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# Why does this work? .collect() automatic conversion to function return type

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I'm completing the rustlings exercises as part of self-teaching Rust. While working on the third iterators exercise, I solved the exercise but don't quite understand why my solution works.

Specifically, I made two functions that are identical except for their name and return type, but the function body is the same in each:

// Complete the function and return a value of the correct type so the test
// passes.
// Desired output: Ok([1, 11, 1426, 3])
fn result_with_list() -> Result<Vec<i32>, DivisionError> {
let numbers = vec![27, 297, 38502, 81];
numbers.into_iter().map(|n| divide(n, 27)).collect()
}

// Complete the function and return a value of the correct type so the test
// passes.
// Desired output: [Ok(1), Ok(11), Ok(1426), Ok(3)]
fn list_of_results() -> Vec<Result<i32, DivisionError>> {
let numbers = vec![27, 297, 38502, 81];
numbers.into_iter().map(|n| divide(n, 27)).collect()
}


Some sort of automatic conversion must be occuring, likely due to Iterator::collect() having implemented the std::iter::FromIterator trait. Is that right or am I on the wrong track? How would I make the final type more explicit even though I don't need to, so that I can see how to do so (likely via some turbofish notation)?

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You're very much on the right track.

# std::iter::FromIterator trait

This trait is indeed what makes it work. In particular, both functions' return types implement the trait:

std::result::Result<A,E> implements it if A implements it.[1] And here A = std::Vec.

std::Vec implements it as long as the iterated type matches the type contained in the Vec.[2]

# Turbofish operator

Yes you can force collect's output type with the turbofish operator:

someIter.collect::<Vec<Result<i32, DivisionError>>()


Or you don't have to specify the whole type:

someIter.collect::<Vec<Result<_>>()
someIter.collect::<Vec<_>()


If not on the returning line, you could also use type annotations

let vec_of_results: <Vec<Result<i32, DivisionError>> = someIter.collect();


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