Data.Functor.Adjunction

adjunctions-4.3: Adjunctions and representable functors

Copyright | 2008-2013 Edward Kmett
License | BSD
Maintainer | Edward Kmett <ekmett@gmail.com>
Stability | experimental
Portability | rank 2 types, MPTCs, fundeps
Safe Haskell | Trustworthy
Language | Haskell98

Data.Functor.Adjunction

Description

Synopsis

- class (Functor f, Representable u) => Adjunction f u | f -> u, u -> f where
- adjuncted :: (Adjunction f u, Profunctor p, Functor g) => p (a -> u b) (g (c -> u d)) -> p (f a -> b) (g (f c -> d))
- tabulateAdjunction :: Adjunction f u => (f () -> b) -> u b
- indexAdjunction :: Adjunction f u => u b -> f a -> b
- zapWithAdjunction :: Adjunction f u => (a -> b -> c) -> u a -> f b -> c
- zipR :: Adjunction f u => (u a, u b) -> u (a, b)
- unzipR :: Functor u => u (a, b) -> (u a, u b)
- unabsurdL :: Adjunction f u => f Void -> Void
- absurdL :: Void -> f Void
- cozipL :: Adjunction f u => f (Either a b) -> Either (f a) (f b)
- uncozipL :: Functor f => Either (f a) (f b) -> f (Either a b)
- extractL :: Adjunction f u => f a -> a
- duplicateL :: Adjunction f u => f a -> f (f a)
- splitL :: Adjunction f u => f a -> (a, f ())
- unsplitL :: Functor f => a -> f () -> f a

# Documentation

class (Functor f, Representable u) => Adjunction f u | f -> u, u -> f where #

An adjunction between Hask and Hask.

Minimal definition: both `unit` and `counit` or both `leftAdjunct` and `rightAdjunct`, subject to the constraints imposed by the default definitions that the following laws should hold.

    unit = leftAdjunct id
    counit = rightAdjunct id
    leftAdjunct f = fmap f . unit
    rightAdjunct f = counit . fmap f

Any implementation is required to ensure that `leftAdjunct` and `rightAdjunct` witness an isomorphism from `Nat (f a, b)` to `Nat (a, g b)`

    rightAdjunct unit = id
    leftAdjunct counit = id

Methods

unit :: a -> u (f a) #

counit :: f (u a) -> a #

leftAdjunct :: (f a -> b) -> a -> u b #

rightAdjunct :: (a -> u b) -> f a -> b #

Instances

Adjunction Identity Identity # |
Methods unit :: a -> Identity (Identity a) # counit :: Identity (Identity a) -> a # leftAdjunct :: (Identity a -> b) -> a -> Identity b # rightAdjunct :: (a -> Identity b) -> Identity a -> b #
Adjunction ((,) e) ((->) e) # |
Methods unit :: a -> e -> (e, a) # counit :: (e, e -> a) -> a # leftAdjunct :: ((e, a) -> b) -> a -> e -> b # rightAdjunct :: (a -> e -> b) -> (e, a) -> b #
Adjunction f u => Adjunction (Free f) (Cofree u) # |
Methods unit :: a -> Cofree u (Free f a) # counit :: Free f (Cofree u a) -> a # leftAdjunct :: (Free f a -> b) -> a -> Cofree u b # rightAdjunct :: (a -> Cofree u b) -> Free f a -> b #
Adjunction f g => Adjunction (IdentityT * f) (IdentityT * g) # |
Methods unit :: a -> IdentityT * g (IdentityT * f a) # counit :: IdentityT * f (IdentityT * g a) -> a # leftAdjunct :: (IdentityT * f a -> b) -> a -> IdentityT * g b # rightAdjunct :: (a -> IdentityT * g b) -> IdentityT * f a -> b #
Adjunction m w => Adjunction (WriterT s m) (TracedT s w) # |
Methods unit :: a -> TracedT s w (WriterT s m a) # counit :: WriterT s m (TracedT s w a) -> a # leftAdjunct :: (WriterT s m a -> b) -> a -> TracedT s w b # rightAdjunct :: (a -> TracedT s w b) -> WriterT s m a -> b #
Adjunction w m => Adjunction (EnvT e w) (ReaderT * e m) # |
Methods unit :: a -> ReaderT * e m (EnvT e w a) # counit :: EnvT e w (ReaderT * e m a) -> a # leftAdjunct :: (EnvT e w a -> b) -> a -> ReaderT * e m b # rightAdjunct :: (a -> ReaderT * e m b) -> EnvT e w a -> b #
(Adjunction f g, Adjunction f' g') => Adjunction (Sum * f f') (Product * g g') # |
Methods unit :: a -> Product * g g' (Sum * f f' a) # counit :: Sum * f f' (Product * g g' a) -> a # leftAdjunct :: (Sum * f f' a -> b) -> a -> Product * g g' b # rightAdjunct :: (a -> Product * g g' b) -> Sum * f f' a -> b #
(Adjunction f g, Adjunction f' g') => Adjunction (Compose * * f' f) (Compose * * g g') # |
Methods unit :: a -> Compose * * g g' (Compose * * f' f a) # counit :: Compose * * f' f (Compose * * g g' a) -> a # leftAdjunct :: (Compose * * f' f a -> b) -> a -> Compose * * g g' b # rightAdjunct :: (a -> Compose * * g g' b) -> Compose * * f' f a -> b #

adjuncted :: (Adjunction f u, Profunctor p, Functor g) => p (a -> u b) (g (c -> u d)) -> p (f a -> b) (g (f c -> d)) #

`leftAdjunct` and `rightAdjunct` form two halves of an isomorphism.

This can be used with the combinators from the `lens` package.

    adjuncted :: Adjunction f u => Iso' (f a -> b) (a -> u b)

tabulateAdjunction :: Adjunction f u => (f () -> b) -> u b #

Every right adjoint is representable by its left adjoint applied to a unit element

Use this definition and the primitives in Data.Functor.Representable to meet the requirements of the superclasses of Representable.

indexAdjunction :: Adjunction f u => u b -> f a -> b #

This definition admits a default definition for the `index` method of 'Index", one of the superclasses of Representable.

zapWithAdjunction :: Adjunction f u => (a -> b -> c) -> u a -> f b -> c #

zipR :: Adjunction f u => (u a, u b) -> u (a, b) #

A right adjoint functor admits an intrinsic notion of zipping

unzipR :: Functor u => u (a, b) -> (u a, u b) #

Every functor in Haskell permits unzipping

unabsurdL :: Adjunction f u => f Void -> Void #

A left adjoint must be inhabited, or we can derive bottom.

absurdL :: Void -> f Void #

cozipL :: Adjunction f u => f (Either a b) -> Either (f a) (f b) #

And a left adjoint must be inhabited by exactly one element

uncozipL :: Functor f => Either (f a) (f b) -> f (Either a b) #

Every functor in Haskell permits `uncozipping`

extractL :: Adjunction f u => f a -> a #

duplicateL :: Adjunction f u => f a -> f (f a) #

splitL :: Adjunction f u => f a -> (a, f ()) #

unsplitL :: Functor f => a -> f () -> f a #