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How do I use Option and the '?' operator with a for loop? (feddit.it)

submitted 2 weeks ago by [email protected] to c/[email protected]

19 comments fedilink hide all child comments

fn get_links(link_nodes: Select) -> Option<String> {

        let mut rel_permalink: Option<String> = for node in link_nodes {
            link = String::from(node.value().attr("href")?);

            return Some(link);
        };

        Some(rel_permalink)
    }

This is what I'm trying to do, and I've been stuck with this code for an hour, I simply don't know how to put this function togheter.. Essentially I would like to take some link_nodes and just return the link String, but I'm stuck in the use of Option with the ? operator.. Pheraps trying to write it with match would clear things out(?)

Also I come from JavaScript in which expressions do not have their own scope, meaning I'm having troubles to understand how to get out a variable from a for loop, should I initialize the rel_permalink variable as the for loop result?

This are the errors i get:

error[E0308]: mismatched types
  --> src/main.rs:55:49
   |
55 |           let mut rel_permalink: Option<String> = for node in link_nodes {
   |  _________________________________________________^
56 | |             link = String::from(node.value().attr("href")?);
57 | |
58 | |             return Some(link);
59 | |         };
   | |_________^ expected `Option<String>`, found `()`
   |
   = note:   expected enum `Option<String>`
           found unit type `()`
note: the function expects a value to always be returned, but loops might run zero times
  --> src/main.rs:55:49
   |
55 |         let mut rel_permalink: Option<String> = for node in link_nodes {
   |                                                 ^^^^^^^^^^^^^^^^^^^^^^ this might have zero elements to iterate on
56 |             link = String::from(node.value().attr("href")?);
   |                                                          - if the loop doesn't execute, this value would never get returned
57 |
58 |             return Some(link);
   |             ----------------- if the loop doesn't execute, this value would never get returned
   = help: return a value for the case when the loop has zero elements to iterate on, or consider changing the return type to account for that possibility

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[–] [email protected] 0 points 2 weeks ago (1 children)

@dontblink e.g.

enum OsResponseType {  
 Message { message: String },  
 Bool(bool),  
}

fn find\_os(os: OperativeSystem) -\> OsResponseType {  
 match os {  
 OperativeSystem::Linux =\> OsResponseType::Message {  
 message: "This pc uses Linux".into(),  
 },  
 OperativeSystem::Windows =\> OsResponseType::Bool(false),  
 OperativeSystem::Mac =\> OsResponseType::Bool(true),  
 \_ =\> OsResponseType::Bool(false),  
 }  
}

Note the logic here is nonsense

[–] [email protected] 1 points 2 weeks ago (1 children)

@dontblink It feels like you should never OsResponseType::Bool(true) so you could probably encode that in the enum just as the same Unrecognised

enum OsResponseType {  
 Message { message: String },  
 Unrecognised  
}

[–] [email protected] 1 points 1 week ago

Thank you so much, You are taking a lot of effort to answer my doubts and I really appreciate!

So essentially match can return different types, but of course I have to specify it in the function signature, wheter using an enum or other ways, this makes sense! This was a missing piece in my puzzle, I didn't consider the fact that the match return here was the function return type as well, and i had encoded -> String as return type.