[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
bug#62117: 29.0.60; cl-letf on a map place has side-effects
From: |
Michael Heerdegen |
Subject: |
bug#62117: 29.0.60; cl-letf on a map place has side-effects |
Date: |
Sun, 12 Mar 2023 01:00:12 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) |
Augusto Stoffel <arstoffel@gmail.com> writes:
> Consider this example:
>
> (require 'cl-lib)
> (defun f (map)
> (cl-letf (((map-elt map 'a) 1))
> map))
>
> (let ((map '(b 2)))
> (f map)
> map)
> => (b 2 a nil)
>
> (let ((map (make-hash-table)))
> (f map)
> (map-length map))
> => 1
>
>
> I would expect `f' to have no side effects, so get (b 2) and 0
> respectively in the two examples.
This is a symptom of a general limitation of `cl-letf'. Currently you can't
rely on a "no side effect" behavior. There are other examples like that
(`alist-get') and cases that are worse (binding `buffer-local-value' of
a variable in a buffer with no buffer local binding doesn't remove the
buffer-localness - that's one reason why that gv had been deprecated).
> Of course it's usual to treat a nil entry and no entry as equivalent in
> Lisp, but this behavior can be a problem e.g. when constructing data to
> pass to other programs.
I would say: if it is a problem, map.el is the wrong abstraction for
your case. That's the genuine idea of map.el: that the inner structure
of a map doesn't matter.
So I would close this one - unless you have some enlightening idea that
would be an obvious improvement with no downsides and backward
compatibility problems.
Thanks,
Michael.