Null type in Truffle

[Dain Sundstrom]

Another long question :)

How do I add a null type into the type system (e.g., Ruby nil, Python none, or SQL unknown)?  

Here is what I have tried for SQL unknown.  In SQL you can have expressions like "42 + null" where the null doesn't really have a type, but the full expression is numeric type with a null value.  To make this construct work, I created a class Unknown with no implementations. I added Unknown as the first element in @TypeSystem.  Then I was forced to add a @TypeCheck:

    @TypeCheck
    public boolean isUnknown(Object value)
    {
        return value == null;
    }

Then added this method to my base ExpressionNode:


    public Unknown executeUnknown(VirtualFrame frame)
            throws UnexpectedResultException
    {
        return SqlTypesGen.SQLTYPES.expectUnknown(execute(frame));
    }

I added a UnknownLiteral node:

    public static class UnknownLiteral
            extends LiteralNode
    {
        public Unknown execute(VirtualFrame frame)
        {
            return null;
        }
    }

And finally I added these methods to my BinaryNode:

    @Specialization(order = 100)
    public Unknown unknownAndAnything(Unknown left, Object right)
    {
        return null;
    }

    @Specialization(order = 101)
    public Unknown anythingAndUnknown(Object left, Unknown right)
    {
        return null;
    }

    @Specialization(order = 102)
    public Unknown unknownAndUnknown(Unknown left, Unknown right)
    {
        return null;
    }

The order parameter is there because the DSL is now forcing met to add order to all of my methods like this in my Add node:

    @Specialization(order = 1, rewriteOn = ArithmeticException.class)
    public long addLong(long left, long right)
    {
        return ExactMath.addExact(left, right);
    }

In general, the isUnknown() type check method doesn't seem right, and the order declaration is really annoying. Is there an easier way?

-dain