Do you have some specific special effects in mind?
something I'd sometimes like to do when I'm profiling complex code will be to have an (essentially) global variable tracking the sum of how long a function took to execute over all invocations.
I am guessing that mutating the global counter would count as an effect, and I wouldn't really want to add the effect all the way through the call graph. I think this is something where the handling ought to be similar to how you're handling Debug.
The team can’t seem to make up its minds of the language is intended for high performance or not. They talk about the importance of purity for automatic optimizations but in the real world there’s all sorts of practical reasons for needing to debug production compiled code (eg imagine something like a browser and you’re trying to figure out some weird behavior that’s difficult to catch in a debugger but too slow to reproduce in a debug build or even not reproducible due to different timings resulting in different race conditions)
Also blaming the users of your language for your language not being able to meet their needs isn’t a good look. It suggests the language is probably attracting the wrong users or positioning itself incorrectly in the market place.
> They talk about the importance of purity for automatic optimizations but in the real world there’s all sorts of practical reasons for needing to debug production compiled code
I imagine they're talking about their defaults. One can commonly reconfigure how different build profiles work.
> Also blaming the users of your language for your language not being able to meet their needs isn’t a good look.
Isn't that what the whole post is about though? They even say the following.
> Returning to earth: we may be academics, but we are trying to build a real programming language. That means listening to our users and that means we have to support print debugging. The question is how?
Things like this - they're painting users of programming languages as the ones being unreasonable.
> I imagine they're talking about their defaults. One can commonly reconfigure how different build profiles work.
From the article:
> We don’t want published packages to (a) lie to the type and effect system, or (b) contain print debugging statements > As a result, using dprintln in production mode causes a compilation error.
There is no documentation about the existence of build profiles or how they might work. I think you're reading too charitably.
For systems in production, we have the `Logger` effect and associated handlers.
And yes, Rust doesn’t have an effect system yet, but others have mentioned Haskell and how it handles tracing and logging and the limitations of effect systems interplaying with such things.
Either way, extremely well explained both in motivate and implementation!
Perhaps there is two or three more, but I do think this is a finite set that we can write a language around and just sort of consider them "ambient effects". While metrics and logging are nominally impure, in that they are certainly mutations, if you can't read the logs or the metrics without an effect, you still retain the really important aspect of purity, which is that the pure code can't cause a change that is observable by that or other code, with the very specific exception of the logging stream and metrics.
I wouldn't be quite ready to put all my chips on this, but I think "the inability to create changes that can be witnessed" is actually the true goal, not "the inability to create changes" with no qualifications. Pure code already necessarily creates changes in a system, the key is that while the CPU registers may change and other parts of the system may mutate, the code can't witness those changes and conditionalize future execution on it. All effects-based systems have already agreed that there are things that are mutations in something real in the physical world they aren't going to consider effects, adding a couple more categories is not going from 0 to 1 but 12 to 15. It's not a strict purity question but a cost/benefits question.
It occurs to me as I type this that a really ambitious language with a strong enough type system might even be able to turn this into a completely safe proposition, allowing users to declare effects with some sort of very safe "sink" associated with them that the type system checks can not escape out into the rest of the code in any visible way and constraining the visibility somewhere that is contained in the conventional effects system. All these things I'm talking about here and in the blog post are taking the form of values that simply disappear into the ether from the point of view of the creating function. I'd like the language to make it easy to query a function for which ambient effects it uses (as a development-time operation, not a run-time one), and I think that would clean up most of the rest of the practical problems.
It's proper (but goofy) to pass in a `logger` object to every function, but the practicality of plumbing a "logger" everywhere is disgusting. There's a ton of value to be able to "capture" what was logged by a function, but as you mentioned with "metrics", the invasiveness of plumbing a metrics object "through" your code is really gross.
Maybe a straw man syntax like:
this.__classHooks__.logger
= new AbstractLogger(...)
this.__classHooks__.metrics
= new AbstractMetrics(...)
It already is, kinda. In my practice very often you have global singleton values that are either defined as static variables, or passed as arguments to nearly all functions in a module. Since implicit presence of `Debug` effect is already a compilation parameter, it could be generalized to support any sets of implicit effects. Thus you might design a module that has implicit Logger and Database effects in all its functions.
Logging does seem like a very similar case to debugging, only that you expect to leave it on in production. On the other hand an implicit Database effect kind of defeat the point of an effect system.
I think the key is that Debug and Logger effects don't really affect the rest of the code - if you remove all debug/log statements, the only thing that changes is the debug/log output (and slightly faster execution probably).
At some point you're composing your pure functions and _have_ to call them by some effectful function (otherwise they're never executed or you're doing computations that aren't consumed by anything).
The only sane alternative is to have a debug effectful variant where you turn off these checks. But then why would `stdout` debugging fine, and not say writing to a different stream or file?
I'm curious what other languages are trying to achieve such?
1. Initially there was no way to do effects in Haskell, everything was pure.
2. Then it was realized that IO can be modeled with monads, so the IO type and do notation were added.
3. Gradual realization that monads can be used to also constrain effects, ie you can construct a type of "stateful computations" that can read and write to a specific state, but not touch other states or write to disk or something.
4. Monad transformers are invented, which allow stacking monads on top of eachother to support multiple effects. Together with type classes, this gets us pretty close to extensible effects (the approach used in Flix, if I understand it correctly). So for example you can express that your function needs to write to a log and may exit early with an error message with the constraints `(MonadWriter w m, MonadError e m) => ... -> m resultType`, and you can then use monad transformers to build a stack that provides both of these effects.
5. Monad transformers have some issues though: they affect performance significantly and the interaction between effects is tricky to reason about. So an alternative is sought and found in extensible effects. The initial proposals were, iirc, based on free monads, but those aren't great for performance either, so ever since there has been a whole zoo of different effects and handlers implementations that all make different trade-offs and compromises, of which I think the `effectful` library is now the de facto default, and I think what it offers is quite similar to the Flix language's effect system (I'm not sure on what finer points it differs).
You can see my talk "A History of Effect Systems" for a synopsys of the history. I gave it at Zurihac this year. It's very close to the history you gave (though I think point 1 is not right: Haskell always had a way to do IO)
- https://koka-lang.github.io/
It's not just print debugging. It's also logging. Oh, you want/have to add useful/required logging? Good luck refactoring your entire code to thread IO everywhere.
[1] https://hackage.haskell.org/package/base-4.21.0.0/docs/Debug...
This is bc of non strict eval in Haskell, but still the core is that the lang is designed to not eval stuff when it’s not necessary.
If printing- or logging statements have no effect, the compiler might reorder them or even remove them.
Imagine saying that certain memory stores are allowed to cross a memory barrier operation, you'd completely lose the ability to reason about concurrency around those stores.