Haskell's type system is both expressive and flexible enough to allow most of the benefits of polymorphism, via type classes, while being rigid enough to both c

Haskell's type system is both expressive and flexible enough to allow most of the benefits of polymorphism, via type classes, while being rigid enough to both catch coarse-grained logic errors and enable efficient compilation. And the syntax is gorgeous! I only sketch type signatures out in Haskell syntax.