So then the OP shouldn't have made the claim. It ain't my job to prove it. :)
That said, I fundamentally disagree with the idea that one can't measure developer productivity. I agree that there are many potential measures, all of which have the potential to be flawed or distorted. But the idea that the productivity of a developer can't be measured at all is something I've never heard anyone seriously claim.
> So then the OP shouldn't have made the claim
Come now, it was you who moved those goalposts and you moved them half a field or more. OP claimed REPLs "have benefits". Your response was (paraphrased) "demonstrable measurement of productivity improvement or GTFO". That sort of ante-upping just isn't that nice for good conversation, and certainly not when you're demanding a standard none of us can meet about anything.
> If your language doesn't have a REPL, your language is less productive. Deal with it.
That seems exactly like the kind of claim that everyone should view with a healthy dose of skepticism, and I think zzalpha's initial response[2] hit that right on the nose.
I repeat: there are no perfect measures of productivity. But there are imperfect measures that have some utility. The idea that we cannot measure at all because we cannot measure perfectly is simply untrue.
In this case we could, for example, take 1000 individuals with similar years of programming experience and given them two languages and a non-trivial problem to solve and see how long they take to come to a working and correct solution.
Is that perfect? No. But it's still something that's measurable. Do that with a reasonably large sample and you can probably start making inferences.
Our industry suffers from a horrible lack of concrete, scientific studies to back common wisdom. We would all benefit from ignoring Fowler, here, and getting down to the nuts and bolts of finding solutions to that problem. Until we do, while we'll have lots of fun having pointless debates on HN that cannot be resolved because we have no real data, the industry simply won't move forward.
No one can demonstrate productivity gains because productivity can't be measured. Lines of code can, and those are often a bad thing. Some of our most productive efforts make programs smaller.