read-delimited-list char {&optional input-stream recursive-p} => list
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 whitespace_2 characters and comments) is char.
read-delimited-list looks ahead at each step for the next non-whitespace_2 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 whitespace_2 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*.
@xref{read; read-preserving-whitespace} , section peek-char [Function] , section read-char [Function] , section unread-char [Function] .
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.
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