How do I run elisp code on a filtered email in Gnus?

Question

I am downloading mail using fetchmail from a pop account and using gnus to read mails from the maildir.

I would like to run elisp code on emails that are matched by fancy split rules, or something similar.

My end goal would be to match all mails from [email protected] and run elisp code to extract the data and upload it to a server.

I have read the linked manual and got the idea after reading this portion

(! func split)
If the split is a list, and the first element is !, 
then split will be processed, and func will be called as a 
function with the result of split as argument. func should return a split.

Would I be able to use the function to run code for the side effects and still return the split, or am I on the completely wrong track?

Answer 1

I spent a few hours reading the docs and testing a solution. I have now solved this as per below.

1. Set the nnmail split method to "nnmail-split-fancy"

   (setq nnmail-split-methods 'nnmail-split-fancy)
   

   This allows one to match incoming emails to groups.

2. Set the var nnmail-split-fancy to define the rules to filter emails

   (setq nnmail-split-fancy '(|
              (any "deploy@client" (: script-send-mail))
              "mail.misc"))
   

   The "|" char can be thought of as an OR and the ":" as "execute function and return group". The rule above is the equivalent of saying "If any incoming mail's headers match 'deploy@client' then file according to the return of the function 'script-send-mail', else file in 'mail.misc'"

3. Write function script-send-mail to operate on the current mail's buffer-string

   (defun script-send-mail ()
     (save-excursion
        (save-restriction
         ;; (widen) is necessary to get body of email
         (widen)
         (progn
           ;; Extract data and send to server
           (script-extract-data (buffer-string))
           ;; final return is a gnus group
           (format "mail.deploy")))))
   

   Now, whenever an incoming mail is received matching [email protected] The function will run and do what it needs to with the email before it is sent to a group.

*ps: Not sure if Answering my own question is acceptable. Please close if I have crossed any guidelines.