Constructions on Typeclasses, Part 1: F-Algebras

This post is rendered from literate Haskell. I recommend doing the exercises inline, so use the source.

> {-# LANGUAGE DeriveFunctor
>            , DeriveFoldable
>            , DeriveTraversable
>            , TypeOperators #-}
>
> import Control.Applicative
> import Data.Foldable
> import Data.Traversable

Certain kinds of typeclasses have some very regular instances. For example, it is obvious how to implement (Num a, Num b) => Num (a,b) and (Monoid a, Monoid b) => Monoid (a,b), and similarly if F is some applicative functor, (Num a) => Num (F a) and (Monoid a) => (Monoid F a) are obvious. Furthermore, these instances (and many others) seem to be obvious in the same way.

(+) a b     = (+)     <$> a <*> b
mappend a b = mappend <$> a <*> b

fromInteger n = pure (fromInteger n)
mempty        = pure mempty

And take them on pairs:

(x,x')     +     (y,y')  = (x     +     y, x'     +     y')
(x,x') `mappend` (y,y')  = (x `mappend` y, x' `mappend` y')

fromInteger n = (fromInteger n, fromInteger n)
mempty        = (mempty       , mempty)

It would be straightforward for these cases to derive the necessary implementations from the type signature. However, it would be nice if there were a more abstract perspective, such that we didn’t have to inspect the type signature to find the operations – that they could arise from some other standard construction. Further, it is not quite as obvious from the the type signature how to automatically instantiate methods such as

mconcat :: (Monoid m) => [m] -> m

without making a special case for [], whereas hopefully a more abstract perspective would inform us what kinds of type constructors would be supported.

In this post, we will see such an abstract perspective. It comes from (surprise!) category theory. I disclaim that I’m still a novice with category theory (but in the past few weeks I have gained competence by studying). So we will not get very deep into the theory, just enough to steal the useful concept and leave the rest behind. I welcome relevant insights from the more categorically educated in the comments.

F-Algebras

The unifying concept we will steal is the F-algebra. An F-algebra is a Functor f and a type a together with a function f a -> a. We can make this precise in Haskell:

> type Algebra f a = f a -> a

I claim that Num and Monoid instances are F-algebras over suitable functors. Look at the methods of Monoid:

mempty :: m
mappend :: m -> m -> m

We need to find a functor f such that we can recover these two methods from a function of type f m -> m. With some squinting, we arrive at:

> data MonoidF m
>     = MEmpty
>     | MAppend m m
>
> memptyF :: Algebra MonoidF m -> m
> memptyF alg = alg MEmpty
>
> mappendF :: Algebra MonoidF m -> (m -> m -> m)
> mappendF alg x y = alg (MAppend x y)

Exercise 1: work out the functor NumF over which Num instances are F-algebras, and write the methods of Num in terms of it.

Exercise 2: for each of the standard classes Eq, Read, Show, Bounded, and Integral, work out whether they are expressible as F-algebras. If so, give the functor; if not, explain or prove why not.

Exercise 3: write a function toMonoidAlg which finds the MonoidF-algebra for a given instance m of the Monoid class.

Combining Instances

Motivated by the examples in the introduction, we can find the “instance” for pairs given instances for each of the components.

> pairAlg :: (Functor t) => Algebra t a -> Algebra t b -> Algebra t (a,b)
> pairAlg alga algb tab = (alga (fmap fst tab), algb (fmap snd tab))

Also, we hope we can find the instance for an applicative functor given an instance for its argument

applicativeAlg :: (Functor t, Applicative f)
               => Algebra t a -> Algebra t (f a)

but there turns out to be trouble:

applicativeAlg alg tfa = ...

We need to get our hands on an t a somehow, and all we have is a t (f a). This hints at something from the standard library:

sequenceA :: (Traversible t, Applicative f) => t (f a) -> f (t a)

which indicates that our functor needs more structure to implement applicativeAlg.

> applicativeAlg :: (Traversable t, Applicative f)
>                => Algebra t a -> Algebra t (f a)
> applicativeAlg alg tfa = fmap alg (sequenceA tfa)

Now we should be able to answer the query from the beginning:

Exercise 4: For what kinds of type constructors c is it possible to automatically derive instances for (a) pairs and (b) Applicatives for a typeclass with a method of type c a -> a. (e.g. mconcat :: [a] -> a). Demonstrate this with an implementation.

Combining Classes

Intuitively, joining the methods of two classes which are both expressible as F-algebras should give us another class expressible as an F-algebra. This is demonstrated by the following construction:

> data (f :+: g) a = InL (f a) | InR (g a)
>     deriving (Functor, Foldable, Traversable)
>
> coproductAlg :: (Functor f, Functor g)
>              => Algebra f a -> Algebra g a -> Algebra (f :+: g) a
> coproductAlg falg _ (InL fa) = falg fa
> coproductAlg _ galg (InR ga) = galg ga

So now we can model a subclass of both Num and Monoid by type NumMonoidF = NumF :+: MonoidF.

Exercise 5: We hope to be able to recover Algebra NumF a from Algebra NumMonoidF a, demonstrating that the latter is in fact a subclass. Implement the necessary function(s).

Exercise 6: Given the functor product definition

> data (f :*: g) a = Pair (f a) (g a)
>     deriving (Functor, Foldable, Traversable)

find a suitable combinator for forming algebras over a product functor. It may not have the same form as coproduct’s combinator! What would a typeclass formed by a product of two typeclasses interpreted as F-algebras look like?

Free Constructions

One of the neat things we can do with typeclasses expressed as F-algebras is form free monads over them – i.e. form the data type of a “syntax tree” over the methods of a class (with a given set of free variables). Begin with the free monad over a functor:

> data Free f a
>     = Pure a
>     | Effect (f (Free f a))
>     deriving (Functor, Foldable, Traversable)
>
> instance (Functor f) => Monad (Free f) where
>     return = Pure
>     Pure a >>= t = t a
>     Effect f >>= t = Effect (fmap (>>= t) f)

(Church-encoding this gives better performance, but I’m using this version for expository purposes)

Free f a can be interpreted as a syntax tree over the typeclass formed by f with free variables in a. This is also called an “initial algebra”, a term you may have heard thrown around in the Haskell community from time to time. We demonstrate that a free construction over a functor is a valid F-algebra for that functor:

> initialAlgebra :: (Functor f) => Algebra f (Free f a)
> initialAlgebra = Effect

And that it is possible to “interpret” an initial algebra using any other F-algebra over that functor.

> initiality :: (Functor f) => Algebra f a -> Free f a -> a
> initiality alg (Pure a) = a
> initiality alg (Effect f) = alg (fmap (initiality alg) f)

Exercise 7: Give a monoid isomorphism (a bijection that preserves the monoid operations) between Free MonoidF and lists [], ignoring that Haskell allows infinitely large terms. Then, using an infinite term, show how this isomorphism fails.

Next time: F-Coalgebras