What is the idiomatic way to propagate error from value inside map statement in Rust?

Question

I have a function which returns a Result<u32, SomeError>. I want to collect four values from this function, and collect them to sum them up. How can you propagate the error inside the map operator? I want to stop on the first error, and propagate the error up to the function which called it

Works

(0..4) .map(|_| self.consume().unwrap()) .collect::<Vec<u32>>() .iter() .sum::<u32>();

Does not work

(0..4) .map(|_| self.consume()?) .collect::<Vec<u32>>() .iter() .sum::<u32>();

The compiler error is

.map(|_| self.consume()?) ----^^^^^^^^^^^^^^^---------------------- | | | cannot use the `?` operator in a closure that returns `u32` this function should return `Result` or `Option` to accept `?`

When you encounter an error, what do you want to do? Skip it? Collect into vec of Results? Stop on the first error? Collect into a Result<Vec> and fail completely if there was an error? — Chayim Friedman
– Chayim Friedman, Commented Jun 15, 2022 at 16:15

Chayim Friedman · Accepted Answer · 2022-06-16 02:01:58Z

3

map() is just an iterator adapater. It cannot propagate the error itself, since it doesn't run the code that can produce it. The executor of the code here is sum() (actually, collect(), but it is redundant here) and thus it should deal with the error (or a replacement).

In this case, you can exploit the fact that Result implements Sum, allowing you to sum into Result<T, E>:

(0..4).map(|_| self.consume()).sum::<Result<u32, _>>()?;

collect() also does the same, so you can collect into Result<Vec<T>, E> for example.

edited Jun 16, 2022 at 2:01

answered Jun 15, 2022 at 16:29

Chayim Friedman

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4 Comments

Kevin Anderson Over a year ago

For both myself and the OP, does this occur as you go, or are all of the map calls done before the sum is attempted? That was one of the OP's requirements, that it stops on the first error.

Chayim Friedman Over a year ago

@KevinAnderson As I said, map() does nothing but produces an iterator. It is lazy (otherwise, it would have to keep the results in a Vec). The real work is done in sum(), and it processes the element until the first error, by that time it discards the result and returns the error.

jevoso5871 Over a year ago

@ChayimFriedman Thank you very much! Is it possible to do something similar also if consume returns Result<char, _>, and then apply .to_digit(x) (.unwrap() on each of these chars? (this self.consume().unwrap().to_digit(16).unwrap())

Chayim Friedman Over a year ago

@jevoso5871 If they return the same error type, it's as easy as self.consume()?.to_digit(). If not, you may need to be more explicit and maybe specify the error type. But nonetheless, inside the closure you can use the normal ? operator and all error handling facilities - you just need to return Result from it.

Aplet123 · Accepted Answer · 2022-06-15 16:29:10Z

There's actually a Sum instance for Result<T, E> that fits this usecase perfectly:

(0..4) .map(|_| self.consume()) .sum::<Result<u32, _>>()?;

spdrman · Accepted Answer · 2022-06-21 16:49:00Z

Here you go:

(0..4) .map(|_| self.consume()) .collect::<Result<Vec<u32>, _>>()? .iter() .sum::<u32>();

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