bug#47992: [External] : bug#47992: 27; 28; Phase out use of `equal` in `add-hook`, `remove-hook`

[Daniel Mendler]

On 4/24/21 10:12 PM, Drew Adams wrote:
> So instead of just advising users not to use lambda forms
> (which makes sense), you'd make it no longer work at all
> for interpreted lambda forms (except rare cases where
> they might actually be `eq' - e.g., same list structure)?

I agree that it makes sense to use symbols in case you want to add a 
plain function as hook. However often you want to add closures; 
`minibuffer-with-setup-hook`, `set-transient-map` and `eval-after-load` 
are examples where this happens. In order to improve the support for 
closures as hooks, this change is necessary.

It is not reasonable to require every `add-hook` user, who wants to add 
a closure, to introduce a symbol indirection. This is neither obvious 
nor easy. Even a very commonly used macro like 
`minibuffer-with-setup-hook` got this wrong.

Furthermore I would argue there are no plausible scenarios where you 
want to add a closure or lambda as hook and then remove or add it again 
afterwards, but not using the identical object, but only an object which 
is `equal`.

This is more than enough motivation for a change to `eq`.

Daniel