Edit a cl-struct by copy, or, slot-value for cl-structs

Edit a cl-struct by copy, or, slot-value for cl-structs

[Yuan Fu]

I’m trying to make the cl-structs that I define immutable, and only edit them by copying. One way to do it is just by convention: I promise myself that I never setf a struct without making a copy. A better way is to use :read-only attribute for the slot to enforce it. But then there is not convenient way to edit by copy: I need to manually construct a new struct, copy slot values from the old one, like so:

(make-xxx :slot1 (xxx-slot1 obj)
          :slot2 (xxx-slot2 obj)
          :slot3 newvalue)

It gets tiring very quickly, I’m hoping for something like

(copy-with obj :slot3 newvalue)

Anyone knows how to define such a copy-with function without resorting to EIEIO? I can get the slots of a struct by 

(mapcar #'cl--slot-descriptor-name
        (cl--struct-class-slots (get (type-of obj) 'cl--class)))

But I can’t get any further as I can’t find any generic getter and setter for cl-struct’s. Something like slot-value. So close!

Thanks,
Yuan

Re: Edit a cl-struct by copy, or, slot-value for cl-structs

[Emanuel Berg via Users list for the GNU Emacs text editor]

Yuan Fu wrote:

> (make-xxx :slot1 (xxx-slot1 obj)
>           :slot2 (xxx-slot2 obj)
>           :slot3 newvalue)

How do you create the initial struct?

> Anyone knows how to define such a copy-with function without
> resorting to EIEIO?

"Enforce In-order Execution of I/O" ... and a joke (?)
<https://en.wikipedia.org/wiki/Enforce_In-order_Execution_of_I/O>

-- 
underground experts united
https://dataswamp.org/~incal

Re: Edit a cl-struct by copy, or, slot-value for cl-structs

[Yuan Fu]

> On Oct 29, 2021, at 11:43 AM, Emanuel Berg via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org> wrote:
> 
> Yuan Fu wrote:
> 
>> (make-xxx :slot1 (xxx-slot1 obj)
>>          :slot2 (xxx-slot2 obj)
>>          :slot3 newvalue)
> 
> How do you create the initial struct?

I don’t think enforcing read-only when creating an object makes much sense ;-)

> 
>> Anyone knows how to define such a copy-with function without
>> resorting to EIEIO?
> 
> "Enforce In-order Execution of I/O" ... and a joke (?)
> <https://en.wikipedia.org/wiki/Enforce_In-order_Execution_of_I/O>

That would have been a half-decent joke, but I was actually referring to eieio.el, which implements parts of CLOS from Common Lisp.

Yuan

Re: Edit a cl-struct by copy, or, slot-value for cl-structs

[Stefan Monnier via Users list for the GNU Emacs text editor]

> (mapcar #'cl--slot-descriptor-name
>         (cl--struct-class-slots (get (type-of obj) 'cl--class)))
>
> But I can’t get any further as I can’t find any generic getter and
>  setter for cl-struct’s. Something like slot-value. So close!

The position of the slot descriptor in `cl--struct-class-slots` mirrors the
position of the slot in the actual objet, so you just need `aref/aset`.

It would also make a lot of sense to replace/extend the useless
`:copier` thingy of `cl-defstruct` with something like the
`:constructor`, so you could say

    (cl-defstruct (my-foo
                   (:copier my-foo-with)
                   (:copier my-foo-with-a (b a)))
      a b c)

and then use (my-foo-with foo :c 42) and (my-foo-with-a foo 153 "42")


        Stefan

Re: Edit a cl-struct by copy, or, slot-value for cl-structs

[Yuan Fu]

> On Oct 29, 2021, at 12:15 PM, Stefan Monnier via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org> wrote:
> 
>> (mapcar #'cl--slot-descriptor-name
>>        (cl--struct-class-slots (get (type-of obj) 'cl--class)))
>> 
>> But I can’t get any further as I can’t find any generic getter and
>> setter for cl-struct’s. Something like slot-value. So close!
> 
> The position of the slot descriptor in `cl--struct-class-slots` mirrors the
> position of the slot in the actual objet, so you just need `aref/aset`.

Ah, silly me!

> 
> It would also make a lot of sense to replace/extend the useless
> `:copier` thingy of `cl-defstruct` with something like the
> `:constructor`, so you could say
> 
>    (cl-defstruct (my-foo
>                   (:copier my-foo-with)
>                   (:copier my-foo-with-a (b a)))
>      a b c)
> 
> and then use (my-foo-with foo :c 42) and (my-foo-with-a foo 153 "42”)

That’s cool. But my lazy fingers like better to write a single function and use it for everything ;-)

Yuan

Re: Edit a cl-struct by copy, or, slot-value for cl-structs

[Stefan Monnier]

>>> (mapcar #'cl--slot-descriptor-name
>>>        (cl--struct-class-slots (get (type-of obj) 'cl--class)))
>>> 
>>> But I can’t get any further as I can’t find any generic getter and
>>> setter for cl-struct’s. Something like slot-value. So close!

FWIW, I just pushed to `master` a patch which makes `slot-value` work to
read slots from structs.  It doesn't help you, tho, because it still
doesn't let you set them, and in your case it really shouldn't since
they are defined as read-only.

>> and then use (my-foo-with foo :c 42) and (my-foo-with-a foo 153 "42”)
> That’s cool. But my lazy fingers like better to write a single function and
> use it for everything ;-)

But that tends to come at a performance cost.


        Stefan

Re: Edit a cl-struct by copy, or, slot-value for cl-structs

[Michael Heerdegen]

Yuan Fu <casouri@gmail.com> writes:

> (make-xxx :slot1 (xxx-slot1 obj)
>           :slot2 (xxx-slot2 obj)
>           :slot3 newvalue)

Sorry for the dumb question, but isn't that more or less the same as
`xxx-copy' plus modifying slot3 of the copy?


Michael.

Re: Edit a cl-struct by copy, or, slot-value for cl-structs

[Emanuel Berg via Users list for the GNU Emacs text editor]

Michael Heerdegen wrote:

>> (make-xxx :slot1 (xxx-slot1 obj)
>>           :slot2 (xxx-slot2 obj)
>>           :slot3 newvalue)
>
> Sorry for the dumb question, but isn't that more or less the
> same as `xxx-copy' plus modifying slot3 of the copy?

But you are not allowed to do that :)

-- 
underground experts united
https://dataswamp.org/~incal

Re: Edit a cl-struct by copy, or, slot-value for cl-structs

[Michael Heerdegen]

Emanuel Berg via Users list for the GNU Emacs text editor
<help-gnu-emacs@gnu.org> writes:

> > Sorry for the dumb question, but isn't that more or less the
> > same as `xxx-copy' plus modifying slot3 of the copy?
>
> But you are not allowed to do that :)

Ah ok - forget it.  As I said, a dumb question :-)

Michael.