Lecture 8: Polymorphism and System F

In Lecture 7 we saw how the simply typed lambda calculus, and type systems in general, can be powerful tools for guaranteeing safety properties about programs. Robin Milner tells us that "well typed programs cannot go wrong", and in the case of the lambda calculus, that meant that we could statically guarantee that a program could evaluate successfully.

While the simply typed lambda calculus is powerful, its type system—and the generalization of it to a real functional programming language—can often be inconvenient. Consider this combinator that applies a function to an argument twice:

double = λf. λx. f (f x)

What type should double have? In some sense, double is "generic"; as long as f is an abstraction that both takes as input and returns the same type that x has, double will be well typed. But our type systems thus far have had no way to express this idea of being "generic".

In this lecture we'll introduce the idea of polymorphism, a type system feature that allows a single piece of code to be used with multiple types. We'll see a few ad-hoc examples to build some intuition, and then introduce a particular polymorphic type system called System F for the lambda calculus.

Type variables

Let's consider double more concretely:

double = λf. λx. f (f x)

We can copy and paste this definition each time we want to use it with a different type, like this (imagining that we've extended the simply typed lambda calculus with Nats, which we didn't cover in Lecture 7 but follows the same idea as with Bool):

doubleBool = λf:(Bool -> Bool). λx:Bool. f (f x)
doubleNat = λf:(Nat -> Nat). λx:Nat. f (f x)

But this is a bit frustrating! It's also not great as a software engineering practice: it's just one piece of code, but repeated multiple times, which suggests we are missing some kind of abstraction.

One intuitive idea for solving this problem is to have some notion of a type variable. We could then define double like this:

double = λf:(T -> T). λx:T. f (f x)

But this doesn't quite work either. Consider this code:

if (double not true) then (double succ 1) else (double succ 2)

Is this expression well typed? Certainly it appears that way on the surface: not has type Bool -> Bool and true has type Bool, while succ has type Nat -> Nat and 1 and 2 have type Nat. But the two uses of double conflict: the first one requires that T must be Bool, while the second requires that T must be Nat.

What's missing from our idea of type variables is some way to separate them—to soundly instantiate them multiple times within the same program. Now, intuitively, this is not really a difficult idea: we somehow want "each use" of T to be allowed to be different, and as long as they are "locally" correct it's fine to give T a different value at each position. But implementing this idea turns out to be extremely challenging. In fact, people have gotten it wrong: Java's type system was recently discovered to be incorrect in exactly this way.

As PL people, we know that the answer to getting these sorts of problems right is to formalize what seems "obvious". Rather than this hand-waving notion of "type variables", we need a formal notion of polymorphism.

Varieties of polymorphism

There are actually quite a few different ideas bottled up into the same word "polymorphism":

• The idea we're going to study today is parametric polymorphism. This allows a single piece of code to have a "generic" type using type variables. The key feature of parametric polymorphism is that all instances of the generic piece of code have the same behavior, regardless of type. In the case of double, the function always does the same thing, it just does it to different types of values. You've seen parametric polymorphism before as generic functions in Java (also ML).
  • In particular, in this lecture we'll see a style of parametric polymorphism known as impredicative or first-class polymorphism.
  • There is also a more limited style of parametric polymorphism known as let-polymorphism or ML-style polymorphism. These restrict where type variables and generic functions can appear in a term, and in return get a simpler and decidable algorithm for type inference.
• Another form of polymorphism is ad-hoc polymorphism, which allows the same name to be used to refer to different functions. This appears to be polymorphism to the programmer, but in reality, it's just a dispatch mechanism: the types are used to select (either at compile time or run time) which of the different functions to invoke. An example of ad-hoc polymorphism you've probably seen before is operator overloading in C++ or Python: the + operator, for example, is just one "name", but there can be many different implementations of it, and the types are used to decide which implementation to execute.
• Object-oriented languages often have a notion of subtype polymorphism, which allows a single term to masquerade as multiple different types that are related by some kind of subtyping relation. Subtype polymorphism is what allows us to talk about "subclasses" (like Cats) and pass them around as if they were their superclass (like Animal).

These notions are easy to confuse with each other, and often we (somewhat inaccurately) say "polymorphism" to mean any or all of these ideas. For example, Java has always had subtype polymorphism (through classes and inheritance), and ad-hoc polymorphism (through method overloading), but did not have parametric polymorphism (generics) at release; those came later.

In the double example above, what we were really looking for was parametric polymorphism—we wanted to be able to use the same double code with multiple different (unrelated) types. So that's what we'll study today.

System F

System F is a parametrically polymorphic type system for the lambda calculus, first discovered by Jean-Yves Girard in 1972, and somewhat contemporaneously by John Reynolds in 1974. In the same way that the lambda calculus is the essence of computation, and the simply typed lambda calculus is the essence of type systems, so to is System F the essence of parametric polymorphism. It's a convenient vehicle for studying more complex polymorphism, and also the basis for numerous programming language designs.

The big idea is to abstract out the type of a term, and then instantiate the abstract term with concrete type annotations once we have them. We're going to bake this abstraction and instantiation into the language itself—we will be adding new constructs not just to the type system but to the lambda calculus itself. We'll do this in three parts: first, we'll add new polymorphic types to the type system syntax, then we'll add new polymorphic terms to the lambda calculus syntax and semantics, and then finally we'll add polymorphic typing rules to the type system semantics.

Polymorphic types

We can start by defining some new polymorphic types in our type system. We'll keep the same Bool and -> constructors from Lecture 7, but add two new constructors:

T := bool
   | T -> T
   | α
   | ∀α. T

Here, α is a type variable. The two new constructors give us a way to talk about generic types. For example, here's (almost; see the next section) the type of a generic identity function:

∀α. α -> α

We can read this as saying that for any type α, we can get a function that takes as input something of type α and returns something of type α. Similarly, here's (almost) a generic type for our double function from before:

∀α. (α -> α) -> α -> α

For any type α, it takes as input a function on αs and a base α and returns another α.

Type abstractions and applications

Recall that in the lambda calculus we called functions like λx. t abstractions. The idea was that the function abstracts a term t to work for any x, and we can get back the term t for a specific x by applying it. In parametric polymorphism, we similarly want some idea of abstracting out the type of a term, and applying the resulting abstraction to get back the concrete term. We're therefore going to add type abstractions and type applications to our lambda calculus.

Let's start with the syntax, which adds two new constructors for lambda calculus terms:

t := x
   | λx:T. t
   | t t
   | true
   | false
   | if t then t else t
   | Λα. t
   | t T

The first new constructor is the type abstraction Λα. t. Type abstractions take as input a type and have a term as their body. The second constructor is type application or type instantiation, which applies a type abstraction to get back a concrete term t in which the type variable has been instantiated with the given type T.

With this syntax, we can actually write generic functions. For example, this term is the generic identity function:

id = Λα. λx:α. x

To use this identity function, we first have to instantiate it with a concrete type. For example, the identity function for Bools is id Bool. The term id has type ∀α. α -> α, while the term id Bool has type Bool -> Bool. Similarly, we can now write the generic double function:

double = Λα. λf. λx. f (f x)

which has type ∀α. (α -> α) -> α -> α. As a concrete instantiation, double Nat has type (Nat -> Nat) -> Nat -> Nat.

Of course, since we've added new syntax, we need to add new semantics too. There is nothing too surprising here: the rules are the same as for abstractions and applications, except that the right-hand side T of an application can't step because it needs to be a literal type. That means that, in some sense, this semantics is automatically "call by value". $$ \frac{t_1 \rightarrow t_1'}{t_1 \: T \rightarrow t_1' \: T} \: \textrm{STAppLeft} $$ $$ \frac{}{(\Lambda\alpha. \: t) \: T \rightarrow t[\alpha \mapsto T]} \: \textrm{STApp} $$

One point to note here is that our substitution notion is slightly different, and we're being lazy with syntax. In $\textrm{STApp}$ we're substituting a type into a term, which is different to the term-into-term substitution we've been doing before. The only place this type-into-term substitution happens is in the type annotations of abstractions: for an abstraction λx:α. x, if α is the type variable we are substituting for, then we need to replace it with the concrete type T.

Polymorphic typing rules

Finally, how do we decide when a term in our new language has a type in our new type system? We need to extend our typing judgment to include rules for the type abstraction and type application terms. Here's the judgment for type abstractions: $$ \frac{\Gamma, \alpha \: \vdash \: t \: : \: T} {\Gamma \vdash \Lambda \alpha. \: t \: : \: \forall \alpha. T} \: \textrm{TTAbs} $$ One little thing to note here is that we need to extend our typing context $\Gamma$ to include the type variable $\alpha$ that is newly in scope. But unlike the rule for typing regular abstractions, we don't need to remember anything about $\alpha$, because type variables are only used in System F for universal types ∀α. T. In other words, the actual type assigned to $\alpha$ is irrelevant to the typing judgment. We're remembering $\alpha$ in the context only to do capture-avoiding substitution if type variables are reused.

Finally, here's the judgment for type applications: $$ \frac{\Gamma \: \vdash \: t_1 \: : \: \forall \alpha. T_{12}} {\Gamma \: \vdash \: t_1 \: T_2 \: : \: T_{12}[\alpha \mapsto T_2]} \: \textrm{TTApp} $$ Here we have yet another form of substitution in the conclusion! $T_{12}[\alpha \mapsto T_2]$ is doing type-in-type substitution: in the type $T_{12}$, replace any instances of type variable $\alpha$ with the type $T_2$. Hopefully it now makes sense why we spent so much time talking about substitution in Lecture 6—it comes up everywhere!

Programming with System F

As we've hinted at a few times, System F lets us write truly generic types for some of the functions we've seen before. For example, let's validate that id Bool has type Bool -> Bool, as we hoped for, where id = Λα. λx:α. x: $$ \frac{ \Large \frac{ \LARGE \frac{ \huge \frac{x:\alpha \, \in \, \alpha, x:\alpha} {\alpha, x:\alpha \: \vdash \: x \: : \: \alpha} \textrm{TVar} }{\alpha \: \vdash \: \lambda x:\alpha. \: x \: : \: \alpha \rightarrow \alpha} \: {\Large \textrm{TAbs}} }{\vdash \: \texttt{id} \: : \: \forall \alpha. \: \alpha \rightarrow \alpha} \: {\small \textrm{TTAbs}} } {\vdash \: \texttt{id Bool} \: : \: \texttt{Bool} \rightarrow \texttt{Bool}} \: \textrm{TTApp} $$ The same proof structure will tell us that id Nat will have type Nat -> Nat, and so on for any other type.

One thing we can do with our new polymorphism is give types to the Church encodings we saw before. The type CBool of Church booleans (to distinguish them from the Bool we've baked into our calculus) is ∀α. α -> α -> α: A Church boolean takes two arguments of the same type and returns one of them. As another example, CNat is really just the type ∀α. (α -> α) -> α -> α: a Nat in the Church encoding takes as input a function and a base term, and returns the same type. Then, for example, 1 is the term Λα. λf:α->α. λx:α. f x.

A natural question might be: can we give a type to the Y combinator for general recursion? Unfortunately, the answer is no: all well-typed programs in System F terminate. This property of a type system—that well-typed programs terminate—is called normalization. The proof of this is very difficult—it was one of the major parts of Girard's PhD thesis.

Properties

As with the simply typed lambda calculus, we can prove that System F is sound, in the sense that well-typed programs cannot get stuck. The proof via syntactic type safety is very close to the one we saw last lecture: we prove progress and preservation, and those compose to give type safety.

Type erasure

One thing you might find unusual or inelegant about System F is that it extends the syntax of the base language itself. There are two ways to look at this quirk. One is that it's not too different to how generics make their way into other languages you've seen: those show up in the syntax too, when we write things like LinkedList<T>. Under the hood, we have some idea that the compiler or runtime is "instantiating" these type abstractions with the right concrete types when we try to use them. This isn't too different to the type abstractions and applications we have in System F.

Another way to look at it is that all these type annotations and abstractions can be erased from the program if we want to—they don't really have any effect on the behavior of the program; they exist only to extend the strength of our well-typed guarantees.

The type erasure $\operatorname{erase}(t)$ of a term t just replaces any type abstraction Λα. t with its body t, and deletes the type annotation T from any abstraction λx:T. t. We're left with a lambda calculus term that is functionally equivalent, which we can capture in an adequacy theorem:

Theorem (adequacy): $t \rightarrow t'$ if and only if $\operatorname{erase}(t) \rightarrow \operatorname{erase}(t')$.

A natural question to ask about type erasure is: if we can delete the types from a typed term, is there also a way to add the types to an untyped term? We say that an untyped term $m$ is typable if there exists some well-typed term $t$ such that $\operatorname{erase}(t) = m$. The question of deciding whether a term is typable is the type reconstruction problem. We'll study this problem in more detail in a later lecture, but for now, the answer for System F is not quite: type reconstruction for System F is undecidable, although this was an open problem until 1994. This is the motivation for various restrictions of System F, such as let polymorphism.

Parametricity

Finally, let's take a brief look at a surprising and interesting property of programs that are parametrically polymorphic.

There are many different functions that have type Bool -> Bool. Here are a few:

λx:Bool. x 
λx:Bool. true
λx:Bool. false
λx:Bool. if x then false else true
...

These are (mostly) all different functions, in the sense that they evaluate to (two) different values. More generally, there are many functions of type Bool -> (Bool -> Bool), etc.

By contrast, try to write down a list of all the functions that have type ∀α. α -> α. There is only one: Λα. λx:α. x. Every program of type ∀α. α -> α must behave identically to this one. This is very surprising! Informally, it says that if you give me only a parametrically polymorphic type, I can tell you quite a bit about the behavior of any term of that type. Intuitively, the idea is that because the term with type ∀α. α -> α is polymorphic in α, it can't do anything interesting with its argument: whatever it wants to do needs to work for every possible type α, and the lambda calculus is so simple that the only such thing it can do is return the argument.

This phenomenon is called parametricity: the idea that polymorphic terms behave uniformly on their type variables. Note that it depends very centrally on how simple the lambda calculus is: language features like reflection break parametricity.

As another example, consider the type CBool = ∀α. α -> α -> α we saw before. There are only two terms that have this type (up to $\alpha$-equivalence): Λα. λx:α. λy:α. x and Λα. λx:α. λy:α. y. We can figure this out rather mechanically, again because any term with this type can't do anything except return one of the two αs it has access to: it has no way to "invent" another α because it has no idea what α is. These two terms are exactly the terms true and false.