What is it, specifically, that makes Haskell’s type system
It has been engineered for the past decade to be both flexible — as a logic for property verification — and powerful.
Haskell’s type system has been developed over the years to encourage a relatively flexible, expressive static checking discipline, with several groups of researchers identifying type system techniques that enable powerful new classes of compile-time verification. Scala’s is relatively undeveloped in that area.
That is, Haskell/GHC provides a logic that is both powerful and designed to encourage type level programming. Something fairly unique in the world of functional programming.
Some papers that give a flavor of the direction the engineering effort on Haskell’s type system has taken:
- Fun with type functions
- Associated types with class
- Fun with functional dependencies