Mixin evaluators for the expression problem.

These evaluators are based on variant-reuse.pdf:

Jacques Garrigue: Code reuse through polymorphic variants. In Workshop on Foundations of Software Engineering. Sasaguri, Japan, November 2000.

• mixev.ml: The evaluator presented in the paper.
• mixev2.ml: Full source of the sum type version of the evaluator, requires -rectypes.
• mixin2.ml: Larger example, using objects to structure code.
• mixmod.ml: Original example, encoded using polymorphic variants and recursive modules (ocaml 3.07). Much more verbose, but does not use polymorphism.
• mixmod2.ml: Same thing using normal sum types, without -rectypes. Does not work anymore with recent versions of OCaml.
• mixmod5.ml: A tentative solution(?), using polymorphic variant, recursive modules, and private rows. Quite verbose, but each extension can be writeen with only O(n m) boilerplate, where n is the number of types involved in the recursion, and m the number of operation.
• mixobj.ml: A purely object-oriented version (without variants), using immediate objects (ocaml 3.08).