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Re: [PATCH] Add cl-map-into

From: Stefan Monnier
Subject: Re: [PATCH] Add cl-map-into
Date: Mon, 27 Sep 2021 13:18:09 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)

> Absense of Common Lisp's map-into in cl-lib is an unfortunate omission.

[ As someone who likes to avoid side-effects, I have a slightly different
  opinion ;-) ]

> The proposed patch adds it.

Looks pretty good.  I have a code suggestion below, but my main
comments would be:

- Could you provide some corresponding tests for test/emacs-lisp/cl-tests.el?
- Since we're (weakly) promoting seq.el over cl-lib.el, I wonder if you
  think it would be worthwhile to try and do something similar in seq.el?

> +       (or (and small basic-cache)
> +           (and (not small) general-cache)
> +           (let ((mapper (cl--make-map-into-mapper sig)))
> +             (if small (setf basic-cache mapper)
> +               (setf general-cache mapper))))))

This can turn into

          (or (if small basic-cache general-cache)
              (let ((mapper (cl--make-map-into-mapper sig)))
                (setf (if small basic-cache general-cache)


          (or (if small basic-cache general-cache)
              (setf (if small basic-cache general-cache)
                    (cl--make-map-into-mapper sig)))

which thus looks very much like what I used in
`cl--generic-with-memoization`, so you could replace the symbol-macrolet
with something like:

        (if small
            (aref cl--map-into-mappers-array sig)
          ;; TODO: Order alist entries for faster lookup
          ;; (note that we'll have to abandon alist-get then).
          (alist-get sig cl--map-into-mappers-alist nil nil #'=))
      (cl--make-map-into-mapper sig))

-- Stefan

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