[rimo]

The approach is interesting but the syntax makes it hard to use. Why not make the first arguments the types like one would expect?

[mneary]

I went back and forth with that. I think I went with postfix because there exists another syntax which looks prefix:

    T([Number, Number])(function(x) { return x; })

For this method, a function type is defined and then serves as a constructor that accepts a function as argument.