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Re: Is `eval' allowed to modify its argument?


From: Stefan Monnier
Subject: Re: Is `eval' allowed to modify its argument?
Date: Thu, 12 Nov 2020 17:13:07 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)

> is `eval' allowed to modify a list specified as FORM argument?

There's nothing that says it's no allowed, I think.  It's probably not
very desirable in general, tho I would expect some macros to do it
(e.g. in a previous life I played with a version of `macroexpand` which
modified its argument in place to replace the original code with its
macro-expanded form: I was considering it as an optimization to speed up
execution of non-byte-compiled loops by avoiding the re-expansion of
macro invocations in the body).

> It seems to happen to me in one case.

I can imagine cases where one might think it's a good idea to do so, but
I suspect that in practice it probably always ends up being a bad idea,
so you might like to report it as a ... let's not call it a bug but
a "sign that something's fishy"?


        Stefan




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