lookahead-char, peek-char - peek at the next character from a port
(import (rnrs)) ;R6RS
(import (rnrs io ports)) ;R6RS
(import (rnrs)) ;R6RS
(import (rnrs io simple)) ;R6RS
(import (scheme r5rs)) ;R7RS
(import (scheme base)) ;R7RS
(lookahead-char textual-input-port) ;R6RS
Read a character from
without updating the port to point past the character.
This procedure blocks as necessary until a character is available, or
the data that are available cannot be the prefix of any valid
encoding, or an end of file is reached.
If a complete character is available before the next end of file, this
procedure returns that character.
If an end of file is reached before any data are read, this procedure
returns the end-of-file object.
is omitted, it defaults to the value returned by
With the standard UTF-8, UTF-16 and UTF-32 transcoders, up to four
bytes of lookahead may be needed.
They may additionally skip over the BOM at the start of a file, in
which case two to four bytes are skipped over.
Nonstandard transcoders may need even more lookahead.
Returns a single value; a character or an end of file object.
;; This demonstrates one way to parse /etc/passwd
;; using peek-char.
(import (scheme base) (scheme file) (scheme write))
(define (get-string-until-chars p terminators)
(let lp ((chars '()))
(let ((c (peek-char p)))
(cond ((or (eof-object? c)
(memv c terminators))
;; Take the characters we found up until
;; the terminator and return them as
;; a string.
(list->string (reverse chars)))
(lp (cons (read-char p) chars)))))))
(define (get-record p)
(let lp ((fields '()))
(let* ((field (get-string-until-chars p '(#\: #\newline)))
(terminator (read-char p)))
(cond ((and (eof-object? terminator) (null? fields))
;; Returns the end-of-file object
((or (eof-object? terminator)
(eqv? terminator #\newline))
;; The record was terminated with or
;; without a newline.
(reverse (cons field fields)))
;; Save the field from the record.
(lp (cons field fields)))))))
;; Read records from the port and write them as lists to standard
;; output until the end of the file.
(let lp ()
(let ((record (get-record p)))
(unless (eof-object? record)
;; The R7RS program above prints something like this:
;; ("root" "x" "0" "0" "root" "/root" "/bin/bash")
;; ("daemon" "x" "1" "1" "daemon" "/usr/sbin" "/usr/sbin/nologin")
;; ("bin" "x" "2" "2" "bin" "/bin" "/usr/sbin/nologin")
These procedures are commonly used in a certain style of parsers,
which dispatch on the next character from the input.
An alternative to providing lookahead would be to provide a procedure
which "unreads" a character from a port. This latter type of operation
raises questions of how many characters it should be possible to unread.
Normally in Scheme one would not want there to be a limit for this case,
but such a requirement would demand a less efficient implementation of
which is also not in line with how Scheme is normally designed.
Except for R7RS's variations in which types of ports and characters
are supported, these procedures work the same everywhere. The
name is unique to R6RS.
This procedure can raise exceptions with the following condition types:
- &assertion (R6RS)
The wrong number of arguments was passed or an argument was outside its domain.
- &i/o-port (R6RS)
This condition is included with the conditions below and contains
- &i/o-read-error (R6RS)
There was a read error during the I/O operation.
- &i/o-decoding (R6RS)
The input from a port cannot be translated into a character by
the port's transcoder and the transcoder's
The assertions described above are errors.
Implementations may signal an error, extend the procedure's
domain of definition to include such arguments,
or fail catastrophically.
is new to R6RS's reworked I/O system.
procedure first showed up in R4RS.
This page is part of the
It includes materials from the RnRS documents.
More information can be found at
These procedures are prone to a type of bug where you forget to
consume data from the port following a peek, resulting in an infinite
loop of peeking.
Markup created by unroff 1.0sc, March 04, 2023.