Dear Cafe, I wanted to benchmark (part of) an API of a package, by enumerating all possible (nested) calls by size of their AST, much as smallcheck does it for data. So I made this straightforward AST representation and (generically) got its Serial instances. That last part has been in smallcheck for a long time, and works nicely. But I do wonder if there is a way to get the AST (and corresponding interpreter) with less boilerplate code (than this: https://github.com/jwaldmann/pretty-test/blob/master/src/Lib.hs#L29 ) If you read the code: it also has a type for "AST contexts" because I wanted to iterate them (to produce larger trees). Again - easy to write down, but boring. - J.