Add or_try_* variants for HashMap Entry API by peterjoel · Pull Request #120708 · rust-lang/rust (original) (raw)
These methods are technically unneeded as you can implement them yourself by matching on Entry::Occupied and Entry::Vacant, but this makes it more ergonomic to use the Entry API in cases where construction of a value can fail.
I realise that the same methods can also be added to BTreeMap's Entry, but I thought I'd wait to see if this was accepted first.
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@Jules-Bertholet You mean something like this?
pub fn or_try_insert_with<F, R>(self, default: F) -> R
where:
F: FnOnce() -> R,
R: Try<Output = V>,
Actually I think this level of genericity is not possible without HKT because the type of Try::Output is different in the return types of the default function and or_try_insert_with itself.
The default function can still have a generic return type, but I think or_try_insert_with's return type needs to be fixed as a Result.
pub fn or_try_insert_with<F, R>(self, default: F) -> Result<&'a mut V, R::Residual>
where
F: FnOnce() -> R,
R: Try<Output = V>,
My understanding of Try was out of date. In the try_trait_v2 implementation, the residual of, e..g, Result<T, E> is Result<!, E> (not E as I has assumed), and there doesn't appear to be a way to retrieve the E at either the type or value level.
@Jules-Bertholet Please let me know if I'm missing something, but I think my original signature is what we're stuck with, without larger changes to the error-handling traits.
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Thanks @Nilstrieb, I thought previously this wasn't necessary for small changes, and didn't think to check! I'll take a look at that now.
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