messages: Provide a simpler 'header' procedure and export it.

* jupyter/messages.scm (<header>): Rename contructor to '%header'.
(%protocol-version): New variable.
(current-date-string, header, make-header): New procedures.
(reply): Adjust to new 'header' API.
(string->header): Likewise.

jupyter/messages.scm

@@ -38,6 +38,8 @@
             message-parts
             parts->message
 
+            header
+            make-header
             header?
             header-id
             header-user
@@ -57,7 +59,7 @@
 ;; Jupyter message header as defined at
 ;; <https://jupyter-client.readthedocs.io/en/stable/messaging.html#general-message-format>.
 (define-record-type <header>
-  (header id user session date type version sender)
+  (%header id user session date type version sender)
   header?
   (id           header-id)                         ;bytevector (UUID)
   (user         header-user)                       ;string
@@ -70,6 +72,24 @@
   ;; itself.
   (sender       header-sender))                    ;bytevector (UUID) | #f
 
+
+(define %protocol-version "5.0")
+
+(define (current-date-string)
+  (date->string (time-utc->date (current-time time-utc))
+                "~5.~N"))
+
+(define* (header type user session
+                 #:key
+                 (id (make-id))
+                 (date (current-date-string))
+                 (version %protocol-version)
+                 sender)
+  "Return a new Jupyter message header."
+  (%header id user session date type version sender))
+
+(define make-header header)
+
 ;; Jupyter message.
 (define-record-type <message>
   (%message header parent-header metadata content buffers)
@@ -122,14 +142,8 @@ This is a low-level procedure for internal use."
 (define (reply message type content)
   "Return a Jupyter message that is a reply to MESSAGE."
   (let ((parent (message-header message)))
-    (make-message (header (make-id) (header-user parent)
-                          (header-session parent)
-                          (date->string (time-utc->date
-                                         (current-time time-utc))
-                                        "~5.~N")
-                          type
-                          (header-version parent)
-                          #f)
+    (make-message (header type (header-user parent)
+                          (header-session parent))
                   content
                   #:parent-header parent)))
 
@@ -147,13 +161,13 @@ record.  Return #f if STR is the empty dictionary."
   (let ((table (json-string->scm str)))
     (if (zero? (hash-count (const #t) table))
         #f
-        (header (hash-ref table "msg_id")
+        (header (hash-ref table "msg_type")
                 (hash-ref table "username")
                 (hash-ref table "session")
-                (hash-ref table "date")
-                (hash-ref table "msg_type")
-                (hash-ref table "version")
-                sender))))
+                #:id (hash-ref table "msg_id")
+                #:date (hash-ref table "date")
+                #:version (hash-ref table "version")
+                #:sender sender))))
 
 (define (header->string header)
   "Return HEADER as a JSON string."