From 277a836aa2e9fca708b8860533ef68227a4c9308 Mon Sep 17 00:00:00 2001
From: Arun Isaac
Date: Sat, 8 Sep 2018 17:40:37 +0530
Subject: Initial commit.

---
 email/email.scm | 804 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 804 insertions(+)
 create mode 100644 email/email.scm

(limited to 'email/email.scm')

diff --git a/email/email.scm b/email/email.scm
new file mode 100644
index 0000000..ccda3ac
--- /dev/null
+++ b/email/email.scm
@@ -0,0 +1,804 @@
+;;; guile-email --- Guile email parser
+;;; Copyright © 2018 Arun Isaac <arunisaac@systemreboot.net>
+;;;
+;;; This file is part of guile-email.
+;;;
+;;; guile-email is free software; you can redistribute it and/or modify
+;;; it under the terms of the GNU Affero General Public License as
+;;; published by the Free Software Foundation; either version 3 of the
+;;; License, or (at your option) any later version.
+;;;
+;;; guile-email is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;; Affero General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU Affero General Public
+;;; License along with guile-email.  If not, see
+;;; <http://www.gnu.org/licenses/>.
+
+(define-module (email email)
+  #:use-module (ice-9 iconv)
+  #:use-module (ice-9 match)
+  #:use-module (ice-9 peg)
+  #:use-module (ice-9 regex)
+  #:use-module (rnrs io simple)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-2)
+  #:use-module (srfi srfi-9)
+  #:use-module (srfi srfi-11)
+  #:use-module (srfi srfi-19)
+  #:use-module (srfi srfi-26)
+  #:use-module (sxml transform)
+  #:use-module (email base64)
+  #:use-module (email quoted-printable)
+  #:use-module ((email utils)
+		#:select (get-line-with-delimiter
+			  read-objects read-while
+			  acons* alist-delete*))
+  #:export (<email>
+	    make-email
+	    email?
+	    email-headers
+	    email-body
+	    <mime-entity>
+	    make-mime-entity
+	    mime-entity?
+	    mime-entity-headers
+	    mime-entity-body
+	    email->headers+body
+	    parse-email
+	    parse-email-headers
+	    parse-email-body
+	    parse-email-address
+	    interpret-address
+	    mbox->emails))
+
+(define (flatten-and-filter terms tree)
+  (filter list? (keyword-flatten terms tree)))
+
+(define-record-type <email>
+  (make-email headers body)
+  email?
+  (headers email-headers)
+  (body email-body))
+
+(define-record-type <mime-entity>
+  (make-mime-entity headers body)
+  mime-entity?
+  (headers mime-entity-headers)
+  (body mime-entity-body))
+
+(define string->lcase-symbol
+  (compose string->symbol string-downcase))
+
+;;; PEG parser implementing the ABNF grammar specified in RFC5322
+;;; (Internet Message Format), RFC6854 (Update to Internet Message
+;;; Format to Allow Group Syntax in the "From:" and "Sender:" Header
+;;; Fields)
+
+;;; Obsolete syntax has not been implemented.
+
+;;; Core ABNF rules from RFC5234
+
+(define-peg-pattern alpha body
+  (or (range #\A #\Z) (range #\a #\z)))
+
+;; Though line endings should be crlf (\r\n), we also tolerate bare
+;; line feeds (\n)
+(define-peg-pattern crlf none
+  (or "\r\n" "\n"))
+
+(define-peg-pattern digit body
+  (range #\0 #\9))
+
+(define-peg-pattern dquote body
+  "\"")
+
+;; Printable ASCII characters and UTF-8 characters > \x7f (RFC6532)
+(define-peg-pattern vchar body
+  (and (not-followed-by (or (range #\Nul #\Space)
+			    "\x7f"))
+       peg-any))
+
+(define-peg-pattern wsp body
+  (or " " "\t"))
+
+(define-peg-pattern lwsp body
+  (* (or wsp (and crlf wsp))))
+
+(define-syntax-rule (define-printable-ascii-character-pattern name . exceptions)
+  (define-peg-pattern name body
+    (and (not-followed-by (or . exceptions))
+	 vchar)))
+
+;;; Quoted characters
+
+(define-peg-pattern quoted-pair body
+  (and (ignore "\\") (or vchar wsp)))
+
+;;; Folding white space and comments
+
+(define-peg-pattern fws body
+  (and (? (and (* wsp) crlf)) (+ wsp)))
+
+(define-printable-ascii-character-pattern ctext "(" ")" "\\")
+
+(define-peg-pattern comment none
+  (and "(" (* (and (? fws) ccontent)) ")"))
+
+(define-peg-pattern ccontent body
+  (or ctext quoted-pair comment))
+
+(define-peg-pattern cfws body
+  (or (and (+ (and (? fws) comment)) (? fws)) fws))
+
+;;; Atom
+
+(define-printable-ascii-character-pattern atext
+  "\"" "(" ")" "," "." ":" ";" "<" ">" "@" "[" "\\" "]")
+
+(define-peg-pattern atom body
+  (and (? cfws) (+ atext) (? cfws)))
+
+(define-peg-pattern dot-atom-text body
+  (and (+ atext) (* (and "." (+ atext)))))
+
+(define-peg-pattern dot-atom body
+  (and (? cfws) dot-atom-text (? cfws)))
+
+;;; Quoted strings
+
+(define-printable-ascii-character-pattern qtext "\\" "\"")
+
+(define-peg-pattern qcontent body
+  (or qtext quoted-pair))
+
+;; TODO: Remove workaround guile peg bug for ignore
+(define-peg-pattern quoted-string body
+  (and (? cfws) (ignore (and dquote))
+       (* (and (? fws) qcontent))
+       (? fws)
+       (ignore (and dquote)) (? cfws)))
+
+;;; Miscellaneous tokens
+
+(define-peg-pattern word body
+  (or atom quoted-string))
+
+(define-peg-pattern phrase body
+  (+ word))
+
+;; ABNF modified to ignore leading whitespace
+;; ABNF modified to allow for blank lines in folded field
+(define-peg-pattern unstructured body
+  (and (ignore (? fws))
+       (* (and (? fws) (? vchar)))
+       (ignore (* wsp))))
+
+;;; Date and time specification
+
+(define-peg-pattern day-name body
+  (or "Mon" "Tue" "Wed" "Thu" "Fri" "Sat" "Sun"))
+
+(define-peg-pattern day-of-week all
+  (and (ignore (? fws)) day-name))
+
+;; TODO: Remove workaround guile peg bug for ignore
+(define-peg-pattern day all
+  (and (ignore (? fws)) digit (? digit) (ignore (and fws))))
+
+(define-peg-pattern month all
+  (or "Jan" "Feb" "Mar" "Apr" "May" "Jun"
+      "Jul" "Aug" "Sep" "Oct" "Nov" "Dec"))
+
+;; TODO: Remove workaround guile peg bug for ignore
+(define-peg-pattern year all
+  (and (ignore (and fws)) digit digit digit digit (ignore (and fws))))
+
+(define-peg-pattern date all
+  (and day month year))
+
+(define-peg-pattern hours all
+  (and digit digit))
+
+(define-peg-pattern minutes all
+  (and digit digit))
+
+(define-peg-pattern seconds all
+  (and digit digit))
+
+(define-peg-pattern time-of-day all
+  (and hours (ignore ":") minutes (? (and (ignore ":") seconds))))
+
+;; zone-sign, zone-hours and zone-minutes do not exist in the ABNF
+;; specified in RFC5322. But, we have it here since it eases parsing
+;; the zone and converting it to a number of seconds as required by
+;; the make-date procedure of SRFI-19.
+(define-peg-pattern zone-sign all
+  (or "+" "-"))
+
+(define-peg-pattern zone-hours all
+  (and digit digit))
+
+(define-peg-pattern zone-minutes all
+  (and digit digit))
+
+;; TODO: Remove workaround guile peg bug for ignore
+(define-peg-pattern zone all
+  (and (ignore (and fws)) zone-sign zone-hours zone-minutes))
+
+(define-peg-pattern time all
+  (and time-of-day zone))
+
+(define-peg-pattern date-time all
+  (and (? (and day-of-week (ignore ","))) date time (? cfws)))
+
+;;; Address specification
+
+(define-peg-pattern display-name all
+  phrase)
+
+(define-peg-pattern local-part body
+  (or dot-atom quoted-string))
+
+(define-printable-ascii-character-pattern dtext "[" "]" "\\")
+
+(define-peg-pattern domain-literal body
+  (and (? cfws) "[" (* (and (? fws) dtext)) (? fws) "]" (? cfws)))
+
+(define-peg-pattern domain body
+  (or dot-atom domain-literal))
+
+(define-peg-pattern addr-spec body
+  (and local-part "@" domain))
+
+(define-peg-pattern angle-addr all
+  (and (ignore (? cfws))
+       (ignore"<") addr-spec (ignore">")
+       (ignore (? cfws))))
+
+(define-peg-pattern name-addr body
+  (and (? display-name) angle-addr))
+
+(define-peg-pattern mailbox all
+  (or name-addr addr-spec))
+
+(define-peg-pattern mailbox-list all
+  (and mailbox (* (and (ignore ",") mailbox))))
+
+(define-peg-pattern group all
+  (and display-name (ignore ":")
+       (? group-list) (ignore ";") (? cfws)))
+
+(define-peg-pattern group-list all
+  (or mailbox-list cfws))
+
+(define-peg-pattern address body
+  (or mailbox group))
+
+(define-peg-pattern address-list all
+  (and address (* (and (ignore ",") address))))
+
+;;; Fields
+
+(define-syntax-rule (define-field-pattern name header pattern)
+  (define-peg-pattern name all
+    (and (ignore (string-ci header)) (ignore ":") pattern crlf)))
+
+;;; Origination date field
+
+(define-field-pattern orig-date "Date" date-time)
+
+;;; Originator fields
+
+(define-field-pattern from "From" (or mailbox-list address-list))
+(define-field-pattern sender "Sender" (or mailbox address))
+(define-field-pattern reply-to "Reply-To" address-list)
+
+;; Destination address fields
+
+(define-field-pattern to "To" address-list)
+(define-field-pattern cc "Cc" address-list)
+(define-field-pattern bcc "Bcc" (? (or address-list cfws)))
+
+;;; Identification fields
+
+(define-peg-pattern no-fold-literal body
+  (and "[" (* dtext) "]"))
+
+(define-peg-pattern id-left body
+  dot-atom-text)
+
+(define-peg-pattern id-right body
+  (or dot-atom-text no-fold-literal))
+
+(define-peg-pattern msg-id all
+  (and (ignore (? cfws)) (ignore "<")
+       id-left "@" id-right
+       (ignore ">") (ignore (? cfws))))
+
+(define-field-pattern message-id "Message-ID" msg-id)
+(define-field-pattern in-reply-to "In-Reply-To" (+ msg-id))
+(define-field-pattern references "References" (+ msg-id))
+
+;;; Informational fields
+
+(define-field-pattern subject "Subject" unstructured)
+(define-field-pattern comments "Comments" unstructured)
+(define-field-pattern keywords "Keywords" (and phrase (* (and "," phrase))))
+
+;;; Resent fields
+
+(define-field-pattern resent-date "Resent-Date" date-time)
+(define-field-pattern resent-from "Resent-From" (or mailbox-list address-list))
+(define-field-pattern resent-sender "Resent-Sender" (or mailbox address))
+(define-field-pattern resent-to "Resent-To" address-list)
+(define-field-pattern resent-cc "Resent-Cc" address-list)
+(define-field-pattern resent-bcc "Resent-Bcc" (? (or address-list cfws)))
+(define-field-pattern resent-msg-id "Resent-Message-ID" msg-id)
+
+;;; Trace fields
+
+;; word is given last priority in the ordered choice
+(define-peg-pattern received-token body
+  (or angle-addr addr-spec domain word))
+
+;; TODO: Do not discard the internal structure of received
+(define-field-pattern received "Received"
+  (and (* received-token) (ignore ";") date-time))
+
+(define-peg-pattern path body
+  (or angle-addr (and (? cfws) (ignore "<") (? cfws) (ignore ">") (? cfws))))
+
+(define-field-pattern return "Return-Path" path)
+
+(define-peg-pattern trace all
+  (and (? return) (+ received)))
+
+;;; Optional fields
+
+(define-printable-ascii-character-pattern ftext ":")
+
+(define-peg-pattern field-name all
+  (+ ftext))
+
+(define-peg-pattern optional-field all
+  (and field-name (ignore ":") unstructured crlf))
+
+;;; MIME version
+
+;; I have prepended optional cfws to account for leading whitespace.
+(define-field-pattern mime-version "MIME-Version"
+  (and (ignore (? cfws)) (+ digit) "." (+ digit)))
+
+;;; Content type
+
+;; TODO: What is <"> in RFC2045?
+(define-peg-pattern tspecials body
+  (or "(" ")" "<" ">" "@"
+      "," ";" ":" "\\"
+      "/" "[" "]" "?" "="))
+
+;; TODO: What is a CTL in RFC2045?
+(define-peg-pattern token body
+  (+ (and (not-followed-by (or " " crlf tspecials))
+	  peg-any)))
+
+;; TODO: Implement iana-token, ietf-token and x-token
+(define-peg-pattern iana-token body
+  token)
+
+(define-peg-pattern ietf-token body
+  token)
+
+(define-peg-pattern x-token body
+  token)
+
+(define-peg-pattern extension-token body
+  (or ietf-token x-token))
+
+(define-peg-pattern discrete-type body
+  (or "text" "image" "audio" "video"
+      "application" extension-token))
+
+(define-peg-pattern composite-type body
+  (or "message" "multipart" extension-token))
+
+(define-peg-pattern type all
+  (or discrete-type composite-type))
+
+(define-peg-pattern subtype all
+  (or extension-token iana-token))
+
+(define-peg-pattern attribute all
+  token)
+
+;; quoted-string is given higher precedence
+(define-peg-pattern value all
+  (or quoted-string token))
+
+(define-peg-pattern parameter all
+  (and attribute (ignore "=") value))
+
+;; I have prepended optional cfws to account for leading whitespace.
+(define-field-pattern content "Content-Type"
+  (and (ignore (? cfws)) type (ignore "/") subtype
+       (* (and (ignore ";") (ignore (? cfws)) parameter))))
+
+;;; Content Disposition as defined in RFC2183
+
+(define-peg-pattern disposition-type body
+  (or "inline" "attachment" extension-token))
+
+(define-peg-pattern quoted-date-time all
+  quoted-string)
+
+(define-syntax-rule (define-parameter-pattern name header pattern)
+  (define-peg-pattern name all
+    (and (ignore header) (ignore "=") pattern)))
+
+(define-parameter-pattern filename-parm "filename" value)
+(define-parameter-pattern creation-date-parm "creation-date" quoted-date-time)
+(define-parameter-pattern modification-date-parm "modification-date" quoted-date-time)
+(define-parameter-pattern read-date-parm "read-date" quoted-date-time)
+(define-parameter-pattern size-parm "size" (+ digit))
+
+(define-peg-pattern disposition-parm body
+  (or filename-parm creation-date-parm modification-date-parm
+      read-date-parm size-parm parameter))
+
+;; I have prepended optional cfws to account for leading whitespace.
+(define-field-pattern disposition "Content-Disposition"
+  (and (ignore (? cfws)) disposition-type
+       (* (and (ignore ";") (ignore (? cfws)) disposition-parm))))
+
+;;; Content transfer encoding
+
+(define-peg-pattern mechanism body
+  (or "7bit" "8bit" "binary"
+      "quoted-printable" "base64"
+      ietf-token x-token))
+
+(define-field-pattern encoding "Content-Transfer-Encoding"
+  (and (ignore (? cfws)) mechanism))
+
+;;; Fields
+
+;; The ABNF specified for fields in RFC5322 does not make sense. With
+;; it, all headers are eaten up by optional-field. So, as a temporary
+;; workaround, I am going with the following much simpler (but
+;; possibly incorrect) ABNF.
+
+;; TODO: Try to understand and implement the actual ABNF specified by
+;; RFC5322.
+(define-peg-pattern fields all
+  (* (or trace
+	 resent-date
+	 resent-from
+	 resent-sender
+	 resent-to
+	 resent-cc
+	 resent-bcc
+	 resent-msg-id
+	 orig-date
+	 from
+	 sender
+	 reply-to
+	 to
+	 cc
+	 bcc
+	 message-id
+	 in-reply-to
+	 references
+	 subject
+	 comments
+	 keywords
+	 mime-version
+	 content
+	 disposition
+	 encoding
+	 optional-field)))
+
+(define-peg-pattern mime-extension-field-name all
+  (and "Content-" (+ ftext)))
+
+(define-peg-pattern mime-extension-field all
+  (and mime-extension-field-name (ignore ":") unstructured crlf))
+
+(define-peg-pattern mime-entity-fields all
+  (* (or content
+	 disposition
+	 encoding
+	 mime-extension-field
+	 optional-field)))
+
+(define (decode-mime-encoded-word word)
+  (regexp-substitute/global
+   #f "=\\?([^?]*)\\?([^?]*)\\?([^?]*)\\?=" word
+   'pre (lambda (match-record)
+	  (let ((charset (match:substring match-record 1))
+		(encoding (string->lcase-symbol (match:substring match-record 2)))
+		(encoded-text (match:substring match-record 3)))
+	    (bytevector->string
+	     ((case encoding
+		((b) base64-decode)
+		((q) q-encoding-decode)
+		(else (error "Encoding of MIME word unknown" word)))
+	      encoded-text)
+	     charset)))
+   'post))
+
+(define (body->mime-entities body boundary)
+  "Split BODY into a list of mime entities separated by BOUNDARY (as
+explained in RFC2045), and return that list."
+  (define (read-till-boundary port)
+    (read-while port get-line-with-delimiter
+		(negate (cut string-prefix? (string-append "--" boundary) <>))))
+
+  (define (read-mime-entity port)
+    (if (string-prefix? (string-append "--" boundary "--")
+			(get-line-with-delimiter port))
+	(eof-object)
+	(read-till-boundary port))) 
+  
+  (call-with-input-string body
+    (lambda (port)
+      (read-till-boundary port)
+      (read-objects read-mime-entity port))))
+
+(define (email->headers+body email)
+  "Split EMAIL into headers and body. Return as multiple values."
+  (call-with-input-string email
+    (lambda (port)
+      (let ((headers (read-while port get-line-with-delimiter
+				 (lambda (line)
+				   (not (or (string= line "\n")
+					    (string= line "\r\n")))))))
+	(get-line-with-delimiter port)
+	(values headers
+		(read-while port get-line-with-delimiter identity))))))
+
+(define (post-process-content-transfer-encoding _ value)
+  (list 'content-transfer-encoding (string->lcase-symbol value)))
+
+(define post-process-content-type
+  (match-lambda*
+    (`(content (type ,type)
+	       (subtype ,subtype)
+	       . ,parameters)
+     (let ((type (string->lcase-symbol type))
+	   (subtype (string->lcase-symbol subtype)))
+       `(content-type
+	 ,(acons* 'type type
+		  'subtype subtype
+		  (let ((parameters
+			 (map (match-lambda
+				(`(parameter (attribute ,attribute)
+					     (value ,value))
+				 (cons (string->lcase-symbol attribute) value)))
+			      (flatten-and-filter '(parameter) parameters))))
+		    (if (and (eq? type 'text)
+			     (not (assoc-ref parameters 'charset)))
+			;; UTF-8 is specified as the default charset in RFC6657
+			(acons 'charset "utf-8" parameters)
+			parameters))))))))
+
+(define post-process-content-disposition
+  (match-lambda*
+    (`(disposition ,type . ,parameters)
+     `(content-disposition
+       ,(acons 'type (string->lcase-symbol type)
+	       (map (match-lambda
+		      (('filename-parm ('value filename))
+		       (cons 'filename (basename filename)))
+		      (((? (lambda (date-parm)
+			     (member date-parm '(creation-date-parm modification-date-parm read-date-parm)))
+			   date-parm) value)
+		       ;; TODO: Convert to SRFI-19 datetime
+		       (cons date-parm value))
+		      (('size-parm value)
+		       (cons 'size (string->number value)))
+		      (`(parameter (attribute ,attribute)
+				   (value ,value))
+		       (cons (string->lcase-symbol attribute) value)))
+		    (flatten-and-filter
+		     '(filename-parm creation-date-parm modification-date-parm
+				     read-date-parm size-parm parameter)
+		     parameters)))))))
+
+(define post-process-optional-field
+  (match-lambda*
+    (`(optional-field
+       (field-name ,field-name)
+       ,field-value)
+     (list (string->lcase-symbol field-name)
+	   field-value))
+    (`(optional-field (field-name ,field-name))
+     (list field-name ""))))
+
+(define* (macro-process-address-list _ . addresses)
+  (flatten-and-filter '(address mailbox) addresses))
+
+(define (parse-email-address address)
+  "Parse ADDRESS as an email address and return an association list
+with keys being the symbols name and address, and values being the
+display-name and addr-spec respectively. display-name and addr-spec
+are as defined in RFC5322.
+
+For example,
+(parse-email-address \"Foo <foo@example.org>\")
+=> ((name \"Foo\") (address \"foo@example.org\"))
+(parse-email-address \"foo@example.org\")
+=> ((address \"foo@example.org\"))"
+  (cond
+   ((string-match "([^<]*)<([^>]*)>" address)
+    => (lambda (match-record)
+	 (let ((name (string-trim-both (match:substring match-record 1)))
+	       (address (match:substring match-record 2)))
+	   (if (string-null? name)
+	       `((address . ,address))
+	       `((name . ,name)
+		 (address . ,address))))))
+   (else `((address . ,address)))))
+
+(define interpret-address
+  (match-lambda
+    ((('name . name)
+      ('address . address))
+     (format #f "~a <~a>" name address))
+    ((('address . address)) address)))
+
+(define (parse-email-body headers body)
+  "Parse BODY as email body where HEADERS is an association list of
+header keys and values as returned by parse-email-headers. Return a
+list of <mime-entity> records if the body is a multipart
+message. Else, return a single <mime-entity> record."
+  (let ((content-type (assoc-ref headers 'content-type)))
+    (case (assoc-ref content-type 'type)
+      ((multipart)
+       (map parse-mime-entity
+	    (body->mime-entities body (assoc-ref content-type 'boundary))))
+      ((text)
+       (string-trim-both
+	(decode-body body (assoc-ref headers 'content-transfer-encoding)
+		     (assoc-ref content-type 'charset))))
+      (else (decode-body body (assoc-ref headers 'content-transfer-encoding))))))
+
+(define (add-default-headers headers)
+  ;; Default Content-Type and Content-Transfer-Encoding headers as
+  ;; specified in RFC2045
+  (let ((default-headers
+	  (acons* 'content-type '((type . text)
+				  (subtype . plain)
+				  ;; UTF-8 is specified as the default
+				  ;; charset in RFC6657
+				  (charset . "utf-8"))
+		  'content-transfer-encoding '#{7bit}#)))
+    (append (alist-delete* (append (if (assoc-ref headers 'content-type)
+				       (list 'content-type) (list))
+				   (if (assoc-ref headers 'content-transfer-encoding)
+				       (list 'content-transfer-encoding) (list)))
+			   default-headers)
+	    headers)))
+
+(define (parse-mime-entity text)
+  (let-values (((headers body) (email->headers+body text)))
+    (let ((headers
+	   (pre-post-order
+	    (peg:tree
+	     (match-pattern mime-entity-fields text))
+	    `((content . ,post-process-content-type)
+	      (encoding . ,post-process-content-transfer-encoding)
+	      (disposition . ,post-process-content-disposition)
+	      (optional-field . ,post-process-optional-field)
+	      (mime-entity-fields . ,(lambda (_ . mime-entity-fields)
+				       (add-default-headers
+					(map (match-lambda
+					       ((mime-entity-field value)
+						(cons mime-entity-field value))
+					       ((mime-entity-field . values)
+						(cons mime-entity-field values)))
+					     mime-entity-fields))))
+	      (*text* . ,(lambda (_ text) text))
+	      (*default* . ,(lambda tree tree))))))
+      (make-mime-entity headers (parse-email-body headers body)))))
+
+(define (parse-email email)
+  "Parse string EMAIL and return result as a <email> record."
+  (let-values (((headers body) (email->headers+body email)))
+    (let ((headers-alist (parse-email-headers headers)))
+      (make-email headers-alist (parse-email-body headers-alist body)))))
+
+(define (parse-email-headers headers)
+  "Parse string HEADERS as email headers and return an association
+list of header keys and values."
+  (define (extract-value _ value) value)
+
+  (pre-post-order
+   (peg:tree
+    (match-pattern fields headers))
+   `((date-time . ,(lambda node
+		     (match-let
+			 ((`((day ,day) (month ,month) (year ,year)
+			     (hours ,hours) (minutes ,minutes) (seconds ,seconds)
+			     (zone-sign ,zone-sign) (zone-hours ,zone-hours) (zone-minutes ,zone-minutes))
+			   (flatten-and-filter
+			    '(day month year hours minutes seconds
+				  zone-sign zone-hours zone-minutes)
+			    node)))
+		       (make-date 0
+				  (string->number seconds)
+				  (string->number minutes)
+				  (string->number hours)
+				  (string->number day)
+				  (1+ (list-index
+				       (cut equal? <> month)
+				       (list "Jan" "Feb" "Mar" "Apr" "May" "Jun"
+					     "Jul" "Aug" "Sep" "Oct" "Nov" "Dec")))
+				  (string->number year)
+				  (* (case (string->symbol zone-sign)
+				       ((+) 1)
+				       ((-) -1))
+				     (+ (* 60 60 (string->number zone-hours))
+					(* 60 (string->number zone-minutes))))))))
+     (orig-date . ,(lambda (_ date) (list 'date date)))
+     (angle-addr . ,extract-value)
+     (mailbox . ,(match-lambda*
+		   (`(mailbox (display-name ,name) ,address)
+		    `((name . ,(decode-mime-encoded-word
+				(string-trim-both name)))
+		      (address . ,address)))
+		   (`(mailbox ,address)
+		    `((address . ,(string-trim-both address))))
+		   (_ (error "Failed to parse mailbox"))))
+     (address-list *macro* . ,macro-process-address-list)
+     (mailbox-list *macro* . ,macro-process-address-list)
+     (optional-field . ,post-process-optional-field)
+     (msg-id . ,(match-lambda* (`(msg-id ,msg-id) msg-id)))
+     (content . ,post-process-content-type)
+     (encoding . ,post-process-content-transfer-encoding)
+     (disposition . ,post-process-content-disposition)
+     (fields . ,(lambda (_ . fields)
+		  (add-default-headers
+		   (filter-map (match-lambda
+				 (('trace . _) #f)
+				 ((field value)
+				  (cons field value))
+				 ((field . values)
+				  (cons field values))
+				 (_ #f))
+			       fields))))
+     (*text* . ,extract-value)
+     (*default* . ,(lambda tree tree)))))
+
+(define* (decode-body body encoding #:optional charset)
+  (let ((octets
+	 (case encoding
+	   ((base64)
+	    (base64-decode 
+	     (string-filter
+	      (char-set-union
+	       (ucs-range->char-set (char->integer #\a) (1+ (char->integer #\z)))
+	       (ucs-range->char-set (char->integer #\A) (1+ (char->integer #\Z)))
+	       (ucs-range->char-set (char->integer #\0) (1+ (char->integer #\9)))
+	       (char-set #\+ #\/ #\=))
+	      body)))
+	   ((quoted-printable) (quoted-printable-decode body))
+	   ((#{7bit}# #{8bit}# binary) body)
+	   (else (error "Body decoding failed. Unknown encoding" encoding)))))
+    (if charset
+	(case encoding
+	  ((base64 quoted-printable) (bytevector->string octets charset))
+	  ((#{7bit}# #{8bit}# binary) octets))
+	octets)))
+
+(define (read-next-email-in-mbox port)
+  ;; Read and discard From_ line
+  (get-line-with-delimiter port)
+  ;; Read the actual email
+  (read-while port get-line-with-delimiter
+	      (negate (cut string-prefix? "From " <>))))
+
+(define (mbox->emails port)
+  (read-objects read-next-email-in-mbox port))
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