How do you know if they are productive before hiring them?

Someone who is "a seasoned java dev who trained themselves in Scala already" will have a track record of Getting Stuff Done of course. Also see http://www.paulgraham.com/pypar.html

That's only sometimes true. There are plenty of intelligent developers learning the newest and coolest languages who'd I'd never hire.

Why? Because learning cool tech and Getting Stuff Done are two very different things.

While it is true that most developers suck at writing code, knowing Scala doesn't tell me whether you:

1. Have enough discipline to do the boring, tedious parts of your job 2. Know how to prioritize tasks 3. Can write easily maintainable code 4. Can work well with others.

Technical ability is only one part of an employee.

I quoted the word "seasoned" too for a reason!

far easier said than done. Have you hired a lot of people? What form does their track record come in? A list of projects on their resume?

Yes, actually :-) Most people's CVs (we call them) are pretty poor indicators of their abilities (because, no fault of their own, they take other people's bad advice on CVs) but all I talk about in interviews is, what have you done and what difference did it make. I'm not perfect at it by any means, but my hires usually work out pretty well...

I wasn't trying to give you a hard time. I've been hiring for years and it's really hard.