Weird let behavior (and where to ask for support)

Weird let behavior (and where to ask for support)

[dalanicolai]

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I am trying again to improve/clean up the implementation of continuous
scroll in doc-view.
 But I find some strange behavior, for which I have no explanation.

 So let me first explain the essence of the problem. I am using the
following code, in a file with lexical-binding enabled (in case that
would matter).
I will provide the full function later also, but here is the essence:

 (defun book-image-positions (image-sizes)
  (let ((positions '(0)))
    (message "POS %s" positions)))

I really use the variable name 'positions' only within this function
(I bind the result to
a variable named 'image-positions'

I would expect that 'positions' would always get initialized as '(0).
But when I print the value after the let, then it shows me a large list.
I guess I am missing something here, and also I guess you probably
know what it is.

A little more context, I have a list with image-sizes (heights), and I
am creating
a list with image positions from it.  For that I am using the following function
(incl. quite some print forms for debugging).

 (defun book-image-positions (image-sizes)
  (let ((sum 0)
        (positions '(0)))
    (print sum)
    (message "POS %s" positions)
    (dolist (s image-sizes)
      (setq sum (+ sum (cdr s)))
      (print (push sum positions)))
    (message "RES %s" (reverse positions))
    (nreverse positions)))

this function gets called multiple times and 'positions' does have the
'(0) value the first time.

Then finally, I was trying to search for some related issue in the
devel archive, but the
 results always get  'cut off', because there are too many search
results. Also, I think
 users will more likely be able to find the question and answer on
stack overflow, so
would you agree it is often best to ask these kind of 'elisp' questions there?

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Re: Weird let behavior (and where to ask for support)

[Pierpaolo Bernardi]

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You are modifying the constant '(0). Modifying constants has an unspecified effect, one manifestation of which you are observing.

Change '(0) to (list 0).


Il giorno 23 gennaio 2022, alle ore 01:34, dalanicolai <dalanicolai@gmail.com> ha scritto:

I am trying again to improve/clean up the implementation of continuous scroll in doc-view.
 But I find some strange behavior, for which I have no explanation.

 So let me first explain the essence of the problem. I am using the following code, in a file with lexical-binding enabled (in case that would matter).
I will provide the full function later also, but here is the essence:

 (defun book-image-positions (image-sizes)
  (let ((positions '(0)))
    (message "POS %s" positions)))

I really use the variable name 'positions' only within this function (I bind the result to
a variable named 'image-positions'
I would expect that 'positions' would always get initialized as '(0).
But when I print the value after the let, then it shows me a large list.
I guess I am missing something here, and also I guess you probably 
know what it is.

A little more context, I have a list with image-sizes (heights), and I am creating
a list with image positions from it.  For that I am using the following function
(incl. quite some print forms for debugging).

 (defun book-image-positions (image-sizes)
  (let ((sum 0)
        (positions '(0)))
    (print sum)
    (message "POS %s" positions)
    (dolist (s image-sizes)
      (setq sum (+ sum (cdr s)))
      (print (push sum positions)))
    (message "RES %s" (reverse positions))
    (nreverse positions)))

this function gets called multiple times and 'positions' does have the '(0) value the first time.

Then finally, I was trying to search for some related issue in the devel archive, but the
results always get 'cut off', because there are too many search results. Also, I think
users will more likely be able to find the question and answer on stack overflow, so
would you agree it is often best to ask these kind of 'elisp' questions there?


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Re: Weird let behavior (and where to ask for support)

[Stefan Monnier]

> You are modifying the constant '(0). Modifying constants has an unspecified
> effect, one manifestation of which you are observing.
> Change '(0) to (list 0).

Exactly.  IOW never use `nreverse`, `sort`¸ `nconc`, etc... on a list
unless you know for sure all the `conses` in that list are all freshly
made and not shared with anything else.

    '(0)

does not return a freshly made list, but instead return the same old
and tired list created when the code was loaded into Emacs.


        Stefan

Re: Weird let behavior (and where to ask for support)

[dalanicolai]

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Thanks for the answers. I must admit that I did not study the full elisp
manual (far from it),
and also I did not understand your answers immediately. Anyway, now that I
knew what
to look for I have found the section about 'self evaluating forms'
<https://www.gnu.org/software/emacs/manual/html_node/elisp/Self_002dEvaluating-Forms.html>
in the elisp manual and
reread the quote and mutability sections. I think it is clear now...

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