char—a character.
input-stream—an input stream designator. The default is standard input.
recursive-p—a generalized boolean. The default is false.
list—a list of the objects read.
read-delimited-list
reads objects from input-stream
until the next character after an object's
representation (ignoring whitespace2 characters and comments) is char.
read-delimited-list
looks ahead at each step
for the next non-whitespace2 character
and peeks at it as if with peek-char
.
If it is char,
then the character is consumed and the list of objects is returned.
If it is a constituent or escape character,
then read
is used to read an object,
which is added to the end of the list.
If it is a macro character,
its reader macro function is called;
if the function returns a value,
that value is added to the list.
The peek-ahead process is then repeated.
If recursive-p is true,
this call is expected to be embedded in a higher-level call to read
or a similar function.
It is an error to reach end-of-file during the operation of
read-delimited-list
.
The consequences are undefined if char has a syntax type of whitespace2
in the current readtable.
(read-delimited-list #\]) 1 2 3 4 5 6 ]
→ (1 2 3 4 5 6)
Suppose you wanted #
{a b c ...
z}
to read as a list of all pairs of the elements a, b, c,
..., z, for example.
#{p q z a} reads as ((p q) (p z) (p a) (q z) (q a) (z a))
This can be done by specifying a macro-character definition for #
{
that does two things: reads in all the items up to the },
and constructs the pairs. read-delimited-list
performs
the first task.
(defun |#{-reader| (stream char arg)
(declare (ignore char arg))
(mapcon #'(lambda (x)
(mapcar #'(lambda (y) (list (car x) y)) (cdr x)))
(read-delimited-list #\} stream t))) → |#{-reader|
(set-dispatch-macro-character #\# #\{ #'|#{-reader|) → T
(set-macro-character #\} (get-macro-character #\) nil
))
Note that true is supplied for the recursive-p argument.
It is necessary here to give a definition to the character } as well to prevent it from being a constituent. If the line
(set-macro-character #\} (get-macro-character #\) nil
))
shown above were not included, then the } in
#{ p q z a}
would be considered a constituent character, part of the symbol named
a
}. This could be corrected by putting a space before
the }, but it is better to call
set-macro-character
.
Giving } the same
definition as the standard definition of the character )
has the
twin benefit of making it terminate tokens for use with
read-delimited-list
and also making it invalid for use in any
other context. Attempting to read a stray } will signal an error.
*standard-input*
,
*readtable*
,
*terminal-io*
.
read, peek-char, read-char, unread-char.
read-delimited-list
is intended for use in implementing reader macros.
Usually it is desirable for char to be a terminating macro character
so that it can be used to delimit tokens; however, read-delimited-list
makes no attempt to alter the syntax specified for char by the current
readtable. The caller must make any necessary changes to the readtable syntax
explicitly.