[pron]

A property P is compositional with respect to terms A and B and composition ∘ if P(A) ∧ P(B) ⇔ P(A∘B); the equivalence is sometimes replaced with implication in one of the directions (this is a special case for a predicate, i.e. a boolean-valued function; a function F is compositional if F(A∘B) = F(A) ⋆ F(B)).

What you're saying is that proofs can be composed. That is true, but unhelpful, as it doesn't make proofs easy. All proofs work this way. Compositionality does (in fact, all properties that are easily proven by type inference are compositional; type safety itself is compositional). I've seen somewhere compositionality described as the opposite of emergence[1], which makes sense. A property is emergent in a composition if it is not a property of the components.

[1]: https://julesh.com/2017/04/22/on-compositionality/

[ghettoimp]

Thanks, this seems like a perfectly coherent definition of compositional. I think my earlier confusion was a purely terminological matter.

Regarding "What you're saying is that proofs can be composed. That is true, but unhelpful, as it doesn't make proofs easy." I agree that the ability to compose proofs does not make proof easy. But I would push back somewhat against the idea that this is unhelpful.

The ability to prove, in isolation, the relevant properties about some entity (e.g., a function, a hardware module, or what have you), and then to make use of these properties, without ever looking inside the entity again, to prove properties of some larger system, seems to me to be absolutely essential for scaling proofs to large systems.

This ability is of course completely common in tools like interactive proof assistants, but is less common in more automated procedures.

[pron]

> The ability to prove, in isolation, the relevant properties about some entity (e.g., a function, a hardware module, or what have you), and then to make use of these properties, without ever looking inside the entity again, to prove properties of some larger system, seems to me to be absolutely essential for scaling proofs to large systems.

I understand the motivation, and of course you're right that it would be "essential", it's just that it has been mathematically proven (see my post) that this is not the case. Results in computational complexity theory, that studies how hard it is to do things, show that "making use of these properties, without ever looking inside the entity again" does not, in fact, make verification easier. More precisely, the difficulty of proving a property of A_1 ∘ ... ∘ A_n is not any function of n, because we cannot hide the inner details of the components in a way that would make the effort lower even though that is what we want to do and even though it is essential for success.

In practice, we have not been able to scale proofs to large systems, which means that maybe there is some truth to those mathematical results.