sunet/rfc822.scm

219 lines
9.5 KiB
Scheme

;;; RFC 822 field-parsing code -*- Scheme -*-
;;; Copyright (c) 1995 by Olin Shivers.
;;; <shivers@lcs.mit.edu>
;;;
;;; Imports and non-R4RS'isms
;;; downcase-string upcase-string (string->symbol conversion)
;;; read-crlf-line
;;; let-optionals, :optional
;;; receive values (MV return)
;;; "\r\n" in string for cr/lf
;;; ascii->char (defining the tab char)
;;; index
;;; join-strings (reassembling body lines)
;;; error
;;; ? (COND)
;;; RFC 822 is the "Standard for the format of ARPA Internet text messages"
;;; -- the document that essentially tells how the fields in email headers
;;; (e.g., the Subject: and To: fields) are formatted. This code is for
;;; parsing these headers. Here are two pointers to the document:
;;; Emacs/ange /ftp@ftp.internic.net:/rfc/rfc822.txt
;;; URL ftp://ftp.internic.net/rfc/rfc822.txt
;;; RFC 822 parsing is useful in other contexts as well -- the HTTP protocol
;;; uses it, and it tends to pop up here and there.
;;;
;;; RFC 822 header syntax has two levels: the general syntax for headers,
;;; and the syntax for specific headers. For example, once you have figured
;;; out which chunk of text is the To: line, there are more rules telling
;;; how to split the To: line up into a list of addresses. Another example:
;;; lines with dates, e.g., the Date: header, have a specific syntax for
;;; the time and date.
;;;
;;; This code currently *only* provides routines for parsing the gross
;;; structure -- splitting the message header into its distinct fields.
;;; It would be nice to provide the finer-detail parsers, too. You do it.
;;; -Olin
;;; A note on line-terminators:
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Line-terminating sequences are always a drag, because there's no agreement
;;; on them -- the Net protocols and DOS use cr/lf; Unix uses lf; the Mac
;;; uses cr. One one hand, you'd like to use the code for all of the above,
;;; on the other, you'd also like to use the code for strict applications
;;; that need definitely not to recognise bare cr's or lf's as terminators.
;;;
;;; RFC 822 requires a cr/lf (carriage-return/line-feed) pair to terminate
;;; lines of text. On the other hand, careful perusal of the text shows up
;;; some ambiguities (there are maybe three or four of these, and I'm too
;;; lazy to write them all down). Furthermore, it is an unfortunate fact
;;; that many Unix apps separate lines of RFC 822 text with simple linefeeds
;;; (e.g., messages kept in /usr/spool/mail). As a result, this code takes a
;;; broad-minded view of line-terminators: lines can be terminated by either
;;; cr/lf or just lf, and either terminating sequence is trimmed.
;;;
;;; If you need stricter parsing, you can call the lower-level procedure
;;; %READ-RFC-822-FIELD and %READ-RFC822-HEADERS procs. They take the
;;; read-line procedure as an extra parameter. This means that you can
;;; pass in a procedure that recognises only cr/lf's, or only cr's (for a
;;; Mac app, perhaps), and you can determine whether or not the terminators
;;; get trimmed. However, your read-line procedure must indicate the
;;; header-terminating empty line by returning *either* the empty string or
;;; the two-char string cr/lf (or the EOF object).
;;; (read-rfc822-field [port])
;;; (%read-rfc822-field read-line port)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Read one field from the port, and return two values [NAME BODY]:
;;; - NAME Symbol such as 'subject or 'to. The field name is converted
;;; to a symbol using the Scheme implementation's preferred
;;; case. If the implementation reads symbols in a case-sensitive
;;; fashion (e.g., scsh), lowercase is used. This means you can
;;; compare these symbols to quoted constants using EQ?. When
;;; printing these field names out, it looks best if you capitalise
;;; them with (CAPITALIZE-STRING (SYMBOL->STRING FIELD-NAME)).
;;; - BODY List of strings which are the field's body, e.g.
;;; ("shivers@lcs.mit.edu"). Each list element is one line
;;; from the field's body, so if the field spreads out
;;; over three lines, then the body is a list of three
;;; strings. The terminating cr/lf's are trimmed from each
;;; string. A leading space or a leading horizontal tab
;;; is also trimmed, but one and only one.
;;; When there are no more fields -- EOF or a blank line has terminated the
;;; header section -- then the procedure returns [#f #f].
;;;
;;; The %READ-RFC822-FIELD variant allows you to specify your own
;;; read-line procedure. The one used by READ-RFC822-FIELD terminates
;;; lines with either cr/lf or just lf, and it trims the terminator
;;; from the line. Your read-line procedure should trim the terminator
;;; of a line so an empty line is returned just as an empty string.
(define htab (ascii->char 9))
;;; Convert to a symbol using the Scheme implementation's preferred case,
;;; so we can compare these things against quoted constants.
(define string->symbol-pref
(if (char=? #\a (string-ref (symbol->string 'a) 0)) ; Is it #\a or #\A?
(lambda (s) (string->symbol (downcase-string s)))
(lambda (s) (string->symbol (upcase-string s)))))
(define (read-rfc822-field . maybe-port)
(let-optionals maybe-port ((port (current-input-port)))
(%read-rfc822-field read-crlf-line port)))
(define (%read-rfc822-field read-line port)
(let ((line1 (read-line port)))
(if (or (eof-object? line1)
(zero? (string-length line1))
(string=? line1 "\r\n")) ; In case read-line doesn't trim.
(values #f #f) ; Blank line or EOF terminates header text.
(? ((string-index line1 #\:) => ; Find the colon and
(lambda (colon) ; split out field name.
(let ((name (string->symbol-pref (substring line1 0 colon))))
;; Read in continuation lines.
(let lp ((lines (list (substring line1
(+ colon 1)
(string-length line1)))))
(let ((c (peek-char port))) ; Could return EOF.
;;; RFC822: continuous lines has to start with a space or a htab
(if (or (eqv? c #\space) (eqv? c htab))
(lp (cons (read-line port) lines))
(values name (reverse lines))))))))
(else (error "Illegal RFC 822 field syntax." line1)))))) ; No :
;;; (read-rfc822-headers [port])
;;; (%read-rfc822-headers read-line port)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Read in and parse up a section of text that looks like the header portion
;;; of an RFC 822 message. Return an alist mapping a field name (a symbol
;;; such as 'date or 'subject) to a list of field bodies -- one for
;;; each occurence of the field in the header. So if there are five
;;; "Received-by:" fields in the header, the alist maps 'received-by
;;; to a five element list. Each body is in turn represented by a list
;;; of strings -- one for each line of the field. So a field spread across
;;; three lines would produce a three element body.
;;;
;;; The %READ-RFC822-HEADERS variant allows you to specify your own read-line
;;; procedure. See notes above for reasons why.
(define (read-rfc822-headers . maybe-port)
(let-optionals maybe-port ((port (current-input-port)))
(%read-rfc822-headers read-crlf-line port)))
(define (%read-rfc822-headers read-line port)
(let lp ((alist '()))
(receive (field val) (%read-rfc822-field read-line port)
(? (field (? ((assq field alist) =>
(lambda (entry)
(set-cdr! entry (cons val (cdr entry)))
(lp alist)))
(else (lp (cons (list field val) alist)))))
;; We are done. Reverse the order of each entry and return.
(else (for-each (lambda (entry)
(set-cdr! entry (reverse (cdr entry))))
alist)
alist)))))
;;; (rejoin-header-lines alist [separator])
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Takes a field alist such as is returned by READ-RFC822-HEADERS and
;;; returns an equivalent alist. Each body (string list) in the input alist
;;; is joined into a single list in the output alist. SEPARATOR is the
;;; string used to join these elements together; it defaults to a single
;;; space " ", but can usefully be "\n" or "\r\n".
;;;
;;; To rejoin a single body list, use scsh's JOIN-STRINGS procedure.
(define (rejoin-header-lines alist . maybe-separator)
(let-optionals maybe-separator ((sep " "))
(map (lambda (entry)
(cons (car entry)
(map (lambda (body) (join-strings body sep))
(cdr entry))))
alist)))
;;; Given a set of RFC822 headers like this:
;;; From: shivers
;;; To: ziggy,
;;; newts
;;; To: gjs, tk
;;;
;;; We have the following definitions:
;;; (get-header-all hdrs 'to) -> ((" ziggy," " newts") (" gjs, tk"))
;;; - All entries, or #f
;;; (get-header-lines hdrs 'to) -> (" ziggy," " newts")
;;; - All lines of the first entry, or #f.
;;; (get-header hdrs 'to) -> "ziggy,\n newts"
;;; - First entry, with the lines joined together by newlines.
(define (get-header-all headers name)
(let ((entry (assq name headers)))
(and entry (cdr entry))))
(define (get-header-lines headers name)
(let ((entry (assq name headers)))
(and entry
(pair? entry)
(cadr entry))))
(define (get-header headers name . maybe-sep)
(let ((entry (assq name headers)))
(and entry
(pair? entry)
(join-strings (cadr entry)
(:optional maybe-sep "\n")))))
;;; Other desireable functionality
;;; - Unfolding long lines.
;;; - Lexing structured fields.
;;; - Unlexing structured fields into canonical form.
;;; - Parsing and unparsing dates.
;;; - Parsing and unparsing addresses.