binding question

binding question

[Eric Abrahamsen]

I've got a variable binding scenario that I thought would be easy to
resolve, but is giving me trouble. I have the feeling that this scenario
represents some sort of Lesson 1 in How Bindings Work, but I'm still not
sure of the proper solution.

The general situation is: I want to start with a list template stored in
a variable. Another function in this library starts with the template,
modifies it a few times, then does something with the modified version.

Every time this function is called, it should start with a "fresh" copy
of the template.

Clearly the following doesn't work: it actually modifies the original
list-template.

(defvar list-template
  '(item ((with . "some")
	  (sub . "lists"))
	 nil (other things will go here)))

(defun main-entry-function ()
  ;; obviously this doesn't work as defvars are always special
  (let ((list-template list-template))
  ;; list-template is accessed dynamically in the following functions
    (modify-list-template)
    (modify-list-template-again)
    (do-something-with-modified-list-template)))

The other option would be to pass list-template as an argument to each
of these functions. I guess each call would then have to look like:

(setq list-template (modify-list-template list-template))

This seems ugly, and it won't necessarily be easy to ensure the
modifying functions end by returning the new value of list-template.

What's the best way of handling this? And is there one "right" solution
for both dynamic and lexical binding?

Thanks!
Eric


PS: The wiki[1] mentions pretty much exactly my scenario, but doesn't
actually say what should be done.

[1]
http://www.emacswiki.org/emacs/DynamicBindingVsLexicalBinding#toc4

Re: binding question

[Thien-Thi Nguyen]

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() Eric Abrahamsen <eric@ericabrahamsen.net>
() Sun, 06 Oct 2013 17:35:30 +0800

   What's the best way of handling this?

Check out ‘copy-tree’, e.g.:

 (let ((list-template (copy-tree list-template)))
   ...)

See also:

 http://www.emacswiki.org/emacs/ElispCookbook

which compares ‘copy-sequence’ and ‘copy-tree’ in section "Copying".

WRT, lexical vs dynamic binding, the code is written to assume dynamic
binding, so you might want to explicitly set ‘lexical-binding’ to nil
somewhere.

-- 
Thien-Thi Nguyen
   GPG key: 4C807502
   (if you're human and you know it)
      read my lisp: (responsep (questions 'technical)
                               (not (via 'mailing-list)))
                     => nil

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Re: binding question

[Michael Heerdegen]

Hi Eric,

in LISP, lists (cons cells) are passed by reference.  Thus, different
bindings can refer to the same list object, like here the variables a
and b:

(setq a '(1 2))

(setq b a) ; copies the reference to the list, not the list itself

(setcar a 'foo)

b

==> (foo 2)

> (defun main-entry-function ()
>   ;; obviously this doesn't work as defvars are always special
>   (let ((list-template list-template))

This creates a new local binding, but it references the same list object
in memory.  If you change the structure of the referenced object, the
change is "visible" for all variables which are bound to this object.
You can create a real copy of the object with `copy-tree'.  Or don't
destructively alter the list, but process the value with
non-destructive functions instead.

I'm sure there is a good explanation somewhere in the manual.


Regards,

Michael.

Re: binding question

[Eric Abrahamsen]

Thien-Thi Nguyen <ttn@gnu.org> writes:

> () Eric Abrahamsen <eric@ericabrahamsen.net>
> () Sun, 06 Oct 2013 17:35:30 +0800
>
>    What's the best way of handling this?
>
> Check out ‘copy-tree’, e.g.:
>
>  (let ((list-template (copy-tree list-template)))
>    ...)
>
> See also:
>
>  http://www.emacswiki.org/emacs/ElispCookbook
>
> which compares ‘copy-sequence’ and ‘copy-tree’ in section "Copying".
>
> WRT, lexical vs dynamic binding, the code is written to assume dynamic
> binding, so you might want to explicitly set ‘lexical-binding’ to nil
> somewhere.

Thanks very much to both of you! Copying sounds like exactly what I
wanted.

I was sort of wondering if there was a one-size-fits-all solution that
would also work in future lexically-bound environments, but I guess that
really would require passing arguments.

Thanks again,
Eric

Re: binding question

[Stefan Monnier]

> WRT, lexical vs dynamic binding, the code is written to assume dynamic
> binding, so you might want to explicitly set ‘lexical-binding’ to nil
> somewhere.

Better than set lexical-binding to nil is to add (defvar <foo>) at the
beginning of the file for every variable <foo> which is used via
dynamic scoping.


        Stefan