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0 pointslolinder2y ago0 comments

The killer feature of Rust's Result isn't actually the monad itself, it's the ? operator. Being able to concisely say "if there's an error, bail out by returning it" gets you pretty close to exception-level convenience with just a bit more explicit syntax showing where an error might come from.

0 comments

kitd2y ago

I've not used Rust. Do you get to add context at the point of bailing?

oneshtein2y ago

No, context is not added by default. With third party libraries like anyhow, it's possible to add context[0] before `?` operator in human-readable or machine-readable form:

    a_method().context("Failed to complete the work")?;
    a_method().context(FailedToCompleteTheWork)?;

[0]: https://docs.rs/anyhow/latest/anyhow/trait.Context.html

dthul2y ago

That depends on the type you use to represent errors. A more "fancy" error type will allow you to add context / backtraces / whatever you want, while a simpler error type, for example just an error code, does not. (The error type is the E in Result<T, E>).

preseinger2y ago

you don't want exception-style "convenience", that's the whole point

you want to be able to read code and see a single control flow

? subverts that core requirement

nulld3v2y ago

The problem with exceptions is that they can come from any line of code and cause a "return". Rust's question mark solves the issue because it marks which lines of code can cause a "return".

Therefore, you can always see see the control flow of a function.

Moreover, you can go even further if you really really really want a single control flow. You can write a clippy lint to disallow early returns (ban "return" keyword) and question marks. That said, I really don't think this is a good idea. The "return" keyword exists for a good reason.

preseinger2y ago

? enables chaining, chaining subverts comprehensibility in exactly the ways i'm describing

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bvrmn2y ago

And what the point to make `if err {return err}` in most of cases? I've read a lot of code and error handling logic almost never in caller location.

kaba02y ago

Except that without try-catch blocks you will have n separate control flow instead of a trivial to see pattern.

preseinger2y ago

huh? it's exactly the opposite

ideally, control flow goes 'down' via func calls, and 'up' via return statements

this is the "trivial to see pattern" -- the code as written

exceptions subvert those simple rules, they say any expression can potentially be a return statement, and recursively so!

j / k navigate · click thread line to collapse

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kitd2y ago

I've not used Rust. Do you get to add context at the point of bailing?

oneshtein2y ago

No, context is not added by default. With third party libraries like anyhow, it's possible to add context[0] before `?` operator in human-readable or machine-readable form:

    a_method().context("Failed to complete the work")?;
    a_method().context(FailedToCompleteTheWork)?;

[0]: https://docs.rs/anyhow/latest/anyhow/trait.Context.html

dthul2y ago

preseinger2y ago

you don't want exception-style "convenience", that's the whole point

you want to be able to read code and see a single control flow

? subverts that core requirement

nulld3v2y ago

The problem with exceptions is that they can come from any line of code and cause a "return". Rust's question mark solves the issue because it marks which lines of code can cause a "return".

Therefore, you can always see see the control flow of a function.

preseinger2y ago

? enables chaining, chaining subverts comprehensibility in exactly the ways i'm describing

1 more reply

bvrmn2y ago

And what the point to make `if err {return err}` in most of cases? I've read a lot of code and error handling logic almost never in caller location.

kaba02y ago

Except that without try-catch blocks you will have n separate control flow instead of a trivial to see pattern.

preseinger2y ago

huh? it's exactly the opposite

ideally, control flow goes 'down' via func calls, and 'up' via return statements

this is the "trivial to see pattern" -- the code as written

exceptions subvert those simple rules, they say any expression can potentially be a return statement, and recursively so!

j / k navigate · click thread line to collapse