Allow types of kind
Constraintto 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.
Implicit parameterconstraints, e.g.
- Equality constraints, e.g.
a ~ Int(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
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 kind
Anything whose form is not yet known, but the user has declared to have kind
Constraint(for which they need to import it from
Data.Kind). So for example
type Foo (f :: Type -> Constraint) = forall b. f b => b -> bis 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.