Hi I have a bunch of data types which are used to represent a JavaScript program. The data types and a lexer and a parser have all been generated using BNFC. So basically an entire JavaScript program is represented as a tree using these data types. Ideally I'd like to be able to "fold" over this data structure, but I can't instantiate Foldable for my data types, since the data types all have kind *, if I'm not completely lost. Here's an example data type: data Statement = StmtFunDecl JIdent [JIdent] ExprOrBlock | StmtVarDecl [VarDecl] | StmtLetDecl [VarDecl] | StmtWhile Expr Statement | ... So, basically I'd like some sort of folding functionality for these data types, without having to hack the lexer/parser myself (parameterising the data types), because as I said they're being generated by BNFC. I noticed that you can make BNFC generate GADTs instead of normal ADTs, which would allow me to instantiate Foldable, but I'm not entirely sure that this is the best way to do this. Any help is appreciated, Deniz Dogan