This article describes a fixpoint combinator for Haskell types, with some justification of why such a thing would be useful. It's meant to be an accessible introduction to material which is already covered elsewhere.

You'll need some basic Haskell knowledge, such as how to define and use polymorphic data types. And you might want to first study the value-level `fix`

function. An excellent introduction can be found here.

This code is standard Haskell 98, except in the "GHC tricks" section.

# Recursive types

Many useful data types are recursive:

`data Natural = Zero | Succ Natural -- natural numbers`

data List a = Nil | Cons a (List a) -- lists

data Tree a = Node a [Tree a] -- rose trees

In each case, the type declared on the left-hand side of "`=`

" is mentioned on the right-hand side. This is fine, but just for fun let's see if there's another way.

We can imagine a language with a keyword "`self`

", which refers to the type being defined:

`data Natural = Zero | Succ self`

data List a = Nil | Cons a self

data Tree a = Node a [self]

Haskell doesn't have this keyword, so we'll make `self`

into a type parameter:

`data NaturalF self = Zero | Succ self`

data ListF a self = Nil | Cons a self

data TreeF a self = Node a [self]

We've changed the names because these aren't the types we wanted. They're non-recursive and each has an extra parameter.

To build the recursive type `Natural`

using the non-recursive `NaturalF`

, we need to "tie the knot":

`type Natural = NaturalF (NaturalF (NaturalF (NaturalF ...)))`

i.e.

`type Natural = NaturalF Natural`

But this is disallowed:

```
foo.hs:3:0:
Cycle in type synonym declarations:
foo.hs:3:0-30: type Natural = NaturalF Natural
```

Since `type`

defines a transparent synonym, this would generate an infinitely-large type-expression. Haskell disallows those. We need to hide the recursion behind a data constructor:

`newtype Natural = MkNat (NaturalF Natural)`

When we use this type, the "wrapper" constructor `MkNat`

alternates with the constructors of `NaturalF`

:

`toNatural :: Integer -> Natural`

toNatural 0 = MkNat Zero

toNatural n = MkNat . Succ $ toNatural (n-1)

# Type-level `Fix`

So far this is just pointless complexity. We've replaced one type:

`data Natural = Zero | Succ Natural`

with two types:

`data NaturalF self = Zero | Succ self`

newtype Natural = MkNat (NaturalF Natural)

Notice that the definition of `Natural`

doesn't depend on the structure of natural numbers. Let's rename `Natural`

to `Fix`

and `MkNat`

to `In`

:

`newtype Fix = In (NaturalF Fix)`

Then we'll abstract out `NaturalF`

as a type parameter `f`

:

`newtype Fix f = In (f (Fix f))`

Now `Natural`

is just a synonym:

`type Natural = Fix NaturalF`

When we create values of type `Natural`

, we still have "wrapper" constructors:

`toNatural :: Integer -> Natural`

toNatural 0 = In Zero

toNatural n = In . Succ $ toNatural (n-1)

# Properties of `Fix`

Recall that the value-level `fix`

function can be defined as

`fix f = f (fix f)`

This looks a lot like our type

`newtype Fix f = In (f (Fix f))`

The similarity is reflected at the type and kind levels:

```
GHCi> :t fix
fix :: (a -> a) -> a
GHCi> :k Fix
Fix :: (* -> *) -> *
```

`fix`

is a function which takes another function. `Fix`

is a type constructor which takes another type constructor.

The types `Fix f`

and `f (Fix f)`

are isomorphic, and we'll want to convert between them. In one direction we'll use the data constructor

`In :: f (Fix f) -> Fix f`

and the other direction is easily defined as

`out :: Fix f -> f (Fix f)`

out (In x) = x

# Folding lists

So why bother writing types this way? Well, we usually don't. But factoring the recursion out of our data type enables us to factor the recursion out of our functions.

Consider taking the sum of a list:

`sumF :: (Num a) => List a -> a`

sumF (In Nil) = 0

sumF (In (Cons x xs)) = x + sumF xs

We traverse the structure with the combining function `(+)`

, making a recursive call at each recursive position of the data type. How can we capture this pattern?

If we're reducing a value of type `List T`

to get a result of type `R`

, then a value of type `ListF T R`

represents a list node with the recursive call's result already in place. So we might look for a function like

`cata :: (ListF a b -> b) -> List a -> b`

If we had this, we could write

`sumF :: (Num a) => List a -> a`

sumF = cata f where

f Nil = 0

f (Cons x xs) = x + xs

The function `f`

specifies one step of reduction, and the magical function `cata`

handles the rest.

The recursion pattern captured by `cata`

is very similar to `foldr`

on the built-in list type. In fact, we can implement `foldr`

with `cata`

:

`foldrF :: (a -> b -> b) -> b -> List a -> b`

foldrF f z = cata g where

g Nil = z

g (Cons x xs) = f x xs

If `foldr`

is good enough, then what's so special about `cata`

? The answer is that `cata`

actually has this very general type:

`cata :: (Functor f) => (f a -> a) -> Fix f -> a`

I'll give the one-line definition of `cata`

later. Writing it yourself is a fun puzzle.

We can see that we'll need this instance:

`instance Functor (ListF a) where`

fmap _ Nil = Nil

fmap f (Cons x xs) = Cons x (f xs)

This `fmap`

just takes a function and applies it to the `self`

-typed field, if any. It's not recursive and has no relation to the `[]`

instance of `Functor`

, for which `fmap = map`

.

# Folding natural numbers

Let's put our generic `cata`

to good use on another type. Every natural number is either zero, or the successor of another natural number:

`data NaturalF self = Zero | Succ self`

type Natural = Fix NaturalF

Again, `fmap`

applies a function to each `self`

-typed field:

`instance Functor NaturalF where`

fmap _ Zero = Zero

fmap f (Succ v) = Succ (f v)

We can then use `cata`

to define conversion to `Integer`

:

`fromNatural :: Natural -> Integer`

fromNatural = cata f where

f Zero = 0

f (Succ n) = 1 + n

as well as addition and multiplication:

`add :: Natural -> Natural -> Natural`

add n = cata f where

f Zero = n

f (Succ m) = In $ Succ m

mul :: Natural -> Natural -> Natural

mul n = cata f where

f Zero = In Zero

f (Succ m) = add n m

Testing it:

```
GHCi> fromNatural (add (toNatural 2) (mul (toNatural 3) (toNatural 5)))
17
```

# Folding rose trees

Let's try one more structure, namely rose trees:

`data TreeF a self = Node a [self]`

type Tree a = Fix (TreeF a)

instance Functor (TreeF a) where

fmap f (Node v xs) = Node v (map f xs)

As before, `fmap`

applies a function to each `self`

-typed field. Since we have a list of these, we use `map`

.

We define some traversals:

`valuesF :: Tree a -> [a]`

valuesF = cata (\(Node x xs) -> x : concat xs)

depthF :: Tree a -> Integer

depthF = cata (\(Node _ xs) -> 1 + maximum (0:xs))

and test them:

```
GHCi> let n x = In . Node x
GHCi> let t1 = n 'a' []
GHCi> valuesF t1
"a"
GHCi> depthF t1
1
GHCi> let t2 = n 'a' [n 'b' [], n 'c' [n 'd' []], n 'e' []]
GHCi> valuesF t2
"abcde"
GHCi> depthF t2
3
```

# The definition of `cata`

As promised:

`cata :: (Functor f) => (f a -> a) -> Fix f -> a`

cata g = g . fmap (cata g) . out

# Generic programming

We usually manipulate data because we care about its *meaning*. But some operations only depend on the data's *structure*. If we have some complex nested data value, we might want to pretty-print it, or serialize it with binary, or collect all the `String`

s buried within. These are concepts which apply to any data type you like. Shouldn't our code be similarly adaptable?

This idea is called *generic programming*. Implementations in Haskell include syb, uniplate, multiplate, instant-generics, GHC's built-in support, and many other efforts.

Each of these libraries demands certain information about each data type, and in return provides some generic operations. In this view, our above development of `Fix`

constitutes a really tiny generics library. We require

- that you write recursive types as
`Fix F`

, where`F`

is non-recursive, and - that you provide
`instance Functor F`

.

In return, you get the single generic operation `cata`

, which reduces a recursive structure to a "summary" value, by replacing each constructor with an arbitrary computation.

Our toy generics library is simple, but it doesn't actually save much work. Using `Fix`

adds noise to the code, and our `Functor`

instances aren't much shorter than a custom fold function for each type. Conversely, our uses of `cata`

aren't much simpler than explicit recursive traversals.

Practicality aside, I think it's cool to define folding "once and for all". I'm interested to learn (from you!) what else can be done with `Fix`

'd data or similar techniques.

For a non-trivial generics library built around similar ideas, see multirec and the accompanying paper.

# Odds and ends

## Nomenclature

`Fix`

is sometimes named `Mu`

, because the Greek letter μ is used to represent least fixed points.

`cata`

stands for catamorphism.

Some of this stuff shows up in "Evolution of a Haskell Programmer". They build a factorial function using not only catamorphisms but also zygomorphisms and paramorphisms. Tell your friends!

In *TaPL* there's a discussion of equirecursive versus isorecursive type systems. Haskell is in the latter category. `Fix f`

is isomorphic to, but not equivalent to, `f (Fix f)`

. The functions `In`

and `out`

provide the isomorphism. If we defined `Fix`

using `data`

instead of `newtype`

, the isomorphism would fail because of the extra value `In ⊥`

.

## Isomorphisms

`NaturalF`

is isomorphic to `Maybe`

:

`data NaturalF self = Zero | Succ self`

data Maybe a = Nothing | Just a

The `Functor`

instances are also the same.

`NaturalF`

is also isomorphic to `ListF ()`

, ignoring undefined values.

## GHC tricks

While the above is standard Haskell, we can pull a few more tricks by using a couple of GHC extensions.

GHC could help us reduce boilerplate by deriving `Functor`

instances.

With some GHC extensions, we can write `Show`

for `Fix`

:

`instance (Show (f (Fix f))) => Show (Fix f) where`

showsPrec p (In x) = showParen (p >= 11) (showString "In " . showsPrec 11 x)

Testing it:

```
GHCi> toNatural 3
In (Succ (In (Succ (In (Succ (In Zero))))))
```

This requires `-XFlexibleContexts`

`-XUndecidableInstances`

. The latter is required because `f (Fix f)`

is a syntactically larger type than `Fix f`

, so GHC can't convince itself that instance resolution will definitely terminate. For well-behaved arguments to `Fix`

, it works out. We'll have something like

`-- from above`

instance (Show (f (Fix f)))

=> Show (Fix f)

-- from 'deriving (Show)' on NaturalF

instance (Show self)

=> Show (NaturalF self)

which instantiates to

`instance (Show (NaturalF (Fix NaturalF)))`

=> Show (Fix NaturalF)

instance (Show (Fix NaturalF))

=> Show (NaturalF (Fix NaturalF))

and GHC is clever enough to resolve the mutual recursion.

Remarkably, GHC can *derive* this instance with `-XStandaloneDeriving`

:

`deriving instance (Show (f (Fix f))) => Show (Fix f)`

That's how I got the precedence-correct code above, via `-ddump-deriv`

.