Signal exception if input isn’t vCard

… with a somewhat naïve check. Also, also count
horizontal-tab characters as starters of folded
lines (as per spec).

vcarded.scm

@@ -21,6 +21,7 @@
     (chicken base)
     (chicken condition)
     (chicken io)
+    (chicken irregex)
     (srfi 1)
     (srfi 4)
     (srfi 13)
@@ -223,24 +224,53 @@
               (parse-vcard-value property value-elements))))
 
 
-  ;; Reader thunk. Read/parse an entire vcard into a “vcard alist.”
-  (define (read-vcard)
-    (let [(element (read-vcard-element))]
+  ;; Reader thunk. Read/parse an entire vCard into a “vCard alist.”
+  ;; Ignore the IGNORED parameter.
+  (define (read-vcard . ignored)
+    (let* [(first-element? (optional ignored #t))
+           (element (read-vcard-element first-element?))]
       (if (not (eof-object? (peek-char)))
-          (append (list element) (read-vcard))
+          (append (list element) (read-vcard #f))
           (list element))))
 
 
-  ;; Read a single unfolded line into a vcard “element” list.
-  (define (read-vcard-element)
-    (parse-vcard-line (read-folded-line)))
+  ;; Read a single unfolded line into a vCard “element” list.
+  ;; The optional FIRST-ELEMENT? argument simply makes sure the next line is a
+  ;; BEGIN:VCARD element (as the first element in any valid vCard file should
+  ;; be. It is used internally by READ-VCARD.
+  (define (read-vcard-element . first-element?)
+    (let* [(first-element? (optional first-element? #f))
+           (line (read-folded-line (if first-element? 100 #f)))]
+      (cond
+       ;; Ignore blank lines, they’re not doing anyone any harm!
+       [(or (string-null? line)
+            (irregex-search "^[[:whitespace:]]+$" line))
+        (read-vcard-element first-element?)]
+       ;; If we’ve previously established this is a vCard stream we’re dealing
+       ;; with, just go ahead and parse. If we haven’t established that, make
+       ;; sure it’s a BEGIN:VCARD line first.
+       [(or (not first-element?)
+            (and first-element?
+                 (string-contains line "BEGIN")
+                 (string-contains line "VCARD")))
+        (parse-vcard-line (read-folded-line))]
+       ;; If we’re still making sure the file is vCard, and the first non-blank
+       ;; line *isn’t* a BEGIN:VCARD line, then the file probably isn’t a vCard.
+       [#t
+        (signal
+         (condition '(exn location vcarded message
+                          "Not a valid vCard file.")
+                    '(file)
+                    '(vcard)))])))
+
 
 
   ;; 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)
-    (let [(line (read-line))]
-      (if (eq? (peek-char) #\space)
+  (define (read-folded-line #!optional limit)
+    (let [(line (read-line (current-input-port) limit))]
+      (if (or (eq? (peek-char) #\space)
+              (eq? (peek-char) #\tab))
           (string-concatenate
            (list line
                  (string-drop (read-folded-line) 1)))