Re: A fundamental problem with defcustoms that are lists

[Lennart Borgman (gmail)]

Drew Adams wrote:
>> If you have a defcustom which uses variable length lists
>>   (defcustom my-option
>>     :type '(repeat ...))
>>
>> then you may want the default values from the list but also 
>> add some of your own.
>>
>> If you customize such an option and the default values are changed you
>> will not notice because when your setting is used Emacs never sees the
>> changes in default.
>>
>> This mean that any enhancements or bug corrections to the default will
>> never be used.
>>
>> Wouldn't it be worthwhile to give some way to add entries at the
>> begining or end of the list but add those to the default value instead
>> of just replacing the default value?

> The default value of a list option is typically not just a starting point to 
> add
> to but is a default value to replace.

Why do you think so?

There are certainly situation when this is the case, but I can without
doubt point to other where instead the default value is useful and it is
natural to complement it, not replace it.

> What you raise is the difference between a
> default value that is a list and a list that includes a minimum set of values 
> by
> default, that is, a list of default values.
> 
> In the latter case, a library can do something like this:
> 
> (defvar foo-defaults '(a b c) "List of default values")
> (defcustom foo-extras () "Your added values"
>   :type '(repeat ...))
> (setq foo (append foo-defaults foo-extras))

I can see your idea (but it should not be handled the way you propose
above, use :set instead).

But that would not give the same flexibility as what I propose.

Here is my proposal a bit more concretely expressed:

- In the values stored for a list (ie type 'repeat) for a defcustom
allow a value 'custom-insert-default to insert the default value for the
'repeat list.

Is that clear? Of course the UI have to provide functionality for this too.