GNU bug report logs - #67669
29.1; Drop text suggesting using `and' to replace `if'

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From: Xiyue Deng <manphiz <at> gmail.com>
To: Drew Adams <drew.adams <at> oracle.com>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 67669-done <at> debbugs.gnu.org
Subject: bug#67669: 29.1; Drop text suggesting using `and' to replace `if'
Date: Thu, 07 Dec 2023 12:00:42 -0800

Hi Drew,

Drew Adams <drew.adams <at> oracle.com> writes:

>> I quite like this suggestion[1] that `if' and `when' are used to guard side
>> effects,
>
> More clearly, `when' and `unless' (not `if', which is general - no
> special suggestion) are used to suggest (to humans) that they're used
> _only_ to perform side effects.  With this convention, code shouldn't
> depend on their (always `nil') return value.
>
>> whereas `and' and `or' are used for pure functions.
>
> That too isn't really the convention (though it is what's said in that emacs.SE answer).
>
> What should be said is that you use `and' and `or' when _the return value matters_, i.e., when it's used somewhere.
>
> That does _not_ preclude the use of `and' and `or' to perform side
> effects.  It's simply that the _return value matters_.  Unlike
> `progn', the (Boolean) return value of each of their "steps" (sexps)
> determines whether subsequent ones are evaluated.

Agreed here.  I was quoting the SE answer for the "pure function" part,
but indeed I also think what matters is whether we use the results.

>
> I've added another answer to the emacs.SE question you cited:
>
> https://emacs.stackexchange.com/a/79744/105
>
> But as you correctly noted, such conventions are a question of personal (or group) coding style.  Lisp doesn't care or recognize any such meaning.

Also agreed.  And I've since closed this.  Thanks for the discussion!

-- 
Xiyue Deng

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