Currently no validation is done upon the form type configured, while this is actually a mandatory configuration.

The handlers are stateful, that's because it contains setters and the data object is integrated inside. There's no real way to prevent this without losing information about types. While the form handlers are services, you never use them like this as they get passed along with the action arguments.

Due to their stateful nature, I'd like to have the collector compiler pass add the shared option as false to all handlers during container compilation. It might be a slight behavioural change, but I consider this a bug because it gives unexpected results if the handlers are shared.

Would there be any implications when adding this option?

This issue ask for quite a big change, but I just wanted to share my thoughts about this cool package. It's quite hard to grasp at first and I think naming is part of this.

The name that confused me most is the FormProviderInterface. It doesn't actually provide anything, you pass the handler (and optionally the form) to it. In fact, this is the actual form handler.

On the other hand, FormHandlerInterface isn't about handling at all (it's delegated to both FormProviderInterface and FormSuccesHandlerInterface/FormFailureHandlerInterface). In fact, this interface is more about providing the form. It's more like a form manager or the like.

I think renaming these, especially the FormProviderInterface could help in understanding the responsibilities of the classes.

Please note that I'm just starting with this bundle. Maybe the names do make lots of sense and I just don't understand the internals good enough yet.