Re: cond*

[Adam Porter <adam@alphapapa.net>]

> To finish the design of cond*, we need to find what features
> needed to be added so that it is rare to encounter a pcase
> that can't be replaced cleanly.    Would people please tell me
> about possible examples?

Well, one of the most common things I do with pcase is to destructure 
alists, plists, and structs, often nested at multiple levels.  For example:

(defun ement-ignore-user (user-id session &optional unignore-p)
   "Ignore USER-ID on SESSION.
If UNIGNORE-P (interactively, with prefix), un-ignore USER."
   (interactive (list (ement-complete-user-id)
                      (ement-complete-session)
                      current-prefix-arg))
   ;; NOTE: Destructures account-data slot of ement-session struct.
   (pcase-let* (((cl-struct ement-session account-data) session)
                ;; NOTE: Destructures alist nested in another alist and
                ;; binds to a variable of a different name than the key.
                ((map ('content (map ('ignored_users ignored-users))))
                 (cl-find "m.ignored_user_list" account-data
                          :key (lambda (event) (alist-get 'type event)) 
:test #'equal)))
     ...))

(defun ement--room-in-space-p (room space)
   "Return non-nil if ROOM is in SPACE on SESSION."
   ;; NOTE: Destructures slots in two structs to alternative variable
   ;; names, and destructures an alist key in each struct's `local'
   ;; slot to a variable of the same name.
   (pcase-let* (((cl-struct ement-room (id parent-id) (local (map 
children))) space)
                ((cl-struct ement-room (id child-id) (local (map 
parents))) room))
     ...))

No alternative to pcase that omitted these features could take its place 
for me.  In a large package like Ement.el, where function after function 
destructures similar objects in similar ways, the benefit of the 
consistency, conciseness, and flexibility offered by pcase can't be 
understated.

As well, pcase is easily extended with its built-in flexibility, e.g. 
how easily support was added for the cl-type based pattern.  It's a very 
well-designed system.

Then there are pcase-lambda, pcase-dolist, and pcase-setq, all of which 
use the same underlying facilities and patterns.  It's a coherent 
system: learning pcase opens up this world of useful forms.  I would not 
want to give up these powerful tools which make my programs better and 
my programming more enjoyable.