Calling a function that returns Result<_,_> from a map method

Question

I'm learning Rust and I am struggling with optional URL parsing, I've got the following code:

use hyper::Uri; fn main() -> Result<(), String> { let url = Some("http://www.stackoverflow.com"); let works = if let Some(url) = url { Some(parse_url(url)?) } else { None }; let does_not = url.map(|u| parse_url(u)?); Ok(()) } fn parse_url(url_str: &str) -> Result<Uri, String> { unimplemented!(); }

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Documentation says that calling ? can be used as a syntactic sugar to simplify error handling.

So what bothers me is that I can perfectly fine map the inner value of Option<T> manually, using if let syntax and return early from the method but cannot just .map() the inner value of Option<T>.

So my question is actually how can I easily parse an optional value by avoiding if/else conditions in Rust?

John Kugelman · Accepted Answer · 2020-05-21 16:01:02Z

? handles errors in the current function. When you call map, you create a closure (using |_|), which is a different function.

You'd have to use ? outside of that closure:

let does_not = url.map(|u| parse_url(u))?;

Unfortunately, this won't quite work as at this point you have a Option<Result<_, _>> rather than a Result<Option<T>, _>. You can fix that using transpose:

let does_not = url.map(|u| parse_url(u)).transpose()?;

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