Constraint kinds

also: constraintkinds, constraint synonym

The extension that gives constraints their own kind, so the thing left of the arrow becomes a first-class type that can be named, parameterised, and computed by a type family. It is how a container class expresses whatever its elements require without fixing it in the class head.

With ConstraintKinds, the thing to the left of $\Rightarrow$ becomes a first-class type of kind Constraint, so constraints can be named, parameterised, and abstracted over like any other type.

{-# LANGUAGE ConstraintKinds, TypeFamilies #-}
 
type Serialisable a = (Show a, Read a, Eq a)   -- one name for three
 
type family Elem c :: Constraint where
  Elem [a]      = Ord a
  Elem (Set a)  = Ord a
 
-- and a constraint as a class parameter
class HasDict (c :: Constraint) where withDict :: (c => r) -> r

What it buys is a way out of the combinatorial growth of constraint lists, and the ability to let an instance decide what its own constraint is, which is how container classes express “whatever this container needs of its elements” without fixing it in the class head.

Kinds and types are the same thing here, which is what makes it work: Constraint is just another kind alongside $\ast$, so everything the type system already does with types applies to constraints unchanged.

Higher-kinded type

• Typeclasses in DepthMINIMAL pragmas, superclasses, multi-param classes and fundeps, overlapping/incoherent instances, orphans, and constraint kinds.