Which is still extremely important.
I think this is an aspect that the original article fails to take into account. Eliding tests can feel very liberating, and it allows you to plow ahead adding new features faster. Particularly in small, or at least new, projects. But over time reality catches up, and the lack of tests becomes a burden. You start avoiding adding new features, and particularly improving existing code, out of fear of breaking something. And so you end up more constrained than if you had added the right balance of tests along the way.
Writing software that is maintainable, with staying power of years or decades, requires the sacrifice of some up-front productivity.
http://docs.python.org/library/doctest.html
Just copy paste the REPL interaction into a doc string, and you're done. Apparently, it recognizes ">>>" as the REPL prompt, and the following line as the expected output. The equivalent for Clojure would be a neat addition.
The "code-as-data" homomorphic semantics of lisp would make building something like this quite interesting, as it would probably need to transform some of the code you've REPLled from statements into assertions, etc.
Then you wouldn't using a REPL for what a REPL can offer, but writing test code.
Unless it was really magical and would cover 100% of anything that someone could type in the REPL. If you were only using a subset of the REPL/languages features because you know your to-test-converter doesn't like some stuff, then you're writing the tests anyway.
Currenly, using the clojure REPL to test things comes with a twinge of guilt, as it is not being captured for regression tests. (and I am too lazy to write unit tests separately)
This would make it so that using the REPL would cycle between two "styles", ad-hoc experimentation and then, when you've found some repeatable behavior you want codified in a test, capture mode. These can in some cases be distinct processes (physically and mentally) and in other cases overlap so much as to look and feel like the same thing.
The reason TDD works and is fun is that you are using tests to learn and explore. It just so happens the artifact of that learning ends up living forever as a test. In a REPL, I'm doing that same learning and exploration already. The act of writing a test becomes as exciting as filing a TPS report.
Here's an example. I've just implemented a new function, and REPL'ed it to solidity. There were about 4-5 ad-hoc calls I made to the function to prove that it worked. I just finally got to the point where I can call it using my 4-5 different arguments, and it always outputs the right thing. Using readline, my arrow keys, and my enter key, I'm repeating the same series of steps over and over until the function works. We all do this. Win.
Now, I'm at a crossroads. Do I just start working on the next piece of the project? I know this piece works, I'm happy with it.
But wait! What if something changes. I need to write a test don't I. Sadness consumes me, since testing is slowing me down. I've already exercised the code, I already know it works, and I've already written the tests, albeit sloppily, in the REPL. Why do I have to switch gears now and start writing a file, running a test runner, and so on?
The truth is: I won't. I'll move onto the next thing, not breaking my flow and not doing something boring, something I already know the outcome of, instead of doing something fun: the next feature.
Maybe those who do switch off and go through the motions to write a test, repeating themselves, are more noble and careful in their programming. But I humbly suspect most of us are more lazy than noble :)
If you can't automate 100% of the REPL-into-test feature, if you need two mindsets/styles/etc, if you still need to "find the behaviour codified in a test", then you're just duplicating in the REPL the same workflow and results of writing the tests in a file. They need to overlap 100%.
Now, if they do, then it's awesome.
It seems to me the trick is to set sensible limits on the context of the current REPL state preserved at test time, in a way that works for most kinds of common unit tests. I believe this is the "magic" of which you speak.
I don't mean to pour cold water on the idea, though; if someone figures out a way of doing it that's useful I'd happily change my mind.
It's the main thing I miss when programming in Haskell. GHCi doesn't quite measure up to Lisp (or Ruby, or Python) REPLs.
What someone needs to do (and I'll do eventually) is implement a REPL enhancement that allows "test capture". Namely, if you have recently evaluated one or more expressions, execute a "capture" command that extracts the working environment (locals, globals), the previous statements you've run, and their evaluated results, and outputs one or more unit tests. For example, a session with clojure might look like this:
> (def x 1)
> (def y 2)
> (+ x y)
3
> capture!
-- Saved capture001.clj
(1 test passed, 0 tests failed.)
Where capture.clj contains a test similiar to:
(def x 1)
(def y 2)
(def expected-result 3)
(assert-equal (+ x y) expected-result)
Fuzzy, and I apologize as I don't know the clojure unit testing syntax, but you get the idea. Of course, this is a baseline case, you'd need more functionality such as allowing the user to specify which parts of the environment to capture, and what results to assert, but these should fall out naturally as the tool is dogfooded.