Effing-mad, an effect library for Rust(github.com)

    effect fn read_to_string<R>(reader: &mut R) -> io::Result<String>
    where
        R: effect Read
    {
        let mut string = String::new();
        reader.read_to_string(&mut string).do;  // note the .do here
        string
    }

  fn foo<T,E>(closure: impl FnMut(&T) -> bool | E) -> Option<T> | E

    // very complex and powerful API
    effing_mad::effects! {
        Read {
            fn read_to_string(bug: &mut String) -> Result<usize>;
        }
    }

    #[effectful(Read)]
    fn read_to_string() -> io::Result<String> {
        let mut string = String::new();
        (yield read_to_string(&mut string))?;
        Ok(string)
    }

    let handler = handler! {
        Read {
            read() => reader.read(),
        }
    };

    let action = handle_group(read_to_string(), handler);
    let result = run(action);