There are nicer ways to do structural typing. Here's just one example: I have a post here of how you can partially define structure to be filled in by the con

There are nicer ways to do structural typing. Here's just one example: I have a post here of how you can partially define structure to be filled in by the consumer of the API: http://bitemyapp.com/posts/2014-04-11-aeson-and-user-created-types.html http://bitemyapp.com/posts/2014-04-11-aeson-and-user-created... Can't do this without polymorphism and higher-kinded types.