try_or_else

try_or_else.rs

#![feature(try_trait_v2)]

use anyhow::Result; // 1.0.57
use core::ops::Try;

trait TryOrElse<T> {
    fn try_or_else<R: Try<Output = T>, F: FnOnce() -> R>(self, f: F) -> R;
}

// Proposed new API for Option https://github.com/rust-lang/libs-team/issues/59
// This would actually use `self` there of course.
impl<T> TryOrElse<T> for Option<T> {
    fn try_or_else<R: Try<Output = T>, F: FnOnce() -> R>(self, f: F) -> R {
        if let Some(v) = self {
            Try::from_output(v)
        } else {
            f()
        }
    }
}

impl<T, E> TryOrElse<T> for Result<T, E> {
    fn try_or_else<R: Try<Output = T>, F: FnOnce() -> R>(self, f: F) -> R {
        match self {
            Ok(t) => R::from_output(t),
            Err(_) => f(),
        }
    }
}

#[test]
fn test_option() -> Result<()> {
    let o = Some("foo".to_string());
    let n: Option<String> = None;

    // Tests with Result; https://github.com/rust-lang/libs-team/issues/59
    assert_eq!(
        o.clone().try_or_else(|| std::fs::read_to_string("/"))?,
        "foo"
    );
    assert_eq!(
        n.clone()
            .try_or_else(|| -> anyhow::Result<_> { Ok("fallback".to_string()) })?,
        "fallback"
    );
    assert!(n.clone().try_or_else(|| anyhow::bail!("oops")).is_err());

    // Tests that replicate or_else https://doc.rust-lang.org/std/option/enum.Option.html#method.or_else

    assert_eq!(
        o.clone().try_or_else(|| Some("fallback".to_string())),
        Some("foo".to_string())
    );
    // Fallback must be lazy
    assert_eq!(
        o.clone()
            .try_or_else(|| -> Option<String> { unreachable!() }),
        Some("foo".to_string())
    );
    assert_eq!(
        n.clone().try_or_else(|| Some("fallback".to_string())),
        Some("fallback".to_string())
    );
    assert_eq!(n.clone().try_or_else(|| None), None);
    Ok(())
}

#[test]
fn test_result() -> Result<()> {
    let s = Ok::<_, std::io::Error>("foo".to_string());
    let e = Err(anyhow::anyhow!("Oops"));

    // Tests with Result; https://github.com/rust-lang/libs-team/issues/59
    assert_eq!(s.try_or_else(|| None), Some("foo".to_string()));
    assert_eq!(e.try_or_else(|| Some("foo")), Some("foo"));
    Ok(())
}