refactor(notes): generalize citar-has-note

Revert the keys-with-notes defcustom and code, and instead generalize
'citar-has-note', which now iterates through 'citar-note-backends'.

Fix #623

README.org

@@ -272,7 +272,7 @@ Since ~citar-open-note-functions~ is a list, you can also include multiple funct
 Please note that if you choose to use org-roam-bibtex using the above configuration, you will need to set ~:immediate-finish t~ in the template that you use for bibliography notes in ~org-roam-capture-templates~.
 Since ~citar-open-note-functions~ attempts multiple functions one after the other, this is needed to ensure the ~org-capture~ returns immediately without waiting for further user input.
 
-Citar also includes a ~citar-keys-with-notes-functions~ variable, which specifies a list of functions, each of which returns a list of keys that have associated notes.
+Citar also includes a ~citar-has-notes-functions~ variable, which specifies a list of functions, each of which returns a predicate function to test whether a reference has associated notes.
 This function allows Citar to correctly format the completion UI candidates.
 The default function only supports one-file-per-key notes.
 

citar-file.el

@@ -253,6 +253,11 @@ need to scan the contents of DIRS in this case."
   "Return a list of keys with file notes."
   (hash-table-keys (citar-file--has-library-files-hash)))
 
+(defun citar-file-has-notes ()
+  "Return a predicate testing whether a reference has associated notes."
+  (citar-file--has-file citar-notes-paths
+                        citar-file-note-extensions))
+
 (defun citar-file--has-file (dirs extensions &optional entry-field)
   "Return predicate testing whether a key and entry have associated files.
 

citar.el

@@ -239,9 +239,13 @@ If nil, single resources will open without prompting."
   :group 'citar
   :type '(function))
 
-(defcustom citar-keys-with-notes-functions
-  '(citar-file--keys-with-file-notes)
-  "Functions, without argument, to return a list of keys with notes."
+(defcustom citar-has-note-functions
+  '(citar-file-has-notes)
+  "Functions to return a predicate testing whether a reference has
+associated notes.
+
+Such functions must take arguments KEY and ENTRY and return
+non-nil when the reference has associated notes."
   :group 'citar
   :type '(function))
 
@@ -556,14 +560,6 @@ HISTORY is the 'completing-read' history argument."
              (category . multi-category))
          (complete-with-action action (delete-dups resources) string predicate))))))
 
-(defun citar-keys-with-notes ()
-  "Return a list of keys with associated notes."
-  (seq-uniq
-   (seq-mapcat
-    (lambda (fn)
-      (funcall fn))
-    citar-keys-with-notes-functions)))
-
 (defun citar--select-group-related-resources (resource transform)
   "Group RESOURCE by type or TRANSFORM."
     (let ((extension (file-name-extension resource)))
@@ -691,9 +687,12 @@ non-nil when the entry has associated notes in `citar-notes-paths`.
 Note: for performance reasons, this function should be called
 once per command; the function it returns can be called
 repeatedly."
-  (citar-file--has-file citar-notes-paths
-                        citar-file-note-extensions))
-
+  ;; Call each function in `citar-has-note-functions` to get a list of predicates
+  (let ((preds (mapcar #'funcall citar-has-note-functions)))
+    ;; Return a predicate that checks if `citekey` and `entry` have a note
+    (lambda (citekey entry)
+      ;; Call each predicate with `citekey` and `entry`; return the first non-nil result
+      (seq-some (lambda (pred) (funcall pred citekey entry)) preds))))
 
 (defun citar--format-candidates (bib-files &optional context)
   "Format candidates from BIB-FILES, with optional hidden CONTEXT metadata.
@@ -703,15 +702,15 @@ key associated with each one."
          (raw-candidates
           (parsebib-parse bib-files :fields (citar--fields-to-parse)))
          (hasfilep (citar-has-file))
-         (hasnote (citar-keys-with-notes))
+         (hasnotep (citar-has-note))
          (main-width (citar--format-width (citar--get-template 'main)))
          (suffix-width (citar--format-width (citar--get-template 'suffix)))
          (symbols-width (string-width (citar--symbols-string t t t)))
          (star-width (- (frame-width) (+ 2 symbols-width main-width suffix-width))))
     (maphash
      (lambda (citekey entry)
        (let* ((files (when (funcall hasfilep citekey entry) " has:files"))
-              (notes (when (member citekey hasnote) " has:notes"))
+              (notes (when (funcall hasnotep citekey entry) " has:notes"))
               (link (when (citar--field-with-value '("doi" "url") entry) "has:link"))
               (candidate-main
                (citar--format-entry
@@ -1158,7 +1157,7 @@ With prefix, rebuild the cache before offering candidates."
   (or (seq-some
        (lambda (opener)
          (funcall opener key entry)) citar-open-note-functions)
-      (funcall citar-file--open-note key entry)))
+      (funcall citar-create-note-function key entry)))
 
 ;;;###autoload
 (defun citar-open-entry (key-entry)