A Tour of Some Useful Recursive Types

(This article is also published at Medium)

I’m presently at NIPS and so felt like writing about some appropriate machine learning topic, but along the way I wound up talking about parameterized recursive types, and here we are. Enjoy!

One starts to see common ‘shapes’ in algebraic data types after working with them for a while. Take the natural numbers and a standard linked list, for example:

data Natural =
    One
  | Succ Natural

data List a =
    Empty
  | Cons a (List a)

These are similar in some sense. There are some differences - a list has an additional type parameter, and each recursive point in the list is tagged with a value of that type - but the nature of the recursion in each is the same. There is a single recursive point wrapped up in a single constructor, plus a single base case.

Consider a recursive type that is parameterized by a functor with kind ‘* -> *’, such that the kind of the resulting type is something like ‘(* -> *) -> *’ or ‘(* -> *) -> * -> *’ or so on. It’s interesting to look at the ‘shapes’ of some useful types like this and see what kind of similarities and differences in recursive structure that we can find.

In this article we’ll look at three such recursive types: ‘Fix’, ‘Free’, and ‘Cofree’. I’ll demonstrate that each can be viewed as a kind of program parameterized by some underlying instruction set.

Fix

To start, let’s review the famous fixed-point type ‘Fix’. I’ve talked about it before, but will go into a bit more detail here.

{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE UndecideableInstances #-}

newtype Fix f = Fix (f (Fix f))

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

Note: I’ll omit interpreter output for examples throughout this article, but feel free to try the code yourself in GHCi. I’ll post some gists at the bottom. The above code block also contains some pragmas that you can ignore; they’re just there to help GHC derive some instances for us.

Anyway. ‘Fix’ is in some sense a template recursive structure. It relies on some underlying functor ‘f’ to define the scope of recursion that you can expect a value with type ‘Fix f’ to support. There is the degenerate constant case, for example, which supports no recursion:

data DegenerateF r = DegenerateF
  deriving (Functor, Show)

type Degenerate = Fix DegenerateF

degenerate :: Degenerate
degenerate = Fix DegenerateF

Then you have the case like the one below, where only an infinitely recursive value exists:

newtype InfiniteF r = InfiniteF r
  deriving (Functor, Show)

type Infinite = Fix InfiniteF

infinite :: Infinite
infinite = Fix (InfiniteF infinite)

But in practice you’ll have something in between; a type with at least one recursive point or ‘running’ case and also at least one base or ‘terminating’ case. Take the natural numbers, for example:

data NatF r =
    OneF
  | SuccF r
  deriving (Functor, Show)

type Nat = Fix NatF

one :: Nat
one = Fix OneF

succ :: Nat -> Nat
succ = Fix . SuccF

Here ‘NatF’ provides both a ‘running’ case - ‘SuccF’ - and a ‘terminating’ case in - ‘OneF’. ‘Fix’ just lets ‘NatF’ do whatever it wants, having no say of its own about termination. In fact, we could have defined ‘Fix’ like this:

data Program f = Running (f (Program f))

Indeed, you can think of ‘Fix’ as defining a program that runs until ‘f’ decides to terminate. In turn, you can think of ‘f’ as an instruction set for the program. The whole shebang of ‘Fix f’ may only terminate if ‘f’ contains a terminating instruction.

Here’s a simple set of instructions, for example:

data Instruction r =
    Increment r
  | Decrement r
  | Terminate
  deriving (Functor, Show)

increment :: Program Instruction -> Program Instruction
increment = Running . Increment

decrement :: Program Instruction -> Program Instruction
decrement = Running . Decrement

terminate :: Program Instruction
terminate = Running Terminate

And we can write a sort of stack-based program like so:

program :: Program Instruction
program =
    increment
  . increment
  . decrement
  $ terminate

Richness of ‘Fix’

It’s worthwhile to review two functions that are useful for working with ‘Fix’, unimaginatively named ‘fix’ and ‘unfix’:

fix :: f (Fix f) -> Fix f
fix = Fix

unfix :: Fix f -> f (Fix f)
unfix (Fix f) = f

You can think of them like so: ‘fix’ embeds a value of type ‘f’ into a recursive structure by adding a new layer of recursion, while ‘unfix’ projects a value of type ‘f’ out of a recursive structure by peeling back a layer of recursion.

This is a pretty rich recursive structure - we have a guarantee that we can always embed into or project out of something with type ‘Fix f’, no matter what ‘f’ is.

Free

Next up is ‘Free’, which is really just ‘Fix’ with some added structure. It is defined as follows:

data Free f a =
    Free (f (Free f a))
  | Pure a
  deriving Functor

deriving instance (Show a, Show (f (Free f a))) => Show (Free f a)

The ‘Free’ constructor has an analogous definition to the ‘Fix’ constructor, meaning we can use ‘Free’ to implement the same things we did previously. Here are the natural numbers redux, for example:

type NatFree = Free NatF

oneFree :: NatFree a
oneFree = Free OneF

succFree :: NatFree a -> NatFree a
succFree = Free . SuccF

There’s also another branch here called ‘Pure’, though, that just bluntly wraps a value of type ‘a’, and has nothing to do with the parameter ‘f’. This has an interesting consequence: it means that ‘Free’ can have an opinion of its own about termination, regardless about what ‘f’ might decree:

type NotSoInfinite = Free InfiniteF

notSoInfinite :: NotSoInfinite ()
notSoInfinite = Free (InfiniteF (Free (InfiniteF (Pure ()))))

(Note that here I’ve returned the value of type unit when terminating under the ‘Pure’ branch, but you could pick whatever else you’d like.)

You’ll recall that ‘InfiniteF’ provides no terminating instruction, and left to its own devices will just recurse endlessly.

So: instead of being forced to choose a branch of the underlying functor to recurse on, ‘Free’ can just bail out on a whim and return some value wrapped up in ‘Pure’. We could have defined the whole type like this:

data Program f a =
    Running (f (Program f a))
  | Terminated a
  deriving Functor

Again, it’s ‘Fix’ with more structure. It’s a program that runs until ‘f’ decides to terminate, or that terminates and returns a value of type ‘a’

As a quick illustration, take our simple stack-based instruction set again. We can define the following embedded language terms:

increment :: Program Instruction a -> Program Instruction a
increment = Running . Increment

decrement :: Program Instruction a -> Program Instruction a
decrement = Running . Decrement

terminate :: Program Instruction a
terminate = Running Terminate

sigkill :: Program f Int
sigkill = Terminated 1

So note that ‘sigkill’ is independent of whatever instruction set we’re working with. We can thus write another simple program like before, except this time have ‘sigkill’ terminate it:

program :: Program Instruction Int
program =
    increment
  . increment
  . decrement
  $ sigkill

Richness of ‘Free’

Try to define the equivalent versions of ‘fix’ and ‘unfix’ for ‘Free’. The equivalent to ‘fix’ is easy:

free :: f (Free f a) -> Free f a
free = Free

You’ll hit a wall, though, if you want to implement the (total) analogue to ‘unfix’. One wants a function of type ‘Free f a -> f (Free f a)’, but the existence of the ‘Pure’ branch makes this impossible to implement totally. In general there is not going to be an ‘f’ to pluck out:

unfree :: Free f a -> f (Free f a)
unfree (Free f) = f
unfree (Pure a) = error "kaboom"

The recursion provided by ‘Free’ is thus a little less rich than that provided by ‘Fix’. With ‘Fix’ one can always project a value out of its recursive structure - but that’s not the case with ‘Free’.

It’s well-known that ‘Free’ is monadic, and indeed it’s usually called the ‘free monad’. The namesake ‘free’ comes from an algebraic definition; roughly, a free ‘foo’ is a ‘foo’ that satisfies the minimum possible constraints to make it a ‘foo’, and nothing else. Check out the slides from Dan Piponi’s excellent talk from Bayhac a few years back for a deeper dive on algebraic freeness.

Cofree

‘Cofree’ is also like ‘Fix’, but again with some extra structure. It can be defined as follows:

data Cofree f a = Cofree a (f (Cofree f a))
  deriving Functor

deriving instance (Show a, Show (f (Cofree f a))) => Show (Cofree f a)

Again, part of the definition - the second field of the ‘Cofree’ constructor - looks just like ‘Fix’. So predictably we can do a redux-redux of the natural numbers using ‘Cofree’:

type NatCofree = Cofree NatF

oneCofree :: NatCofree ()
oneCofree = Cofree () OneF

succFree :: NatCofree () -> NatCofree ()
succFree f = Cofree () (SuccF f)

(Note that here I’ve again used unit to fill in the first field - you could of course choose whatever you’d like.)

This looks a lot like ‘Free’, and in fact it’s the categorical dual of ‘Free’. Whereas ‘Free’ is a sum type with two branches, ‘Cofree’ is a product type with two fields. In the case of ‘Free’, we could have a program that either runs an instruction from a set ‘f’, or terminates with a value having type ‘a’. In the case of ‘Cofree’, we have a program that runs an instruction from a set ‘f’ and returns a value of type ‘a’.

A ‘Free’ value thus contains at most one recursive point wrapping the value with type ‘a’, while a ‘Cofree’ value contains potentially infinite recursive points - each one of which is tagged with a value of type ‘a’.

Rolling with the ‘Program’ analogy, we could have written this alternate definition for ‘Cofree’:

data Program f a = Program {
    annotation :: a
  , running    :: f (Program f a)
  } deriving Show

A ‘Cofree’ value is thus a program in which every instruction is annotated with a value of type ‘a’. This means that, unlike ‘Free’, it can’t have its own opinion on termination. Like ‘Fix’, it has to let ‘f’ decide how to do that.

We’ll use the stack-based instruction set example to wrap up. Here we can annotate instructions with progress about how many instructions remain to execute. First our new embedded language terms:

increment :: Program Instruction Int -> Program Instruction Int
increment p = Program (remaining p) (Increment p)

decrement :: Program Instruction Int -> Program Instruction Int
decrement p = Program (remaining p) (Decrement p)

terminate :: Program Instruction Int
terminate = Program 0 Terminate

Notice that two of these terms use a helper function ‘remaining’ that counts the number of instructions left in the program. It’s defined as follows:

remaining :: Program Instruction Int -> Int
remaining = loop where
  loop (Program a f) = case f of
    Increment p -> succ (loop p)
    Decrement p -> succ (loop p)
    Terminate   -> succ a

And we can write our toy program like so:

program :: Program Instruction Int
program =
    increment
  . increment
  . decrement
  $ terminate

Evaluate it in GHCi to see what the resulting value looks like.

Richness of ‘Cofree’

If you try and implement the ‘fix’ and ‘unfix’ analogues for ‘Cofree’ you’ll rapidly infer that we have the opposite situation to ‘Free’ here. Implementing the ‘unfix’ analogue is easy:

uncofree :: Cofree f a -> f (Cofree f a)
uncofree (Cofree _ f) = f

But implementing a total function corresponding to ‘fix’ is impossible - we can’t just come up with something of arbitrary type ‘a’ to tag an instruction ‘f’ with, so, like before, we can’t do any better than define something partially:

cofree :: f (Cofree f a) -> Cofree f a
cofree f = Cofree (error "kaboom") f

Just as how ‘Free’ forms a monad, ‘Cofree’ forms a comonad. It’s thus known as the ‘cofree comonad’, though I can’t claim to really have any idea what the algebraic notion of ‘cofreeness’ captures, exactly.

Wrapping Up

So: ‘Fix’, ‘Free’, and ‘Cofree’ all share a similar sort of recursive structure that make them useful for encoding programs, given some instruction set. And while their definitions are similar, ‘Fix’ supports the richest recursion of the three in some sense - it can both ‘embed’ things into and ‘project’ things out of its recursive structure, while ‘Free’ supports only embedding and ‘Cofree’ supports only projecting.

This has a practical implication: it means one can’t make use of certain recursion schemes for ‘Free’ and ‘Cofree’ in the same way that one can for ‘Fix’. There do exist analogues, but they’re sort of out-of-scope for this post.

I haven’t actually mentioned any truly practical uses of ‘Free’ and ‘Cofree’ here, but they’re wonderful things to keep in your toolkit if you’re doing any work with embedded languages, and I’ll likely write more about them in the future. In the meantime, Dave Laing wrote an excellent series of posts on ‘Free’ and ‘Cofree’ that are more than worth reading. They go into much more interesting detail than I’ve done here - in particular he details a nice pairing that exists between ‘Free’ and ‘Cofree’ (also discussed by Dan Piponi), plus a whack of examples.

You can also find industrial-strength infrastructure for both ‘Free’ and ‘Cofree’ in Edward Kmett’s excellent free library, and for ‘Fix’ in recursion-schemes.

I’ve dumped the code for this article into a few gists. Here’s one of everything excluding the running ‘Program’ examples, and here are the corresponding ‘Program’ examples for the Fix, Free, and Cofree cases respectively.

Thanks to Fredrik Olsen for review and great feedback.