How to correctly highlight it?

[ghost]

The keyword Nil on the language has two meanings. It's both a constant and a type (the language doesn't use Void and reused Nil for Void type). TextAdept doesn't have the power to parse the syntax of the language to correctly determine when Nil is a constant and when Nil is a type. This is really difficult to do.

[orbitalquark]

You could try a contextual match. I don't know your exact syntax, but if a `Nil` type is followed by whitespace and then a word, you could match that and highlight it was a type. Otherwise, highlight it was a constant.