# Polymorphism

In the last chapter, we discussed some constructions on types, namely the product, sum, as well as two particular types Unit and Void. A product type was equivalent for asking for both types, while a sum type was equivalent to asking for either type. We saw that Unit, when combined with a product did nothing (because you always have one if you need it), and we saw the same thing when combining Void in a sum type (because you can never have one, by definition).

Today, we want to cover the last thing we’ll need in our typesystem for quite some time, and that’s a more general notion of what it means for something to be polymorphic. Recall that when we were discussing multiwires, our annotations could be polymorphic, which was understood to mean, “we don’t really care what shape wire you try to plug into here.”

Such a thing proved to be a useful notion – it allowed us to build machines like Snap* and Demux – machines which performed a “semantic” operation in the sense of “doing something useful for our purposes”. They were abstract enough that we didn’t really care what the low-level inputs and outputs were, because we were using these machines to serve a particular purpose we had in mind. Because we knew how to create a device which would take a snapshot of 2 wires, and one which would take a snapshot of 3 wires, and one which would take a snapshot of 4 wires, and so on and so forth, we could bundle all of that knowledge up into one “schema” diagram and give you instructions for building a machine that would take a snapshot of however many wires you so desired. Instead of giving you machines for building circuits directly, we were giving you blueprints to build machines to build circuits.

By discussing a less specific (polymorphic) machine, we found that we actually had a more useful tool to work with. Because it was less specific, it worked in more situations, at the expense of making us think a little harder about what it all means. This strikes me as being a fantastic trade-off, because it’s sort of the definition of “work smarter, not harder”. We spent some time looking at patterns in the things we were painstakingly building, and we came up with a better way of doing it. It saved us effort in the long run, which saved us time, which not only is money, but also time that you now have to reinvest in making smarter tools to save more time to make more money to invest harder in all of this stuff.

Deliciously vicious!

## Value Polymorphism

There are two (related) kinds of polymorphism we need to discuss in this chapter. The first, for lack of a better term, we will call value-level polymorphism, which is the kind of polymorphism we are familiar with from our experiences with multiwires.

An example of this was in our Snap* machine, which you might remember looked like this:

An earnest (if naive) attempt to encode this machine as a symbolic computation might look something like this:

snap* : Bool -> n -> n

Here, snap* has no opinion on what type n must be, other than that the input to this machine sure as hell better be the same as the output of this machine.

How do we know that n is polymorphic, here? Well, we decide by fiat (and a matter of convention) that any “type” which starts with a lowercase letter is polymorphic. That means n, z, cat, machinery and zzyzxyz are all polymorphic types, and moreover, if we had a machine:

silly : n -> z -> cat -> machinery -> zzyzxyz

we’d know that all of these polymorphic types could (but need not) be different types. Recall that if a single machine refers to the same polymorphic type more than once, we must use the same type to fill in all of the “holes” for that polymorphic type.

With that one rule in mind, we can substitute types at will into silly. The following are both valid type signatures for silly:

• silly : Void -> Unit -> Bool -> Strange -> Something
• silly : Unit -> Unit -> Unit -> Unit -> Unit

Of course, there are infinity different ways you could fill in these polymorphic types, so this is not an exhaustive list. Go nuts!

These things we’ve been referring to as “polymorphic types” are more often known as type variables. t is a type variable, and it is distinct from x. Furthermore the type variable t is distinct from all other type variables that are not named t. This argument holds in generality, for any name you can think of to call your type variable.

Just so we really and truly drive the point home, given a machine with polymorphic type variables:

odd : a -> a

then it’s perfectly fine to “instantiate” odd with the following signatures:

• odd : Bool -> Bool
• odd : Void -> Void
• odd : (Unit, Bool) -> (Unit, Bool)

but the following are not well-typed instantiations of odd:

• odd : Bool -> Void
• odd : Unit -> (Unit, Bool)

because we must replace all as in odd with the same type.

Sorry for stressing that point so so much, but it really and truly is very important.

## Type Polymorphism

The other sort of polymorphism we need to discuss is “type polymorphism”, which is really just the same idea as value polymorphism, except in a different place. Behold:

type Either a b = Left  a
| Right b

This should be read as “Either is a type constructor which takes two types, a and b, both polymorphic. The values of Either a b are either tagged with a Left if they are an a, or with a Right if they are a b.”

Quite a mouthful, but this is still just the same idea. We call Either a type constructor because it must be given types before it “gives you back” a type. What does that mean? That means that Either is not a type by itself, but Either Unit Bool is a type.

You can thin of each type variable on the left of a type equality as being a “hole”, which need to be plugged before you can do anything useful with the type constructor. Type constructors follow the same rules as value-polymorphism – you have to replace a type variable with the same type everywhere. You get the spiel by now, so I’ll stop lecturing you on it.

A point to notice is that anything after the type name, but to the left of the equals sign in a type definition must be a type variable. What does that mean? It means that it’s meaningless to say things like type Bad Unit = What. Unit is a type, not a type variable, so it has no meaning to be put on the left side of the equals sign. Again, common-sense stuff, but it bears mentioning.

We’ll keep this Either a b thing around from here on out, it turns out that all sum types can be generated by nesting Either a b inside of itself and instantiating the type variables with whatever you please. In a sense, Either a b is the “universal” sum type, because it allows us to do this.

Point of order: If you’re filling out an Either a b, and you want to fill the first type variable itself with Either a b, now you have something of the form Either (Either a b) c, not Either (Either a b) b. The two eithers get different type variables. You can avoid this problem if you replace all of the type variables in a type constructor at the same time, or by just remembering this fact. Type variables only need to be consistent within a single instance of looking at a type constructor, not when composing types together.

You might be wondering now if there’s a “universal” product type, and indeed, there is. We call it (a, b), where – you guessed it – a and b are both type variables.

## Doing Some Type-Foo

Now that we’re equipped with polymorphic types, we are capable of defining a few types which will make our lives much easier. The only one we’ll look at today is the Maybe a type, defined like this:

type Maybe a = Just a
| Nothing

What is this crazy thing?” you are probably asking yourself. What indeed? Maybe a can be thought of as “an a which may or may not be there.” Consider the following values of Maybe Bool:

• Just On : Maybe Bool
• Just Off : Maybe Bool
• Nothing : Maybe Bool

The Just tag tells us that we do, in fact, have a Bool hiding inside of our Maybe Bool. However, if instead we have a Nothing we know that we do not have a Bool (because there is nowhere to put it, all we have is Nothing!).

But, why is this thing useful? And finally, for the first time in this book, I can say “it actually is!” Finally! Remember, when we were defining our Mem machine:

we wanted to be able to choose between Reading and Writing, so we needed a wire for that. But we also only wanted this machine to output the value at its Destination if we wanted to read things. We solved this problem by adding a Go wire, whose only job it was was to tell us when to actually do something.

If we had Maybe a available to us back then, we could have gotten rid of G entirely, and instead just made RW : Maybe Bool, where a value of Nothing represented the fact that we didn’t want to read or write. That’s exactly the semantics we wanted, but we didn’t have a great way to do it, so we did something inelegant and cheated by adding that G wire in.

(where, of course, :1 is the multiwire annotation for a single wire – ie. it is just Bool in disguise.)

Perhaps now you see why all of this type stuff might be useful – it allows us to specify more exact and, perhaps more importantly, more elegant semantics for the tools we’re trying to build.

## Something More Natural

Before we conclude this chapter, there’s one more interesting type we should look at before getting back to our humdrum task of building a device capable of computing for us.

One type that is conspicuously missing from our pantheon is that of numbers. Sure, we can kind of cheat by representing a binary number in terms of a product type with lots of Bools, but that is inelegant, and worse, has a “largest number” representable (eg. the largest number you can represent with (Bool, Bool) is 3).

No, we want something that is closer to the numbers that we work with everyday, not some well-argued thing that “a bunch of wires beside one another is just as good as having numbers.” We want real numbers, if for no other reason than to feel impressed with ourselves for having done it. As an added bonus, the construction of numbers is actually quite interesting. Without further ado:

type Nat = Zero
| S Nat

Such a type is called Nat because it is equivalent to the natural numbers, which is to say, the numbers that you can count with. 0, 1, 2, 3, and so on. Notably, there are no “negative” natural numbers, nor are there any “fractional” natural numbers. That means $$-5$$, $$0.3$$ and $$\frac{8}{7}$$ are all out. Anything else though, anything else is fair game.

But how does this Nat thing work? Well, a natural number is either Zero, or it is the Successor to some other Natural number. Let’s look at a few values of Nat so you get the hang of out:

• $0 =$ Zero : Nat
• $1 =$ S Zero : Nat
• $2 =$ S (S Zero) : Nat
• $3 =$ S (S (S Zero)) : Nat

In general, if you want to represent a number as a Nat, you just need to stick that many Ss in front of a Zero, and make sure you wrap your parentheses properly. If you want to decipher what number a value of type Nat actually refers to, you just need to count how many Ss make it up.

As a matter of fact, this Nat type is exactly equivalent to how counting works. Such a fact was noticed by a guy named Giuseppe Peano, and so sometimes you’ll see Nat referred to as peano arithmetic. Neat, right?

However, needing to count a potentially huge number of Ss in order to determine what number we’re describing sounds like it would get pretty tedious pretty quickly. And so, as a convention, we allow ourselves to write a number in its natural decimal form, and just say that that numeral is just a short-hand for the peano construction of the same number.

All of this boils down to the fact that we can say 7 : Nat as being completely equivalent to S (S (S (S (S (S (S Zero)))))) : Nat. It’s a good convention, and one that we should all celebrate.

In the next chapter, we’ll actually get back to business towards building our computer. And we’ll bring our fancy new types along to help.

## Exercises

1. How many values are there of type Maybe (Bool, Either Bool Bool)?
2. Is x : Bool -> Bool -> Bool -> Unit a valid instance of the polymorphic machine x : a -> b -> a -> c?
3. Write out the peano representation of Just 4 : Maybe Nat. Don’t forget the Maybe part!
4. Define your own type that has exactly 5 values. Use only Bool, Either a b and Maybe a.