Side note that IPersistentStack extends IPersistentCollection, and I don't think we'd want to have TransientVector implement IPC (as it's not persistent). So, would either need to introduce some custom logic to RT.peek()/pop() or introduce an ITransientStack interface line.

Retargeting this as new feature rather than bug.

I think this would mean PersistentVector.TransientVector would need to support IPersistentStack but that has two methods - peek and pop. Interestingly, ITransientVector and TransientVector both have pop() already. Seems like the pop() impl has all the smarts you'd need to implement peek() too.

I just ran into this issue today. I agree it would be convenient if peek worked on a transient vector. Other functions such as nth and count work on transient vectors, so it's natural to want peek.

As a work-around (not a suggested fix), I use this:

Admittedly, that's a misnomer but it fits the bang pattern for transient transformation of regular collection code. I also have a convenience function update! which calls assoc!. I offer the work-around just for users who run into this problem and want to get back to work.