ThingModels should not define "required" affordances in derived TDs, but "optional" ones

[thjaeckle]

During discussion in #1384 several opinions agreed upon that defining optional affordances in ThingModels would be a more intuitive way than defining required ones (as currently defined with tm:required).

To summarize the discussion in #1384:

• proposal: by default every affordance defined in a TM should be required to be present in a TD following that model
• the current default (if tm:required is omitted/forgotten) is, that everything is optional
  • everything being optional means that a TD can "implement" a TM without providing any affordance of the TM - making the TM kind of "useless"
  • analogy to "interfaces" in programming languages: implementing an interface / abstract class without providing any abstract method in it
• by default, modelers of TMs will likely do not consider adding required affordances to tm:required

So a simple proposal would be to have an element tm:optional with the same syntax than the current tm:required.

[sebastiankb]

tm:required is more less inspired from JSON Schema which also not have an optional definition. I would be open to adopting a new default assumption, if tm:required is missing, everything should be mandatory. Lets talk about this in one of the next TD meetings.

[egekorkan]

Call of 23.02:

• The group says that by default everything should be required
• The default behavior should be explained more clearly
• The word stays tm:required but the default value (in case it is not present) has all the affordances
• We should contact the JSON Schema community to see what is the history behind using required instead of optional

[egekorkan]

I got replies from the JSON Schema slack with some issues that talk about this. Some discussions are even less than 1 year old:

• Suggestion: optional keyword to complement required json-schema-org/json-schema-spec#1112
• Add requireAllExcept keyword json-schema-org/json-schema-spec#1144
• "requiredProperties" keyword json-schema-org/json-schema-spec#846
• Object properties should be required by default json-schema-org/json-schema-spec#1096

Some further replies by ether of JSON Schema slack:

• in general though, json schema keywords are only additive, rather than subtractive, in the sense that a keyword can add a new error, but it can’t take away an error that something else generated. so if we made all properties required by default, except for those that optional listed, then that keyword would have to somehow remove the “property is missing” error that properties generated.. or, make the implementation of properties aware of the existence of optional, which we also try to avoid
• there are a few exceptions though, where a keyword is aware of another keyword’s existence and modifies itself accordingly — e.g. contains doesn’t generate an error when it would normally error while minContains: 0 is present.
• I think if we were designing contains today, we would try to do something different there

[thjaeckle]

Something else came into my mind:
It must also be specified how "extended" tm:required relations must be handled.

I assume that a model B which tm:extends model A must also provide all tm:required affordances from model A, as an instance of B can only be also an instance of A, if all required affordances from A were provided.

For tm:ref this can not be defined as there is no "is a" relationship here.

[egekorkan]

Discussions about its default value can be discussed at #1581 . Closing