All toplevel expressions are evaluated within a module which is a first class object. Other than that you always need to place the expression into its environme

All toplevel expressions are evaluated within a module which is a first class object. Other than that you always need to place the expression into its environment (to construct top-level expressions). If you just place macros the environment is not accessible (i.e. macros operate solely on their arguments), however, you can distinguish whether a symbol will be evaluated within the defining scope (of the macro) or the scope in which it is used. Regarding types, all expressions are of type Expr or Symbol (there's a few other things that can be in the AST, but that's mostly it). What kind of expression it is is determined by the head field. There as some discussion about making the Expr type parametric on its head field so you can dispatch on that, but nothing in that direction recently.