# Generalizing generic fold

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## Uniform folding

Suppose we’ve created a new container data structure in Haskell, such as Data.Sequence.Seq. We now want to write a Data instance for it, to allow generic queries and transforms inside our container.

Let’s recall gfoldl definition for lists:

gfoldl f z [] = z []
gfoldl f z (x:xs) = z (:) f x f xs

We can define gfoldl similarly for sequences (in fact, this is the precise definition from Data.Sequence):

gfoldl f z s = case viewl s of
EmptyL  -> z empty
x :< xs -> z (<|) f x f xs

There are, however, downsides to this definition:

1. Generic map over our container would be less efficient than hand-written map, even if we ignore the costs associated with run-time type casts.

Indeed, in the generic definition we do not take advantage of the fact that we already have a proper container structure for our elements. Instead, we construct the structure from scratch, adding elements one by one. This overhead may be small for Seq, whose cons is O(1). But in case of an array-based structure, such as Vector, we’re out of luck.

2. If we only care about the closed recursion combinators, such as everywhere, we wouldn’t know the difference. But when open recursion is needed (e.g. gmapT), the semantics is not the one we’d expect. The intuition is that gmapT maps over the immediate «subterms» of an expression. One might expect that the subterms of Seq a are all of its elements (of type a). But no, we’re forced into this recursive list mindset, where the immediate subterms of a sequence are its head and tail.

(To be fair, it does make sense if we imagine data structures really being inductively-defined terms. But it is a very limited worldview.)

Can we define gfoldl in the uniform fashion, folding over all the elements at once? Try this yourself to understand better why it’s not easy.

I haven’t actually done it, but I think it’s achievable using some amount of type class hackery. Anyway, having to come up with such hacks defeats the point of an easy-to-use generic programming library.

In essence, the limitation of gfoldl is that it forces us to know the number of elements in advance. We cannot (easily) construct the result of the fold by recursive computation.

## Generic traversals

In order to resolve the problem stated above, we need to restrict the type of gfoldl1.

Recall its type:

gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b)
-> (forall g. g -> c g)
-> a -> c a

What might the first functional argument of gfoldl do? It probably transforms d to something like c d by using its Data instance, and then combines c (d -> b) with c d to obtain c b. This is not a sure thing, of course: the caller of gfoldl knows what c exactly is, so it might combine them in unexpected ways. Let us proceed with this assumption, however; as I said above, we need to restrict the caller in order to give more power to the callee.

So, if our guess is correct, we can separate the two activities into two different functions:

gfoldl :: (forall d. Data d => d -> c d)
-> (forall d b. c (d -> b) -> c d -> c b)
-> (forall g. g -> c g)
-> a -> c a

The whole thing now suddenly becomes very familiar: c is an applicative functor! The second and third arguments are <*> and pure, respectively. And the applicative laws had better hold — there are now many ways to express the same thing, and we don’t expect there to be a difference.

Using the Applicative class, we can rewrite the type as

gtraverse :: Applicative c => (forall d . GTraversable d => d -> c d) -> a -> c a

Note that I took this chance to rename our restricted gfoldl to gtraverse and Data to GTraversable. The reason is that it is a generic version of traverse from Data.Traversable, whose type is

traverse :: Applicative c => (a -> c b) -> t a -> c (t b)

Read the type of gtraverse as «traverse this structure, applying the supplied function to the immediate subterms; then use the Applicative instance to reconstruct the original structure».

We can now write the uniform instance for lists (and any other algebraic data type) without a hassle:

gtraverse f [] = pure []
gtraverse f (x:xs) = (:) <\$> f x <*> gtraverse f xs

But how does it restrict the caller of gfoldl? Not much. All the usual combinators, such as gmapT, gmapQ, gmapM etc. are expressible with gtraverse. I’d be curious to see an application of gfoldl for which gtraverse is too restrictive2.