| Copyright | (c) Ashley Yakeley 2007 |
|---|---|
| License | BSD-style (see the LICENSE file in the distribution) |
| Maintainer | ashley@semantic.org |
| Stability | stable |
| Portability | portable |
| Safe Haskell | Trustworthy |
| Language | Haskell2010 |
GHC.Internal.Control.Category
Description
Documentation
class Category (cat :: k -> k -> Type) where Source #
A class for categories.
In mathematics, a category is defined as a collection of objects and a collection
of morphisms between objects, together with an identity morphism id for every
object and an operation (.) that composes compatible morphisms.
This class is defined in an analogous way. The collection of morphisms is represented
by a type parameter cat, which has kind k -> k -> for some kind variable Type k
that represents the collection of objects; most of the time the choice of k will be
Type .
Examples
Expand
As the method names suggest, there's a category of functions:
instance Category (->) where
id = \x -> x
f . g = \x -> f (g x)
Isomorphisms form a category as well:
data Iso a b = Iso (a -> b) (b -> a) instance Category Iso where id = Iso id id Iso f1 g1 . Iso f2 g2 = Iso (f1 . f2) (g2 . g1)
Natural transformations are another important example:
newtype f ~> g = NatTransform (forall x. f x -> g x) instance Category (~>) where id = NatTransform id NatTransform f . NatTransform g = NatTransform (f . g)
Using the TypeData language extension, we can also make a category where k isn't
Type, but a custom kind Door instead:
type data Door = DoorOpen | DoorClosed data Action (before :: Door) (after :: Door) where DoNothing :: Action door door OpenDoor :: Action start DoorClosed -> Action start DoorOpen CloseDoor :: Action start DoorOpen -> Action start DoorClosed instance Category Action where id = DoNothing DoNothing . action = action OpenDoor rest . action = OpenDoor (rest . action) CloseDoor rest . action = CloseDoor (rest . action)
Methods
id :: forall (a :: k). cat a a Source #
The identity morphism. Implementations should satisfy two laws:
These essentially state that id should "do nothing".
(.) :: forall (b :: k) (c :: k) (a :: k). cat b c -> cat a b -> cat a c infixr 9 Source #