Free and Forgetful Functors

Jul 20, 2017

Today I'm going to continue the previous topic of Adjunctions, last time we talked about how you can build a sensible adjunction from any Representable functor, this time we're going to talk about a (semantically) different form of adjunction, one formed by a pair of Free and Forgetful Functors. First I'll describe the relationship of Free and Forgetful Functors, then we'll see how an Adjunction can making translating between them slightly easier.

Let's define our terms, hopefully you already know what a Functor is, it's any type with a map method (called fmap in Haskell). A Free Functor is a functor which can embed any element "for free". So any Functor where we could just 'inject' a value into is considered a Free Functor. If the functor has an Applicative instance then inject is called pure.

inject :: a -> f a

To do this maybe it means we make up some of the structure, or have some default values we use in certain parts. Let's see some contrived examples of Free Functors.

Simple single slot functors like Identity:

inject a = Identity a

Simple structures like List or Maybe or Either:

inject a = [a]
inject a = Just a
inject a = Right a
inject a = Pair a a
inject a = repeat a

Or even anything paired with a monoid, since we can 'make up' the monoid's value using mempty.

inject :: Monoid t => a -> Tagged t a
inject a = Tagged mempty a

Note however that some of these Free functors are unsuitable for use with adjunctions since Sum types like Maybe, List and Either aren't Distributive because the number of a slots in the functor can change between values.

Next we need the forgetful functor, this is a functor which 'loses' or 'forgets' some data about some other functor when we wrap it. The idea is that for each pair of Free and Forgetful functors there's a Natural Transformation to the Identity Functor: Forget (Free a) ~> Identity a; and since there's an isomorphism Identity a ≅ a we end up with something like Forget (Free a) ~> a. This expresses that when we forget a free functor we end up back where we started.

Let's see what 'forgetting' the info from a Free functor looks like by implementing forget :: Free a -> a for different Free functors.

-- Identity never had any extra info to begin with
forget :: Identity a -> a
forget (Identity a) = a

-- The extra info in a nonempty list is the extra elements
forget :: List.NonEmpty a -> a
forget (a:|_) = a

-- The extra info in a 'Tagged' is the tag
forget :: Tagged t a -> a
forget (Tagged _ a) = a

-- The extra info in a Pair is the duplication
forget :: Pair a -> a
forget (Pair a _) = a

You can imagine this sort of thing for many types; for any Comonad type we have forget = extract. Implementations for Maybe or Either or List are a bit trickier since it's possible that no value exists, we'd have to require a Monoid for the inner type a to do these. Notice that these are the same types for which we can't write a proper instance of Distributive, so we'll be avoiding them as we move forwards.

Anyways, enough chatting, let's build something! We're going to do a case study in the Tagged type we showed above.

{-# language DeriveFunctor #-}
{-# language TypeFamilies #-}
{-# language MultiParamTypeClasses #-}
{-# language FlexibleInstances #-}

module Tagged where

import Data.Distributive
import Data.Functor.Rep
import Data.Functor.Adjunction
import Data.Char

newtype Forget a = Forget { getForget :: a } deriving (Show, Eq, Functor)
data Tagged t a = Tagged
  { getTag :: t
  , untag :: a
  } deriving (Show, Eq, Functor)

Okay so we've got our two functors! Tagged promotes an 'a' to a 'a' which is tagged by some tag 't'. We'll need a Representable instance for Forget, which need Distributive, these are pretty easy to write for such simple types. Notice that we have a Monoid constraint on our tag which makes Distributive possible.

instance Distributive Forget where
  distribute fa = Forget (getForget <$> fa)

instance Representable Forget where
  type Rep Forget = ()
  index (Forget a) () = a
  tabulate describe = Forget (describe ())

Hopefully this is all pretty easy to follow, we've chosen () as the representation since each data type has only a single slot.

Now for Adjunction! We'll unfortunately have to choose a concrete type for our tag here since the definition of Adjunction has functional dependencies. This means that for a given Left Adjoint there can only be one Right Adjoint. We can see it in the class constraint here:

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

It's a shame, but we'll just pick a tag type; how about Maybe String, a Just means we've tagged the value and a Nothing means we haven't. Maybe String is a monoid since String is a Monoid.

type Tag = Maybe String

instance Adjunction (Tagged Tag) Forget where

  unit :: a -> Forget (Tagged Tag a)
  unit a = Forget (Tagged Nothing a)
  counit :: Tagged Tag (Forget a) -> a
  counit (Tagged _ (Forget a)) = a

  -- leftAdjunct and rightAdjunct have default implementations in terms of unit & counit
  leftAdjunct :: (Tagged a -> b) -> a -> Forget b
  rightAdjunct :: (a -> Forget b) -> Tagged a -> b

There we go! Here we say that Forget is Right Adjoint to Tagged, which roughly means that we lose information when we move from Tagged to Forget. unit and counit correspond to the inject and forget that we wrote earlier, they've just got that extra Forget floating around. That's okay though, it's isomorphic to Identity so anywhere we see a Forget a we can pull it out into just an a and vice versa if we need to embed an a to get Forget a.

We now have access to helpers which allow us to promote and demote functions from one functor into the other; so if we've got a function which operates over tagged values we can get a function over untagged values, the same goes for turning functions accepting untagged values into ones taking tagged values. These helpers are leftAdjunct and rightAdjunct respectively! We're going to wrap them up in a small layer to perform the a ≅ Forget a isomorphism for us so we can clean up the signatures a little.

overUntagged :: (Tagged Tag a -> b) -> a -> b
overUntagged f = getForget . leftAdjunct f 

overTagged :: (a -> b) -> Tagged Tag a -> b
overTagged f = rightAdjunct (Forget . f)

To test these out let's write a small function which takes Strings which are Tagged with a String annotation and appends the tag to the string:

applyTag :: Tagged Tag String -> String
applyTag (Tagged Nothing s) = s
applyTag (Tagged (Just tag) s) = tag ++ ": " ++ s

λ> applyTag (Tagged (Just "Book") "Ender's Game")
"Book: Ender's Game"
λ> applyTag (Tagged Nothing "Steve")

Using our helpers we can call applyTag over untagged strings too, though the results are expectedly boring:

λ> overUntagged applyTag "Boring"

Now let's see the other half of our adjunction, we can define a function over strings and run it over Tagged strings!

upperCase :: String -> String
upperCase = fmap toUpper

λ> upperCase "Steve"
λ> overTagged upperCase (Tagged (Just "Book") "Ender's Game")

Notice that we lose the tag when we do this, that's the price we pay with a lossy Adjunction! The utility of the construct seems pretty limited here since fmap and extract would pretty much give us the same options, but the idea is that Adjunctions represent a structure which we can generalize over in certain cases. This post was more about understanding adjunctions and Free/Forgetful functors than it was about programming anyways :)