6.10.3. The Constraint
kind¶

ConstraintKinds
¶ Since: 7.4.1 Status: Included in GHC2021
Allow types of kind
Constraint
to be used in contexts.
Normally, constraints (which appear in types to the left of the =>
arrow) have a very restricted syntax. They can only be:
 Class constraints, e.g.
Show a
Implicit parameter
constraints, e.g.?x::Int
(with theImplicitParams
extension) Equality constraints, e.g.
a ~ Int
(with theTypeFamilies
orGADTs
extensions)
With the ConstraintKinds
extension, 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
:
 Anything which is already valid as a constraint without the flag: saturated applications to type classes, implicit parameter and equality constraints.
 Tuples, all of whose component types have kind
Constraint
. So for example the type(Show a, Ord a)
is of kindConstraint
.  Anything whose form is not yet known, but the user has declared to
have kind
Constraint
(for which they need to import it fromData.Kind
). For exampletype Foo (f :: Type > Constraint) = forall b. f b => b > b
is allowed.
Note, however, that the TypeFamilies
and GADTs
extensions
also allow the manipulation of things with kind Constraint
, without necessarily
requiring the ConstraintKinds
extension:
 With XTypeFamilies XNoConstraintKinds
type T :: Type > (Type > Constraint)
type family T a where
T Int = Num
T Double = Floating
 With XGADTs XNoConstraintKinds
type Dict :: Constraint > Type
data Dict c where
MkDict :: c => Dict c
With the ConstraintKinds
extension, constraints are just handled as
types of a particular kind. This 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
UndecidableInstances
to signal that you don’t mind if the type
checker fails to terminate.