I think what the parent means is that even if you have data in your object, still the (OOP) language doesn't have discriminate unions nor match expressions. Al

I think what the parent means is that even if you have data in your object, still the (OOP) language doesn't have discriminate unions nor match expressions. Also, if you call that method on the (OOP) object then "it still does the thing" - rather than returning a description of the thing. The author talks about a different (meta) way of programming where you create data types for all the actions the program should be able to do, then have your functions modify these descriptions, and, as the very last step interpret the description/run the program.