lookahead-char, peek-char - peek at the next character from a port

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LIBRARY

;; lookahead-char
(import (rnrs))                     ;R6RS
(import (rnrs io ports))            ;R6RS

;; peek-char
(import (rnrs))                     ;R6RS
(import (rnrs io simple))           ;R6RS
(import (scheme r5rs))              ;R7RS
(import (scheme base))              ;R7RS

SYNOPSIS

(lookahead-char textual-input-port) ;R6RS
(peek-char)
(peek-char textual-input-port)

DESCRIPTION

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.

If textual-input-port is omitted, it defaults to the value returned by current-input-port(3scm).

IMPLEMENTATION NOTES

RETURN VALUES

EXAMPLES

;; 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)))
            (else
             (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
            terminator)
           ((or (eof-object? terminator)
                (eqv? terminator #\newline))
            ;; The record was terminated with or
            ;; without a newline.
            (reverse (cons field fields)))
           (else
            ;; Save the field from the record.
            (lp (cons field fields)))))))

(call-with-input-file "/etc/passwd"
  (lambda (p)
    ;; 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)
          (write record)
          (newline)
          (lp))))))

;; 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")
;; ...

APPLICATION USAGE

RATIONALE

COMPATIBILITY

ERRORS

&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 textual-input-port.

&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 error-handling-mode(7scm) is raise.

R7RS

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.

SEE ALSO

STANDARDS

HISTORY

AUTHORS

BUGS