λ. Aaron Weiss
Reasoning with Types in Rust

Rust is a modern programming language which is marketed primarily on the basis of its very nice type system, and I’d like to tell you about how you can use this type system to reason about your programs in interesting ways. Most of the time when its type system is discussed, the focus is on its guarantee of data race freedom and ability to enable so-called fearless concurrency (and rightfully so—this is a place where Rust truly shines!). Today, I have a different focus in mind, characterized perhaps most succinctly as follows:

From the type of a polymorphic function we can derive a theorem that it satisfies. Every function of the same type satisfies the same theorem. This provides a free source of useful theorems.

Philip Wadler, Theorems for Free!

If you’re not the most mathematically inclined, don’t be scared off by the word theorem! The quote is telling us that—with the right property of our type system—we can learn useful properties about generic (i.e. polymorphic) functions solely by inspecting their types. In the rest of this post, we’ll cover this type system property, and a number of example properties we can derive from types as as result.1 Much of what’s covered can be generalized to languages aside from Rust, but (most) examples will be in Rust with Rust-specific aspects highlighted.

A Principal Property for Reasoning

The property at the heart of this style of type-based reasoning with generics is known as parametricity. Parametricity can be formulated as a mathematical theorem,2 but it’s best thought of intuitively as the notion that all instances of a polymorphic function act the same way. With this intuition in mind, you can imagine determining whether or not a particular function is parametric. For example, we can determine that the following Java function is parametric:

And that the following almost-Java function is not:3

The reason for this is that the latter function has chosen to specialize its behavior based on the type of its parameter, rather than acting the same on all types. This cuts to the essence of parametricity: to write parametric functions, we must treat parametric types opaquely! While Java does not enforce parametricity (and in fact often encourages otherwise), other type systems like that of Haskell and Rust require all functions to be parametric.4 When all polymorphic functions are parametric, the type system is said to be parametrically polymorphic—though in practice, many parametrically polymorphic type systems support some degree of ad hoc (that is, type-dependent polymorphism). In this case, we know that all polymorphic functions are parametric and we’re able to learn some of their properties solely from their type. So, let’s look at some examples in Rust.

Who am I? or: Reasoning about Identity

Consider the following function type, and try to imagine as many implementations as possible:

This type describes a function that for any type T, takes an argument of type T and returns a result of type T. If you’re already familiar with Rust, I’m sure it wouldn’t take long to come up with the following implementation, the identity function:

In fact, since there are no operations we can actually perform on x, it’s the only possible return value for this function. Of course, since Rust is effectful, we could print something before we return like so:

And Rust is also partial, meaning we could error (called panicking in Rust) or otherwise diverge:

These various implementations all tell us something about what the type means, which we can phrase like so:

A function of type fn<T>(T) -> T must:

  • return its argument or
  • panic or abort or
  • never return

Additionally, since we still know nothing about the type T, we can conclude that any effects that occur during the function are not dependent on the argument. With these two properties, we can then conclude the more general properties that functions of the type fn<T>(T) -> T behave “like an identity function”:

Given a function id of type fn<T>(T) -> T, a total function f of the form fn(A) -> B where A and B are both concrete types, and a value a of type A, then either:

  • id can be composed arbitrarily (e.g. id(f(a)) = f(id(a))) or
  • id(f(a)) and f(id(a)) both panic or diverge.

In order to conclude this, we can consider each of the cases we previously described. If the function returns its argument, then we know both that id(a) = a and id(f(a)) = f(a) and we can combine these two equalities to conclude the first result. If the function does not return its arguments, we know it either panics or never returns but we also know that this cannot be dependent on the argument in any way. Thus if id(f(a)) panics, then f(id(a)) must panic as well.

With that, we’ve intuited (but have not formally proven)5 our first “useful theorem” about a family of functions based solely on their type. While it’s nice to know that identity-looking functions behave like an identity function, there’s certainly nothing earth-shattering about the result. But the fact that we can apply this style of reasoning to every type ought to be compelling.

Vectors Abound

Let’s look at a slightly more complicated type now, involving Rust’s Vec<T> type for dynamically-sized buffers. We’ll again follow the same formula of enumerating some possible implementations before trying to conclude a general property. Given the type:

We can come up with implementations such as:

We can then try to capture a sense of what this type means as we did before:

A function m (for mystery) of type fn<T>(Vec<T>) -> Vec<T> must:

  • return a Vec<T> that contains a subset of the contents of its argument Vec<T> in any order. (i.e. ∀v. {e | e ∈ m(v)}{e | e ∈ v}) or
  • panic or abort or
  • never return

The process of concluding this is more complicated, but the general gist is that such a function can only perform the operations defined on Vec<T> and as usual cannot inspect the types of its elements. From there, we know that we cannot create new values of type T or perform any operations dependent on values within the vector. This also leverages the Rust-specific fact that values (in this case, of type T) cannot be copied without knowing that they implement Clone and/or Copy (whereas in other languages with parametricity, this typically is not the case). We can then conclude that all functions at this type must yield a permutation (or possibly a subset of a permutation) of the input vector. Of course, the same exceptions about panics and divergence apply. Interestingly, we can reach a similar general conclusion to the one we reached for fn<T>(T) -> T:

Given a function m of type fn<T>(Vec<T>) -> Vec<T>, a total function f of the form fn(A) -> B where A and B are both concrete types, and a is a value of type Vec<A>, then either:

  • mystery(map_f(a)) = map_f(mystery(a)) where map_f is defined as |x| { x.iter().map(f).collect() } or
  • at least one of mystery(map_f(a)) and map_f(mystery(a)) panic or diverge.

Noninterference for Free

Thus far, we’ve looked at rather simple properties of programs because it is easier to imagine the proof in your head. But now, let’s take the opportunity to explore a security property called noninterference for which a number of tailored type systems have been built. The idea behind these type systems is typically that you annotate types and values in your program with labels indicating whether a value should be public or secret (some systems expand this with further labels, but just the two are enough for the basics). Noninterference then says that functions with public output cannot depend on private inputs. Fortunately, using parametricity, we can have this property for free in Rust!6

To do so, first, we have to define a notion of secret (we’ll treat all unannotated types as public, though we could choose to introduce a public type as well for symmetry):

Strictly speaking, we’ve now achieved noninterference! That was probably easier than you expected, but the intuition should be clear: since we can perform no operations whatsoever on values of the type Secret<T>, it is impossible for public outputs to depend on secret data! However, there is a caveat: because of how access modifiers work in Rust, code in the same module can violate noninterference like so:

To avoid this, we can place our implementation of secret types inside of its own module with no additional code:

Now, we have noninterference enforced in any downstream code, but in real security type systems, you can still use secret values to compute other secret values. To do this, we can use Rust’s trait system to add common functionality. We can use this to define a lot of operations, but some of the operator-overloading traits (std::ops) are not currently general enough making some code less pleasant.7 Here is our example with some ability to use secret values to compute other secret values:

Now, we have some ways of using our secret data to construct other secret data. It’s limited, but many other extensions should follow similar patterns and we could also add other operations implemented directly on Secret<T> types that compose secret values without going through a trait like so:

With all these extensions, the argument that parametricity is still enforcing noninterference is now dependent on the exact set of operations that have been implemented for Secret<T>, but as long as they always return an argument of the form Secret<T>, Rust will enforce noninterference. We can even include operations that combine Secret<T> and T as long as their results are themselves secret. We could even imagine building a simple static analysis tool that runs atop Rust to audit a crate providing such a secret type to ensure that every function it implements returns a secret marked type.

Bountiful Properties with Bounded Parametricity

Though we used traits to extend the functionality of our Secret<T> type, they played a somewhat limited role in our argument for noninterference via parametricity, but we can do more. Fundamentally, traits allow us to bound type parameters with a specific interface that can be used within functions. This allow us to weaken our notion of parametricity from type parameters and values at those types being completely opaque to values at those types being usable in a controlled fashion. Correspondingly, we can derive even more interesting properties from the types. For a simple example, consider this extended version of our original identity example:

Previously, we said that any side-effects of this function could not depend on the argument. By adding the Display bound on T, we’ve allowed the argument to be displayed in output effects like println!. In a sense, this new ability to display the argument is expanding the allowed set of side-effects. This expansion is most evident from the fact that all of our old implementations are still legal at this bounded type, but new implementations are also legal. For example:

You may have noticed as we went through our earlier noninterference example that this property seems almost useless by virtue of being overly strict. In particular, since public outputs cannot depend on secret values in any way, there’s really no reason to use secret values at all. In practice, security type systems offer escape hatches (much like Rust’s unsafe) to selectively reveal secret information in a way that is readily auditable. With traits, we can build a principled escape hatch giving us a weakened property known as relaxed noninterference.8 Relaxed noninterference can be understood intuitively as the property that public outputs can only depend on secret values according to predetermined rules known as declassification policies.

In our formulation in Rust, we will record these policies as traits and use trait bounds to decide what policies are available within a function. Consequently, the type signatures of our functions will necessarily have to tell us how they plan on using the secret data we give them giving us strong, local reasoning principles for security. At the heart of this approach is our previous definition of Secret<T> with a trait representing the empty declassification policy:

Our private module here is used to seal the Sec<T> trait preventing it from being implemented on any additional types beyond Secret<T>. With just this, we can now specify functions like before that have noninterference:

We can then specify a number of declassification policies that enable us to make selective use of our secret values:

And then we can use these declassification policies to discern legal implementations of specific types as we’ve done before. Consider the type:

We know that there are some trivial implementations (e.g. comparing the u64 against 0) that don’t make use of the secret value, but what about implementations that do? We can come up with something like:

Now, if we connected this to a web framework (like the amazing Rocket), we could imagine having our forms always providing passwords as secret values. Then, by using traits as declassification policies, we can use the type system to ensure that we never accidentally misuse the password. However, we should be wary: we used Hash in this example because it’s provided by std and includes already-implemented hash algorithms, but it’s actually overly-permissive for this purpose. We could write a custom hasher that would allow us to leak information or even completely reveal the value. For a real implementation, we would instead provide a more constrained trait that allows you to compute a specific cryptographic hash such as bcrypt or scrypt.

Some Final Words

If you’ve made it this far, you’ve seen a bunch of “crazy academic concepts” like parametricity, free theorems, and noninterference. You’ve also seen how traits can be used to relax parametricity and give us even more useful free theorems. Hopefully, this endeavor has convinced you of the strength of type-based reasoning in Rust. The small examples that you’ve seen throughout the post are really just scratching the surface of this kind of reasoning: we can go further by using the added constraints from the ownership system to produce even more interesting theorems (such as that a cryptographic nonce is only used once). The extent of these reasoning capabilities is one of my personal favorite features of strong type systems, and subsequently one of my favorite things about Rust. Maybe it’ll be one of yours now too!


  1. Owing to their presentation in the paper Theorems for Free!, these properties are known in the academic world as free theorems—though I suspect that some will be unhappy with my liberal application of this term to intuited properties.

  2. And indeed, it was originally presented as the abstraction theorem in John C. Reynolds’ Types, abstraction, and parametric polymorphism.

  3. Actual Java does not allow the cast from Integer to T as such, but there are more complicated examples involving subtyping that can produce similar specialized-by-type behavior. This simpler example nevertheless captures the essence of non-parametric functions.

  4. Strictly speaking, Haskell’s seq breaks general parametricity, as do Rust’s various reflection capabilities (including Sized) and the upcoming impl specialization feature. Fortunately, like in the Haskell paper, we can always refine our notion of parametricity. Though this does have some consequences for precisely what properties you can glean from types.

  5. Though our argument is somewhat proofy, we would require a formal semantics for Rust. There exists one in the form of RustBelt, and as part of my research, I hope to produce an alternative formal backing for these free theorems in Rust, particularly the latter ones related to security.

  6. A connection between parametricity and noninterference was commonly held wisdom in the programming languages community, but was not proven until Bowman and Ahmed’s Noninterference for Free.

  7. The consequence of this is that we would need to define methods instead of operators which would make secret code look weirder and be less ergonomic, but is not a fundamental limitation to this approach. If the trait definitions were made more general, this would be a nonissue, and we could use macros instead to offer some improvements.

  8. In Type Abstraction for Relaxed Noninterference, we see a related presentation of relaxed noninterference as a consequence of object-oriented type abstraction capabilities. Since Rust uses parametric polymorphism with traits for type abstraction, we are developing an analogue here.

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