Fold serialized vCards’ lines to ~70 characters

The spec recommends splitting at 75 octets, but
that’s hard. We assume that 70 characters is
more-or-less 75 octets (even though with
UTF-8 chars this might be passed).

vcarded.scm

@@ -84,9 +84,7 @@
   ;; description, but unfortunately I’m coming up empty! =w=,
   (define (write-vcard vcard-alist #!optional (port (current-output-port)))
     (for-each (lambda (element)
-                (write-string (serialize-vcard-element element) #f port)
-                (write-char #\return port)
-                (write-char #\newline port))
+                (write-folded-line (serialize-vcard-element element)))
               vcard-alist))
 
 
@@ -185,7 +183,7 @@
 
 
 
-  ;;; ~ Property-value parsers ~
+  ;;; ~ Property-value parsers/serializers ~
 
   ;; List of all properties with semicoloned-structured strings.
   (define vcard-semicoloned-properties
@@ -328,16 +326,37 @@
 
   ;; Reader-thunk. Read a “logical” folded-line, where a line beginning with a
   ;; space is a continuation of the previous line — like with vcards.
-  (define (read-folded-line #!optional (limit 100) (port (current-input-port)))
-    (if (not (eof-object? (peek-char port)))
-        (let [(line (read-line port limit))]
-          (if (or (eq? (peek-char port) #\space)
-                  (eq? (peek-char port) #\tab))
-              (string-concatenate
-               (list line
-                     (string-drop (read-folded-line limit port) 1)))
-              line))
-        ""))
+  ;; LIMIT is the maximum amount of characters it will read before giving up
+  ;; on finding a newline.
+  ;; It will default to (current-input-port) as the PORT.
+  (define (read-folded-line #!optional (limit #f) (port (current-input-port)))
+    (let [(port (or port (current-input-port)))]
+      (if (not (eof-object? (peek-char port)))
+          (let [(line (read-line port limit))]
+            (if (or (eq? (peek-char port) #\space)
+                    (eq? (peek-char port) #\tab))
+                (string-concatenate
+                 (list line
+                       (string-drop (read-folded-line limit port) 1)))
+                line))
+          "")))
+
+
+  ;; Write a line to a port as a “logical” folded-line string, in the style of
+  ;; vCard and vCalendar. If the line is too long (passed FOLD-LENGTH [70]), it
+  ;; will be written as several lines of FOLD-LENGTH, with each such line
+  ;; prepended by the CREASE-CHAR (#\tab).
+  ;; It will default to (current-output-port), if PORT is #f or unspecified.
+  (define (write-folded-line string #!optional
+                             (port (current-output-port))
+                             (fold-length 70)
+                             (crease-char #\tab))
+    (write-string
+     (string-concatenate
+      (list (string-join (string-split-by-lengths string fold-length)
+                         (list->string (list #\return #\newline crease-char)))
+            (list->string '(#\return #\newline))))
+     #f (or port (current-output-port))))
 
 
   ;; Split a STRING into strings of a size of (at most) LENGTH signs.