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by ThatGeoGuy 1516 days ago
This is correct (in that the most simplistic case is try-catch).

However, the difference between a try-catch and conditions / restarts is that when one signals a condition (exception), the restart (catch) has a continuation from the condition. This allows you to inject an expression into the location where an exception occurred and "restart" your code from that point.

Whether you do such a thing or not depends on the code, on the type of condition raised, and on what expressions are valid. So you get a lot more flexibility in how errors are handled across the system. But likewise: more complexity in having to make that choice in the first place.

Going farther than this, conditions and restarts are really just a fancy way of packaging delimited continuations. I don't personally know any non-Lisp language that has attempted to package these concepts (maybe Dylan, which is a Lisp-like in its own way but without the syntax?). Going back to the original thought regarding error handling - I think Result<T, Err> type handling is fine and that most languages would be better served by that than having different types of exceptions. Conditions and restarts are powerful but your language has to be very expression focused (i.e. does not use a lot of statements) and it's not really clear that there's been a lot of work on making restarts nice to use. Exceptions in all languages that have them have their own set of associated problems, for what its worth, and it's not as easy to move Lisp features into a non-Lisp as one might believe...
1 comments
mananaysiempre 1516 days ago
> Going farther than this, conditions and restarts are really just a fancy way of packaging delimited continuations. I don't personally know any non-Lisp language that has attempted to package these concepts (maybe Dylan, which is a Lisp-like in its own way but without the syntax?).

Dylan does have a condition system, but it’s basically a Lisp without the parens, so probably doesn’t count. On the other hand, algebraic effects are another fancy way of packaging delimited continuations, so arguably the research languages Eff[1] and Koka[2] tried. (I don’t think either one explored the connection with condition systems, but I’m not sure.)

> I think Result<T, Err> type handling is fine and that most languages would be better served by that than having different types of exceptions. Conditions and restarts are powerful but your language has to be very expression focused (i.e. does not use a lot of statements) [...]

Huh? I don’t know why you’d say that, if anything I think it’s the Either err t / Result<T, Err> style that is more expression-focused (I mean, it even originates in Haskell :). I wouldn’t even call Common Lisp particularly expression-oriented, honestly, not unless we’re comparing with plain old C and not Rust.

[1] https://www.eff-lang.org/

[2] https://koka-lang.github.io/

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ThatGeoGuy 1512 days ago
> Huh? I don’t know why you’d say that, if anything I think it’s the Either err t / Result<T, Err> style that is more expression-focused (I mean, it even originates in Haskell :). I wouldn’t even call Common Lisp particularly expression-oriented, honestly, not unless we’re comparing with plain old C and not Rust.

I think that's exactly what I mean. The vast majority of languages (including golang, in TFA) use statements for dealing with exceptions. Rust also had try-catch, but has long since removed that syntax.

Anyways, the reason I said it is because it is not clear what to do when one wants to restart a statement. There are plenty of non-expressions that can throw, and usually it's not thought about deeply, but from a language semantics point of view one does need to have an idea of how to engage with it. For example, if you wrote:

    with open('somefile') as f:
        for line in f: 
            # ...
in Python, and had to deal with a restart during `open`, how do you manage this? The naive answer is to just return the continuation at `open`, but the "with" statement may have contextual setup. For example, `open` might be fine during `__init__`, but may have failed in `__enter__`. If you "restart" in `__enter__`, you need to deal with the partial state. Expression-based languages don't really have this issue because the call stack is usually clear (there's no magic under the hood). Similar analogues would be the `using` keyword in C#, or perhaps even lambda-expressions in C++. The abstraction in the code is separated from the execution of the restart, so it gets kind of gross as a language implementer in terms of not having to have very specific places where restarts can and cannot be.

This is a good reason why Rust / Haskell don't package these and just use Either / Result instead. If you have a bunch of types that you didn't write, injecting a restart into any failing code now brings a question of: "Can you safely inject types into a restart for code that you do not have access to?" and the answer is often no. The visibility rules in Rust make this a non-starter, and in Haskell you have a problem of mutability as well. A condition may be triggered at a point where IO could be injected, and so many of the language semantics would be in question. I suspect the type definitions for a restart in any arbitrary location in the code would be pretty hard to write, so maybe this is an open research area in Haskell already, but I doubt it'd be as ergonomic.

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