I think this is a case where the Free Monad could help a lot. I've only ever built toy-examples but I really liked the way it allows you to structure the progra

I think this is a case where the Free Monad could help a lot. I've only ever built toy-examples but I really liked the way it allows you to structure the program's plan separately from interpreting it. A dry-run mode could be implemented as a side-effect free interpreter .. without ever having to touch the plan or the program's grammar.