Everything seems to be working OK now.

flycheck-vale.el

@@ -4,7 +4,7 @@
 ;; Author: Austin Bingham <austin.bingham@gmail.com>
 ;; Version: 0.1
 ;; URL: https://github.com/abingham/flycheck-vale
-;; Package-Requires: ((emacs "24") (flycheck "0.22") (cl-lib "0.5"))
+;; Package-Requires: ((emacs "24") (flycheck "0.22"))
 ;;
 ;; This file is not part of GNU Emacs.
 ;;
@@ -56,24 +56,39 @@
   :type '(string)
   :group 'flycheck-vale)
 
+(defcustom flycheck-vale-modes '(text-mode markdown-mode rst-mode)
+  "List of major modes in which to apply this checker."
+  :type '(repeat function))
+
 (defconst flycheck-vale--level-map
   '(("error" . error)
     ("warning" . warning)))
 
 (defun flycheck-vale--issue-to-error (result)
+  "Parse a single vale issue into a flycheck error struct.
+
+We only fill in what we can get from the vale issue directly. The
+rest (e.g. filename) gets filled in elsewhere."
   (let-alist result
     (flycheck-error-new
      :line .Line
      :column (elt .Span 0)
-     ;; :buffer (current-buffer)
-     ;; :filename "TODO"
      :message .Message
-     ;; :checker checker
      :level (assoc-default .Severity flycheck-vale--level-map 'string-equal 'error))))
 
 (defun flycheck-vale--output-to-errors (output)
+  "Parse the full JSON output of vale into a sequence of flycheck
+error structs."
   (let* ((full-results (json-read-from-string output))
+
+         ;; Get the list of issues for each file.
          (result-vecs (mapcar 'cdr full-results))
+
+         ;; Chain all of the issues together. The point here, really, is that we
+         ;; don't expect results from more than one file, but we should be
+         ;; prepared for the theoretical possibility that the issues are somehow
+         ;; split across multiple files. This is basically a punt in lieu of
+         ;; more information.
          (issues (apply 'concatenate 'list (mapcar 'cdr full-results))))
     (mapcar 'flycheck-vale--issue-to-error issues)))
 
@@ -82,6 +97,7 @@
 
   (let ((orig-buf (current-buffer))
         (outbuf (get-buffer-create "*flycheck-vale-output*")))
+
     ;; Clear the output buffer
     (with-current-buffer outbuf
       (read-only-mode 0)
@@ -98,31 +114,29 @@
      "--output"
      "JSON")
 
+    ;; Parse the content of the output buffer into flycheck error structures,
+    ;; passing them to the provided callback.
     (with-current-buffer outbuf
       (let ((errors (flycheck-vale--output-to-errors (buffer-string))))
+        ;; Fill in the rest of the error struct data.
         (loop for err in errors do
-              (setf (flycheck-error-buffer err) orig-buf)
-              (setf (flycheck-error-filename err) (buffer-file-name orig-buf))
-              (setf (flycheck-error-checker err) checker))
+              (setf
+               (flycheck-error-buffer err) orig-buf
+               (flycheck-error-filename err) (buffer-file-name orig-buf)
+               (flycheck-error-checker err) checker))
         (funcall callback 'finished errors)))))
 
-(defun flycheck-vale--in-supported-mode ()
-  "Determines if buffer is in a mode where vale linting is appropriate."
-  text-mode)
-
 ;;;###autoload
 (defun flycheck-vale-setup ()
   "Convenience function to setup the vale flycheck checker.
 
-This adds a hook to watch for ycmd parse results, and it adds the
-ycmd checker to the list of flycheck checkers."
+This adds the vale checker to the list of flycheck checkers."
   (add-to-list 'flycheck-checkers 'vale))
 
 (flycheck-define-generic-checker 'vale
   "A flycheck checker using vale natural language linting."
   :start #'flycheck-vale--start
-  ;; :predicate #'flycheck-vale--in-supported-mode
-  :modes '(text-mode))
+  :modes flycheck-vale-modes)
 
 (provide 'flycheck-vale)