Testing whether an arbitrary proof is circular?

Question

I was thinking about proofs and ran into an interesting observation. So proofs are equivalent to programs via the Curry-Howard Isomorphism, and circular proofs correspond to infinite recursion. But we know from the halting problem that in general testing whether an arbitrary program recurses forever is undecidable. By Curry-Howard, does that mean there is no "proof checker" that can determine if a proof uses circular reasoning?

I've always thought that proofs are supposed to be composed of easily-checkable steps (which correspond to applications of inference rules), and checking all the steps gives you confidence that the conclusion follows. But now I'm wondering: maybe it is actually impossible to write such a proof checker, because there is no way for it to get around the halting problem and detect circular reasoning?

Answer 1

The vast majority of proof systems don't allow for infinite, circular proofs, but they do so by making their langauges non-Turing complete.

In a normal functional language, the only way to make a program go on forever is with recursion, and in terms of theory, usually we look at recursion as the $Y$ combinator, a program of type $\forall a \ldotp (a \to a) \to a$: that is, it takes a function which makes calls to some other "self" argument, and turns it into a single recursive function.

Now, apply the Curry-Howard isomorphism to this: we now have a proof that, for any proposition $a$, if $a$ implies itself, then we can prove $a$. We can prove anything this way!

The key here is that the Y-combinator is built-in to a language, it's taken as an axiom. So if you want it not to cause you problems, just get rid of it as an axiom!

Most formal proof systems, because of this, require your recursion to be well founded. They only accept functions that they can prove will halt. And as a result, they reject some programs that do halt, but which they can't prove it for.

Coq does this in a fairly limited way: it just requires that any recursive functions have an argument where any recursive calls only use strictly smaller versions of that argument. Agda does something similar, but with a little more fancy checking to accept a few more programs.