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From: Mark Oteiza <mvoteiza@udel.edu>
To: Stefan Monnier <monnier@iro.umontreal.ca>
Cc: emacs-devel@gnu.org
Subject: Re: [Emacs-diffs] master 1b4442b: Don't expand body inside a let-binding when there are no bindings
Date: Thu, 16 Feb 2017 23:44:22 -0500	[thread overview]
Message-ID: <878tp58lzt.fsf@holos> (raw)
In-Reply-To: <jwvtw7umfqk.fsf-monnier+emacsdiffs@gnu.org> (Stefan Monnier's message of "Thu, 16 Feb 2017 08:32:06 -0500")

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> +  (if vars
>> +      `(let ,(mapcar (lambda (b) (list (car b) (cdr b))) vars)
>> +         ,@code)
>> +    `(progn ,@code)))
>
> Just curious here:
>
> I resisted the temptation to do that when working on that code because
> I thought that it should (both in the sense that I expect the current
> code to behave that way, and in the sense that if it doesn't behave that
> way, we should fix it) make no difference to the byte-code we generate.
>
> So, I'm wondering if you've found a concrete case where it makes
> a difference, or it's just that you wanted the output to look
> more pretty.

Nope, AFAICT the bytecode remains the same.

In the past I noticed that pcase expanded things to (let nil ...), which
I think is a bit annoying wrt readability, yes; however just recently I
was poking at elint.el, which complains about such forms with

  Empty varlist in let: (let nil <snip>)

so I was more inclined to change it.

Between vars possibly being expensive, and--one step beyond that--using an
uninterned symbol to ameliorate evalling vars twice incurring a cost
with dynamic binding, I suppose this change was short sighted.



  reply	other threads:[~2017-02-17  4:44 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20170216014306.20854.5128@vcs0.savannah.gnu.org>
     [not found] ` <20170216014308.12DC326007@vcs0.savannah.gnu.org>
2017-02-16 13:32   ` [Emacs-diffs] master 1b4442b: Don't expand body inside a let-binding when there are no bindings Stefan Monnier
2017-02-17  4:44     ` Mark Oteiza [this message]
2017-02-18  0:05       ` Stefan Monnier
2017-02-18  0:10         ` Mark Oteiza

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