eieio defclass evaluate :initform value

eieio defclass evaluate :initform value

[Joe Riel]

According to the info page for eieio for Emacs 23, one
could do

(defclass foo-class
   ((bar :initform (lambda () my-var)
         :type string)))

When make-instance was called, the lambda expression
for :initform would be evaluated and the expression
assigned to my-var would be used for the slot bar.

The info page for eieio Emacs 24 does not mention this 
functionality and testing shows it no longer exists.

Is there nice way, in 24, to create a class with an :initform
that, upon instantiation, evaluates to the value of a variable?
  

-- 
Joe Riel

Re: eieio defclass evaluate :initform value

[Eric Abrahamsen]

Joe Riel <joer@san.rr.com> writes:

> According to the info page for eieio for Emacs 23, one
> could do
>
> (defclass foo-class
>    ((bar :initform (lambda () my-var)
>          :type string)))
>
> When make-instance was called, the lambda expression
> for :initform would be evaluated and the expression
> assigned to my-var would be used for the slot bar.
>
> The info page for eieio Emacs 24 does not mention this 
> functionality and testing shows it no longer exists.
>
> Is there nice way, in 24, to create a class with an :initform
> that, upon instantiation, evaluates to the value of a variable?

Depending on what exactly you want, you might be able to solve it with
`oset-default', which lives outside the class definition, and can
evaluate variables. See if that does what you want?

Eric

Re: eieio defclass evaluate :initform value

[Stefan Monnier]

> (defclass foo-class
>    ((bar :initform (lambda () my-var)
>          :type string)))

As the name implies, :initform expects a *form* rather than a function.
The above will simply initialize `bar` by default to have as value
a function of no arguments that returns the value of `my-var`.

> When make-instance was called, the lambda expression
> for :initform would be evaluated and the expression
> assigned to my-var would be used for the slot bar.

You can do

    (defclass foo-class ()
       ((bar :initform (progn my-var)
             :type string)))

[ For backward compatibility reasons, just using `my-var` doesn't work,
  because the EIEIO code treated symbols as unevaluated.  ]

> Is there nice way, in 24, to create a class with an :initform
> that, upon instantiation, evaluates to the value of a variable?
  
Does the above work for you?


        Stefan

Re: eieio defclass evaluate :initform value

[Joe Riel]

On Wed, 04 Jan 2017 12:45:22 -0800
Eric Abrahamsen <eric@ericabrahamsen.net> wrote:

> Joe Riel <joer@san.rr.com> writes:
> 
> > According to the info page for eieio for Emacs 23, one
> > could do
> >
> > (defclass foo-class
> >    ((bar :initform (lambda () my-var)
> >          :type string)))
> >
> > When make-instance was called, the lambda expression
> > for :initform would be evaluated and the expression
> > assigned to my-var would be used for the slot bar.
> >
> > The info page for eieio Emacs 24 does not mention this 
> > functionality and testing shows it no longer exists.
> >
> > Is there nice way, in 24, to create a class with an :initform
> > that, upon instantiation, evaluates to the value of a variable?
> 
> Depending on what exactly you want, you might be able to solve it with
> `oset-default', which lives outside the class definition, and can
> evaluate variables. See if that does what you want?

Thanks, Eric; I wasn't aware of oset-default and could use it, however,
am currently taking a different approach.  The bigger picture is that
I'm using a buffer-local object to store configuration data for a
major mode.  The object is normally updated when the mode is invoked,
but the global defaults should be configurable.  The current approach
is to clone a default object whose values can be assigned via a 
customization.  The documentation for using defcustom with an eieio object
is slim, but I got it working.


-- 
Joe Riel

Re: eieio defclass evaluate :initform value

[Eric Abrahamsen]

Joe Riel <joer@san.rr.com> writes:

> On Wed, 04 Jan 2017 12:45:22 -0800
> Eric Abrahamsen <eric@ericabrahamsen.net> wrote:
>
>> Joe Riel <joer@san.rr.com> writes:
>> 
>> > According to the info page for eieio for Emacs 23, one
>> > could do
>> >
>> > (defclass foo-class
>> >    ((bar :initform (lambda () my-var)
>> >          :type string)))
>> >
>> > When make-instance was called, the lambda expression
>> > for :initform would be evaluated and the expression
>> > assigned to my-var would be used for the slot bar.
>> >
>> > The info page for eieio Emacs 24 does not mention this 
>> > functionality and testing shows it no longer exists.
>> >
>> > Is there nice way, in 24, to create a class with an :initform
>> > that, upon instantiation, evaluates to the value of a variable?
>> 
>> Depending on what exactly you want, you might be able to solve it with
>> `oset-default', which lives outside the class definition, and can
>> evaluate variables. See if that does what you want?
>
> Thanks, Eric; I wasn't aware of oset-default and could use it, however,
> am currently taking a different approach.  The bigger picture is that
> I'm using a buffer-local object to store configuration data for a
> major mode.  The object is normally updated when the mode is invoked,
> but the global defaults should be configurable.  The current approach
> is to clone a default object whose values can be assigned via a 
> customization.  The documentation for using defcustom with an eieio object
> is slim, but I got it working.

Yup, that would be awkward to do with `oset-default'. Glad you got
something working.

Eric