Allow Try Trait to convert to whole return value

language design
1

The current implementation of the Try trait (the one that is used with ?) currently does support the following transformation.

fn func_1(...) -> Result<T1, E1>;

fn func_2(...) -> Result<T2, E2> {
    ...
    let res = func_1(...)?;
    ...
}

If the following bound is the only one that exists:

impl From<E2> for Result<T1, E1>;

I think that this could be a valid use case for this operator.

2

You can do this with map:

fn func_1(...) -> Result<T1, E1>;

fn func_2(...) -> Result<T2, E2> {
    ...
    let res = func_1(...).map(T2::from)?;
    ...
}

Is there a reason why this is not sufficient?

3

Yes, if part of the From impl is for an instance of E2 to become an instance of T1.

4

The function you want is Result::or_else, except both Ok types must equal. The particular control flow you want is specialized enough that you may want to just write it out as a match.

Adding more implicit conversions to ? seems poorly advised (and may break existing programs, but I don't know off the top of my head if that's true).

1 Like
5

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