Re: pcase generates an unprintable expansion for a form in test erc--restore-initialize-priors

[Michael Heerdegen]

Alan Mackenzie <acm@muc.de> writes:

> The form being compared using pcase, although not tiny, is not all that
> big, and it would be easy to increase its size to cause it to violate any
> reasonable value of PRINT_CIRCLE.

Even worse: if you try to compile a file consisting of only this defun
(this is exactly one part of that test factored out):

#+begin_src emacs-lisp
(defun erc--restore-initialize-priors-1 (expansion)
  (pcase expansion
    (`(let* ((,p (or erc--server-reconnecting erc--target-priors))
             (,q (and ,p (alist-get 'erc-my-mode ,p))))
        (unless (local-variable-if-set-p 'erc-my-mode)
          (error "Not a local minor mode var: %s" 'erc-my-mode))
        (setq foo (if ,q (alist-get 'foo ,p) (ignore 1 2 3))
              bar (if ,q (alist-get 'bar ,p) #'spam)
              baz (if ,q (alist-get 'baz ,p) nil)))
     t)))
#+end_src

with a slightly reduced `max-lisp-eval-depth' (from 1600 to 1000)
compilation fails.


> Would it be possible and a good idea to amend pcase such that it
> generates less deeply nested expansions for forms such as we have here?

I think this todo in pcase.el is related:

;; - try and be more clever to reduce the size of the decision tree, and
;;   to reduce the number of leaves that need to be turned into functions:


> Or does anybody have any ideas how better to resolve the problem?

Apart from trying to work around it somehow, I don't.


Michael.