configuration: Have define-configuration add just one handler per slot.

[Ambrevar]

Description

Currently calling define-configuration multiple times over the same slot has no effect.
This patch ensures there is only one handler per slot.

Discussion

• The add-hook of nhooks should probably do this handler replacement for us.
• Why are handler named uniquely? Shouldn't they be named after the slot, actually?

Checklist:

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• I have pulled from master before submitting this PR
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  • See Norvig & Pitman's Tutorial on Good Lisp Programming Style (PDF) and Google Common Lisp Style Guide.
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EDIT: Added more discussion.

[Ambrevar]

@aartaka OK with you?

[aadcg]

Why are handler named uniquely? Shouldn't they be named after the slot, actually?

I'd say they should indeed.

If there's a single handler per slot, how could the user change the value of a slot at runtime?

I don't remember now, but don't we generate new objects when using define-configuration?

[Ambrevar]

I don't remember now, but don't we generate new objects when using define-configuration?

True in the 2-series but no longer the case since our user-classes have hooks :) See user-classes.lisp.

[aartaka]

No, I don't agree!

The change with %slot-value% opened for a new workflow: composing several handlers to change the value several times. I consider this a step towards a more intuitive model of configuration. If one puts several configuration forms for the same slot in their config -- they want all of those, together, modifying the value one by one!

[Ambrevar]

If one puts several configuration forms for the same slot in their config -- they want all of those, together, modifying the value one by one!

I'm confused, what's the point of setting all the values for the same slot? I mean

(setf slot-foo 17)
(setf slot-foo 18)

is the same as

(setf slot-foo 18)

[aartaka]

What about this?

(setf slot-foo (append 17 %slot-value%))
(setf slot-foo (append 18 %slot-value%))

This use-case is enabled by the current implementation, and I don't want to let it go!

[Ambrevar]

Oh right...

Then I suggest one change: to have one handler per slot and to name the handler after the slot: this way it makes hook introspection and editing much easier.

[aartaka]

"one handler per slot"?

[Ambrevar]

Currently define-configuration defines only one handler for all the slot assignments. I suggest we add one handler for each slot assignment.

Does that make sense?

[aartaka]

Yes, I see. Then I totally agree with this!

[Ambrevar]

You mean you agree with my last proposal, but not this pull request as it is, right?

[aartaka]

Yes, I don't agree with deleting the same slot handlers, but I agree with splitting slots into their own handlers!

So the course I'd be content with is removing the :place and :value from the generated handler and not deleting the handler with matching place.

[Ambrevar]

Superseded by #2480.