(Then they seem to also have added their own kind of dependency management thing to update components, which seems to me to kind of defeat the purpose..)
You also run into trouble if your language has side effects (ie, almost all of them). A leftPad that launches a fiber to mine cryptocurrency or sends an http call that fires nuclear missiles can still pass tests. It's hard to guarantee hygiene via tests alone.
These are problems that are hairy enough that I don't want to write my own solution, yet tractable enough that there ought to be a solution that never needs to be touched again. Maybe someone finds a better way of doing it, but the way they're currently doing it will never be wrong.
Like leftpad?
Where it can get slightly hairy is that to do it well, you need to have a LOT of seams between layers.
> but the idea of providing tests and types without implementation is a pretty interesting one.
I feel like in my head, you need to have -some- baseline/example implementation; e.x. Akka/Pekko/Akka.NET have Plugin specs for Persistence but there's still a Memory-only implementation of Persistence as a reference/baseline; after all you need to make sure the spec is possible at all.