[barrkel]

You can invert the box; instead of putting the state inside the struct, put the struct inside the state. So instead of having a Foo with a Yellow state, have a Yellow Foo. You can then call methods on the Yellow state to get a Foo in the next state.

To make it work better, you'd need type-level functions, or some other kind of compile-time function, which can be called when instantiating something with compile-time arguments (like generics). Anything less and you lose the static checking when try and treat your state as a first-class value. The computation of the type (state) needs to happen at compile time. Otherwise you're just back in dynamic land and you might as well put in assertions.