OCaml has been getting a lot of mileage from its polymorphic variants (which allow structural subtyping on sum types) especially on problems relating to AST transformations and the infamous "expression problem". Has there been any work on extending Haskell's type system with structural subtyping? What is the canonical solution to the expression problem in Haskell? What techniques do Haskellers use to simulate subtyping where it is appropriate? I bring this up because I have been working on a Scheme compiler in Haskell for fun, and something like polymorphic variants would be quite convinent to allow you to specify versions of the AST (input ast, after closure conversion, after CPS transform, etc.), but allow you to write functions that work generically over all the ASTs (getting the free vars, pretty printing, etc.). -- Alan Falloon