As far as I know, currently the non-julia-types type lattice is hardcoded. But even if that's the case, that's not fundamental to the design.
> If [you make the type lattice user extendable], how do you get code reuse?
What kind of code reuse do you mean? This alternative type lattice is not supposed to change the semantics of any Julia program, aside from rejecting programs that would otherwise have semantics in Julia.
As far as I know, currently the non-julia-types type lattice is hardcoded. But even if that's the case, that's not fundamental to the design.
> If [you make the type lattice user extendable], how do you get code reuse?
What kind of code reuse do you mean? This alternative type lattice is not supposed to change the semantics of any Julia program, aside from rejecting programs that would otherwise have semantics in Julia.