Re: [Emacs-diffs] master 1b4442b: Don't expand body inside a let-binding when there are no bindings

Re: [Emacs-diffs] master 1b4442b: Don't expand body inside a let-binding when there are no bindings

[Stefan Monnier]

> +  (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.


        Stefan

Re: [Emacs-diffs] master 1b4442b: Don't expand body inside a let-binding when there are no bindings

[Mark Oteiza]

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.

Re: [Emacs-diffs] master 1b4442b: Don't expand body inside a let-binding when there are no bindings

[Stefan Monnier]

> 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.

OK, thanks.

> 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.

Sorry, I didn't understand this part of your message.


        Stefan

Re: [Emacs-diffs] master 1b4442b: Don't expand body inside a let-binding when there are no bindings

[Mark Oteiza]

On 17/02/17 at 07:05pm, Stefan Monnier wrote:
> > 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.
> 
> OK, thanks.
> 
> > 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.
> 
> Sorry, I didn't understand this part of your message.

I was pondering not sending the last part because I was suddenly getting
really tired.  As long as the change is okay I guess it doesn't matter.
Woops.