I almost feel it's a mistake to make all the Cause operations stack safe, because of the overhead in allocation.

But anyway, this seems a good change. 👍

I thought about the same thing. Unfortunately I do think that users sometimes create very large nested causes, for example using collectAllPar. Hopefully the check for reference equality with the empty cause covers the vast majority of "happy path" cases and we are really only doing this when a failure has occurred.

One alternative would be to ensure that we construct a balanced tree so that we never reach a sufficient depth where we need to be stack safe similar to what we do with Chunk. This would have some complexities with the fact that Cause has both a Both and a Then case but it is something we could explore.