Data.Functor.Day

Eitan Chatav first introduced me to this construction

The Day convolution of two covariant functors is a covariant functor.

Day convolution is usually defined in terms of contravariant functors, however, it just needs a monoidal category, and Hask^op is also monoidal.

Day convolution can be used to nicely describe monoidal functors as monoid objects w.r.t this product.

http://ncatlab.org/nlab/show/Day+convolution

Documentation

data Day f g a

The Day convolution of two covariant functors.

Constructors

forall b c . Day (f b) (g c) (b -> c -> a)

Instances

Functor (Day f g)

(Typeable1 f, Typeable1 g) => Typeable1 (Day f g)

day :: f (a -> b) -> g a -> Day f g b

Construct the Day convolution

dap :: Applicative f => Day f f a -> f a

Collapse via a monoidal functor.

 
 dap (day f g) = f <*> g

assoc :: Day f (Day g h) a -> Day (Day f g) h a

Day convolution provides a monoidal product. The associativity of this monoid is witnessed by assoc and disassoc.

 assoc . disassoc = id
 disassoc . assoc = id
 fmap f . assoc = assoc . fmap f

disassoc :: Day (Day f g) h a -> Day f (Day g h) a

Day convolution provides a monoidal product. The associativity of this monoid is witnessed by assoc and disassoc.

 assoc . disassoc = id
 disassoc . assoc = id
 fmap f . disassoc = disassoc . fmap f

swapped :: Day f g a -> Day g f a

The monoid for Day convolution on the cartesian monoidal structure is symmetric.

 fmap f . swapped = swapped . fmap f

intro1 :: f a -> Day Identity f a

Identity is the unit of Day convolution

 intro1 . elim1 = id
 elim1 . intro1 = id

intro2 :: f a -> Day f Identity a

Identity is the unit of Day convolution

 intro2 . elim2 = id
 elim2 . intro2 = id

elim1 :: Functor f => Day Identity f a -> f a

Identity is the unit of Day convolution

 intro1 . elim1 = id
 elim1 . intro1 = id

elim2 :: Functor f => Day f Identity a -> f a

Identity is the unit of Day convolution

 intro2 . elim2 = id
 elim2 . intro2 = id

trans1 :: (forall x. f x -> g x) -> Day f h a -> Day g h a

Apply a natural transformation to the left-hand side of a Day convolution.

This respects the naturality of the natural transformation you supplied:

 fmap f . trans1 fg = trans1 fg . fmap f

trans2 :: (forall x. g x -> h x) -> Day f g a -> Day f h a

Apply a natural transformation to the right-hand side of a Day convolution.

This respects the naturality of the natural transformation you supplied:

 fmap f . trans2 fg = trans2 fg . fmap f