Add-to-list or setq append...

Add-to-list or setq append...

[Andrea Crotti]

Just a curiosity, I was wondering why in so many README and doc they
suggest to add elements to lists with something like:

--8<---------------cut here---------------start------------->8---
(setq list (append list element))
--8<---------------cut here---------------end--------------->8---

instead of doing simply
--8<---------------cut here---------------start------------->8---
(add-to-list list-va element)
--8<---------------cut here---------------end--------------->8---

Is there any good reason or is just historical heritage?
I think add-to-list is even more smart adding only if not there already,
then what's the point in using that ugly form?

Re: Add-to-list or setq append...

[Richard Riley]

Andrea Crotti <andrea.crotti.0@gmail.com> writes:

> Just a curiosity, I was wondering why in so many README and doc they
> suggest to add elements to lists with something like:
>
>
>
> (setq list (append list element))
>
>
>
> instead of doing simply
>
>
> (add-to-list list-va element)
>
>
>
> Is there any good reason or is just historical heritage?
> I think add-to-list is even more smart adding only if not there already,
> then what's the point in using that ugly form?
>


With append you can append a list. e.g

(setq load-path (append load-path '("/usr/local/emacs/site-lisp" "/usr/share/emacs/site-lisp")))

I don't know if add-to-list can do that?

Re: Add-to-list or setq append...

[Andrea Crotti]

Richard Riley <rileyrg@gmail.com> writes:

> With append you can append a list. e.g
>
> (setq load-path (append load-path '("/usr/local/emacs/site-lisp" "/usr/share/emacs/site-lisp")))
>
> I don't know if add-to-list can do that?

Sure but I was talking about simpler cases, for example

--8<---------------cut here---------------start------------->8---
(setq auto-mode-alist (cons '("\\.ml\\w?" . tuareg-mode) auto-mode-alist))
--8<---------------cut here---------------end--------------->8---

is equivalent to
--8<---------------cut here---------------start------------->8---
(add-to-list 'auto-mode-alist '("\\.ml\\w?" . tuareg-mode))
--8<---------------cut here---------------end--------------->8---

right?
Maybe some years ago add-to-list was not present?

RE: Add-to-list or setq append...

[Drew Adams]

> (setq auto-mode-alist (cons '("\\.ml\\w?" . tuareg-mode) auto-mode-alist))
> is equivalent to
> (add-to-list 'auto-mode-alist '("\\.ml\\w?" . tuareg-mode))

No, it is not.  The former unconditionally adds the entry at the beginning of
the alist.  The latter adds the entry to the alist only if an `equal' entry is
not already present. 

The former ensures that the entry is at the start of the list.  The latter does
not.  

In the former case the alist can end up with multiple entries with the given
key, the first such entry shadowing all others with the same key.  If you then
pop off that first entry, the previously first of all entries with that key is
still present and now becomes the one that matters (shadows any others with that
key).

Re: Add-to-list or setq append...

[Andrea Crotti]

"Drew Adams" <drew.adams@oracle.com> writes:
>
> No, it is not.  The former unconditionally adds the entry at the beginning of
> the alist.  The latter adds the entry to the alist only if an `equal' entry is
> not already present. 
>
> The former ensures that the entry is at the start of the list.  The latter does
> not.  
>
> In the former case the alist can end up with multiple entries with the given
> key, the first such entry shadowing all others with the same key.  If you then
> pop off that first entry, the previously first of all entries with that key is
> still present and now becomes the one that matters (shadows any others with that
> key).

Sure but
--8<---------------cut here---------------start------------->8---
ELISP> my-l
((1 . 3)
 (1 . 2))

ELISP> (add-to-list 'my-l '(2 . 3))
((2 . 3)
 (1 . 3)
 (1 . 2))
--8<---------------cut here---------------end--------------->8---

it actually not add it ONLY if they're really equal, which is desirable
in my opinion.
What's the point in having twice the same cons in auto-mode-alist?

Then the real difference is that I add in the end instead of at the
beginning, and in that case I don't shadow an already present value...

Re: Add-to-list or setq append...

[Andrea Crotti]

Andrea Crotti <andrea.crotti.0@gmail.com> writes:

> it actually not add it ONLY if they're really equal, which is desirable
> in my opinion.
> What's the point in having twice the same cons in auto-mode-alist?
>
> Then the real difference is that I add in the end instead of at the
> beginning, and in that case I don't shadow an already present value...

And also why not using some kind of union when merging two lists.
There is a union defined in cl-seq, that for example would suffice the
ugly double elements in the same list...

RE: Add-to-list or setq append...

[Drew Adams]

> And also why not using some kind of union when merging two lists.
> There is a union defined in cl-seq, that for example would suffice the
> ugly double elements in the same list...

Why not take the bus to Milwaukee?  Why not collect stamps?

You use a set union function when you want a set union.  End of story.

You can use a list for anything.  Sometimes you want duplicate elements;
sometimes you do not; sometimes you don't care.  Sometimes you want the elements
to be sorted and remain sorted.  Sometimes you do not.  It all depends on your
needs.

Re: Add-to-list or setq append...

[Pascal J. Bourguignon]

Andrea Crotti <andrea.crotti.0@gmail.com> writes:

> Andrea Crotti <andrea.crotti.0@gmail.com> writes:
>
>> it actually not add it ONLY if they're really equal, which is desirable
>> in my opinion.
>> What's the point in having twice the same cons in auto-mode-alist?
>>
>> Then the real difference is that I add in the end instead of at the
>> beginning, and in that case I don't shadow an already present value...
>
> And also why not using some kind of union when merging two lists.
> There is a union defined in cl-seq, that for example would suffice the
> ugly double elements in the same list...

If union in cl-seq is implemented as specified by clhs, it doesn't
ensure that the resulting list will have unique elements (for the
given union criteria, which by the way, when involving :test-not
(inequalities in general) is rather hairy and un-obvious).

What is ensured is that for each pair (r,s) in R×S, only one of the
two elements of is in the result, which doesn't  mean much, because
the other element may be included in the result because of the result
of the test on another pair.


If you want a list with unique elements, you should use remove-duplicates.


-- 
__Pascal Bourguignon__                     http://www.informatimago.com/

RE: Add-to-list or setq append...

[Drew Adams]

> ELISP> my-l
> ((1 . 3)
>  (1 . 2))
> 
> ELISP> (add-to-list 'my-l '(2 . 3))
> ((2 . 3)
>  (1 . 3)
>  (1 . 2))
> 
> it actually not add it ONLY if they're really equal, which is 
> desirable in my opinion.

`add-to-list' adds an element ONLY if it is NOT `equal' to any existing element.

`add-to-list' is for any list, not just an alist.  `equal is used to test
whether an _entry_ is already present in the list.  `equal' is applied to the
potentially new entry and to each list entry (aka item, element).

This is not a comparison of the cars or cdrs of the entry; it is a comparison of
the entry as a whole: both its car and cdr.  The cons (2 . 3) is not `equal' to
any item in the list, so it is added.

> What's the point in having twice the same cons in auto-mode-alist?

None.

I misspoke in mentioning shadowing wrt the examples in your second mail.  I was
thinking of something like this, where only the alist keys are compared:

(add-to-list 'auto-mode-alist '("\\.ml\\w?" . tuareg-mode)
             nil (lambda (x y) (equal (car x) (car y))))

That would allow for only one entry with the given key.  I was speaking to why
you might want to allow more than one such - which I think got closer to your
original question.

Your original question was not just about `auto-mode-alist'.  You asked why
someone might use (setq list (append (list element))) instead of (add-to-list
list-va element).  Any list can be used for anything you like.  Multiple
occurrences of `equal' items or even `eq' items can often be useful.

It is often the case in using alists, in particular, that you do not care
whether there are also other items with the same key, because only the first one
is seen (using the usual alist operations).  If you don't care about removing
duplicates (wrt the key) then, well, you don't care about removing them.

And in some cases, as I said, you want to preserve the older alist entries so
that you can retrieve them or make them active at some point.  You might want to
use the alist as a stack of definitions or in some other way where old items can
be useful.

One program, in one context, adds an entry to `auto-mode-alist'.  In a different
context, another adds an entry that shadows the first one (it has the same key).
Upon leaving the second context you might well want to make the previous
association active again.  Removing the shadowing entry is one way to do that.
If the original entry was deleted then you cannot retrieve it (but you could add
it again).

> Then the real difference is that I add in the end instead of at the
> beginning,

That is not the real difference.  That is not even a difference - you can add at
either the end or the beginning with either `add-to-list' or `setq'.  The real
differences are those I outlined previously.

When I said that `add-to-list' does not assure that the entry is at the start of
the list I was referring to the case where the entry is already present
somewhere down the list, so it is not added.  In that case, it is not at the
head of the list.  You might or might not care that the item is first in the
list.  It all depends how you use the list.

> and in that case I don't shadow an already present value...

You never shadow an `equal' entry with a new one if you use `add-to-list' -
there are no occurences of `equal' items.

If you in some other way add an `equal' item at the end or otherwise after some
`equal' occurrence in the list, it is itself shadowed.

You can do anything you want with a list.  You can put an item at any position,
including an item that is `equal' to some other item.  You can put it in any
position relative to any number of such `equal' items, thus determining its
"shadowing depth" (for lack of a better term), which determines at what point it
would be unshadowed if you pop items of the list head.

Both of the cliches you showed are useful (`add-to-list', `setq').  They are
different.

Re: Add-to-list or setq append...

[David Kastrup]

Richard Riley <rileyrg@gmail.com> writes:

> Andrea Crotti <andrea.crotti.0@gmail.com> writes:
>
>> Just a curiosity, I was wondering why in so many README and doc they
>> suggest to add elements to lists with something like:
>>
>>
>>
>> (setq list (append list element))
>>
>>
>>
>> instead of doing simply
>>
>>
>> (add-to-list list-va element)
>>
>>
>>
>> Is there any good reason or is just historical heritage?
>> I think add-to-list is even more smart adding only if not there already,
>> then what's the point in using that ugly form?
>
> With append you can append a list. e.g
>
> (setq load-path (append load-path '("/usr/local/emacs/site-lisp" "/usr/share/emacs/site-lisp")))
>
> I don't know if add-to-list can do that?

It would have taken you less time to ask Emacs than it takes a hundred
readers to read your unnecessary message.

C-h f add-to-list RET

add-to-list is a compiled Lisp function in `subr.el'.

(add-to-list LIST-VAR ELEMENT &optional APPEND COMPARE-FN)

[...]

If ELEMENT is added, it is added at the beginning of the list,
unless the optional argument APPEND is non-nil, in which case
ELEMENT is added at the end.

[...]

-- 
David Kastrup

Re: Add-to-list or setq append...

[Pascal J. Bourguignon]

Andrea Crotti <andrea.crotti.0@gmail.com> writes:

> Just a curiosity, I was wondering why in so many README and doc they
> suggest to add elements to lists with something like:
>
> --8<---------------cut here---------------start------------->8---
> (setq list (append list element))
> --8<---------------cut here---------------end--------------->8---
>
> instead of doing simply
> --8<---------------cut here---------------start------------->8---
> (add-to-list list-va element)
> --8<---------------cut here---------------end--------------->8---
>
> Is there any good reason or is just historical heritage?
> I think add-to-list is even more smart adding only if not there already,
> then what's the point in using that ugly form?

I would rather use:

(require 'cl)
(pushnew element list-va)

than add-to-list.


-- 
__Pascal Bourguignon__                     http://www.informatimago.com/

Re: Add-to-list or setq append...

[Tim X]

Andrea Crotti <andrea.crotti.0@gmail.com> writes:

> Just a curiosity, I was wondering why in so many README and doc they
> suggest to add elements to lists with something like:
>
> (setq list (append list element))
>
> instead of doing simply
> (add-to-list list-va element)
>
> Is there any good reason or is just historical heritage?
> I think add-to-list is even more smart adding only if not there already,
> then what's the point in using that ugly form?
>

Possibly a bit of both. 

Both forms have their use. 

Some will likely argue that setq is more general and can be used to add
items at the end via append or at the beginning using cons etc. However,
they a probably not aware that add-to-list, by default adds the element
at the beginning of the list, but has an optional argument to add it at
the end. some will argue that add-to-list only does an 'equal' test to
determine if the element is already in the list, but this too can be
changed with an optional argument. 

One area where add-to-list is weak is when you want to add the element
to a specific location that is not at the start or end of the list. The
other point to note is that add-to-list modifies its list argument i.e.
is destructive. (append l v) creates a new list and does not modify l,
which is why you need the setq. 

Frequently, you see append, cons etc because it is more 'lispish'. Back
when many places used lisp and scheme to teach, many new emacs users
came to elisp having already learnt a lisp. For them, append, cons,
push etc were/are more familiar idioms while add-to-list is more dialect
specific. 

Tim

-- 
tcross (at) rapttech dot com dot au