If it is done on internal representation - then how errors are supposed to be propagated to the HTML generator (which works on the key value pairs)?

By having a bi-directional mapping between the internal representation and the key-values pairs

How to interface with specialized data validating libraries?

Callbacks might be a solution

When form should be saved to the database? Obviously only when it has valid parameters - but some people also check the HTTP method and other things.

IMHO saving things to a database is not job of the form processor. It could create objects that can be stored into the database with a single method call, but it shouldn't do it - that's the job of the controlling method that called the form processor in the first place.

When values should be loaded from the database?

Same answer as above.

IMHO a form processor shouldn't talk to the database at all, at least not directly. For example the callbacks that validate input data could talk to the database, but having a form processor do that seems like over stretching its task.

Could it be that you have a full CRUD framework in mind, not just a form processor?

In reply to Re: Form Processing by moritz
in thread Form Processing by zby

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