Applicative functors are well-known and well-loved among Haskellers, for their ability to apply functions in an effectful context.
In category-theoretic terms, it can be shown that the methods of Applicative:
pure :: a -> f a
(<*>) :: f (a -> b) -> f a -> f b
are equivalent to having a Functor f with the operations:
unit :: f ()
(**) :: (f a, f b) -> f (a,b)
the idea being that to write pure you just replace the () in unit with the given value, and to write (<*>) you squish the function and argument into a tuple and then map a suitable application function over it.
Moreover, this correspondence turns the Applicative laws into natural monoidal-ish laws about unit and (**), so in fact an applicative functor is precisely what a category theorist would call a lax monoidal functor (lax because (**) is merely a natural transformation and not an isomorphism).
Okay, fine, great. This much is well-known. But that's only one family of lax monoidal functors – those respecting the monoidal structure of the product. A lax monoidal functor involves two choices of monoidal structure, in the source and destination: here's what you get if you turn product into sum:
class PtS f where
unit :: f Void
(**) :: f a -> f b -> f (Either a b)
-- some example instances
instance PtS Maybe where
unit = Nothing
Nothing ** Nothing = Nothing
Just a ** Nothing = Just (Left a)
Nothing ** Just b = Just (Right b)
Just a ** Just b = Just (Left a) -- ick, but it does satisfy the laws
instance PtS [] where
unit = []
xs ** ys = map Left xs ++ map Right ys
It seems like turning sum into other monoidal structures is made less interesting by unit :: Void -> f Void being uniquely determined, so you really have more of a semigroup going on. But still:
Applicative one?The "neat alternative presentation" for Applicative is based on the following two equivalencies
pure a = fmap (const a) unit
unit = pure ()
ff <*> fa = fmap (\(f,a) -> f a) $ ff ** fa
fa ** fb = pure (,) <*> fa <*> fb
The trick to get this "neat alternative presentation" for Applicative is the same as the trick for zipWith - replace explicit types and constructors in the interface with things that the type or constructor can be passed into to recover what the original interface was.
unit :: f ()
Is replaced with pure which we can substitute the type () and the constructor () :: () into to recover unit.
pure :: a -> f a
pure () :: f ()
And similarly (though not as straightforward) for substituting the type (a,b) and the constructor (,) :: a -> b -> (a,b) into liftA2 to recover **.
liftA2 :: (a -> b -> c) -> f a -> f b -> f c
liftA2 (,) :: f a -> f b -> f (a,b)
Applicative then gets the nice <*> operator by lifting function application ($) :: (a -> b) -> a -> b into the functor.
(<*>) :: f (a -> b) -> f a -> f b
(<*>) = liftA2 ($)
To find a "neat alternative presentation" for PtS we need to find
Void into to recover unitEither a b and the constructors Left :: a -> Either a b and Right :: b -> Either a b into to recover **(If you notice that we already have something the constructors Left and Right can be passed to you can probably figure out what we can replace ** with without following the steps I used; I didn't notice this until after I solved it)
This immediately gets us an alternative to unit for sums:
empty :: f a
empty = fmap absurd unit
unit :: f Void
unit = empty
We'd like to find an alternative to (**). There is an alternative to sums like Either that allows them to be written as functions of products. It shows up as the visitor pattern in object oriented programming languages where sums don't exist.
data Either a b = Left a | Right b
{-# LANGUAGE RankNTypes #-}
type Sum a b = forall c. (a -> c) -> (b -> c) -> c
It's what you would get if you changed the order of either's arguments and partially applied them.
either :: (a -> c) -> (b -> c) -> Either a b -> c
toSum :: Either a b -> Sum a b
toSum e = \forA forB -> either forA forB e
toEither :: Sum a b -> Either a b
toEither s = s Left Right
We can see that Either a b ≅ Sum a b. This allows us to rewrite the type for (**)
(**) :: f a -> f b -> f (Either a b)
(**) :: f a -> f b -> f (Sum a b)
(**) :: f a -> f b -> f ((a -> c) -> (b -> c) -> c)
Now it's clear what ** does. It delays fmaping something onto both of its arguments, and combines the results of those two mappings. If we introduce a new operator, <||> :: f c -> f c -> f c which simply assumes that the fmaping was done already, then we can see that
fmap (\f -> f forA forB) (fa ** fb) = fmap forA fa <||> fmap forB fb
Or back in terms of Either:
fa ** fb = fmap Left fa <||> fmap Right fb
fa1 <||> fa2 = fmap (either id id) $ fa1 ** fa2
So we can express everything PtS can express with the following class, and everything that could implement PtS can implement the following class:
class Functor f => AlmostAlternative f where
empty :: f a
(<||>) :: f a -> f a -> f a
This is almost certainly the same as the Alternative class, except we didn't require that the Functor be Applicative.
It's just a Functor that is a Monoid for all types. It'd be equivalent to the following:
class (Functor f, forall a. Monoid (f a)) => MonoidalFunctor f
The forall a. Monoid (f a) constraint is pseudo-code; I don't know a way to express constraints like this in Haskell.