My thoughts on type families: 1) Type families are often too open. I causes "rigid variable" type error messages because when I start writing open type functions, I often realize that what I really intend is not truly open type functions. It happens a lot that I had some assumptions on the arguments or the range of the type function. Then, we need help of type classes to constrain the result of open type functions. For example, try to define HList library using type families instead of type classes with functional dependencies. One will soon need some class constraints. Sometimes, we can use associated type families, but many times it may become tedious when there are multiple arguments and result have certain constraints so that we might end up associating/splitting them over multiple type classes. In such cases, it may be more simple working with functional dependencies alone, rather than using both type classes and type families. I wish we had closed kinds so that we can define closed type functions as well as open type functions. 2) Type families are not good when we need to match types back and forth (e.g. bijective functions), or even multiple ways. We need the help of functional dependencies for these relational definitions. I know that several people are working on the unified implementation for both type families and functional dependencies. Once GHC have common background implementation, type families will truly be syntactic sugar of type classes with functional dependencies, as Mark Jones advocates, or maybe the other way around too.