6.10.3. The Constraint
kind¶

ConstraintKinds
¶ Since: 7.4.1 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
). So for exampletype Foo (f :: Type > Constraint) = forall b. f b => b > b
is allowed, as well as examples involving type families:type family Typ a b :: Constraint type instance Typ Int b = Show b type instance Typ Bool b = Num b func :: Typ a b => a > b > b func = ...
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
UndecidableInstances
to signal that you don’t mind if the type
checker fails to terminate.