How to return a generic struct in Rust

Question

Beginner in Rust, I'm wondering how to return generic data in a function.

pub fn tryparse() -> (CLIAction, ArgSingle) {
    let args = CLI::from_args();
    match args.cmd {
        SubCommand::Add(opt) => {
            (CLIAction::QuantityAdd,opt)
        }
        SubCommand::Del(opt) => {
            (CLIAction::QuantityDel,opt)
        }
    }
}

In this example both Add and Del get a ArgSingle struct type. However I'd like to implement a Search that would use a ArgSearch struct type, and return it... How to tell rust that tryparse() can return either ArgSingle or ArgSearch type?

Answer 1

Sounds like you want to return either ArgSingle or ArgSearch from tryparse() function. There are a few approaches to that, a common one is to use enumerations as the return.

First, define an enum that represents all the possible types that can be returned from the function:

enum CLIResult {
    Single(ArgSingle),
    Search(ArgSearch),
}

Then, modify the function to return this enum inside of the tuple:

pub fn tryparse() -> (CLIAction, CLIResult) {
    let args = CLI::from_args();
    match args.cmd {
        SubCommand::Add(opt) => {
            (CLIAction::QuantityAdd, CLIResult::Single(opt))
        }
        SubCommand::Del(opt) => {
            (CLIAction::QuantityDel, CLIResult::Single(opt))
        }
        SubCommand::Search(opt) => {
            (CLIAction::Search, CLIResult::Search(opt))
        }
    }
}

Now, the function can return either ArgSingle as CLIResult::Single or ArgSearch as CLIResult::Search.

To use the return value of tryparse(), you would need to pattern match on the CLIResult enum to extract the appropriate type of result. For example:

let (action, result) = tryparse();
match result {
    CLIResult::Single(single_arg) => {
        // Handle single argument case
    }
    CLIResult::Search(search_arg) => {
        // Handle search argument case
    }
}