Re: using setq to create lists based on other lists...

[Jean-Christophe Helary]

Thank you for the comments :)

> You're using `setq` on vars you haven't declared/defined yet!

The Introduction doesn't even mention that...

> I recommend you put `-*- lexical-binding:t -*-` somewhere on the first
> line of your file, and that you `M-x byte-compile RET` your file so
> Emacs can help you catch some of those issues.

I will.

> Better write this as (setf (nth 3 myDateLastMonth) myDay)

No reference to setf in the Introduction, and it's buried under tons of 
reference in the Reference...

Thank you again. Now I think there really is too much of a gap between the 
Introduction and the Reference. The exchange today kind of confirms that there 
is a need to overhaul the documentation so as to get beginners to produce 
better code...

Jean-Christophe 

> On Dec 3, 2018, at 2:02, Stefan Monnier <monnier@iro.umontreal.ca> wrote:
> 
>> I see. Eventually I used copy-tree instead to initialize the new lists and
>> then modified them separately with setcar.
> 
> So you're back to using `setcar` :-(
> 
>> Just out of curiosity, let me post what I did. There are probably
>> better ways to do it, but that's the best I could come up with
>> today. I'd love to be able to think in terms closer to what elisp
>> allows though.
> 
> Those date/time thingies are indeed rather annoying to construct.
> copy-sequence + in-place modification is probably the best you can
> use, indeed :-(
> 
>>  (setq now (decode-time (float-time))
>>      myDateLastMonth (copy-tree now)
>>      myDateThisMonth (copy-tree now)
>>      myDateNextMonth (copy-tree now)
>>      now (encode-time now 'integer))
> 
> You're using `setq` on vars you haven't declared/defined yet!
> And theses aren't "trees" but lists/sequences, so better use
> `copy-sequence` which is also more efficient:
> 
>    (let* ((decoded-now (decode-time (float-time)))
>          (myDateLastMonth (copy-tree now))
>          (myDateThisMonth (copy-tree now))
>          (myDateNextMonth (copy-tree now))
>          (encoded-now (encode-time now 'integer)))
> 
> I recommend you put `-*- lexical-binding:t -*-` somewhere on the first
> line of your file, and that you `M-x byte-compile RET` your file so
> Emacs can help you catch some of those issues.
> 
>>  ;; create the data for last month
>>  (setcar (cdr (cdr (cdr myDateLastMonth))) myDay)
> 
> Better write this as (setf (nth 3 myDateLastMonth) myDay)
> It's bad enough that the fields aren't named so you have to refer to
> them by position, but having to count `cdr`s is really annoying IMO.
> 
> 
> -- Stefan
> 
> 

Jean-Christophe Helary
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http://mac4translators.blogspot.com @brandelune