Data.Functor.Kan.Lift

Left Kan lifts for functors over Hask, wherever they exist.

http://ncatlab.org/nlab/show/Kan+lift

Left Kan lifts

newtype Lift g f a

f => g . Lift g f
(forall z. f => g . z) -> Lift g f => z -- couniversal

Here we use the universal property directly as how we extract from our definition of Lift.

Constructors

Lift

Fields

• runLift :: forall z. Functor z => (forall x. f x -> g (z x)) -> z a

Instances

Functor (Lift g h)

Methods

fmap :: (a -> b) -> Lift g h a -> Lift g h b

(<$) :: a -> Lift g h b -> Lift g h a

toLift :: Functor z => (forall a. f a -> g (z a)) -> Lift g f b -> z b

The universal property of Lift

fromLift :: Adjunction l u => (forall a. Lift u f a -> z a) -> f b -> u (z b)

When the adjunction exists

fromLift . toLift ≡ id
toLift . fromLift ≡ id

glift :: Adjunction l g => k a -> g (Lift g k a)

f => g (Lift g f a)

composeLift :: (Composition compose, Functor f, Functor g) => Lift f (Lift g h) a -> Lift (compose g f) h a

composeLift . decomposeLift = id
decomposeLift . composeLift = id

decomposeLift :: (Composition compose, Adjunction l g) => Lift (compose g f) h a -> Lift f (Lift g h) a

adjointToLift :: Adjunction f u => f a -> Lift u Identity a

Lift u Identity a is isomorphic to the left adjoint to u if one exists.

adjointToLift . liftToAdjoint ≡ id
liftToAdjoint . adjointToLift ≡ id

liftToAdjoint :: Adjunction f u => Lift u Identity a -> f a

Lift u Identity a is isomorphic to the left adjoint to u if one exists.

liftToComposedAdjoint :: (Adjunction f u, Functor h) => Lift u h a -> f (h a)

Lift u h a is isomorphic to the post-composition of the left adjoint of u onto h if such a left adjoint exists.

liftToComposedAdjoint . composedAdjointToLift ≡ id
composedAdjointToLift . liftToComposedAdjoint ≡ id

composedAdjointToLift :: Adjunction f u => f (h a) -> Lift u h a

Lift u h a is isomorphic to the post-composition of the left adjoint of u onto h if such a left adjoint exists.

repToLift :: Representable u => Rep u -> a -> Lift u Identity a

repToLift . liftToRep ≡ id
liftToRep . repToLift ≡ id

liftToRep :: Representable u => Lift u Identity a -> (Rep u, a)

liftToComposedRep :: (Functor h, Representable u) => Lift u h a -> (Rep u, h a)

liftToComposedRep . composedRepToLift ≡ id
composedRepToLift . liftToComposedRep ≡ id

composedRepToLift :: Representable u => Rep u -> h a -> Lift u h a