use std::{
    fmt::{Display, Formatter, self},
};

pub struct LazyString<'f, F>
    where F: Fn() -> String + 'f
{
    source: F,
    _lifetime: std::marker::PhantomData<dyn Fn() + 'f>,
}

impl<'f, F> LazyString<'f, F>
    where F: Fn() -> String + 'f
{
    pub fn new(source: F) -> Self {
        LazyString {
            source,
            _lifetime: Default::default(),
        }
    }
}

impl<'f, F> Display for LazyString<'f, F>
    where F: Fn() -> String + 'f
{
    fn fmt(&self, fmt: &mut Formatter) -> fmt::Result {
        let s = (self.source)();
        Display::fmt(&s, fmt)
    }
}

#[test]
fn test_lazy_string() {
    let i = 10;

    let lzstr = LazyString::new(|| format!("the value is {}", i));
    assert_eq!(lzstr.to_string(), "the value is 10");
}