Allow removing front-matter keys using hugo_front_matter_key_replace

Simply set the "newkey" to "nil". See
https://ox-hugo.scripter.co/doc/replace-front-matter-keys/#removing-keys

doc/ox-hugo-manual.org

@@ -2088,13 +2088,13 @@ front-matter key name to accommodate for your favorite Hugo theme.
 The syntax is:
 
 #+begin_src org
-#+hugo_front_matter_key_replace: old_key1>new_key1 old_key2>new_key2
+#+hugo_front_matter_key_replace: oldkey1>newkey1 oldkey2>newkey2
 #+end_src
 
 or, if you want this to apply only for a specific Org tree:
 
 #+begin_src org
-:EXPORT_HUGO_FRONT_MATTER_KEY_REPLACE: old_key1>new_key1 old_key2>new_key2
+:EXPORT_HUGO_FRONT_MATTER_KEY_REPLACE: oldkey1>newkey1 oldkey2>newkey2
 #+end_src
 
 Here are few use cases to better explain this feature.
@@ -2163,6 +2163,23 @@ What do you do? ..
 #+begin_src org
 #+hugo_front_matter_key_replace: tags>categories categories>tags
 #+end_src
+**** Removing front-matter keys during export
+:PROPERTIES:
+:CUSTOM_ID: removing-keys
+:END:
+In the front-matter key replacement syntax:
+#+begin_src org
+#+hugo_front_matter_key_replace: oldkey>newkey
+#+end_src
+If ~newkey~ is a special string *nil*, ~oldkey~ will be _removed_ from
+the exported front-matter.
+
+For example, if you want to remove the ~aliases~ front-matter from
+your exported files (but want to leave them in your Org files), simply
+add this to the top of your Org files:
+#+begin_src org
+#+hugo_front_matter_key_replace: aliases>nil
+#+end_src
 **** Examples
 You can find a bunch of examples if you search for /"Replace
 front-matter keys"/ in {{{ox-hugo-test-file}}}.

ox-hugo.el

@@ -1373,6 +1373,9 @@ The replacement syntax is:
 
     #+hugo_front_matter_key_replace: oldkey>newkey
 
+If newkey is a special string \"nil\", oldkey will be removed
+from the front-matter.
+
 You can also do multiple key replacements:
 
     #+hugo_front_matter_key_replace: oldkey1>newkey1 oldkey2>newkey2
@@ -1422,8 +1425,16 @@ INFO is a plist used as a communication channel."
                 (key-repl (cdr repl)))
             (let ((found-key-cell (assoc key-orig data)))
               (when found-key-cell
-                ;; https://emacs.stackexchange.com/a/3398/115
-                (setf (car found-key-cell) key-repl)))))))
+                ;; (message "[ox-hugo replace-key found-key-cell DBG] %S" found-key-cell)
+                ;; (message "[ox-hugo replace-key key-orig DBG] %S" key-orig)
+                ;; (message "[ox-hugo replace-key key-repl DBG] %S" key-repl)
+                (if (string= "nil" key-repl)
+                    ;; Setting value of a front-matter key to nil will
+                    ;; cause that key to be removed during export.
+                    ;; See `org-hugo--gen-front-matter'.
+                    (setf (cdr found-key-cell) nil)
+                  ;; https://emacs.stackexchange.com/a/3398/115
+                  (setf (car found-key-cell) key-repl))))))))
     data))
 
 ;;;; TODO keywords

test/site/content-org/all-posts.org

@@ -5581,6 +5581,16 @@ there shouldn't be any error.
 :END:
 Some themes might expect the front-matter list variable for authors to
 be ~authors~ instead of the default ~author~ created by ~ox-hugo~.
+*** Remove a front-matter key                                        :remove:
+:PROPERTIES:
+:EXPORT_FILE_NAME: remove-a-front-matter-key
+:EXPORT_HUGO_CUSTOM_FRONT_MATTER: :foo 1 :bar 2
+:EXPORT_HUGO_FRONT_MATTER_KEY_REPLACE: foo>nil bar>zoo
+:END:
+#+begin_description
+This test removes the ~foo~ front-matter key from the exported file
+(and replaces ~bar~ key with ~zoo~).
+#+end_description
 * Date and slug inheritance                           :inheritance:date:slug:
 :PROPERTIES:
 :EXPORT_DATE: 2018-02-12

test/site/content/posts/remove-a-front-matter-key.md

@@ -0,0 +1,10 @@
++++
+title = "Remove a front-matter key"
+description = """
+  This test removes the `foo` front-matter key from the exported file
+  (and replaces `bar` key with `zoo`).
+  """
+tags = ["front-matter", "keys", "replace", "remove"]
+draft = false
+zoo = 2
++++