Hot swapping in OCaml 🥵

Hi, is it possible to do hot swapping in OCaml?
Otherwise, is it possible to add it and how difficult it will be?

What exactly do you mean by hot swapping ? I have no knowledge on that topic, it sounds interesting.

I assume you mean hot code reloading. I think it was possible to some degree using Dynlink, as shown in this blog post I found: Hot-reloading OCaml on Web, Desktop, and Android | Jared However, it might be no longer possible since 4.08: #1063 which if I understand correctly forbids loading module when a module with the same name is already loaded.

I have been using this tool to live-code in OCaml:

It worked successfully to develop medium-sized programs, though OCaml idioms have to be adapted to be a bit more “live” friendly.


Here is an example of hotcaml editing the slideshow of wall presentation:

Steps to reproduce:

opam pin add wall
opam pin add hotcaml
git clone
cd wall/slideshow

A window should open displaying the slideshow.
Open another terminal and edit wall/slideshow/, changes should be reflected in the window.


Thanks it’s exactly what I was looking for!

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Fantastic work @let-def
How feasible would it be to hook this up to a utop style repl?

Speaking of utop, this reminded me of an older experiment I had - It’s not quite the same as HotCaml above, but you can get a Proof-General style interaction mechanism with OCaml with some simple GNU Emacs hackery:


Which seems like it would be a pretty good fit for live-coding.

Code: Interactive Utop Mode ($2170216) · Snippets · Snippets · GitLab


It can be done, at least from an engineering point of view.
I have given thoughts to something similar, not for hot loading but for debugging: interrupting a running program and getting a toplevel in the current execution context, with builtin stack navigation.

So while it is possible to implement, I am not sure what is the right UI/UX to present that kind of features. (This hotcaml proof-of-concept did not need much work on this front: the edition workflow does not change, it is just the runner that pick up changes when they typecheck).

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