[TYPES] union types and overloading

[Joshua Dunfield]

Kwangkeun Yi wrote:
> I would like to extend our ML compiler's type system so that it allow
> functions to be defined over the sum of multiple datatypes. (union types?)

Frank Pfenning and I have a couple of papers on typechecking union 
types (and intersections) that may be of interest: 
 
  http://gs146.sp.cs.cmu.edu:8080/joshuad/papers/union-assignment/ 
  http://gs146.sp.cs.cmu.edu:8080/joshuad/papers/tridirectional-typechecking/ 
 
Important earlier work on unions/intersections includes Reynolds' Forsythe 
papers and Benjamin Pierce's dissertation. 
 
This work is on generic union types, not only unions of algebraic
datatypes; if you only need unions of algebraic datatypes, perhaps
work on extensible datatypes (like SML's `exn') would be more useful.
 
Best,
-j.