# # Recursion Schemes

## # Fixed points

`Fix`

takes a "template" type and ties the recursive knot, layering the template like a lasagne.

```
newtype Fix f = Fix { unFix :: f (Fix f) }
```

Inside a `Fix f`

we find a layer of the template `f`

. To fill in `f`

's parameter, `Fix f`

plugs in **itself**. So when you look inside the template `f`

you find a recursive occurrence of `Fix f`

.

Here is how a typical recursive datatype can be translated into our framework of templates and fixed points. We remove recursive occurrences of the type and mark their positions using the `r`

parameter.

```
{-# LANGUAGE DeriveFunctor #-}
-- natural numbers
-- data Nat = Zero | Suc Nat
data NatF r = Zero_ | Suc_ r deriving Functor
type Nat = Fix NatF
zero :: Nat
zero = Fix Zero_
suc :: Nat -> Nat
suc n = Fix (Suc_ n)
-- lists: note the additional type parameter a
-- data List a = Nil | Cons a (List a)
data ListF a r = Nil_ | Cons_ a r deriving Functor
type List a = Fix (ListF a)
nil :: List a
nil = Fix Nil_
cons :: a -> List a -> List a
cons x xs = Fix (Cons_ x xs)
-- binary trees: note two recursive occurrences
-- data Tree a = Leaf | Node (Tree a) a (Tree a)
data TreeF a r = Leaf_ | Node_ r a r deriving Functor
type Tree a = Fix (TreeF a)
leaf :: Tree a
leaf = Fix Leaf_
node :: Tree a -> a -> Tree a -> Tree a
node l x r = Fix (Node_ l x r)
```

## # Folding up a structure one layer at a time

**Catamorphisms**, or **folds**, model primitive recursion. `cata`

tears down a fixpoint layer by layer, using an **algebra** function (or **folding function**) to process each layer. `cata`

requires a `Functor`

instance for the template type `f`

.

```
cata :: Functor f => (f a -> a) -> Fix f -> a
cata f = f . fmap (cata f) . unFix
-- list example
foldr :: (a -> b -> b) -> b -> List a -> b
foldr f z = cata alg
where alg Nil_ = z
alg (Cons_ x acc) = f x acc
```

## # Unfolding a structure one layer at a time

**Anamorphisms**, or **unfolds**, model primitive corecursion. `ana`

builds up a fixpoint layer by layer, using a **coalgebra** function (or **unfolding function**) to produce each new layer. `ana`

requires a `Functor`

instance for the template type `f`

.

```
ana :: Functor f => (a -> f a) -> a -> Fix f
ana f = Fix . fmap (ana f) . f
-- list example
unfoldr :: (b -> Maybe (a, b)) -> b -> List a
unfoldr f = ana coalg
where coalg x = case f x of
Nothing -> Nil_
Just (x, y) -> Cons_ x y
```

Note that `ana`

and `cata`

are **dual**. The types and implementations are mirror images of one another.

## # Unfolding and then folding, fused

It's common to structure a program as building up a data structure and then collapsing it to a single value. This is called a **hylomorphism** or **refold**. It's possible to elide the intermediate structure altogether for improved efficiency.

```
hylo :: Functor f => (a -> f a) -> (f b -> b) -> a -> b
hylo f g = g . fmap (hylo f g) . f -- no mention of Fix!
```

Derivation:

```
hylo f g = cata g . ana f
= g . fmap (cata g) . unFix . Fix . fmap (ana f) . f -- definition of cata and ana
= g . fmap (cata g) . fmap (ana f) . f -- unfix . Fix = id
= g . fmap (cata g . ana f) . f -- Functor law
= g . fmap (hylo f g) . f -- definition of hylo
```

## # Primitive recursion

**Paramorphisms** model primitive recursion. At each iteration of the fold, the folding function receives the subtree for further processing.

```
para :: Functor f => (f (Fix f, a) -> a) -> Fix f -> a
para f = f . fmap (\x -> (x, para f x)) . unFix
```

The Prelude's `tails`

can be modelled as a paramorphism.

```
tails :: List a -> List (List a)
tails = para alg
where alg Nil_ = cons nil nil -- [[]]
alg (Cons_ x (xs, xss)) = cons (cons x xs) xss -- (x:xs):xss
```

## # Primitive corecursion

**Apomorphisms** model primitive corecursion. At each iteration of the unfold, the unfolding function may return either a new seed or a whole subtree.

```
apo :: Functor f => (a -> f (Either (Fix f) a)) -> a -> Fix f
apo f = Fix . fmap (either id (apo f)) . f
```

Note that `apo`

and `para`

are **dual**. The arrows in the type are flipped; the tuple in `para`

is dual to the `Either`

in `apo`

, and the implementations are mirror images of each other.

#### # Remarks

Functions mentioned here in examples are defined with varying degrees of abstraction in several packages, for example, `data-fix`

and `recursion-schemes`

(more functions here). You can view a more complete list by searching on Hayoo.