nREPL and ClojureScript: Demystifying Piggieback

I can’t carry it for you… but I can carry you! – Samwise Gamgee, on the virtues of piggyback If you’ve ever fired up a ClojureScript REPL from your editor and it just worked, there’s a decent chance Piggieback was quietly doing the heavy lifting behind the scenes. It’s one of those libraries that’s been around forever, that everyone in the CIDER world depends on, and that almost nobody actually understands. For years I counted myself firmly in the “almost nobody” camp. That changed recently. I finally had to do some serious work on Piggieback myself, and to my mild horror I realized I’d forgotten most of how it works internally. So I did what I always do when I need to understand something properly - I dug in, refactored it into a shape I could actually reason about, and then wrote it all down. This article is that write-up. A little backstory Piggieback was created by Chas Emerick back in August 2012. The very first commit is dated August 10th, 2012, which makes the project a few…