Type Checking with Open Type Functions by Tom Schrijvers, Simon Peyton-Jones, Manuel M. T. Chakravarty, and Martin Sulzmann

We report on an extension of Haskell with open type-level functions and equality constraints that unifies earlier work on GADTs, functional dependencies, and associated types. The contribution of the paper is that we identify and characterise the key technical challenge of entailment checking; and we give a novel, decidable, sound, and complete algorithm to solve it, together with some practically-important variants. Our system is implemented in GHC, and is already in active use.

Related to GHC/Type families, which can be an important optimization technique when specializing the types. Type families allow greater control over types. I guess they didn't like having C++ templates having any advantage with associated types. Personally, it reminds me of some of the power afforded by ML parameterized functors.

(via reddit and 2 Minute intro to Associated Types/Type Families).