True unions, revisited

I've spoken about true unions (or ad-hoc unions, or non-disjoint unions) sort of abstractly before, but Typed Scheme has 'em, and they're great.

In my recent experiences with Haskell and ML, the amount of unnecessary and distracting injection and projection that goes on starts to bloat code to the point of illegibility. Disjoint unions in ML and Haskell provide two orthogonal pieces of functionality in one feature. Typed Scheme separates these two: struct types give you disjointness, and unions give you, well, unions. It's up to you to guarantee that you don't create unions with overlap, but so far I haven't seen that cause any problems.

Then you can build disjoint unions on top of that, as I have in my new types.plt PLaneT package:

(define-datatype Expr
 [Var ([name : Symbol])]
 [Abs ([var : Symbol] [body : Expr])]
 [App ([rator : Expr] [rand : Expr])])

macro-expands to:

(begin
 (define-struct: Var ([name : Symbol]))
 (define-struct: Abs ([var : Symbol] [body : Expr]))
 (define-struct: App ([rator : Expr] [rand : Expr]))
 (define-type-alias Expr (U Var Abs App)))