basic help evaluating function in an alist

basic help evaluating function in an alist

[Matt Price]

Hi,

i'm trying to figure out what's wrong with this simple code:

(add-to-list 'my-winlist
             '(guide . (selected-window) )
             )
(windmove-right)
(select-window(cdr(assoc 'guide my-winlist)) )

So, what I would hope happens here is that I add an element (guide .
(selected-window))  to an alist, but that the second element would be
the actual window object in question, not the text string
"(selected-window)".  Lisp puzzles me pretty thoroughly so I know I'm
missing a very basic feature, but I can't figure it out by flipping
through the info pages...  Thanks everyone!
Matt

RE: basic help evaluating function in an alist

[Drew Adams]

> i'm trying to figure out what's wrong with this simple code:
> (add-to-list 'my-winlist
>              '(guide . (selected-window)))
> (windmove-right)
> (select-window(cdr(assoc 'guide my-winlist)) )

Break it down.  Forget about `windmove-right'.  Just check the value of
`my-winlist' after you added the element.

(setq my-winlist ())
(add-to-list 'my-winlist
             '(guide . (selected-window)))

`C-h v my-winlist' shows:

((guide selected-window))

That's the same as ((guide . (selected-window)))

Already you can see the problem: no window, just a list with the symbol
`selected-window' as its sole element.

If you want, try `M-: (cdr (assoc 'guide my-winlist))', to see that element:
(selected-window).

You need to evaluate `(selected-window)', not just use that list as the cdr of
your element.  Here's what you need:

(setq my-winlist ())
(add-to-list 'my-winlist
             (cons 'guide (selected-window)))

`C-h v my-winlist' shows a list with one element, which is a dotted pair (aka
cons cell), whose cdr is a window:

((guide . #<window 84 on *scratch*>))

With backquote syntax you can express the same thing this way:

(setq my-winlist ())
(add-to-list 'my-winlist
             `(guide ,(selected-window)))

I recommend that you read this manual that comes with Emacs (use `C-h i'):
`Emacs Lisp Intro'.  HTH.

Re: basic help evaluating function in an alist

[Matt Price]

On Sat, Dec 1, 2012 at 6:12 PM, Drew Adams <drew.adams@oracle.com> wrote:
>> i'm trying to figure out what's wrong with this simple code:
>> (add-to-list 'my-winlist
>>              '(guide . (selected-window)))
>> (windmove-right)
>> (select-window(cdr(assoc 'guide my-winlist)) )
>
> Break it down.  Forget about `windmove-right'.  Just check the value of
> `my-winlist' after you added the element.
>
> (setq my-winlist ())
> (add-to-list 'my-winlist
>              '(guide . (selected-window)))
>
> `C-h v my-winlist' shows:
>
> ((guide selected-window))
>
> That's the same as ((guide . (selected-window)))
>
> Already you can see the problem: no window, just a list with the symbol
> `selected-window' as its sole element.
>
> If you want, try `M-: (cdr (assoc 'guide my-winlist))', to see that element:
> (selected-window).
>
> You need to evaluate `(selected-window)', not just use that list as the cdr of
> your element.  Here's what you need:
>
> (setq my-winlist ())
> (add-to-list 'my-winlist
>              (cons 'guide (selected-window)))
>
> `C-h v my-winlist' shows a list with one element, which is a dotted pair (aka
> cons cell), whose cdr is a window:
>
> ((guide . #<window 84 on *scratch*>))
>
> With backquote syntax you can express the same thing this way:
>
> (setq my-winlist ())
> (add-to-list 'my-winlist
>              `(guide ,(selected-window)))
>
> I recommend that you read this manual that comes with Emacs (use `C-h i'):
> `Emacs Lisp Intro'.  HTH.
>
that helps a lot.  I still don't quite understand what it is in the
syntax that causes the function (selected-window) to be evaluated in
one case and not the other, but I'll try to wrap my head around the
manual a little better  thanks to both of you!
matt

RE: basic help evaluating function in an alist

[Drew Adams]

> > '(guide . (selected-window))
> >
> > (cons 'guide (selected-window))
>
> I still don't quite understand what it is in the
> syntax that causes the function (selected-window) to be evaluated in
> one case and not the other

In the first case, the dotted pair `(guide . (selected-window))' is quoted.
When evaluated, the sexp `(quote (guide . (selected-window)))' returns that
dotted pair: `(guide . (selected-window))'.  `quote' returns its argument
without evaluating it.

In the second case, the sexp `(cons 'guide (selected-window))' is evaluated,
which means applying function `cons' to its two arguments, after evaluating each
of them.

Evaluation of the first argument, which is `(quote guide)', returns the symbol
`guide'.  Evaluation of the second arg, `(selected-window)', returns the
selected window.  Function `cons' then creates a cons cell whose car is the
symbol `guide' and whose cdr is that window.

Re: basic help evaluating function in an alist

[Matt Price]

On Sat, Dec 1, 2012 at 10:44 PM, Drew Adams <drew.adams@oracle.com> wrote:
>> > '(guide . (selected-window))
>> >
>> > (cons 'guide (selected-window))
>>
>> I still don't quite understand what it is in the
>> syntax that causes the function (selected-window) to be evaluated in
>> one case and not the other
>
> In the first case, the dotted pair `(guide . (selected-window))' is quoted.
> When evaluated, the sexp `(quote (guide . (selected-window)))' returns that
> dotted pair: `(guide . (selected-window))'.  `quote' returns its argument
> without evaluating it.
>
> In the second case, the sexp `(cons 'guide (selected-window))' is evaluated,
> which means applying function `cons' to its two arguments, after evaluating each
> of them.
>
> Evaluation of the first argument, which is `(quote guide)', returns the symbol
> `guide'.  Evaluation of the second arg, `(selected-window)', returns the
> selected window.  Function `cons' then creates a cons cell whose car is the
> symbol `guide' and whose cdr is that window.
>
Drew, I forgot to write and say thank you.  this was a big help.
matt

Re: basic help evaluating function in an alist

[WJ]

Matt Price wrote:

> Hi,
> 
> i'm trying to figure out what's wrong with this simple code:
> 
> (add-to-list 'my-winlist
>              '(guide . (selected-window) )
>              )
> (windmove-right)
> (select-window(cdr(assoc 'guide my-winlist)) )
> 
> So, what I would hope happens here is that I add an element (guide .
> (selected-window))  to an alist, but that the second element would be
> the actual window object in question, not the text string
> "(selected-window)".  Lisp puzzles me pretty thoroughly so I know I'm
> missing a very basic feature, but I can't figure it out by flipping
> through the info pages...  Thanks everyone!
> Matt

: (setq mylist '((b . 2)))
((b . 2))
: (add-to-list 'mylist (cons 'c (sqrt 3)))
((c . 1.7320508075688772) (b . 2))

Re: basic help evaluating function in an alist

[WJ]

WJ wrote:

> Matt Price wrote:
> 
> > Hi,
> > 
> > i'm trying to figure out what's wrong with this simple code:
> > 
> > (add-to-list 'my-winlist
> >              '(guide . (selected-window) )
> >              )
> > (windmove-right)
> > (select-window(cdr(assoc 'guide my-winlist)) )
> > 
> > So, what I would hope happens here is that I add an element (guide .
> > (selected-window))  to an alist, but that the second element would be
> > the actual window object in question, not the text string
> > "(selected-window)".  Lisp puzzles me pretty thoroughly so I know I'm
> > missing a very basic feature, but I can't figure it out by flipping
> > through the info pages...  Thanks everyone!
> > Matt
> 
> : (setq mylist '((b . 2)))
> ((b . 2))
> : (add-to-list 'mylist (cons 'c (sqrt 3)))
> ((c . 1.7320508075688772) (b . 2))

Another way:

: (require 'cl)
cl
: (setq mylist '((b . 2)))
((b . 2))
: (setq mylist (acons 'c (sqrt 3) mylist))
((c . 1.7320508075688772) (b . 2))