
|
for use in a potential number. For example,
|
||
|
|
all of the following representations are interpreted as symbols, not numbers:
|
||
|
|
<pre>
|
||
|
|
\256 25\64 1.0\E6 |100| 3\.14159 |3/4| 3\/4 5||
|
||
|
|
</pre>
|
||
|
|
In each case, removing the escape character(s) would allow the token
|
||
|
|
to be treated as a number.
|
||
|
|
<P>
|
||
|
|
If a potential number can in fact
|
||
|
|
be interpreted as a number according to the BNF
|
||
|
|
syntax in table <A HREF="node189.html#NUMBERSYNTAXTABLE">22-2</A>, then a number object of the
|
||
|
|
appropriate type is constructed and returned. It should be noted that in
|
||
|
|
a given implementation it may be that not all tokens conforming to the
|
||
|
|
actual syntax for numbers can actually be converted into number objects.
|
||
|
|
For example, specifying too large or too small an exponent for a floating-point
|
||
|
|
number may make the number impossible to represent in the implementation.
|
||
|
|
Similarly, a ratio with denominator zero (such as <tt>-35/000</tt>)
|
||
|
|
cannot be represented in <i>any</i> implementation.
|
||
|
|
In any such circumstance where
|
||
|
|
a token with the syntax of a number cannot be converted to an internal
|
||
|
|
number object, an error is signaled. (On the other hand, an error
|
||
|
|
must not be signaled for specifying too many significant digits
|
||
|
|
for a floating-point number; an appropriately truncated or rounded
|
||
|
|
value should be produced.)
|
||
|
|
<P>
|
||
|
|
There is an omission in the syntax of numbers
|
||
|
|
as described in table <A HREF="node189.html#NUMBERSYNTAXTABLE">22-2</A>,
|
||
|
|
in that the syntax does not account for the possible
|
||
|
|
use of letters as digits.
|
||
|
|
The radix used for reading integers and ratios is normally decimal.
|
||
|
|
However, this radix is actually determined by the value of
|
||
|
|
the variable <tt>*read-base*</tt>, whose initial value is <tt>10</tt>.
|
||
|
|
<tt>*read-base*</tt> may take on any integral value between <tt>2</tt> and <tt>36</tt>;
|
||
|
|
let this value be <i>n</i>. Then a token <i>x</i> is interpreted as
|
||
|
|
an integer or ratio in base <i>n</i> if it could be properly
|
||
|
|
so interpreted in the syntax <tt>#<i>n</i>R<i>x</i></tt>
|
||
|
|
(see section <A HREF="node191.html#SHARPSIGNMACROCHARACTERSECTION">22.1.4</A>).
|
||
|
|
So, for example, if the value of <tt>*read-base*</tt> is <tt>16</tt>,
|
||
|
|
then the printed representation
|
||
|
|
<P><pre>
|
||
|
|
(a small face in a bad place)
|
||
|
|
</pre><P>
|
||
|
|
would be interpreted as if the following representation had
|
||
|
|
been read with <tt>*read-base*</tt> set to <tt>10</tt>:
|
||
|
|
<P><pre>
|
||
|
|
(10 small 64206 in 10 2989 place)
|
||
|
|
</pre><P>
|
||
|
|
because four of the seven tokens in the list can be interpreted
|
||
|
|
as hexadecimal numbers. This facility is intended to be used
|
||
|
|
in reading files of data that for some reason contain numbers
|
||
|
|
not in decimal radix; it may also be used for reading programs
|
||
|
|
written in Lisp dialects (such as MacLisp) whose default number radix is not
|
||
|
|
decimal. Non-decimal constants in Common Lisp programs
|
||
|
|
or portable Common Lisp data files should be written using
|
||
|
|
<tt>#O</tt>, <tt>#X</tt>, <tt>#B</tt>, or <tt>#<i>n</i>R</tt> syntax.
|
||
|
|
<P>
|
||
|
|
When <tt>*read-base*</tt> has a value greater than <tt>10</tt>, an ambiguity
|
||
|
|
is introduced into the actual syntax for numbers because a letter can serve
|
||
|
|
as either a digit or an exponent marker; a simple example is <tt>1E0</tt>
|
||
|
|
when the value of <tt>*read-base*</tt> is <tt>16</tt>. The ambiguity is resolved
|
||
|
|
in accordance with the general principle that interpretation
|
||
|
|
as a digit is preferred to interpretation as a number marker.
|
||
|
|
The consequence in this case is that
|
||
|
|
if a token can be interpreted as either an integer or a floating-point
|
||
|
|
number, then it is taken to be an integer.
|
||
|
|
<P>
|
||
|
|
If a token consists solely of dots (with no escape characters), then an
|
||
|
|
error is signaled, except in one circumstance: if the token is a single
|
||
|
|
dot and occurs in a situation appropriate to ``dotted list'' syntax,
|
||
|
|
then it is accepted as a part of such syntax. Signaling an error
|
||
|
|
catches not only misplaced dots in dotted list syntax but also
|
||
|
|
lists that were truncated by <tt>*print-length*</tt> cutoff,
|
||
|
|
because such lists end with a three-dot sequence (<tt>...</tt>).
|
||
|
|
Examples:
|
||
|
|
<P><pre>
|
||
|
|
(a . b) ;A dotted pair of <tt>a</tt> and <tt>b</tt>
|
||
|
|
(a.b) ;A list of one element, the symbol named <tt>a.b</tt>
|
||
|
|
(a. b) ;A list of two elements <tt>a.</tt> and <tt>b</tt>
|
||
|
|
(a .b) ;A list of two elements <tt>a</tt> and <tt>.b</tt>
|
||
|
|
(a . b) ;A list of three elements <tt>a</tt>, <tt>.</tt>, and <tt>b</tt>
|
||
|
|
(a |.| b) ;A list of three elements <tt>a</tt>, <tt>.</tt>, and <tt>b</tt>
|
||
|
|
(a ... b) ;A list of three elements <tt>a</tt>, <tt>...</tt>, and <tt>b</tt>
|
||
|
|
(a |...| b) ;A list of three elements <tt>a</tt>, <tt>...</tt>, and <tt>b</tt>
|
||
|
|
(a b . c) ;A dotted list of <tt>a</tt> and <tt>b</tt> with <tt>c</tt> at the end
|
||
|
|
.iot ;The symbol whose name is <tt>.iot</tt>
|
||
|
|
(. b) ;Illegal; an error is signaled
|
||
|
|
(a .) ;Illegal; an error is signaled
|
||
|
|
(a .. b) ;Illegal; an error is signaled
|
||
|
|
(a . . b) ;Illegal; an error is signaled
|
||
|
|
(a b c ...) ;Illegal; an error is signaled
|
||
|
|
</pre><P>
|
||
|
|
<P>
|
||
|
|
In all other cases, the token is construed to be the name of a symbol.
|
||
|
|
If there are any package markers
|
||
|
|
(colons) in the token, they divide the token into pieces used to
|
||
|
|
control the lookup and creation of the symbol.
|
||
|
|
<P>
|
||
|
|
<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
|
||
|
|
If there is a single package marker, and it occurs at the beginning of the
|
||
|
|
token, then the token is interpreted as a keyword, that is, a symbol in the
|
||
|
|
<tt>keyword</tt> package. The part of the token after the package marker must
|
||
|
|
not have the syntax of a number.
|
||
|
|
<P>
|
||
|
|
If there is a single package marker not at the beginning or end of the
|
||
|
|
token, then it divides the token into two parts. The first part
|
||
|
|
specifies a package; the second part is the name of an external symbol
|
||
|
|
available in that package. Neither of the two parts may have the syntax
|
||
|
|
of a number.
|
||
|
|
<P>
|
||
|
|
If there are two adjacent package markers not at the beginning or end of the
|
||
|
|
token, then they divide the token into two parts. The first part
|
||
|
|
specifies a package; the second part is the name of a symbol within
|
||
|
|
that package (possibly an internal symbol).
|
||
|
|
Neither of the two parts may have the syntax of a number.
|
||
|
|
<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
|
||
|
|
<P>
|
||
|
|
<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
|
||
|
|
X3J13 voted in March 1988
|
||
|
|
(COLON-NUMBER) <A NAME=20763> </A>
|
||
|
|
to clarify that, in the situations described in the
|
||
|
|
preceding three paragraphs, the restriction on the syntax of the parts should
|
||
|
|
be strengthened: none of the parts may have the syntax of even
|
||
|
|
a <i>potential</i> number. Tokens such as <tt>:3600</tt>, <tt>:1/2</tt>,
|
||
|
|
and <tt>editor:3.14159</tt> were already ruled out; this clarification further
|
||
|
|
declares that such tokens as <tt>:2^ 3</tt>, <tt>compiler:1.7J</tt>,
|
||
|
|
and <tt>Christmas:12/25/83</tt> are also in error and therefore should not be used
|
||
|
|
in portable programs. Implementations may differ in their treatment of
|
||
|
|
such package-marked potential numbers.
|
||
|
|
<br><img align=bottom alt="change_end" src="gif/change_end.gif">
|
||
|
|
<P>
|
||
|
|
If a symbol token contains no package markers, then the entire token
|
||
|
|
is the name of the symbol. The symbol is looked up in
|
||
|
|
the default package, which is the value
|
||
|
|
of the variable <tt>*package*</tt>.
|
||
|
|
<P>
|
||
|
|
All other patterns of package markers,
|
||
|
|
including the cases where there are more than two
|
||
|
|
package markers or where a package marker appears at the end of the token,
|
||
|
|
at present do not mean anything in Common Lisp (see chapter <A HREF="node111.html#XPACK">11</A>).
|
||
|
|
It is therefore currently an error to use such patterns in a Common Lisp program.
|
||
|
|
The valid patterns for tokens may be summarized as follows:
|
||
|
|
<PRE>
|
||
|
|
<i>nnnnn</i> a number
|
||
|
|
<i>xxxxx</i> a symbol in the current package
|
||
|
|
<tt>:<i>xxxxx</i></tt> a symbol in the keyword package
|
||
|
|
<tt><i>ppppp</i>:<i>xxxxx</i></tt> an external symbol in the <i>ppppp</i> package
|
||
|
|
<tt><i>ppppp</i>::<i>xxxxx</i></tt> a (possibly internal) symbol in the <i>ppppp</i> package
|
||
|
|
</PRE>
|
||
|
|
where <i>nnnnn</i> has the syntax of a number, and <i>xxxxx</i> and <i>ppppp</i> do
|
||
|
|
not have the syntax of a number.
|
||
|
|
<P>
|
||
|
|
<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
|
||
|
|
In accordance with the X3J13 decision noted above
|
||
|
|
(COLON-NUMBER) <A NAME=20789> </A> , <i>xxxxx</i> and <i>ppppp</i> may not have the syntax of even
|
||
|
|
a potential number.
|
||
|
|
<br><img align=bottom alt="change_end" src="gif/change_end.gif">
|
||
|
|
<P>
|
||
|
|
<BR><b>[Variable]</b><BR>
|
||
|
|
<tt>*read-base*</tt><P>The value of <tt>*read-base*</tt> controls the interpretation of tokens
|
||
|
|
by <tt>read</tt> as being integers or ratios. Its value is the radix
|
||
|
|
in which integers and ratios are to be read; the value may be any integer
|
||
|
|
from <tt>2</tt> to <tt>36</tt> (inclusive) and is normally <tt>10</tt> (decimal radix).
|
||
|
|
Its value affects only the reading of integers and ratios.
|
||
|
|
In particular, floating-point numbers are always read in decimal radix.
|
||
|
|
The value of <tt>*read-base*</tt> does not affect the radix for rational numbers
|
||
|
|
whose radix is explicitly indicated by
|
||
|
|
<tt>#O</tt>, <tt>#X</tt>, <tt>#B</tt>, or <tt>#<i>n</i>R</tt> syntax
|
||
|
|
or by a trailing decimal point.
|
||
|
|
<P>
|
||
|
|
Care should be taken when setting <tt>*read-base*</tt> to a value larger
|
||
|
|
than <tt>10</tt>, because tokens that would normally be interpreted as
|
||
|
|
symbols may be interpreted as numbers instead. For example,
|
||
|
|
with <tt>*read-base*</tt> set to <tt>16</tt> (hexadecimal radix), variables
|
||
|
|
with names such as <tt>a</tt>, <tt>b</tt>, <tt>f</tt>, <tt>bad</tt>, and <tt>face</tt>
|
||
|
|
will be treated by the reader as numbers (with decimal values
|
||
|
|
10, 11, 15, 2989, and 64206, respectively). The ability to alter
|
||
|
|
the input radix is provided in Common Lisp primarily for the purpose of
|
||
|
|
reading data files in special formats, rather than for the purpose of altering
|
||
|
|
the default radix in which to read programs. The user is strongly
|
||
|
|
encouraged to use <tt>#O</tt>, <tt>#X</tt>, <tt>#B</tt>, or <tt>#<i>n</i>R</tt> syntax when
|
||
|
|
notating non-decimal constants in programs.
|
||
|
|
<P>
|
||
|
|
<hr>
|
||
|
|
<b>Compatibility note:</b> This variable corresponds to the variable
|
||
|
|
called <tt>ibase</tt> in MacLisp and to the function called <tt>radix</tt>
|
||
|
|
in Interlisp.
|
||
|
|
<hr>
|
||
|
|
<P>
|
||
|
|
<BR><b>[Variable]</b><BR>
|
||
|
|
<tt>*read-suppress*</tt><P>When the value of <tt>*read-suppress*</tt> is <tt>nil</tt>, the Lisp reader
|
||
|
|
operates normally. When it is not <tt>nil</tt>, then most of the interesting
|
||
|
|
operations of the reader are suppressed; input characters are parsed,
|
||
|
|
but much of what is read is not interpreted.
|
||
|
|
<P>
|
||
|
|
The primary purpose of <tt>*read-suppress*</tt> is to support the operation of
|
||
|
|
the read-time conditional constructs <tt>#+</tt> and <tt>#-</tt>
|
||
|
|
(see section <A HREF="node191.html#SHARPSIGNMACROCHARACTERSECTION">22.1.4</A>). It is important for these
|
||
|
|
constructs to be able to skip over the printed representation of a Lisp
|
||
|
|
expression despite the possibility that the syntax of the skipped
|
||
|
|
expression may not be entirely legal for the current implementation; this
|
||
|
|
is because a primary application of <tt>#+</tt> and <tt>#-</tt> is to allow the
|
||
|
|
same program to be shared among several Lisp implementations despite
|
||
|
|
small incompatibilities of syntax.
|
||
|
|
<P>
|
||
|
|
A non-<tt>nil</tt> value of <tt>*read-suppress*</tt> has the following specific
|
||
|
|
effects on the Common Lisp reader:
|
||
|
|
<UL><LI>
|
||
|
|
All extended tokens are completely uninterpreted. It matters not
|
||
|
|
whether the token looks like a number, much less like a valid number;
|
||
|
|
the pattern of package markers also does not matter. An extended token
|
||
|
|
is simply discarded and treated as if it were <tt>nil</tt>; that is, reading
|
||
|
|
an extended token when <tt>*read-suppress*</tt> is non-<tt>nil</tt> simply returns <tt>nil</tt>.
|
||
|
|
(One consequence of this is that the error concerning improper
|
||
|
|
dotted-list syntax will not be signaled.)
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
Any standard
|
||
|
|
<tt>#</tt> macro-character construction that requires, permits, or disallows
|
||
|
|
an infix numerical argument, such as <tt>#<i>n</i>R</tt>, will not enforce
|
||
|
|
any constraint on the presence, absence, or value of such an argument.
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
The <tt>#\</tt> construction always produces the value <tt>nil</tt>.
|
||
|
|
It will not signal an error even if an unknown character name is seen.
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
Each of the <tt>#B</tt>, <tt>#O</tt>, <tt>#X</tt>, and <tt>#R</tt>
|
||
|
|
constructions always scans over a following token and produces the value <tt>nil</tt>.
|
||
|
|
It will not signal an error even if the token does not have the syntax
|
||
|
|
of a rational number.
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
The <tt>#*</tt> construction always scans over a following token and
|
||
|
|
produces the value <tt>nil</tt>.
|
||
|
|
It will not signal an error even if the token does not consist solely
|
||
|
|
of the characters <tt>0</tt> and <tt>1</tt>.
|
||
|
|
</UL>
|
||
|
|
<P>
|
||
|
|
<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
|
||
|
|
<UL><LI>
|
||
|
|
Each of the <tt>#.</tt> and <tt>#,</tt> constructions reads the following
|
||
|
|
form (in suppressed mode, of course) but does not evaluate it.
|
||
|
|
The form is discarded and <tt>nil</tt> is produced.
|
||
|
|
</UL>
|
||
|
|
<img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
|
||
|
|
<P>
|
||
|
|
<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
|
||
|
|
<ul>
|
||
|
|
X3J13 voted in January 1989
|
||
|
|
(SHARP-COMMA-CONFUSION) <A NAME=20861> </A> to remove <tt>#,</tt> from the language.
|
||
|
|
</ul>
|
||
|
|
<img align=bottom alt="change_end" src="gif/change_end.gif">
|
||
|
|
<P>
|
||
|
|
<UL><LI>
|
||
|
|
Each of the <tt>#A</tt>, <tt>#S</tt>, and <tt>#:</tt>
|
||
|
|
constructions reads the following
|
||
|
|
form (in suppressed mode, of course) but does not interpret it in any way;
|
||
|
|
it need not even be a list in the case of <tt>#S</tt>, or a symbol
|
||
|
|
in the case of <tt>#:</tt>. The form is discarded and <tt>nil</tt> is produced.
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
The <tt>#=</tt> construction is totally ignored. It does not read
|
||
|
|
a following form. It produces no object, but is treated as whitespace.
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
The <tt>##</tt> construction always produces <tt>nil</tt>.
|
||
|
|
</UL>
|
||
|
|
Note that, no matter what the value of <tt>*read-suppress*</tt>,
|
||
|
|
parentheses still continue to delimit (and construct) lists;
|
||
|
|
the <tt>#(</tt> construction continues to delimit vectors;
|
||
|
|
and comments, strings, and the quote and backquote constructions continue to be
|
||
|
|
interpreted properly. Furthermore, such situations as
|
||
|
|
<tt>')</tt>,
|
||
|
|
<tt>#<</tt>, <tt>#)</tt>, and <tt>#<b></b>space<b></b></tt> continue to signal errors.
|
||
|
|
<P>
|
||
|
|
In some cases, it may be appropriate for a user-written macro-character
|
||
|
|
definition to check the value of <tt>*read-suppress*</tt> and to avoid certain
|
||
|
|
computations or side effects if its value is not <tt>nil</tt>.
|
||
|
|
<P>
|
||
|
|
<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
|
||
|
|
<BR><b>[Variable]</b><BR>
|
||
|
|
<tt>*read-eval*</tt><P>X3J13 voted in June 1989 (DATA-IO) <A NAME=20886> </A> to add a new reader control variable,
|
||
|
|
<tt>*read-eval*</tt>, whose default value is <tt>t</tt>.
|
||
|
|
If <tt>*read-eval*</tt> is false, the <tt>#.</tt> reader macro signals an error.
|
||
|
|
<P>
|
||
|
|
Printing is also affected. If
|
||
|
|
<tt>*read-eval*</tt> is false and <tt>*print-readably*</tt> is true, any <tt>print-object</tt>
|
||
|
|
method that would otherwise output a <tt>#.</tt> reader macro must either output something
|
||
|
|
different or signal an error of type <tt>print-not-readable</tt>.
|
||
|
|
<P>
|
||
|
|
Binding <tt>*read-eval*</tt> to <tt>nil</tt> is useful when reading data that came from
|
||
|
|
an untrusted source, such as a network or a user-supplied data file; it
|
||
|
|
prevents the <tt>#.</tt> reader macro from being exploited as a ``Trojan horse'' to
|
||
|
|
cause arbitrary forms to be evaluated.
|
||
|
|
<br><img align=bottom alt="change_end" src="gif/change_end.gif">
|
||
|
|
<P>
|
||
|
|
<BR> <HR><A NAME=tex2html3903 HREF="node190.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3901 HREF="node187.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3895 HREF="node188.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3905 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3906 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html3904 HREF="node190.html"> Macro Characters</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html3902 HREF="node187.html"> Printed Representation of </A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html3896 HREF="node188.html"> What the Read </A>
|
||
|
|
<HR> <P>
|
||
|
|
<HR>
|
||
|
|
<P><ADDRESS>
|
||
|
|
AI.Repository@cs.cmu.edu
|
||
|
|
</ADDRESS>
|
||
|
|
</BODY>
|