What is a Constraint kind?
Why would someone use it (in practice)?
What is it good for?
Could you give a simple code example to illustrate the answers to the previous two questions?
Why is it used in this code for example?
Well, I'll mention two practical things it allows you to do:
Maybe it's best to illustrate this with an example. One of the classic Haskell warts is that you cannot make a Functor
instance for types that impose a class constraint on their type parameter; for example, the Set
class in the containers
library, which requires an Ord
constraint on its elements. The reason is that in "vanilla" Haskell, you'd have to have the constraint on the class itself:
class OrdFunctor f where
fmap :: Ord b => (a -> b) -> f a -> f b
...but then this class only works for types that require specifically an Ord
constraint. Not a general solution!
So what if we could take that class definition and abstract away the Ord
constraint, allowing individual instances to say what constraint they require? Well, ConstraintKinds
plus TypeFamilies
allow that:
{-# LANGUAGE ConstraintKinds, TypeFamilies, FlexibleInstances #-}
import Prelude hiding (Functor(..))
import GHC.Exts (Constraint)
import Data.Set (Set)
import qualified Data.Set as Set
-- | A 'Functor' over types that satisfy some constraint.
class Functor f where
-- | The constraint on the allowed element types. Each
-- instance gets to choose for itself what this is.
type Allowed f :: * -> Constraint
fmap :: Allowed f b => (a -> b) -> f a -> f b
instance Functor Set where
-- | 'Set' gets to pick 'Ord' as the constraint.
type Allowed Set = Ord
fmap = Set.map
instance Functor [] where
-- | And `[]` can pick a different constraint than `Set` does.
type Allowed [] = NoConstraint
fmap = map
-- | A dummy class that means "no constraint."
class NoConstraint a where
-- | All types are trivially instances of 'NoConstraint'.
instance NoConstraint a where
(Note that this isn't the only obstacle to making a Functor
instance to Set
; see this discussion. Also, credit to this answer for the NoConstraint
trick.)
This sort of solution hasn't been generally adopted just yet, though, because ConstraintKinds
are still more or less a new feature.
Another use of ConstraintKinds
is to parametrize a type by a class constraint or class. I'll reproduce this Haskell "Shape Example" code that I wrote:
{-# LANGUAGE GADTs, ConstraintKinds, KindSignatures, DeriveDataTypeable #-}
{-# LANGUAGE TypeOperators, ScopedTypeVariables, FlexibleInstances #-}
module Shape where
import Control.Applicative ((<$>), (<|>))
import Data.Maybe (mapMaybe)
import Data.Typeable
import GHC.Exts (Constraint)
-- | Generic, reflective, heterogeneous container for instances
-- of a type class.
data Object (constraint :: * -> Constraint) where
Obj :: (Typeable a, constraint a) => a -> Object constraint
deriving Typeable
-- | Downcast an 'Object' to any type that satisfies the relevant
-- constraints.
downcast :: forall a constraint. (Typeable a, constraint a) =>
Object constraint -> Maybe a
downcast (Obj (value :: b)) =
case eqT :: Maybe (a :~: b) of
Just Refl -> Just value
Nothing -> Nothing
Here the parameter of the Object
type is a type class (kind * -> Constraint
), so you can have types like Object Shape
where Shape
is a class:
class Shape shape where
getArea :: shape -> Double
-- Note how the 'Object' type is parametrized by 'Shape', a class
-- constraint. That's the sort of thing ConstraintKinds enables.
instance Shape (Object Shape) where
getArea (Obj o) = getArea o
What the Object
type does is a combination of two features:
GADTs
), which allows us to store values of heterogeneous types inside the same Object
type.ConstraintKinds
, which allows us to, instead of hardcoding Object
to some specific set of class constraints, have the users of the Object
type specify the constraint they want as a parameter to the Object
type.And now with that we can not only make a heterogeneous list of Shape
instances:
data Circle = Circle { radius :: Double }
deriving Typeable
instance Shape Circle where
getArea (Circle radius) = pi * radius^2
data Rectangle = Rectangle { height :: Double, width :: Double }
deriving Typeable
instance Shape Rectangle where
getArea (Rectangle height width) = height * width
exampleData :: [Object Shape]
exampleData = [Obj (Circle 1.5), Obj (Rectangle 2 3)]
...but thanks to the Typeable
constraint in Object
we can downcast: if we correctly guess the type contained inside an Object
, we can recover that original type:
-- | For each 'Shape' in the list, try to cast it to a Circle. If we
-- succeed, then pass the result to a monomorphic function that
-- demands a 'Circle'. Evaluates to:
--
-- >>> example
-- ["A Circle of radius 1.5","A Shape with area 6.0"]
example :: [String]
example = mapMaybe step exampleData
where step shape = describeCircle <$> (downcast shape)
<|> Just (describeShape shape)
describeCircle :: Circle -> String
describeCircle (Circle radius) = "A Circle of radius " ++ show radius
describeShape :: Shape a => a -> String
describeShape shape = "A Shape with area " ++ show (getArea shape)