I'll copypaste my comment that I've left at the HN discussion of that blogpost, https://news.ycombinator.com/item?id=23333853 https://news.ycombinator.com/item?

I'll copypaste my comment that I've left at the HN discussion of that blogpost, https://news.ycombinator.com/item?id=23333853 https://news.ycombinator.com/item?id=23333853: This is a PEBKAC, not a problem with REPL-driven development. He had all the test bodies written as input in the REPL, he had all the test results printed as output in the REPL. Why didn't he turn these into unit tests as he programmed? That's a simple lack of foresight on the programmer's side that is then blamed on the tools that were used.