Support for primitive types

[prdoyle]

Currently, we don't support primitive types in the state tree. The reason is that it's so very convenient to assume that these three things are all the same:

1. The type of the constructor parameter
2. The type returned by the getter
3. The type returned by Reference.value()

Aside from primitives, these are always the same. For primitives, there's no way to make a Reference<int> whose value() method returns int.

This leaves us with two alternatives:

First, we could use Reference<Integer> for fields of type int. We'd need to beef up a bunch of Bosk, BoskDriver, and PathCompiler logic to become aware that the Reference.targetType() and the return type of Reference.value() could be different.

• Pros
  • Only one kind of Reference
• Cons
  • Boxing and unboxing required, which can harm performance
  • Complexity of having Reference.value() return a type different from that of the field

Second, we could introduce some primitive reference types, like IntReference.

• Pros
  • No boxing/unboxing, so efficient
  • Reference.value() returns the exact same type as that of the field
• Cons
  • Multiple Reference types that need to be supported everywhere, such as in the BoskDriver interface

Supporting separate primitive reference types may be infeasibly cumbersome, so we might be stuck with the first approach.

It's definitely possible to make this work. It just hasn't yet been worth the effort. It would affect Bson and Json serialization, as well as the PathCompiler: they would all need to add special boxing/unboxing cases for fields whose type (eg. int) does not match the type parameter of the Reference (eg. Integer).