Interesting problem! Variants _are_ basically sum types in Nim.
Yeah, I was wondering similar things for some projects. I am not sure what are you using hlists for, is it an AST. To represent an AST you don't really need anything more complicated than data types(variants) in Nim: HList.Cons can contain HValue which is a different data type. HValue can be this heterogen cell. I'd also use directly a seq of children instead of head:tail, but this is not too important: maybe you need it for your case I am also not sure if you need to hold the handler inside the cell: typicall one defines a fun which handles each kind of variant and handles them with a case(or maybe pattern matching) inside (and you can also just dispatch them to other functions gen(value: HValue) to to e.g. gen<KindName>(value..)) Again, if you can give a better example of your usecase it would help, but I doubt you need concepts/existential types
