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There are two types of validation:

• Structural validation of the message (command)
• Business validation requires the current state of the aggregate to check if it can actually handle the command.

It should not be possible to create a structurally invalid message - https://www.47deg.com/blog/functional-domain-modeling-part-2/

Business validation should be done in the decider.decide() function and you have options:

• ignore it by returning an empty flow of events
• throw an exception within the flow, breaking the flow immediately
• model the errors as events, giving the error flow the respect it deserves.

Explain how to model Commands, Events, and State data types.

• how inheritance/polymorphism (Sealed and Open) can be used to model OR/SUM relationship and why is this important for modeling Commands and Events
• what options do we have to model general AND/PRODUCT relationship, and what kotlin idiomatic options do we have (Pairs)
• explain how combine() function is combining two or more Deciders into one big decider that can just run as a whole. Or you can distribute these deciders via Actors for example and run them in parallel. The decider is a Monoid in respect to the combine() function/operator.
• Mention the principle of duality (boolean algebra) that is present here.
• Why do Command and Event have to be nullable (?) in order to be able to use the combine function in the first place.