Report when a required field is missing in the static-pages plugin

Add ASSERT-FIELD to the API as the entry point for plugins to report to the
user when a content field is missing.

plugins/static-pages.lisp

@@ -2,6 +2,7 @@
   (:use :cl)
   (:export #:enable)
   (:import-from :coleslaw #:*config*
+                          #:assert-field
                           #:content
                           #:find-all
                           #:render
@@ -19,7 +20,9 @@
   (:default-initargs :format :md))
 
 (defmethod initialize-instance :after ((object page) &key)
-  (with-slots (coleslaw::url coleslaw::text format) object
+  (assert-field 'title object)
+  (assert-field 'coleslaw::url object)
+  (with-slots (coleslaw::url coleslaw::text format title) object
     (setf coleslaw::url (make-pathname :defaults coleslaw::url)
           format (alexandria:make-keyword (string-upcase format))
           coleslaw::text (render-text coleslaw::text format))))

src/packages.lisp

@@ -46,4 +46,7 @@
            #:add-document
            #:delete-document
            #:write-document
-           #:content-text))
+           #:content-text
+
+	   ;; Error reporting
+           #:assert-field))

src/util.lisp

@@ -1,5 +1,25 @@
 (in-package :coleslaw)
 
+
+(define-condition coleslaw-condition ()
+  ())
+
+(define-condition field-missing (error coleslaw-condition)
+  ((field-name :initarg :field-name :reader missing-field-field-name)
+   (file :initarg :file :reader missing-field-file
+         :documentation "The path of the file where the field is missing."))
+  (:report
+   (lambda (c s)
+     (format s "~A: The required field ~A is missing."
+             (missing-field-file c)
+             (missing-field-field-name c)))))
+
+(defmacro assert-field (field-name content)
+  `(when (not (slot-boundp ,content ,field-name))
+     (error 'field-missing
+            :field-name ,field-name
+            :file (content-file ,content))))
+
 (defun construct (class-name args)
   "Create an instance of CLASS-NAME with the given ARGS."
   (apply 'make-instance class-name args))