Multiple return types in rust

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I have a library function f1 in rust that returns a string and want to update it to optionally return a vector. 

fn f1() -> String {
   "abc"
}

fn f2() -> (String, Vec<usize>) {
   "abc", vec![(0, 1, 2)]
}

fn f3(flag: bool) -> ? {
    if (flag)
        "abc", vec![(0, 1, 2)]
    else
        "abc"
}

Is it possible to have multiple return path like f3?

rust
1 Answers

You can return an enum:

enum StrOrStrAndVec<'a> {
    Str(&'a str),
    StrAndVec(&'a str, Vec<usize>),
}

fn f3(flag: bool) -> StrOrStrAndVec<'static> {
    if flag {
        StrOrStrAndVec::StrAndVec("abc", vec![0, 1, 2])
    } else {
        StrOrStrAndVec::Str("abc")
    }
}

The either crate simplifies this approach:

use either::*;

fn f3(flag: bool) -> Either<&'static str, (&'static str, Vec<usize>)> {
    if flag {
        Right(("abc", vec![0, 1, 2]))
    } else {
        Left("abc")
    }
}

Or, in this case you can use an Option:

fn f3(flag: bool) -> (&'static str, Option<Vec<usize>>) {
    if flag {
        ("abc", Some(vec![0, 1, 2]))
    } else {
        ("abc", None)
    }
}

However, since Vec::new() doesn't allocate memory, returning an empty Vec is similarly efficient:

fn f3(flag: bool) -> (&'static str, Vec<usize>) {
    if flag {
        ("abc", vec![0, 1, 2])
    } else {
        ("abc", Vec::new())
    }
}

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