21.2.16 peek-char [Function]

peek-char &optional peek-type input-stream eof-error-p eof-value recursive-p ⇒ char

Arguments and Values::

peek-type—a character or t or nil.

input-stream—input stream designator. The default is standard input.

eof-error-p—a generalized boolean. The default is true.

eof-value—an object. The default is nil.

recursive-p—a generalized boolean. The default is false.

char—a character or the eof-value.

Description::

peek-char obtains the next character in input-stream without actually reading it, thus leaving the character to be read at a later time. It can also be used to skip over and discard intervening characters in the input-stream until a particular character is found.

If peek-type is not supplied or nil, peek-char returns the next character to be read from input-stream, without actually removing it from input-stream. The next time input is done from input-stream, the character will still be there. If peek-type is t, then peek-char skips over whitespace_2 characters, but not comments, and then performs the peeking operation on the next character. The last character examined, the one that starts an object, is not removed from input-stream. If peek-type is a character, then peek-char skips over input characters until a character that is char= to that character is found; that character is left in input-stream.

If an end of file_2 occurs and eof-error-p is false, eof-value is returned.

If recursive-p is true, this call is expected to be embedded in a higher-level call to read or a similar function used by the Lisp reader.

When input-stream is an echo stream, characters that are only peeked at are not echoed. In the case that peek-type is not nil, the characters that are passed by peek-char are treated as if by read-char, and so are echoed unless they have been marked otherwise by unread-char.

Examples::

 (with-input-from-string (input-stream "    1 2 3 4 5")
    (format t "~S ~S ~S" 
            (peek-char t input-stream)
            (peek-char #\4 input-stream)
            (peek-char nil input-stream)))
 |>  #\1 #\4 #\4
⇒  NIL

Affected By::

*readtable*, *standard-input*, *terminal-io*.

Exceptional Situations::

If eof-error-p is true and an end of file_2 occurs an error of type end-of-file is signaled.

If peek-type is a character, an end of file_2 occurs, and eof-error-p is true, an error of type end-of-file is signaled.

If recursive-p is true and an end of file_2 occurs, an error of type end-of-file is signaled.