I'm actually working in Purescript, but the issue is pretty much the same as one would encounter in Haskell. My application (animation of math-related ideas) is changing constantly. It's research, and I don't know where I'll be with it next week, or even tomorrow. For instance, the kinds of "objects" I'm animating (numbers, equations) and the way they move and interact with each other is a research project, and I'm constantly ripping out parts of my application and scavenging them for variations on a theme. My objects need state. At this point I'm thinking of just putting the state into a map with strings as keys and some kind of sum type to represent each data item I might need. So if I want to represent that an object has a particular color and position and velocity, itemState = M.fromList [("color" , DataColor blue ) ,("position", DataPoint 500.0 100.0) ,("velocity", DataVector 50.0 45.0 ) ] Which starts to look more like a dynamically-typed scripting language. But it's not hard to work with. I can uses lenses/prisms to inspect and update state, and it's okay if my program crashes on encountering malformed state. What this lets me do is create new objects that mix and match parts of old objects, and copy and paste a lot of the code. Is there a better way to do this? Is there a compromise I can make that will allow the compiler to help me out more with finding errors at compile time? D