GNU bug report logs - #73853
Should and-let* become a synonym for when-let*?

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Message #17 received at 73853 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Michael Heerdegen <michael_heerdegen <at> web.de>
Cc: 73853 <at> debbugs.gnu.org
Subject: Re: bug#73853: 31.0.50; and-let* is useless
Date: Fri, 18 Oct 2024 19:42:44 -0400

>> Can we kill/deprecate this?
> `and-let*'s purpose is to express conditions, `when-let*'s is
> conditional evaluation.  We have `and-let*' and `when-let*' for the same
> reason we have `and' and `when'.  See prior discussions.

But there isn't the same "historical" support that justifies having
both, and the syntax&semantics of `and-let*` is just weird:

- Why allow a BODY if the motivation is to mirror the normal `and`?
  If you want a BODY, use `when-let*`.
- What's the use of the final variable binding since (assuming you
  don't use BODY) that variable is never used:

     (and-let* ((a (fooa))
                (b (foob a))
                (i-m-useless (fooc a b))))

- There's a special syntax where the final binding can drop the variable
  name (because of the previous point), which makes for an odd syntax

     (and-let* ((a (fooa))
                (b (foob a))
                ((weird-call a b))))

So the use with BODY is redundant with `when-let*` and the use without
BODY is quirky (and still redundant with `when-let*`, of course).

>> [ I think we have too many (if|when|and)-let(*) for our own good: we
>>   should pick some winners and deprecate the other ones.   ]
> AFAIR the non-star versions exist for backward compatibility only - so I
> would rather get rid of these.  Parallel existence of these non-star
> vs. star versions should be a temporary state, it complicates the matter
> for an epsilon gain.

100% agreement.  Can we `make-obsolete` the non-star versions?

>> I could see a use for something called `and-let(*)` but without a BODY,
>> for the purpose of remove a level of parens and indentation:
>>
>>     (and-let*
>>       (x1 (foo1))
>>       (x2 (foo2)))
>>
>> i.s.o
>>
>>     (and-let*
>>         ((x1 (foo1))
>>          (x2 (foo2))))
>
> Ugh! - I could not imagine anything with more potential for confusion as
> removing the paren around a list of bindings.

FWIW, I agree, I don't like that either.


        Stefan

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