Support custom DataTypes?

[enikao]

We support some built-in primitive types (see #9).

Qustions:

• Can the metamodel author express their own DataTypes, to be used in Propertys?
• Do we support defining structured value types?
• How to serialize them (we're defining a protocol after all)?
• How to express identity?

Example usages:

• Date, represented by the built-in type in each host language
• Lambda, written in host language
• Point, as structured value type of x: integer and y: integer

Decision:
Support typedefs of built-in primitive types (so we know we can serialize them).
Converters for each platform are defined in anotations to the typedef.

For now, we don't expose substructure on M3; this might be added in the future.

Open: How to name this thing?

[ftomassetti]

If we have only typedefs of primitive types, but primitive types are only the built-in ones, then I think that PrimitiveType should not appear in the M3, right?

[ftomassetti]

Or should it appear but users are forgotten to instantiate new ones? In that case should it not be a MetamodelElement as it exists only as built-in and not as part of Metamodels?

[enikao]

In #34, we decided all types are to be encoded as strings.
I think this means all custom types need to (de)serialized from/to string?

[enikao]

On 2022-11-25, we decided to postpone this, as we can have first trials without this capability.