Inductive cycles

Recursive solving without cycles is easy. Solving with cycles is rather more complicated. Before we get into the details of the implementation, let's talk a bit about what behavior we actually expect in the face of possible cycles.

Inductive cycles

By default, Rust trait solving is inductive. What that means is that, roughly speaking, you have to prove something is true without any cycles (i.e., you can't say "it's true because it's true"!).

For our purpose, a "cycle" means that, in the course of proving some canonical goal G, we had to prove that same goal G again.

Consider this Rust program:

fn main() {
trait A { }
impl<T: A> A for Vec<T> { }
impl A for u32 { }

Whether or not we hit a cycle will depend on the goal we are trying to solve. If for example we are trying to prove Implemented(Vec<u32>: A), then we don't hit any cycle:

  • Implemented(Vec<u32>: A) :- Implemented(u32: A) // from the first impl
    • Implemented(u32: A) // from the second impl

But what if we are trying to prove Implemented(?X: A)? This is a bit more interesting. Because we don't know what ?X is, both impls are actually potentially applicable, so we wind up with two ways to prove our goal. We will try them out one after the other.

One possible execution might be:

  • Prove Implemented(?X: A)
    • we find the program clause forall<T> { Implemented(Vec<T>: A) :- Implemented(T: A) } from the first impl
      • we create the variable ?Y to represent T and unify ?X = Vec<?Y>.
      • after unification, we have the subgoal Implemented(?Y: A)
        • when we go to recursively prove this impl, however, we find that it is already on the stack
        • this is because the canonical form of Implemented(?X: A) and Implemented(?Y: A) is the same

What happens if we treat inductive cycles as errors?

So, what do we do when we hit an inductive cycle? Given that we told you that an inductive proof cannot contain cycles, you might imagine that we can just treat such a cycle as an error. But this won't give us the correct result.

Consider our previous example. If we just treat that cycle as an error, then we will conclude that the impl for Vec<T> doesn't apply to ?X: A, and we'll proceed to try the impl for u32. This will let us reason that ?X: A is provable if ?X = u32. This is, in fact, correct: ?X = u32 is a possible answer. The problem is, it's not the only one!

In fact, Implemented(?X: A) has an infinite number of answers. It is true for ?X = u32. It is true for ?X = Vec<u32>. It is also true for Vec<Vec<u32>> and Vec<Vec<Vec<u32>>> and so on.

Given this, the correct result for our query is actually "ambiguous" -- in particular, there is no unique substitution that we can give that would make the query provable.

How we solve cycles: loop and try again

The way we actually handle cycles is by iterating until we reach a fixed point (or ambiguity). We start out by assuming that all cycles are errors and we try to find some solution S. If we succeed, then we can do a loop and iterate again -- this time, for each cycle, we assume the result is S. This may yield some new solution, S1. The key point here is that we now have two possible solutions to the same goal, S and S1. This implies two possibilities:

  • If S == S1, then in fact there is a unique solution, so we can return Provable(S).
  • If S != S1, then we know there are two solutions, which means that there is not one unique solution, and hence the correct result is ambiguous, and in fact we can just stop and return right now.

This technique is very similar to the traditional Prolog technique of handling cycles, which is called tabling. The difference between our approach and tabling is that we are always looking for a unique solution, whereas Prolog (like the SLG solver) tries to enumerate all solutions (i.e., in Prolog, solving a goal is not a function but an iterator that yields solutions, and hence it would yield up S first, and then S1, and then any further answers we might get).

Intuitively, what is happening here is that we're building bigger and bigger "proof trees" (i.e., trees of impl applications). In the first iteration, where we assumed that all recursive calls were errors, we would find exactly one solution, u32: A -- this is the root tree. In the next iteration, we can use this result to build a tree for Vec<u32>: A and so forth.

Inductive cycles with no base case

It is interesting to look at what happens without the base case. Consider this program:

fn main() {
trait B { }
impl<T: B> B for Vec<T> { }

In this case, there is no base case -- this means that in fact there are no solutions at all to the query ?X: B. The reason is that the only type that could match would be a type of infinite size like Vec<Vec<Vec<...>>>: B, where the chain of Vec never terminates.

In our solver, this will work out just fine. We will wind up recursing and encountering a cycle. This will be treated as an error in the first iteration -- and then, at the end, we'll still have an error. This means that we've reached a fixed point, and we can stop.

Inductive cycles: when do we ever terminate

You might be wondering whether there are any examples of inductive cycles that actually terminate successfully and without ambiguity. In fact, there are very few, but you can construct an example like this:

fn main() {
trait C { }
impl<T: C + D> C for Vec<T> { }
impl C for u32 { }

trait D { }

In this case, the only valid result of Implemented(?X: C) is ?X = u32. It can't be Vec<u32> because Implemented(u32: D) is not true.

How does this work out with the recursive solver? In the first iteration, we wind up with ?X = u32, but we do encounter a cycle:

  • proving Implemented(?X: C) has two possibilities...
    • ?X = Vec<?Y> and Implemented(?Y: C), which is a cycle (error, at least in this iteration)
    • ?X = u32, succeeds

So then we try the next iteration:

  • proving Implemented(?X: C) has two possibilities...
    • ?X = Vec<?Y> and Implemented(?Y: C), which is a cycle, so we use our previous result of ?Y = u32
      • we then have to prove Implemented(u32: D), which fails
    • ?X = u32, succeeds