Hot reloading and static types

At Facebook they actually do hot code swapping of Haskell, which is a statically typed language.

The rules they follow is that the swapped in object code must export functions of the same type as the previous version. There is nothing checking these types are correct other than human testing, so it’s effectively dynamically typed at this point.

Alternatively we could define an opaque “any” type and force the user to safely decode their data on code change. This isn’t an option for Haskell as their runtime needs to know the memory layout of the data as it is not designed for dynamic data.

I could imagine a system like this working for a typed Erlang. If I might self publicise a little, I’m working on a typed language for the Erlang VM at the moment, and I plan to experiment with this here GitHub - gleam-lang/gleam: ⭐️ A friendly language for building type-safe, scalable systems! · GitHub

For more info on the work at Facebook the keynote from Simon Marlow at this year’s Haskell exchange was very informative.

Thanks! I just came across Elchemy – how’s Gleam different?

Elchemy is an alternatively compiler for the Elm programming language that outputs Elixir rather than Javascript, Gleam is an entirely new language.

The most obvious differences between Gleam and Elm are that Gleam has side effects, Elm does not, Elm has autocurrying, Gleam does not, Elm has a Haskell like syntax, Gleam has a Rust like syntax, and Gleam has a slightly more expressive (though unfinished) type system as it has typed modules. Elm’s compiler is written in Haskell, Elchemy’s compiler is written in Elm, Gleam’s compiler is written in Rust. Both languages are relatively simple functional programming languages that prioritise safety and clarity.

Elm and Elchemy are more mature than Gleam (which isn’t use useable currently). Gleam was designed to be a BEAM language while Elm was not- because of this I think that once ready Gleam will more suited to writing Erlang/Elixir style OTP programs than Elchemy is, though that’s possibly my personal bias.

Other projects worth checking out are the Alpaca language GitHub - alpaca-lang/alpaca: Functional programming inspired by ML for the Erlang VM · GitHub and the Purescript to Erlang compiler GitHub - purerl/purescript: A small strongly typed language that compiles to Erlang (not JavaScript) · GitHub

It’s interesting to note that in Haskell, there is a library to facilitate upgradable types, called SafeCopy, which is used in some systems, but it does not seem to be entirely painless:

From user ‘Mightybyte’ on the Haskell ∪ Chat Discord group:

Another problem with that approach is that it depends on the old version being known. If you write the incorrect type there or the update is applied to a node running different code to what you expect then your upgrade will fail and likely lose state.