What scenarios do you envision a Result<Result<T, E>, E> having a different meaning than a Result<T, E>? To me, the messy Result type just seems like a case of something that should’ve been handled already (or properly propagated up).
res_res??;
// or
res_res?.map_err(..)?;
// or
res_res.map_err(...)??;
// or
res_res.map_err(...)?.map_err(...)?;
With res_res.flatten()?, you don’t know where you got the error anymore, unless the error type itself is “flatten-aware”, which is a bigger adjustment than the simple ergonomic library addition, and can become itself a problematic pattern with its own disadvantages.
This, it’s not a thing that happens often, but there were a couple of times when flatten would’ve been handy
This was also usually a result of a chain of and_then that could do with some flattening. This could’ve been rewritten as a separate function to make use of ?, but it seems to be a bigger trouble than use
Yeah, I can see your point. It’s certainly not something you should overuse, just because it’s convenient.
I feel like the redeeming points are that it will only be available, if it’s the same error type. And if you use a catch-all error type, like anyhow::Error, which makes it likely for nested results to use the same error type, then it’s likely that you can use ?? already.
So, personally, I feel like it isn’t something that juniors will readily/wrongfully incorporate into their error handling routine and rather it is a tool that’s available for when you know what you’re doing.
What scenarios do you envision a Result<Result<T, E>, E> having a different meaning than a Result<T, E>? To me, the messy Result type just seems like a case of something that should’ve been handled already (or properly propagated up).
(stating the obvious)
You can already :
res_res??; // or res_res?.map_err(..)?; // or res_res.map_err(...)??; // or res_res.map_err(...)?.map_err(...)?;With
res_res.flatten()?, you don’t know where you got the error anymore, unless the error type itself is “flatten-aware”, which is a bigger adjustment than the simple ergonomic library addition, and can become itself a problematic pattern with its own disadvantages.A lot of code doesn’t really care where the error came from. This can be useful when using
anyhowin application code, for example.For library code, I don’t see myself really using it, so it’ll live next to all the other functions I don’t use there I guess.
I think it’s more for cases where you don’t want to return, like
let new_res = old_res.map(func).flatten();This, it’s not a thing that happens often, but there were a couple of times when flatten would’ve been handy
This was also usually a result of a chain of
and_thenthat could do with some flattening. This could’ve been rewritten as a separate function to make use of?, but it seems to be a bigger trouble than useYeah, I can see your point. It’s certainly not something you should overuse, just because it’s convenient.
I feel like the redeeming points are that it will only be available, if it’s the same error type. And if you use a catch-all error type, like
anyhow::Error, which makes it likely for nested results to use the same error type, then it’s likely that you can use??already.So, personally, I feel like it isn’t something that juniors will readily/wrongfully incorporate into their error handling routine and rather it is a tool that’s available for when you know what you’re doing.
Wait, so you say
res_res??gives more information thanres_res.flatten()?, do you?I mean, this is a very trivial case and not best suited for flatten at all, but the information is lost in exactly the same way