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Comment by mercer | Hacker News Reader

I've heard a number of times that static typing would be near impossible for Erlang because of how message-passing works, but just the other day I read a quote where one of the creators (I think it was Joe Armstrong, but not completely certain) indicated that the primary reason they decided against static typing was the hot code reloading, not the feasibility. or not primarily anyways.

Not disagreeing with you, I guess. It was just surprising/interesting to read.

Sadly I can't find the quote...

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I'm the author of Gleam, a statically typed language on the BEAM.

What you've said matches my findings. Messages can be typed, however hot code upgrades cannot be. For Gleam we've sacrified hot code loading in exchange for types, which I think is a good trade for most use cases.

Hi, thank you for contributing! I have gotten into the functional programming paradigm conceptually from working with react/redux. We do all of our development work using TypeScript, rather than JavaScript. Typescript isn’t really static typing, but more like type assertions on top of a dynamic language, but hot reloading works fine and is a core component of the workflow, especially with the ability to maintain state in between loads. I’m new to this world and haven’t yet tried Elixir, but what is it about the type system that prevents hot reloading? Is it because the underlying execution system is using types for something? Could a typescript-style language be built on top of Elixir and BEAM that would allow hot reloading and type assertions together?

Not touching on the reasons why the type system and hot code reloading are complex in the BEAM, but the hot code reloading dev workflows in other languages and the BEAM one are two completely different beasts.

Erlang/OTP was designed for systems where the amount of acceptable downtime is 0, you don't want phone calls failing because you have to fix a bug, the solution is a system where you can replace parts of it running live, if you replace a module all processes currently running it will still work normally but any new call to the module will use the updated version of it.

https://blog.appsignal.com/2018/10/16/elixir-alchemy-hot-cod...

The hot loading of code we're talking about is mutating of running code in production, and it involves running multiple different versions of the same code at the same time in order to do a clean and incremental migration to the new version while continuing to serve traffic. It's a different set of constraints to hot loading JavaScript style and very difficult to get right. The nation problem is that you don't have any way of knowing what the the state of the running system is (it is mutable) so you cannot unify your new types with the existing system.

We could probably do hot loading in development using type assertions fairly happily.

I see, thank you for the response. I haven’t ever heard of something like this being possible in production. We typically have to do phased rollouts at a different level of the stack, it’s not something supported by the runtime!

It came from Ericsson needing to upgrade telephone switch firmware without taking the system down. These telephone switches were often expensive and in physically difficult to reach places so having a standby for a switch-over wasn't viable.

Quite an interesting set of constraints!

I'd really like to read about your finding wrt hot code loading. Is there anything you can point me to?

I'm afraid not, it's not been a fruitful area of research so we've not written anything down about the limitations.

Ok. Thank you and good luck with Gleam!

It’s a combination of message passing to distributed nodes AND hot reloading. The compiler guarantees would only work on your current node, but if you were sending something to another node in the cluster you have zero knowledge of the types without forcing a full contract exchange on connection.

There’s a lot of problems introduced by that.

Joe Armstrong, RIP, also stated they tried to add type to Erlang later on but gave up.

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