7.11. The Constraint kind

Normally, constraints (which appear in types to the left of the => arrow) have a very restricted syntax. They can only be:

With the -XConstraintKinds flag, GHC becomes more liberal in what it accepts as constraints in your program. To be precise, with this flag any type of the new kind Constraint can be used as a constraint. The following things have kind Constraint:

Note that because constraints are just handled as types of a particular kind, this extension allows type constraint synonyms:

type Stringy a = (Read a, Show a)
foo :: Stringy a => a -> (String, String -> a)
foo x = (show x, read)

Presently, only standard constraints, tuples and type synonyms for those two sorts of constraint are permitted in instance contexts and superclasses (without extra flags). The reason is that permitting more general constraints can cause type checking to loop, as it would with these two programs:

type family Clsish u a
type instance Clsish () a = Cls a
class Clsish () a => Cls a where
class OkCls a where

type family OkClsish u a
type instance OkClsish () a = OkCls a
instance OkClsish () a => OkCls a where

You may write programs that use exotic sorts of constraints in instance contexts and superclasses, but to do so you must use -XUndecidableInstances to signal that you don't mind if the type checker fails to terminate.