What makes Haskell's type system more "powerful" than other languages' type systems?

Question

Reading Disadvantages of Scala type system versus Haskell?, I have to ask: what is it, specifically, that makes Haskell's type system more powerful than other languages' type systems (C, C++, Java). Apparently, even Scala can't perform some of the same powers as Haskell's type system. What is it, specifically, that makes Haskell's type system (Hindley–Milner type inference) so powerful? Can you give an example?

Answer 1

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