More efficient representation for datatype instances

[julianhyde]

If a datatype has two type constructors, one with a value and one without, it may be more efficient to represent the one without as Java null. Consider, for example, the builtin option:

datatype 'a option = NONE | SOME of 'a

We currently represent all datatypes as a Java list. option is as follows:

• NONE is Java list with one element: ["NONE"],
• SOME 1 is Java list with two elements: ["SOME", 1].

Suppose that we represented it as follows:

• NONE is Java null;
• SOME 1 is Java Integer(1).

This seems to be more efficient. We have saved a List wrapper in both cases, and saved indirection when handling the datatype.

We cannot use this approach if a datatype has more than one unit constructor. For example builtin datatype order:

datatype order = LESS | EQUAL | GREATER 

Maybe instances of unit constructors can be represented as integers (similar to a C++ enum) and constructors with values can continue to be represented as Java lists.