A few years ago when OCaml 5.0 landed with effects, there was a lot of discussion about making them typed in a way that prevents unhandled-effect runtime errors. Back then, there seemed to be an impression that typed effects were coming soon and they would be a necessary improvement. If you look bac…

A couple years ago I tried to get people to work on typed effects specifically for OCaml, but I failed to gather steam and actually do it (apologies!). There are plenty of people in academia working on type systems for effects, but most of it is not directly applicable to OCaml. I don’t know of acti…

Was there really an impression that typed effects were ‘coming soon’? My understanding was always that they were mostly conceptual at that point and a lot of theoretical and academic work needed to be done for them to even reached POC stage. I don’t think anyone has so much as shown a demo?

Back when effects were first getting released, there was a lot of discussion where people said things like “wait to use effects until they’re typed,” “we don’t want to add a syntax for effects until they’re typed,” or “let’s wait for typed effects before we add features X Y and Z.” I don’t want to g…

The last status I remember is from Next priority for OCaml? - #62 by gasche in 2023.

I’m fairly sure the messaging was more along the lines of ‘don’t use untyped effects directly in application code yet, use them indirectly via libraries like Eio that wrap their functionality’. Maybe there was a communication gap that made people think that meant typed effects were on the way, but …

No, the initial reasoning was very explicitly about waiting for typed effects [image] Tutorial: Roguelike with effect handlers The reasoning for this choice [of not including syntax for effect handlers in 5.0] is that we want to give a chance to a type system for effect handlers, but that still …

Here’s another one: [image] Am I wrong about Effects? I see them as a step back In fact, the current (5.0) recommendation is to use them precisely like that: purely on the implementation side, without exposing them to the user. It was precisely to discourage exposing untyped effects at the API l…

The only thing I would add to this discussion is that Jane Street is making progress on the related front of statically guaranteeing that effects are handled in OxCaml with modes. It’s not fully typed effects, but it is an interesting way of making effects much more usable in application code. They …

Hmm… as a semi-official position, this is disappointing. I also thought untyped effects were sold as a springboard to typed effects somewhere down the line, but it seems like that’s not in the works. That makes me want to avoid effects even more outside of concurrency. In a world where every library…

I appreciate the insight. It’s slightly disappointing that the work doesn’t seem to be going anywhere (for OCaml at least) since typed effects always seemed like an intriguing feature to me. I’m personally happy using the untyped effects we have now, but sometimes wonder about what effects could be…

[image] gasche: some people say that because effects are more pervasives than exceptions in libraries that use them, we catch mistakes earlier in normal testing Sorry for the sarcasm, but this is to me the same as saying the same sentence, but where “effects” are replaced with “bugs”. Yes, le…

What's the status of typed effects?

Ecosystem

welltypedwitch August 12, 2026, 12:33am 6

No, the initial reasoning was very explicitly about waiting for typed effects

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What's the status of typed effects?

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