Hotswapping Haskell

I don't want to knock the technical achievement here -- it's a cool hack -- but I'm really surprised that it was deemed to be the best choice for a production system.

In the first place, "we can't compile our code on every change because it takes too long" is a really awful situation to be in. Are developers not building and testing their changes before deploying them? Can Facebook not afford a continuous integration system that can run builds in parallel? It sounds like this problem is only happening because the application is a giant monolith, but for some reason splitting it up would slow down development even more... I'm not sure I buy that reasoning.

The article says that "Haskell’s strict type system means we’re able to confidently push new code knowing that we can’t crash the server", which is a real stretch. In addition to all of the usual ways a computation can diverge, this hot-swapping system adds a whole new variety of failure modes. The article talks about how the code needs to be carefully audited to prevent memory leaks, but it doesn't even mention the weird things that can happen when mutable state is preserved across code modifications. Debugging is a pain when your data structures can get into states that aren't reachable with any single version of the code. (This is a well-known issue in Linux kernel live-patching, for instance.)

People might find this interesting to compare and constrast to (certainly I noticed parallels with[1]) Netflix's 'serverless' platform, discussed yesterday:

https://medium.com/netflix-techblog/developer-experience-les...

Edit: As a sibling commenter notes this is most eminently doable with e.g. Common Lisp and BEAM (Erlang/Elixir), but more folks are (publicly) attempting this in other environments now (I've experimented with a number of approaches to this the last few years, so I'm trying to keep score - would love to see any comments on other attempts below).

[1]: Quote: "At the core of the redesign is a Dynamic Scripting Platform which provides us the ability to inject code into a running Java application at any time. This means we can alter the behavior of the application without a full scale deployment."

I agree. Fun read and cool hack, but it definitely feels like they are stretching to justify the more fun of the two options (spend time on this or spend time fixing the root cause).

> Common Lisp and BEAM (Erlang/Elixir)

Clojure as well right?

Yup having strong namespaces in general makes this easier I think and in Clojure tools like devcards (CLJS) and Ring reload (CLJ) support this workflow.